Measuring THD in
amplifiers, January 2013.
This page contains :-
Description of gear used for
How to use the gear.
Sheet 1, Block diagram of THD
measurement of amplifier,
Sheet 2, Oscillator, 1kHz Wien
Bridge, 0.004% THD,
Sheet 3, attenuator and buffer
and filter after 1kHz oscillator,
Sheet 4, L&C Bridged T
notch filter for nulling 1kHz, THD amp and filters.
Sheet 5, Hi Zin buffer for use
with hi Z source to be tested.
Sheet 6, THD Measuring unit
Using the test
How do we measure THD in
amplifiers? For the last 13 years I have evolved
my own design of an all analog
set of schematics which you may find useful
if you have a few spare weeks
to build a good measurement tool.
The power supply is not shown
because many of you will have a different spare
transformer than I used. I used
a 10VA transformer with 250Vrms mains input
and 12V-0-12V secondary. The
transformer and its diodes and first reservoir
capacitors were mounted in a
steel sheet cubic box with side dimension = 130mm.
This was screwed to the old
book case where my test gear lives, and about 700mm away
from the box for the
instrument. This box has a 1mm steel sheet bottom which is
on front to make a steel front
panel 130mm high and also turned up on 3 other sides
16mm. A 16mm MDF board is
screwed to steel sheet bottom to allow my DIY boards
to be screwed to the board. The
top of the box is 10mm plywood, with lining of Al-foil
glued on with silicone so when
the box top is screwed to base there is total electrostatic
shielding of all internals.
There is no complete magnetic shielding because it would affect
the bridged T LC notch filter
too much. The coil involved is prone to stray magnetic fields
so equipment nearby with mains
transformers should be turned off, and the amp being
tested or anything else likely
to cause interference be located at least 700mm away from the
All Vdc voltage rails must be
capable of 30mAdc at least and be regulated.
The required rails are +22vdc,
-22Vdc, +16vdc, -16Vdc.
Wires from rails to boards
should be twisted pairs from + and - electrolytic cap rails,
and at each board there is
additional RC filtering with at least 150r + 2,200 uF to
prevent cross talk between
different sections of the instrument. In addition to the electro
caps, 2uF caps are placed close
to op-amps as possible and between rail entry and a
nearby 0V rail. The 0V rail for
the instrument is a short 100mm Cu wire behind the front
panel and it is connected to
the chassis and metal shield of casing via 270r bypassed with
I found there was negligible
hum noise entry via capacitance between power transformer
primary winding and the
Keener constructors would
arrange the PSU so that it acts to charge up batteries while the
unit is turned off, and when
the unit is switched on, AC power and 0V rail is entirely
so that the 0V rail can float
freely and the unit used to measure differentially between two
signal points each with a
common signal. I've managed to never need to do this for THD
measurements, but where there
was a balanced output from an amp it could be useful.
The instrument does not have
its case connected to the mains Earth because of risk of noise
entry, but while used, the 0V
coaxial RCA cables used for tests will refer the test signal 0V
the 0V rail of amp.
Vigilance is needed to ensure
that there is adequate grounding which ever way any gear
is to be tested. Despite
theoretical correct practice, there can always be noise in
used for THD and other
Basic diagram of gear used for
Details of the 1kHz oscillator
Details of the attenuator,
buffer and filter following the 1kHz oscillator :-
The buffer and filter keeps the
input of any device under test well separated from the
C1 and VR1 and VR2 form a
simple 6dB/octave HPF with pole = 160Hz, thus reducing hum
The following LPF has
R12&C2 slightly variable pole for about 20kHz at 0.0dB.
The LPF with R&C between
the op-amps gives -3dB at 1.0kHz and then -18dB/octave
attenuation so that oscillator
THD is reduced to 0.0005%. I had tried to use air cored
L and C for a NFB path for
attenuation of harmonics in oscillator signal but I found the
stray magnetic coupling between
the bridged T LC notch filter and LC filter could not be
reduced to negligible levels
and best LPF is the one above.
The above shows the air cored
inductance with CT used for the bridged T notch filter to
reduce the 1kHz signal in
sample tested signal to about -100dB.
Sample signal from an amp may
be up to 100Vrms and LC filter Rin = 5k0, so the filter has
little loading effect on
A high impedance buffer input
is shown in Sheet 5 below so that higher impedance anode
circuits may be tested with
For high level sample signals,
THD is usually easily viewed and measured using an
oscilloscope and millivolt
meter without any following amp or filtering to make the THD
more easily viewed in an
oscilloscope if the THD is at a very low level.
For testing low level signal
from any amp, say 1Vrms, the X10 amp above amplifies the
recovered THD and filters
unwanted signal and noise below 1.4kHz and above 14kHz,
so that it becomes much easier
to view and measure harmonics between 2kHz and 12kHz.
The Hi Zin buffer :-
The buffer has 2 x HPF with C1
and input R of VR = 400k approx, and C2 and R12,
giving a pole at about 4Hz.
LPF with R13 and C3 give a
variable pole to keep out RF. R13 also offers some series
R to avoid excessive input
current to 2SK369 gate and 1N4148 and 1N4007 act to clamp
gate voltage to less than +/-
17Vpk. So some protection exists for the delicate but low noise
j-fet, 2SK369, and its active
CCS with a BC559.
Distortion of this buffer stage
was found to be negligible, and the j-fet drain supply voltage
bootstrapped to op-amp output.
Layout and size of the unit of
the front panel is flexible, and you may think of a better way
that I have.
Internal Box dimensions = 310mm
wide, 340mm front to back, and 125mm high.
Box material = sheet steel
bottom and front panel, sides, top and rear = 10mm ply lined
with AL foil,
grounded to steel bottom.
Layout of the front panel
controls, sockets and switches :-
HOW TO USE
THE THD MEASURING UNIT.
Turn on unit at mains.
Adjust Fo to centre position
for very close to 1.0kHz.
Adjust fine Vo level at centre
Adjust Course Vo level at
Adjust Hi Zin Level to minimum.
If measuring audio amp output
meant for 8 ohm load, connect RCA lead red active lead
end with alligator clip to
active load voltage.
Connect black 0V coax lead clip
to 0V terminal of amp near input RCA socket.
Turn HiZ-LoZ switch towards Lo
Zin RCA socket, plug in RCA lead from amp load.
Connect RCA lead from DUT (
Device Under Test ) sample to CRO.
Connect RCA lead from Output
THD RCA socket to CRO.
Connect RCA lead from Output
THD RCA socket to Vac voltmeter capable of measuring 2Hz to
and at least 0-1mV range to
Clip an additional Vac meter
across amp load to measure load output voltage, at least up to
Raise Course Level of Vo
Oscillator signal until amp just goes over clip level.
Adjust Fine Level of Vo so amp
Vo is just under clipping seen on CRO, ie, no flats on sine
Adjust CRO Level and Vac meter
level to view and measure THD output. It will seem to be a high
voltage near the amp sample
Adjust Course Notch Filter Null
knob in either direction to reduce THD levels seen.
This should reduce viewed
voltages by at least -20dB. Try adjusting both Course and Fine
a deep as possible Null of the
large signal present which will be 1kHz. You will need to adjust
VM to view and measure THD as
nulling of 1 kHz continues. As 1kHz is reduced, you may see
the wave form become distorted
and see a hum signal appear as the 1kHz is removed, but
leaving behind the amp
distortion and hum or other noise.
Adjust the Oscillator Fine Fo
adjust knob and Notch Filter knobs to achieve the lowest
value of 1kHz present. At this
time, hum levels from power supply artifacts may be greater than
THD and thus invalidate the THD
measurement. Turn the Hum switch to No, and you should see
any signals below 300Hz be
Let us suppose you have amp
load voltage at just under clipping = 15.0Vrms. Let us also
that this isn't a high enough
voltage to overload any meter or CRO input and give a false
The DUT sample voltage when LoZ
input is used will be the same as the load voltage. But
where the amp makes say 50Vrms,
it will be better to use the HiZ input, and turn down the level
input to a lower convenient
voltage, say -12dB, ie, 12.5Vrms.
The reduced level of 12.5Vrms
becomes the DUT sample signal.
Let us suppose the measured and
viewed THD has had the 1kHz nulled maximally.
Where this is seen on the CRO,
you should also achieve a minimum THD measurement on the VM.
There may still be HF noise in
the form of pulses at a rate of 100Hz caused by PSU diode noise
entering the amp signal path
somewhere. The VM will tell you an incorrect reading for THD if
are above the THD level. With
the CRO set for most sensitive position, say 0-10mV, you can
the voltages below 10mV on a
piece of masking tape beside the CRO screen, and even with
pulses or noise the
levels of THD can be read off
within the noise. Don't always rely on what meters say because
gives you a visual picture that
you far more than the meter.
Let us suppose you have
measured the THD at 0.1Vrms. It will most commonly appear as a
ragged wave form
that appears to have perhaps
several frequencies present, usually dominated by 2H and 3H with
difficult to estimate. To
measure levels of each harmonic requires the use of an
additional filter unit not
in this web-page.
But the total of all harmonics
is deemed to be 0.1Vrms.
The THD percentage is
calculated as Dn% = 100 x THD Vrms / DUT Sample Vrms.
In this case, Dn% = 100 x 0.1V
/ 15.0V = 0.66%.
This may be a very good reading
for a PP triode amp before any other loop NFB is applied.
Just finding out the THD at
just under clipping does not tell us what we should want to know
about the amp.
We should want to know what THD
levels are below clipping.
We have so far set up the amp
with a signal input that produces a level just under clipping.
This is the 0.0dB REFERENCE
Adjust the 1kHz oscillator
Level switch down one click and output level should be 0.7 x
15Vrms at the
onset of 0.0dB clipping level.
So Vsample may be 10.5Vrms.
Adjust all 3 nulling knobs for
lowest THD and RECORD YOUR MEASUREMENT IN AN EXERCISE BOOK.
Adjust the 1kHz down another
click for -6dB and Vsample should be 7.5Vrms.
Re-adjust for deepest null,
measure and record it.
Continue down to say 3Vrms and
you may find its difficult to see the THD on the CRO or measure
So turn the switch to THD x 10,
and this amplifies the THD to read easier on CRO, and measure,
BUT YOU NEED TO DIVIDE ALL
MEASURED THD BY 10.
Most class A tube amps have THD
% levels that decrease from 0.0dB in proportion to output load
So if you measure 0.66% at
15Vrms, then at 1.5Vrms, you may find THD = 0.066%.
This means the actual voltage
of the THD = 0.99mVrms, a rather small voltage. If 20dB of
global NFB is
also used with the amp
mentioned here, THD at 1.5Vrms output may be 0.0066%, and THD
= 0.1mV, and THIS IS
DIFFICULT for the average man to measure properly, unless
he has exceedingly
good measurement equipment.
Most very good power amplifiers
have noise levels < 0.25mV under following conditions :-
No signal input present,
Input terminal shorted to 0V
Preamps should have lower levels
of noise at output under the same condition.
Noise is unavoidable in all
amplifiers, but can always be minimized by careful choice of
choice of their operating
conditions, and resistance values, especially at input stages of
Noise originating from power
supplies must always be eliminated with careful design,
placement, positioning, and
Noise should not increase beyond
the low noise measured at the idle condition.
Noise from a power amp may be
expected to measure 0.25mV, and when viewed on the CRO, it looks
AM radio signals, plus some
mains F and harmonics plus diode switching noise. 0.25mV of
noise is inaudible
with average sensitivity
speakers, but often would be audible with headphones. Thus
on power amps often have a
resistance divider to reduce the 8 ohm levels by say at least
This means the the headphone
noise could be less than 0.05mV, and quiet enough.
The amp signal needs to be
raised by +15dB above
the headphone level, but this
will still always be a very low level. The 8 ohm speakers are
when phones are used and the
amp load is usually just the resistance divider for phones, say
which usually halves amount of
THD produced at all levels.
Therefore headphone use usually
gives the lowest amplifier distortion possible.
The use of an exercise book
allows the recorded THD figures to be used to draw a graph of V0
I often prepare THD graphs for
loads 2, 3, 4, 5, 6 , 7, 8, 10, 12, 14, 16 24, 32 ohms. Making
and graphs for each load is a lot of work, but only then does it become clear
what are the effects of
various loads on THD. One should conclude that it is always wise
to never use a load value that is lower
than the nominal load value specified for the amp.
The higher the THD, the higher
the Intermodulation Products will be.
Suppose you have a class A tube
amp with bandwidth at near clipping 0.0dB level from say 14hz to
without any global NFB, and with
THD = 3%, and using RL = 70% of the nominal speaker load value,
and the maximum power level of
the amp is more than 15 times the maximum average power you
If you apply say say 20dB GNFB,
then it usually sounds very well. Bandwidth with GNFB is
GNFB and stability may be
threatened so it is always wise to tailor the phase shift and
gain of the input stages
so that additional BW in excess
of 7Hz to 65kHz is impossible even with GNFB.
20dB of GNFB should reduce 3% of
THD to 0.3% at 0.0dB level and at 1/15 of full PO, output
voltage, Vo will
be reduced by 1/3.9, or about
0.25 of full output voltage. THD will usually be found to be
reduced in proportion to
Vo, so expect 0.075% at 1/15 of
full PO. An amp making 15 Watts at 0.0dB clipping should achieve
0.075% THD at 1 Watt, or less.
Providing there is no slewing of
sine waves and THD does not exceed 1% between 30Hz and 20kHz at
there will be no point to
measure IMD and other artifacts, providing also there is no HF
or LF instability or
OPT saturation effects,
regardless of whether a load is used or if the load is a pure C
or L of any value
which does not cause
overloading. Most amplifiers should be able to work with a pure
C load = 1uF
at 20kHz or pure L load = 80mH
at 20Hz, if the OPT output is meant for 5 ohms. Most amplifiers
will not produce low THD at 1kHz
and 0.0dB level if load = 100uF, or 0.1mH.
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