Vac Meter, discrete solid state. 2015.
The front panel, using retired 1980s obsolete university gear.
Two more pictures at bottom of page.
In 2015 I wanted to improve the RF performance of my AM kitchen radio
so I needed a Vac meter with wider bandwidth than the 2013 meter which
gave 1.5Hz to 300kHz.
The aluminium case, meter, 3 pole 12 position rotary wafer were found
in excellent condition within some old 1980 test gear rescued from a
rubbish bin at ANU.
Contents of this page:-
Sheets 1 to 8 are schematics of my 2015 Vac meter giving 12 Vac ranges for
0.5Hz to 5MHz :-
0.0Vrms to 1.0mV, 3.2mV, 10mV, 32mV, 100mV, 0.32V, 1.00V, 3.2V, 10.0V, 32V,
SHEET 1 :- Basic block diagram of whole unit.
SHEET 2 :- Rotary wafer switch Sw2A,B,C.
SHEET 3 :- Amp 1, gain = x10.00, +20dB.
SHEET 4 :- Amp 2, gain = x10.00, +20dB.
SHEET 5 :- Emitter follower buffers for CRO, F meter, etc.
SHEET 6 :- Amp 3, gain = 1.0, +/- 0.0dB, meter driver.
SHEET 6A :- Amp 3 basic circuit + explanations of GNFB + rectifiers.
Meter dial :- Image for customized analog meter dial to reduce errors.
SHEET 7 :- Power supply, +/-15Vdc regulated.
Fig 1 :- Passive 1:1 or 10:1 R divider probe for CRO or Vac meter.
Fig 2 :- Passive 10:1 capacitance divider probe for CRO or Vac meter.
Fig 3 :- Active probe for CRO or Vac meter.
SHEET 9 :- Switched R divider for Vac range calibration.
SHEET 9A :- Three useful attenuator switches for calibration.
SHEET 8 :- Bandpass filtering for reducing noise.
SHEET 1 BLOCK DIAGRAM for 2015.
SHEET 1 is the overall picture of main element layout. Many R are not
numbered and 3 amp schematics have been reduced to symbols.
The bypassing of Vdc rails to 0V and chassis case floor and including LC
filters prevents RF instability.
There are three cascaded amplifiers with a huge total amount of open loop
gain exceeding 100,000. Each amp has local or GNFB, and is isolated to
prevent any oscillations. The +/- Vdc rails are well grounded to a very low
Z common path of aluminium floor of the box. I used many 2uF polyester
caps rated for 250V and in white plastic boxes. Xc 2uF = 0.08r at 1MHz.
L3 to L6 = 40uH chokes offer low R between amp rails to maintain regulated
+/-15Vdc. The 40uH + 2uF form filter networks to ensure any Vac above
10kHz at +/-Vdc rails cannot find its way to the next amp to cause oscillations.
Each choke has 5 turns of 0.5mm Cu insulated wire taken from Cat-5 cable
and wound through 2mm bore of ferrite tube 20mm long, to make a toroid choke.
Protection against excessive Vac applied is shown after Sheet-2
Amp1 and Sheet-3 Amp2 schematics below.
Manual Vac range selection is easy for 12 Vac ranges from 1mVac to 320Vac.
After turn on, the unit takes 12 seconds for Vdc rails to fully stabilize. I always
try to select a higher Vac range than the Vac I think may be present.
Measuring above 320Vrms could be a problem if you don't know the peak Vac
with a non sine wave. Most Vac in audio amps are sine waves, square waves,
or triangular, but pulse waves and noise Vac may exceed 1,000V easily.
To measure Vac between 100V and 1,000V requires a resistance divider rated
for Vac and Vdc peak levels. Tube amps create peak Vac + Vdc above 3,000V.
A resistance divider shown on page for 2013
meter will withstand 4,000Vdc
for 5 minutes.
If the meter reads below 0.1 x full swing, better accuracy is gained by switching
to a lower Vac range which will swing the needle higher for easy accurate reading.
If the meter reads full scale, switch up to higher Vac ranges until the meter settles
above 0.1 full swing. The Vac range labels tell you which dial to read, and practice
makes perfect !
SHEET 2. Input switching for Vac meter, 2015.
The Vac ranges are approximately 10dB apart. There are three scales
on the dial plate of a 100mm wide analog meter. (more below).
For non standard R values, you MUST use only 1% metal film in series
or parallel to get correct R within 1% or you get errors exceeding 1%.
Every R value shown allows for all combined loading by other R around
it during use.
Trim caps C2 to C8 could be high V rated with adjust screws for
C = 3pF to 8pF. I used turns of insulated wire from Cat-5 cable wound
around 1mm solid copper poles 15mm long soldered to contacts of switch.
Turns of wire are adjusted for flat sine wave response to 6MHz.
Cin of the meter is determined largely by the rotary wafer switch with
unavoidable C < 20pF.
SHEET 3. Amp 1, gain x 10.
Protection for Amp-1. Accidental HV input damage is limited by UF1004
clamping diodes d1-d6 UF4007 across Q1 gate to source, and from source
to 0V. Amp 1 is used for V ranges 0 - 100mV. The max gate V swing =
+/- 2.1Vpk. If 500V is applied to V ranges 1-5, it is applied across R3 470r,
1/4W, current exceeds 1A so R3 rapidly burns open. R3 needs only 23mA for
1/4W. A 50mA fuse could be fitted to limit heat in R3 to 1.2W, but it would
still fuse open after some time. The fuse and holder must be placed for easy
replacement and not increase Cin.
SHEET 4. AMP 2, 2015.
Amp-2 input has Q1 source follower input buffer with Rin = 2M2 and R3 + C1,
C2 form LF pole 0.09Hz. The 2M2 does not cause significant loading of 1k0
network around Sw2C positions 1-5, or for SW2C positions 6 -12. But R10
on Sheet 2 needs to be trimmed carefully to get correct Vac division.
Q1 2SK369 source has CCS dc feed from Q2 PN100 for high open loop gain.
The Q1 follower isolates the input of following gain amp with Q3 to Q8,
preventing instability. R5 200r is prevents oscillations above 10MHz.
Amp Bandwidth is from 0.25Hz to 6MHz.
Protection for Amp-2.
When using the Vac ranges 1-5, Amp-2 is fed by Vac from output of Amp-1
via switch Sw2C and its resistance divider. The highest normal Vac level into
Amp-2 = 10mVac. But during turn on/off, and during gross overload of Amp-1,
maximum possible Amp-2 input is about +/- 7Vpk. Therefore the high Vpk
swing at Amp-2 input is limited to +/- 2Vpk by UF4007 diodes arranged for
least increase of input C.
For Vac ranges 6-12, Amp-1 is not used.
DUT input is fed from outputs SW2C R dividers to Amp 2 Q1 gate.
The dividers all have 3M0 plus smaller R, with the largest small R = 98k,
for V range 0 - 0.32Vac, position 6c. If 3,000V is accidentally applied to input,
maximum Vac output possibly applied from SW2C at position 6c = 100Vac,
but this is limited to +/-2Vpk by UF4007 around Q1 gate and source to 0V.
Maximum Iac in 3M0 with 3,000V applied = 1.0mA, so heat in 3M0 = 3Watts,
and with 2 x 1M5 in series each 0.5W rated, the R will only fuse if the high
Vac is maintained for some time, which is very unlikely. Use of 3 x 1M0 each
1W would be better, but then I'd have more clutter in a small space.
Q1 source drives Q3+5 bases which are non-inverting input to the gain amp.
The Q3+Q5 are in two parallel differential amps (LTP) with PNP and NPN bjts
to give complementary action and best HF response. I have idle Idc = 5.6mA
in Q3,4,5,6 for high gm and high gain. Differential gain is > 50 with collector
loads of less than 2k2. Q7+Q8 have higher gain in a complementary pair in
common emitter mode.
Amp 2 open loop gain > 12,000 at 500Hz, but reduced to just 10.00 with
62dB GNFB. Q7+Q8 collector outputs are loaded by NFB network R21+R21.
The bottom of R21 is connected to 0V via C8+C9 each 8,200uF in series.
These are 10Vdc rated electrolytics which each need 7.5Vdc by divider with
R16+R16, each 10k0. So the effective C from R21 to 0V = 4,100uF, and the
R22 300r + 4,100uF set an LF amp pole = 0.13Hz. The arrangement gives
excellent Vdc stability.
To prevent inevitable RF oscillations with "uncompensated" high gain amps,
the open loop gain is reduced with C7 trim-cap 9-35p in series with VR1 1k0.
With VR1+C7correctly adjusted, there is no sign of oscillation over 6MHz.
Amp 2 Q7 + Q8 collector output is isolated from other stages with 100r to
inputs of monitoring buffers on Sheet 5 and to Amp-3 input on Sheet 6.
The following amp stages have some shunt C which are likely to cause RF
oscillations. Series R between 100r and 220r are used at input or output to
prevent RF oscillations.
THD and noise is negligible.
SHEET 5. Emitter follower buffers. 2015.
Here are two simple emitter followers directly connected to output of Amp 2.
These allow 2 external devices to be connected to the Vac meter such as
Frequency meter, CRO, or alternative Vac meter. Such devices cannot
affect the working of Amps1,2,3.
Protection. I have FR3004 diodes after output caps C1-4 to +/-15Vdc rails.
Q1+Q2 can only be fused with accidental application of HV to the output
terminals. Excessive Iac or Idc current in R4 or R7 from an external HV
source will fuse them open.
HF F2 > 5MHz, and F1 is determined by C1+R5, 5uF + 330k and loading of
a CRO or other Vac meter etc in parallel. If a CRO Rin = 1M0, R = 240k
and -3dB F1 pole = 0.13Hz.
SHEET 6 Meter Amp 3.
Amp 3 is almost identical to amp 2 but without unnecessary input emitter follower
stage because the previous Amp 2 has low output resistance < 150r.
Amp 3 operation is DIFFICULT to understand.
SHEET 6A. Basic action in Amp 3.
Amp-3 on SHEET 6A is drawn here more simply with triangle symbol used for
amplifier and the two + / - input ports have Rin approx 50k, and the output at
Vo is a collector current source. R&C numbers on 6A are same as for SHEET
6 Amp-3 schematic.
The metering function depends on a basic principle :-
Idc flow from charged C in a full wave rectifier circuit = 0.707 x Ia rms flow
in Vrms source.
Alternating current at output of Amp-3 is applied to a diode bridge after to produce
dc flow in the meter coil R // VR3+R17. The Vac flow in the meter R is reduced to
negligible levels with shunt C18+C19 so only DC is applied to the meter.
The action is much like a full wave PSU rectifier where an AC source charges a C
after a diode bridge, and the resulting Vdc without ripple voltage is applied to an
R load. The C18 value must high at 470uF ripple Vac at low F is very low, and to
prevent meter needle wobble exceeding + / - 10% at 0.5Hz. There is negligible
meter wobble at 5Hz. Amp-3 output Iac flows through NFB network which includes
R15, diode bridge with 1N5711, C18+19 and R16.
For meter full swing, Vac across R16 680r = 100mVrms, so Iac =
Idc in VR3+R17 // meter R = 0.14706mAdc.
Meter R = 1k0 and the adjusted total of VR3+R17 = 3.125k, so total R load for DC
flow = 680r, hence Idc = 0.1Vdc / 680r = 0.14706mAdc. The adjustment of VR3 is
fairly sensitive and you could use 1k0 trim pot + 2k7.
Amp-3 output is from Q5+Q6 collectors which are a virtual current source with
Ro > 50k0. If the output produces + 0.147 mA pk for 0.1Vrms at R16, and if open
loop gain = 10,000, then Vac difference between input ports = 10uV.
The transconductance of the amp is transformed by voltage gain to be about 14A / V.
The Vac across R16 680r is made linear to input Vac by GNFB, 80dB max at 500Hz.
Thus current flow from Q5+6 is controlled accurately by GNFB. Vac across R16 is
almost identical in wave shape to the input Vac at Amp-3 input.
The GNFB Vac at R16 contains THD in current flow with diodes and rectifier.
This is amplified to prevent its creation, so that Amp-3 output voltage is varied to
do whatever is needed to reduce THD at R16. This ensures the Idc flow to meter is
linearly proportional to the Iac rms flow in R16, and that the Vdc applied to meter
tells us the True Vrms value for any Vac input wave form.
With sine waves at both input ports, the wave at collector output appears like
a square wave with verticals = +/- 0.5V approx, top and and bottom horizontals
are curved up and down.
The relationship between Vrms, Voltage Root mean square and Vdc is
explained further at Fig 5, 1/2 way down page at 2013 Vac Meter.
Root mean square is also defined better than I could at
Basic units need to be understood.
The Watt is a current flow = 1 Coulomb per second,
= 6.2415 x 10 to power of 18 electrons, which is called 1.0 Joule.
This is the number of electrons in a 1 Farad capacitor charged to 1 Volt.
Where you have 1 Volt applied to 1 Ohm, I = V / R = 1 Amp. For 1 second,
the work done is 1 Joule, or 1Coulomb per second. So where you have
1 amp of current flowing, there are 6.2415 x 10 to power of 18 electrons
flowing per second.
Electrical power is done at a rate per second measured in Watts :-
Power, Watts = V x I, or V squared / R or I squared x R.
The power generates heat in a resistance, causes motion in
an electric motor, removes heat in a refrigerator, creates sound
in air or water.
Electricity bills have power units in Kilowatt Hours, kWh.
My typical winter bill is 16kWh per day. I day = 24hrs, so each hour the average
power = 16 / 24 = 666.6kWh, ie, there is 666.6W average drawn each hour.
Current = P / V = 666.6 / 240V = 2.78 Amps rms. 667Watts is about equal to
1/3 a 2kW rated room heater, equal to 0.89Horse power, and about 30 times
the average power I generate within myself. Civilization is energy hungry.
A sine wave alternating flow of current must have peak +/- Vac = 1.414V to
provide the same heating power in R as 1.0Vdc. The sine wave V and I can be
expressed in terms of Root mean square which equates the Vac and Iac as
equivalent to Vdc and Idc which will generate the same power in an R, called
RL, Resistance Load.
The peak Vac and peak Iac may vary greatly for any electric flow wave form,
but whatever these V & I values may be, the Vrms and Irms can be measured
using the meter I describe here, and in all meters giving "True Vrms" so the
question in your mind, "What is electricity?" need not ruin your day.
It can be proven mathematically that some simple Vac waves of +/-1V peak
at any constant frequency have Vrms values according to a simple table :-
Square wave, 1.0Vrms = Peak Vac / 1,
Sine wave, 0.707Vrms = Peak Vac / sq.rt 2,
Triangular wave, 0.577Vrms = Peak Vac / sq.rt 3.
Vac wave-forms we measure have have very different shapes and may be
usually measured in Vpk, Vpk-pk, or Vrms. Pink noise signals used for testing
speakers sounds like a rumbly big waterfall, and on the CRO it looks like a very
blurry display because of the constant randomly varying amplitude, frequency
and phase. If we measure pink noise Vac as Vrms, the meter may show slow
Vrms changes due to very low F within the noise causing meter needle to wobble.
The Vrms voltage measurement of Vac will be found to generate the same
heating in a load R as would the same applied DC
Vac or Iac waves may be a series of regularly repeating pulses of varying lengths
of time, and may be seen as a stationary wave on a CRO because of the repeating
triggering time of the CRO. The peak value of Vac or Iac change could be many
times the Vrms value. So peak Vac measurements alone do not tell us how much
continuous power that wave will deliver to a load, only the maximum peak current
Engineers find it useful know the Vpk and Ipk as well as the True Vrms and Irms.
If we can see Vpk for a wave on CRO, we can calculate the peak Iac for a given
load R. We can estimate average Iac from the wave shape and its duration as a fraction
of total time for 1 wave, and work out the power liberated in the R load where that
The Iac flow in a transformer winding feeding a diode rectifier and can be viewed
on a dual trace CRO using both channels in differential mode across a 10r0 in
series with winding end and input to diodes before the reservoir C, if one is used.
Amp-3 open loop gain = 12,000 maximum, reduced to very close to 1.000
between Vac input and top of R16 which feeds the GNFB input port of the amp.
The 82dB of NFB ensures the Vdc applied to the meter remains directly
proportional to the input Vrms, so the meter may be calibrated to read Vrms,
and accuracy is good down to less than 0.1mVrms.
Amp-3 output is from high Z current source of Q5+Q6 collectors. During voltage
measurement, the wave form between collectors and 0V looks like a basic
square wave with curved arches instead of straight horizontals. It looks baffling
until you realize the amp is doing all it has to to make the Vac wave form across
R16 and at at NFB port very close to Vac at input.
The analog meter used for this project was made in Australia before 1985 when
we still made good things. However, although the mechanical quality remains
excellent, the needle movement was not linear to the applied Vdc and errors of up
to 15% at low readings and 8% at middle of scale were found.
I have a section of re-calibration of analog meters below......
In Sheet 6 Amp 3, biasing, dc stability, meter LF pole are dependent on R14 82k,
and C14+16 136uF ( 2 x 68uF NP ). This part of GNFB network seemed to work
better at VLF than for the network in Amp-2. The only disadvantage is that any noise
below 0.2Hz generated in R14 82k is amplified by open loop gain and not corrected fully
by NFB. I saw some very slight CRO trace bounce at low Vac levels.
In Amp-3 meter amp, this does not cause any visible meter needle wobble while reading
Vac for any range. Rin at Q2+4 bases is about 50k so R&C network = 82k//50k + 136uF
= 30k + 136uF so F1 = 0.0376Hz in theory. This is below C+R couplings elsewhere so
meter gives F1 at 0.5Hz.
The 82k is a GNFB path from Q5+Q6 collectors to Q2+Q4 bases to maintain
stable Vdc operation without excessive Vdc offset at output.
Base currents to npn Q2 and pnp Q4 flow in opposite directions and are each
approx 0.25mAdc. Only the difference between base currents flow in 82k, about
0.003mAdc. Across 82k, Vdc < 0.24V. It is ideal to operate Amp 3 with Vo close
to 0Vdc, so VR1 is adjusted so Vdc at output < +/- 10mVdc. With VR1 set, there
is enough voltage gain at DC to keep output of the amp close enough 0Vdc under
No PCBs are used. I built Amp1, 2, and 3 on separate pre-drilled boards about
about 120mm long x 85mm wide with all tracks and terminals between parts using
short lengths of 1mm dia solid copper, formed unto a U using long nose pliers,
pushed through two holes of board, then ends folded flat under the board.
The layout of bjts and R parts are copied from schematics as I show them. It is
always easy to know where you are during later service work.
Leads of R or bjts are surface soldered to wire tracks, and most C are under board
with leads up through holes to tracks. With all larger C under the boards and wire
tracks, there is no clutter in the way of measuring Vac on boards.
Only practice makes a nice looking board. It will not be as neat as a PCB, but
the circuitry complete HF and LF stability. After you have done about 10 boards
you should become skilled and make reliable boards very unlikely to ever develop
dry joints even if the unit is dropped to a hard floor.
Each board is mounted off the metal case floor on 4 x 16mm dia x 35mm long
timber dowel spacers at each corner and fastened with 4 gauge x 16mm long
c/s cupboard hinge wood screws down through board and up through case floor.
This allows easy removal of boards, plus gives a short path for 2uF caps from
Vdc rails to case. Such C are shown on SHEET 7 for PSU and necessary for
NOISE could be a huge problem if you build this Vac meter.
To test for noise generated by the 3 amplifiers, the input must be shorted to 0V
using an RCA male plug with short wire shunt. I tried valiantly to build the unit
without a steel sheet shield around Vac range switch and Amp. Noise was only
low enough for me when put Vac range switch and Amp1 inside an additional steel
sheet metal box inside the Aluminium case.
Then I found the equivalent noise at most sensitive Vac range 0-1mVrms < 10uV,
quite OK considering bandwidth of Vac meter is 0.4Hz to 6MHz. The regulated
rails of PSU help keep very low frequencies so low that a CRO used to monitor
Vac does not show any trace movement when set to DC.
SHEET 7. PSU, earthing, feeds to three amp rails.
Sheet 7 PSU for 2015 has 7815 and 7915 regulators for +/-15Vdc rails for the
3 amps. For noise free Vdc output I found C6+C8 470u+u47, and C12+C14
10,000u+u47 were needed. The Sheet 7 AND Sheet 1 arrangements gave me the
lowest noise at all F when viewing the output of buffers on Sheet 5, with input
shorted to 0V and with most sensitive Vac range selected. With amplifiers
used to measure Vac < 0.3Hz, Vdc rails must have very low LF noise, and
only regulation removes the very low F noise generated by random variations
in mains levels into the PSU.
The output resistance from regulators appears to be < 1r0 and low enough
to prevent Vdc rails moving at VLF.
The 0V rail connects to aluminium case for low frequencies near output of PSU
via R5 180r, and with low value C16 0.1uF. The 0V rail of input RCA socket
plus other points on 0V rail are bypassed to Al case floor with 2uF.
The 0V rail is is a solid 1.2mm dia Cu with total length about 350mm long.
There are several 2uF to 0V from points along 0V rail length to prevent the rail
being a tapped inductance with rising Z at HF. The whole total arrangement
works fine with the metal casing and with shielded+LC filtered IEC mains
Sheet 1 shows additional chokes and caps used on each amp board to ensure
Vdc rails remained free of noise or possible RF oscillations. The chokes are
ferrite tubes 20mm long, 6mm oa dia, with 2mm bore dia. I have 6 turns of
0.5mm Cu dia wire, polythene insulated, from Cat-5 cable, to make a tall toroid
coil and which gave 40uH at 1MHz so XL = 251r. This is far more inductance
than a 100mm long piece of 1.0mm dia wire which has L = 0.17uH, and XL =
1r06 at 1MHz. See the calculator for wire inductance at
Rw is < 0.01r. I might assume the Idc flow does not cause significant lowering
of ferrite choke reactance The arrangement of 40uH plus 2uF gives a low pass
filter with pole approx 18kHz. At 400kHz, 40uH + 2uF give XL = 100r, and XC
= 0.2r, so attenuation = 0.2/100 = x0.002 = -54dB. HF in one amp rail cannot
find its way to another amp rail to cause RF oscillations. The exact route and
cause of RF oscillations in this instrument or any other electronic gear may be
difficult to forecast or analyze or cure so its best to try to isolate each rail for
each amp, and but have the common 0V rail bypassed with 2uF several times
along its length to a very low reactance such the aluminium case floor.
ANALOG METER CALIBRATION.
This is the meter dial for an unknown but better brand of analog multi-meter.
Many people will struggle to read it because its so complex and you can see why
DMM have become so popular since 1985. The thick black curve arching across
the dial is not black, really is a mirror so that the image of the needle should be
hidden behind the needle which means you are looking at meter at 90 degrees
and you read the meter correctly - a correction of "parallax error". A quick Google
of "parallax error meter" will bring up countless analog meter images.
Many analog meters have a linearly drawn dial scale of typically 0 - 100.
The one I have needs 0.1Vdc for full scale at 100. But many will be found to be
inaccurate if checked with Vdc = 5mV, 10mV, 25mV, 50mV, 75mV, 100mV.
I found my meter gave 7% error at 50, and 15% at 10, so I thought I needed
to draw a new scale for dial plate. But did I really need to re-calibrate the dial?
I then thought I better measure Vdc applied to meter by Amp 3 by applying a number
of known accurate Vac inputs using the 0 - 1.0 Vac range setting.
I used 1kHz from my low THD oscillator.
First thing needed is a know 1.00Vrms applied to meter input in 1V range, and making
sure Amp 2 was producing a measured 100mVrms at input to meter Amp 3, and that
Amp 3 then produced whatever Vdc was needed for full swing of meter which needs
the meter installed, and adjustment of VR3 seen below meter in SHEET 6 Amp 3.
Amp 2 gain needs to gave gain close to 10.00, but within +/-5% is OK, and Amp 3
gain is 1.0, and VR3 compensates for any errors, and for varying Vdc needed for
full meter swing, slightly different to the nominal amount in spec sheets.
The best way to produce a number of accurate Vac is to use just ONE reference
Vac then divide it with a switched attenuator use the same brand of resistors of
equal value and 1% tolerance, and low enough value to allow loads down to 100k
be connected without change to each Vac at each switch position.
To check my meters I found a suitable aluminium box 210mm x 100mm x100mm long,
and installed this schematic.....
SHEET 9, 3 Useful attenuator switches :-
The schematic shows 3 rotary wafer switches each 1pole x 12 position, all made
before 1980. Nice big 50mm dia types.
S1 and S2 are for testing ranges of Vac or Vdc in -10dB steps and S3 is for
meter dial plate calibration.
All 3 old switches use contact 12 to feed a rotating disc which is the switch pole
which can point to 11 other contacts, and to 12, so 11 different output voltages are
available. Most modern switches have a separate contact 13 to be pole, and you get
12 output voltages.
Consider S3 first, for meter calibration.
S3 uses R12 to R22 = 13 x 270r metal film x 1% x 0.5W arranged so that when 1.000
Vrms is applied to input, you can get 11 output voltages of 1.0V, 0.9V, 0.8, 0.7,
0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.05V.
To make a new calibrated dial :-
1. Make sure work area is clean, and free of any iron particles ( from drilling, filing etc)
2. Remove meter from its mounts, remove perspex front cover. Measure size of new
dial to equal existing for top, and 2 sides.
3. Cut white cardboard template to be exactly equal to existing dial plate.
4. Adjust zero adjust screw to center position.
5. Slide template behind needle and fix with masking tape at top and two sides.
6. The needle must be free to move without touching template. Cardboard must be flat.
7. Mount meter vertically on unit temporarily without front cover.
8. With meter turned on, and with Vac range at 0-10V, and with RCA input grounded
for low noise, and with no Vdc present across meter, the needle position is drawn
in pencil behind the needle near end of needle and at bottom of template.
Use a finely sharpened HB pencil.
9. Connect meter input to S3 output with setting at lowest 0.05V position.
10. Connect low THD sine wave 400 to 1,000Hz from signal gene with low Z output
less than 600r to S3 input.
11. Turn on signal gene and adjust level at S3 input to 1.000Vrms using a known
reference meter, I use my Fluke 117. (Two other DMM give similar readings for
1.00V, with less than +/- 0.4% difference. But if in real doubt, then build a reference
signal generator with guaranteed output level ( Maybe not so easy ). See 100Hz gene
12. Turn up S3 to give full swing of meter. The Vdc across meter should be close to
the nominal Vdc needed for full swing of meter, in my case, close to 0.1Vdc.
13. Mark needle position '100' behind needle in template at end and at bottom of template.
13. Switch S3 down one position and mark behind needle for '90', and all subsequent
positions down to '5'.
Check all 3 times, and an hour later. use enough pencil marking to ensure they appear
well enough when later scanned in black and white.
14. Remove the meter from its mountings, remove template with care.
15. The template is scanned to make a preview scan, then the smaller dial plate area
scanned at 300dpi, black and white. Make sure the outline of dial plate is accurate,
and shows up with vertical and horizontal boundaries.
I've been using ArcSoft PhotoStudio 2000 since about 2002, and a Cannon scanner
from 2001, both still working better than many others.
16. Save the scanned image as "meter-dial-1-2016" and as .bmp in your relevant
"Test gear" folder which is a sub-folder of your larger "Audio Technical" folder.
( I have hundreds of files in many folders in Audio Tech and I need to be able to find
them later easily.)
16. The scan size may be quite large but may be reduced to get an image to fill about
1/2 height a PC screen in MS Paint when "1x size" is used. Thin lines may be drawn in
black over feint/thick/untidy drawn markings. Save the image as a monochrome .bmp,
and that should replace .bmp with grey pixels.
17. Open IrfanView, open .bmp, and increase canvas size enough to plot position of
needle bearing center. Save in original folder.
18. Re-open image in MS Paint with larger canvas size.
Now comes the real work of drawing up a dial worthy of printing.
The size of image on screen will be much larger than the template, and the x2, x6, x8
function will be needed to create a credible dial plate.
19. Draw vertical line down from 1/2 way across the meter plate lower boundary.
20. Draw at least 4 lines through marked needle positions below lower boundary of template
and to intersect vertical line. 2 radii each side of center line will do. You should find an
"average intersection point CP" on center line, then draw a horizontal through vertical,
and remove mess of other lines. Distance from CP to "end of needle marks" should be the
same, within +/- 2mm.
21. Using a ruler on PC screen, measure from CP to plot scale baseline intersections
for the 2 Vac and single dB scales. Plot curve baseline positions along radii at relevant
distances from CP with small cross using a "dot".
22. The dots can be joined to give a multi-faceted baseline curve, with minimum line width
fill in line so at line size steps there is a thicker line, but not more than 2 pixels wide, or high.
23. Tidy up curves without losing essential voltage positions.
24. Between 1 and 2, divide distance in 1/2, and that will be 1.5. plot a dot near curve baseline
The divide each 0.5 into 5 parts with 4 radii lines so distance between each looks equal.
thicken up the lines, make them say 50mm long at 1 and 2, 40mm long at 0.5, and 30mm
at each 0.1 position. Measure and trim line lengths using a ruler. The process here is
interpolation with negligible errors because we know the needle will be at 1 and 2, and at
1.5 if we adjusted Vac input.
23. The process is continued for 0.0 to 0.5, and 0.5 to 1, and then for 2 to 3 and so on
for the whole 0 - 100 scale. I took 8 hours to get that looking acceptable, that's 4.8
minutes per fine line for scale division.
24. The scale 0-32 was plotted by reading off voltage from 0-100 scale and drawing
radii and 0.1 divisions.
25. The dBV scale is far from linear because its based on logarithmic increase of Vac,
but scale marks are drawn from voltages on 0-100 scale, aided by calculation for
1dB reductions of voltage.
26. Lettering for scales is typed in at whatever size is needed to get dial to look right.
Perhaps you can find a meter scale drawing app which automatically can draw a scale
to suit the markings from a template. But I bet you can only find one to make a linear dial
which we do not want; we want THIS meter to tell us what IT measures, which may be
different to the next meter along.
SHEET 6B. Meter Dial plate.
27. This is the image similar to what I finally ended up with in MS Paint.
It is saved as .gif, and then printed. The dial size on paper will be too large because
the printer tries to fill the A4 page. A measurement of length is taken, and size
ratio to real template length calculated.
The overall image size in IrfanView is adjusted by the ratio, and is saved, and then
this printed sheet should show the dial details much smaller, but the same size as
the template, and this can be confirmed if template is laid over image.
28. The image is trimmed to outside boundaries with scissors, and taped to existing
dial plate, and the meter used to check accuracy with varied Vac from S3.
29. I found all was well when I measured, scale had errors < 1% at all meter positions
Tests in several Vac ranges gave less than 1% error of reading, and was better
than all other analog meters I have used or made.
Amp-3 in Sheet 6 has bandwidth of 0.2Hz to 6MHz with GNFB. Meter F response
gives readings -3dB at 0.5Hz and 6MHz.
Checking other Vac ranges.
In SHEET 9, S2 is a Vac attenuator with the same -10dB Vac steps as I have in my
two analog meter Its Rin to R1 = 100k, so the switch has little loading effect on most
signal generator sources. If your generator has Rout < 600r, the loading effect is
negligible. However, there are no C parallel to each R to compensate for HF
losses above 2kHz which are inevitable due to stray circuit C and shunt C of anything
connected to the switch output such as a DVM with high input C well above 100pF.
If the signal source Ro = 500r, the S2 Rout at position 11 is 50r. But at 10 Rout becomes
the maximum possible of 22k. If a meter input resistance = 1M0, then the measured
Vrms at 10 will be 2.3% below the theoretically correct calculated voltage expected.
Rout decreases for lower S2 positions, but divider resistances are a little too high, but
at least the R won't cook if Vin = 320Vrms
S1 is used to switch Vin to 7 positions on S2, thus allowing selection of Rin to be 100k,
32k, 10k, 3k2, 1k0, 320r, 100r. Most signal generators can produce 1.000Vrms
into a 320r load and with wide bandwidth without needing C compensation. with each
step down of Rin, the number of S2 positions reduces and with Rin at 320r, only 6
selectable output levels are available, but this allows loss free testing for lower voltage
levels without noise.
I've used old type rotary 12 position switches but more modern types will have the
switch pole at a "13th terminal" and then the input to position 12 can be 320Vrms with
1.00mV read at position 1 all without cooking anything. If meter Rin = 1M0, a calculation
is needed to correct output at position 11, and I am sure brilliant readers will allow for
My meter here is capable of safely measuring up to 320Vrms, which for sine waves means
+/- 453.3Vpk. This is in dangerous territory for any technician, and may challenge RCA
leads or other coaxial cables. If you must play around with more than 100Vrms anywhere,
make sure you know what you are doing before and during and after.
Some DIYers might just draw up a dial template with pencil and ruler,
and then go over it with black ink pen and ruler, and all including interpolated fine
division marks each fraction of a Volt.
The template can be erased to remove pencil lines, leaving only ink marks.
This is easy, but most ppl end up looking at a mess they should have
done on a PC in a drawing program.
Voltmeter probes and cables.
The simplest voltmeter probes have 2 x 1 meter long read and black cables, well
insulated against a maximum of 3,000Vpk and with very flexible multi strand wire,
with shrouded 4mm plugs one end and plastic probe handles with 2mm dia pointed
metal probes for DUT end.
All DMM are sold with these leads which I found to be safe enough until constant
use fatigues wires and cracks the plastic insulation. Beware the old meter probe
which gets very bitey if measuring +600Vdc.
These cables are very prone to high RF pick up and hum, but are OK for everything
from DC to 1kHz where DUT circuit R < 10k0, and signals are > 10mVac.
Many DMM can only read Vac down to 10Hz and up to only 1kHz if the DUT has
high circuit resistance so that DMM input C shunts signals above 2kHz which
may be the -3dB F2. Most DMM have high Rin usually > 5M0.
For measuring 0.5Hz to 6MHz for low level signals down to 1mVac, the meter
cabling should resemble good probe leads used for a CRO, ie, use coax cable
with good shielding and low shunt C. But the best coax cable has 33pF per meter,
and if meter Cin = 32pF like many CRO then minimum C shunt when probing = 65pF
with a 1M probe lead. Many are 1.5M. so C shunt = 82pF at least.
To get -3dB F2 = 6MHz, and if Cshunt = 82pF, DUT circuit Z should be 320r.
The best coax locally available which will last years without breaking the inner wires
is is RG58CU. It has 67pF per metre, and at least a metre is needed for most
general work to reach between DUT and Vac meter or CRO. With 1M cable
and CRO, total C = 100pF, and DUT circuit impedance should be no more
than 250r to get -3dB F2 = 6MHz.
For measuring an anode circuit of EF86 where R = 100k, and probe Cin = 100pF,
F2 = 16kHz; the act of measuring reduces the working HF Vac, and may cause a
power amp circuit with NFB to oscillate badly at HF. So high shunt C and low
probe input resistance needs to be avoided like the plague.
For most audio work with circuit R < 10k0, RG58CU has good shielding and F2
Standard coaxial cable properties are listed at
The cable with lowest C is RG79A with 10pF per foot, or 33pF per meter.
To avoid the high capacitance of coax cable, a CRO probe with a resistance
divider with capacitor compensation allowing two switchable output levels may
Most switchable CRO probes have a 1:1 ratio where there is no R divider in the
circuit. At the CRO, Rin = 1M0 in parallel with 32pF. If the CRO cable has
C = 67pF, the total 1:1 probe has Zin = 1M0 // 100pF.
When 10:1 ratio is selected, a 9M0 is switched in series with cable output.
A trimmer C = 4-20pF shunts the 9M0 and is adjusted for 1/9 of the Cin to cable
and CRO. This means the probe input C = 10pF plus any C between probe tip
to probe case if used. Some probes have along probe 50mm long which will
allow too much RF entry for low level work. Good probes have a short probe end
and metal case extending out to shield 9M0 and which can be connected to DUT
0V rail or case with a short lead. So many 10:1 probes have 10:1 Cin about 15pF.
Notice that the HF cut off is much higher for low probe Cin.
If a Vac meter input circuit is set up the same as a CRO, many standard CRO
probes may be used for measuring Vac. My Vac meter described here has Rin = 3M0
which can be switched down to 1M0. Cin = 20pF so my Vac meter can be used with
many available CRO probes of 1:1 or 10:1.
Many DMM and other Vac meters have high Rin < 5M0, and high Cin of perhaps
1,000pF and CRO probes are NOT suitable.
Most CRO probes have input voltage rating equal to the CRO, often 600V peak,
or 424Vrms sine waves, and for both switched positions of 10:1 or 1:1.
The 10:1 probe reduces DUT Vac to 1/10 at Vac meter or CRO. My most
sensitive Vac range is 0-1.0mVrms, so the 10:1 probe is not very useful for
DUT Vac < 10mV. Most analysis of circuits are done while measuring Vac > 10mV.
This is OK because the SNR will be better.
Fig 1. Switchable 1:1 or 10:1 Passive CRO probe.
This is a typical 10:1 switched CRO probe used with a generic CRO input circuit
with generic Zin = 1M0 bypassed with 32pF.
I made a non switched 10:1 probe with shielded metal case made from tin plated
steel sheet from olive oil cans. It has the above schematic, but without Sw1.
The probe case is 21mm dia tube 100mm long, capped at both ends with
smaller tube 6mm dia projecting 25mm over the 1.2mm dia wire probe tip, with
about 1mm insulation. This allows reaching to most DUT test points. Shielding is
better than for other manufactured switched probes. The output cable is 1M of
RG58CU with C = 67pF. My CRO has Cin = 33pF, so total Cin to cable = 100pF.
The C1 is adjusted for about 11pF and total Cin to probe is about 15pF. C1 is
adjusted for best square wave for all F between 1kHz and 1MHz, using a flat
sig gene Vac source with Ro < 200r.
With a wide band CRO probe you may find LF noise such as hum or audio Vac
will interfere with low level Vac above 10kHz.
The alternative way to measure or view Vac is to adopt the principle of using
TWO frequency bands, one from DC to 10kHz, and the other from 10kHz to
above the limits of the Vac meter or CRO. The lower can be done with 1:1
probe with high Cin = 100pF, or 10:1 probe with Cin = 15pF.
But as F rises, the reactance of C reduces and can affect viewing waves or
measurements because of loading the DUT circuit Z.
A Capacitance Divider 10:1 probe the best for HF viewing or measurements.
This is an even simpler type of probe and without a switch or 9M0, but with C1
set to give 10:1 ratio at say 1MHz. Without the 9M0, probe bandwidth is
reduced at LF so that F1 pole is at 1.6kHz so that noise below 160Hz is
reduced -20dB below the HF level. 50Hz hum is reduced -30dB, and VLF
trace movement at 1.6Hz or meter wobble is reduced 40dB.
Fig 2. Capacitance divider probe.
C1 for above probe needs to have Vdc rating of 1,000Vdc if possible.
The circuit gives a fixed Vac ratio division above 10kHz if the Rin to a CR0 or Vac
meter is 1M0, and total Cin is between 50pF and 150pF. This extremely simple
probe allows you to measure all Vac in an old radio without much de-tuning of
LC circuits or losses which will alter AVC bias in AM radios where there can
be low level input between 455kHz and 2.2MHz, but also subject to LF noise
in AVC circuit. This probe is very easily made as a metal tube extension to an
RCA male plug with a trim cap inside and access hole to screw adjust.
This allows the 10:1 ratio to be adjusted for 1MHz, and you should find
10:1 ratio remains for all F between 10kHz and 10MHz.
To avoid high Cin and Vac level loss, an active probe may be used with j-fet
& bjt in a high Z input emitter follower.
Fig 2. Active Probe.
One would hope this is better than any passive probe for Vac less than +/- 10Vpeak,
or 7Vrms sine waves. It can be "easily made" with small circuit board 18mm x
100mm long which can slide into 20mm dia steel tube. Cin will be total of 10pF,
with 5pF at Q1 gate plus 5pF to shielding.
The bandwidth should be 0.34Hz to above 6MHz. With input network = 1M0 // 10pF,
Zin at 1.59MHz = 10k0, so the probe is good for all audio work. You should be
able to measure a 1mV signal at DUT if the noise is low enough to permit it.
To exclude LF noise, F1 LF pole is raised by reducing C1 from 0.47uF to whatever
value you choose. If you want F1 = 10Hz, C1 = 0.016uF, and for 1kHz, C = 160pF.
Using C1 = 10pF, total input C = 15pF approx, so F1 = 10.6kHz, and good for
measuring low level RF without the amplitude reduction of a 10:1 probe.
Measuring Vac at any high resistance anode circuit at HF is affected by C of a
probe. The anode circuit of an EF86 may have R = 100k and shunt C between
anode and all other things = 10pF, giving F2 = 159kHz with no meter or CRO
probe connected. If the meter probe C = 10pF, then total shunt C = 20pF and
F2 = 80kHz.
To avoid HF attenuation at anode, you can connect 1k0 between B+ rail and 100k
anode load, then measure Vac across the 1k0. If probe Cin = 10pF, F2 pole is
15.9MHz, and if probe C = 100pF, F2 = 1.59MHz, so the working F response
at anode is not disturbed while you probe it. But this means you have an effective
100:1 probe, so Vac at anode should be at least 1Vrms.
Oscilloscope probes might be purchased.
In Sheet 2 above, I have Sw1 to switch in 1M5 from input to 0V to reduce
max Rin from 3M0 to 1M0 which is the same as both my CROs.
Trying to measure low Vac from an MC phono cartridge using a test record may
be difficult. A Denon MC 103DL has rated output of 0.4mV at 1kHz, with 0.04mV
at 20Hz, 4mV at 20kHz - if RIAA reverse EQ has been applied for cutting grooves.
It is better to use a low noise j-fet phono preamp to increase all F from 20Hz to
20kHz while equalizing relative F levels with RIAA EQ network. If the network is
accurate, and the recorded signal has had accurate reverse RIAA EQ, and the
cartridge response is flat, you should see a flat sine wave response from 20Hz
to 10kHz with -3dB poles just outside these Fo. Three things have to be correct
before you can say the cartridge is GOOD. Deviations from the flat may tell you
about a cartridge. Testing 3 or 4 different MC and MM carts tells you more, and
all will vary slightly, and possibly color the sound like a graphic equalizer using
unknown random settings. My pages on preamps tells you more. Making a phono
preamp for testing is not difficult if a kit with op-amp (OPA2134PA ) & NFB RIAA
is used. Noise can be a problem, and will test your abilities.
Many audio amps may have noise >2.5mVac with no signal present. It is usually
mains related harmonics of 50Hz, 100Hz, 150Hz and 200Hz, plus diode switching
pulses at 100Hz plus hiss or rumble from noisy input devices. Using the low Vac
ranges and a CRO, you can see the truth about amp noise.
To avoid noise above or below the F band you wish to measure, a bandpass filter
(BPF) is connected between DUT and meter or CRO input. This may alter DUT
behaviour and lessen Vac you wish to measure so a simple alternative is to place
an active BPF between Amp-2 output and input to Amp-3 meter amp and buffers
for the CRO.
SHEET 8, BPF, 320Hz to 32kHz.
The bandwidth as shown is 320Hz to 32kHz and excludes most mains related
harmonics, diode noise, and RF noise. If there is radio station RF pick which
is converted to audio by DUT, you may see the AF on CRO with BPF, without
other signals present.
The BPF allows a clearer view of a test signals between 1kHz and 20kHz,
measure low level signals more easily without noise. Noise in an audio amp may
be 50 times higher with no GNFB connected than when GNFB is connected.
Sw1 allows BPF switched in or out so a comparison may be made with BPF
or without BPF.
Additional switching could be used to alter F1 and F2 by altering R&C
values in filter.
Simple bjt emitter followers will produce low enough noise and THD and
produce BW wider than op-amps.
In my page on THD measurement I show use of an LC bridged T notch filter
to remove 1kHz from sample Vac from an audio amp output. This allows
inspection and measurement of THD and noise. At low level signals a switched
hum filter allows me to remove H below 320Hz. At very low levels with THD < 0.1%,
I use op-amps to amplify the THD signal x10 and I have a BPF to pass all HD
between 2kHz and 11kHz so THD of 1kHz may be seen and measured without
too much noise. Where you have more than one F present in any Vac, Total
Vrms = Sq.root of the sum of Vrms squared of each F. If you have 0.1Vrms
2kHz, and 0.033Vrms of 3kHz, total Vrms = sq.rt ( 0.01 + 0.0011 ) = 0.1054Vrms,
so you can see how two Vac with 3:1 amplitude ratio make very little difference
to the Vrms measurement of the largest Vac. If you have 1.0Vrms of 1kHz, and
THD = 10% = 0.1Vrms, total Vrms = 1.005Vrms.
If DUT is high, you should try to eliminate it before making serious measurements.
I lost count of how many audio amps I had to fix or modify before being able to
measure them properly. If noise < 1mVrms, then measuring a 10mVrms test signal
is easy. Most Vac meters will struggle to measure THD signals < 1mV.
But a CRO is useful for measuring below 10mV. I have taped a 1-10mvrms scale
beside CRO screen to allow measurement when using the most sensitive CRO
Vac range. My CROs also have useful switchable amps for x5 or x10.
Well shielded probe cable is essential for wide bandwidth Vac at low levels.
12mm of unshielded probe wire length may allow RF noise pick up to exceed the
signal level you want to measure. Probing a superhet radio near the input stages
may pick up the oscillator signal which obscures the wanted RF signal.
Magnetically induced pick up is not prevented by non ferrous cable shields or
If your CRO has 15MHz bandwidth, then a high level of 20MHz oscillation will be
seen on the CRO as a wide blurry line which the CRO is unable to display as a
wave form. A 250MHz CRO would have no trouble displaying any wave up to 2
50MHz, but usually only if DUT circuit resistance < 50r. Most DIY audio enthusiasts
will not have to deal with anything above 300kHz. But unwanted oscillations up to
100MHz do occur in gear you have made or you have to repair. When I first used
a 2SK369 + triode for a cascode input stage in MC amp, the circuit oscillated
above 20MHz. The measurements of audio Vac and Vdc seemed odd.
The circuit layout included unintended L and C elements forming RF resonant LC
networks. What appeared to be only an audio amp was also an RF oscillator.
Presence of high RF oscillations may be impossible to see on a CRO but their
presence may become obvious by just touching 0V points with a short lead
to the metal chassis/case. The DUT output should be connected to an audio
amp and speaker set for low levels. The touching of points along the 0V rail
with screw driver, or shunting of 0V points to chassis with short wires should
be always inaudible. But if you hear a click during a touch or shunt procedure,
it is because the RF ceases or starts which causes a rapid Vdc change which
is heard as an audible click. The act of measurement of Vo from an wide BW
signal amp will often start HF oscillations because the added 100pF from probe
lead causes 90 degrees phase shift at HF so NFB becomes PFB and it oscillates.
Usually, using a 220r added in series to probe with 100pF prevents the phase
shift at the amp, so no oscillations. The probe F2 pole is 7.2MHz, allowing
high enough F measurement, but loading at 7.2MHz is only 308r. For Vdc
measurement, a 47k resistor between probe end and DUT prevents any
shunt C affecting the circuit.
All coax cabling or cables with a parallel pair of wires have properties not easily
understood because each conductor has inductance and there is distributed
capacitance between conductors along the cable length. This means long lengths
of cables act as "transmission lines" - and you need to Google more about them
because I don't have time to define and explain everything. But short lengths of
coax cable used for probe leads or audio interconnect cables can be considered
to have low inductance, and low resistance, and simple shunt capacitance
between inner wire and outer shielding. Used carelessly, coax cable C can cause
phase shift and oscillations at HF.
Coax cable bandwidth depends on the source impedance feeding the cable input
and the terminating impedance and cable length. Without providing more info about
coax cable properties, you might assume that the lower the source R and
termination R become, the wider the bandwidth. Coax cable is designed for a
"transmission line" and cable losses per 100 metres may be quoted but the
properties are only valid when source and load resistances are 50r or 75r,
and you have cable lengths > 2 metres.
Coax cable data is not relevant to a DIY enthusiast trying to fix an old radio with
fairly high circuit Z throughout, and using the very minimum of test gear.
The BEST description of basic oscilloscope probe properties is at :-
There is much which may be applied to measuring Vac.
Aluminium top cover off, and steel cover off top left box with Amp 1, and input range switch.
Inside the Amp1 box with range switch. It is a bit messy, but is typical DIY wiring
with discrete parts, and final result is after many variations to overcome
many problems to get optimum results.
Back to Education and DIY directory.
Back to Index page.
Perhaps these tables are useful :-
|Vac scale 0-100 Read off|
|Vac scale 0-32 Draw|
|Vrms scale 0-100.0|
|dB scale |
-21 to +3
|DUT Circuit R|
|F2 -3dB 100pF|
|F2 -3dB 15pF|