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patrick-turner patrick-turner is offline
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Default THD testing of amplifiers.

I have added another page to my website which explains the analog gear
needed to test amplifiers, including 5 schematics explained.
http://www.turneraudio.com.au/thd-measurement.html

For those appalled and dimayed by what they see, please find your own vomit bucket. Meanwhile, very keen DIYer types might even build a THD checker like I have and be delighted with results.
Patrick Turner.
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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default THD testing of amplifiers.


"patrick-turner"

I have added another page to my website which explains the analog
gear needed to test amplifiers, including 5 schematics explained.
http://www.turneraudio.com.au/thd-measurement.html

For those appalled and dimayed by what they see, please find your
own vomit bucket. Meanwhile, very keen DIYer types might even
build a THD checker like I have and be delighted with results.



** Making a home brew THD meter that can resolve down to 0.001% is no
trivial task.

The biggest problem is frequency drift in both the sine wave source and the
notch filter - unless both are *rock steady* there is little hope of ever
achieving a deep null for long enough to get a reading in the 0.001% region.

Commercial products use many dodges to get around this - firstly "auto
nulling" is standard so the instrument constantly adjusts the notch
frequency to compensate for drifts anywhere in the circuitry. Plus the sine
wave source is followed by a sharp, low pass, tracking filter that simply
removes any harmonics.

The notch filter typically consists of two cascaded stages, neither of which
is very deep so there is adequate bandwidth to the combined notch to cope
with minor drifts.

Having built an inherently low THD sine wave generator that covered the
audio band and more (published under my name in EA magazine Feb and Mar
1989) I set out to make a precise notch filter to go with it. I soon
discovered that using carbon track pots was out of the question and even
polyester film caps had excessive temperature drift when approaching 0.001%
readings.

The final design operate at three fixed frequencies ( 66, 998 & 5278 Hz )
it employs a single LM308 op-amp in a Wien bridge notch filter followed by a
TL071 x10 gain stage. The caps are all polystyrene, resistors all metal film
and fine adjustment in each leg of the filter is done with a pair of
200ohm, 10 turn WW pots.

The unit is built in a die cast box and has a remote +/- 15V psu.

Attenuation is over 100dB at the notch frequency, less than 0.5dB at double
that frequency and on up to 50kHz. Max direct input is about 8.5 volts rms
and there is a 10:1 divider that allow signals up to 85 volts rms.

With a little patience, readings down to 0.0005% are possible at 998Hz with
signals of 2 volts rms or more.

The only things that have challenged the unit are the low THD bench
oscillator and a Sony CD101 playing a test disk.


.... Phil





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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default THD testing of amplifiers.


"Phil Allison"

The final design operate at three fixed frequencies ( 66, 998 & 5278 Hz )
it employs a single LM308 op-amp in a Wien bridge notch filter followed by
a TL071 x10 gain stage. The caps are all polystyrene, resistors all metal
film and fine adjustment in each leg of the filter is done with a pair of
200ohm, 10 turn WW pots.



** This schem has the same basic circuit.

http://www.tradeofic.com/uploadfile/...7284957652.gif

In my version, resistor values were kept below 50k ohms for low noise and
the op-amp can be the readily available a OP07 type. Cap values were 150nF,
10 nF and 1.8nF.


.... Phil


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patrick-turner patrick-turner is offline
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Default THD testing of amplifiers.

Phil mentioned :-

"The notch filter typically consists of two cascaded stages, neither of which
is very deep so there is adequate bandwidth to the combined notch to cope
with minor drifts."

Electronics World produced a cascaded notch filter which I was tempted to try, but which seemed unecessary with tube circuits which rarely ever achieve 0.001% THD with power amps. I found the LC bridged T notch I have works well enough, and this, with a following x10 amp with a BPF for 1.4kHz to 14kHz seems to work OK and low levels of THD can be displayed on the CRO without excessive amplitude change due to filter or oscillator instability.

The idea of a wien bridge for notch filter also appeared in EW articles.

This all goes to show there are more ways than one to examine THD.

Patrick Turner.

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