Measuring THD in amplifiers, January 2013.

This page contains :-
Description of gear used for THD measurement.
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 front panel. 
Using the test gear.

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 turned up
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 1uF.
I found there was negligible hum noise entry via capacitance between power transformer
primary winding and the secondary winding.

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 disconnected
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 rail
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 voltage samples
used for THD and other measurements.

Basic diagram of gear used for THD :-


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 oscillator.
C1 and VR1 and VR2 form a simple 6dB/octave HPF with pole = 160Hz, thus reducing hum
from oscillator.
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 amplifier outputs.
A high impedance buffer input is shown in Sheet 5 below so that higher impedance anode
circuits may be tested with minimal loading.
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 is

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 :-


Turn on unit at mains.
Adjust Fo to centre position for very close to 1.0kHz.
Adjust fine Vo level at centre position.
Adjust Course Vo level at minimum.
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 500kHz,
and at least 0-1mV range to 0-10V range.

Clip an additional Vac meter across amp load to measure load output voltage, at least up to 1kHz.

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 wave.
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 voltage level.
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 to achieve
a deep as possible Null of the large signal present which will be 1kHz. You will need to adjust CRO and
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 possible
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 largely reduced.

Let us suppose you have amp load voltage at just under clipping = 15.0Vrms. Let us also suppose

that this isn't a high enough voltage to overload any meter or CRO input and give a false reading.
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 of
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 the pulses
are above the THD level. With the CRO set for most sensitive position, say 0-10mV, you can calibrate
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 the CRO
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 others more
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 LEVEL.
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 it.
So turn the switch to THD x 10, and this amplifies the THD to read easier on CRO, and measure,
Most class A tube amps have THD % levels that decrease from 0.0dB in proportion to output load voltages.
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 voltage
= 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 rail.
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 devices,
choice of their operating conditions, and resistance values, especially at input stages of the amp.
Noise originating from power supplies must always be eliminated with careful design, component
placement, positioning, and earthing routes.
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 like
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 headphone outlets
on power amps often have a resistance divider to reduce the 8 ohm levels by say at least -15dB.
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 switched off
when phones are used and the amp load is usually just the resistance divider for phones, say 25 ohms
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 versus THD%.
I often prepare THD graphs for loads 2, 3, 4, 5, 6 , 7, 8, 10, 12, 14, 16 24, 32 ohms. Making all measurements
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 65kHz,
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 need.

If you apply say say 20dB GNFB, then it usually sounds very well. Bandwidth with GNFB is increased with
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 0.0dB,
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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