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Old 22nd December 2001, 07:41 PM   #1
mlloyd1 is offline mlloyd1  United States
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Default DIY Distortion Analyzer/Analysis?

For those of us that don't have access to an Audio Precision set, what are folks out there doing for measuring THD, IMD, SID, whatever? Anything clever for helping to set bias points or actually measuring impacts of changing ground connections, etc?

Here are some excerpts on the topic from another thread ( the Leach amp design thread, I beleive); comments welcome (hint, hint!):

From Damon:

...As for other designs and tweaks, I can only point to Doug Self
and Randy Sloan and their books on amplifier design for hints
as to what might be possible. But we're already into a range of
low distortion that my geriatric collection of Heathkit test
equipment can't possibly measure, and I can't afford an Audio
Precision test set.

Unless I can come up with a "cheap" Audio Precision equivalent,
I'd be shooting completely in the dark with new designs. And
that's daunting....

From me (mlloyd1):

..... I wrestle with the same problem as far as distortion measurements. I usually look at distortion waveforms on my scope. I think we need a group discussion about how to work around this issue for DIYers. There was a REALLY nice project in Audio magazine (RIP) some years ago for a very serious analyzer that could be reworked with current tech (using OPA604 op amps and current buffers instead of TL071, for example) and probably have much more performance than we would need. I've also seen some writings in past issues of The Audio Amateur (now Audio Electronics) by Erno Borbely ( I think) for a distortion analyzer. There was a project in Radio Electronics some years back also. I actually made a PCB and partly built this one. In my opinion, it worked very well - it gave consistent measurements with a Audio Precision I had access to at the time. However, it was VERY tough to control the electronical noise in my DIY environment though - halogen lights, misc dimmers, hair dryers, etc. (it's tough being a married DIYer!) don't make for a clean test environment. Finally, reading through the service manual for some of the HP distortion analyzers (I think the guy that did the Audio magazine project referred to doding this also) suggest a few ideas as well. Alas, I can't seem to find my copies of this anymore :-( I can't recommend enough times that service manuals from GOOD test equipment makers like HP and Tek are EXCELLENT sources of material for study!

I'd be happy with a distortion test box that could spot check with high resolution at about 4 frequencies: 50Hz, 1KHz, 20KHz, 75KHz.

Who's game?

Nelson, what do you do when you have something to test (say an idea at home late at night) and the Audio Precision is nowhere around? Wait until later? :-)

Maybe we could even ask an analog Guru like Jim Williams at Linear Technology to design a simple, high peformance THD analyzer circuit (I single him out because I remember and oscillator circuit he designed that was claimed to have THD specs in the single digit parts per million. This is incredible!). They might already have such a design sitting around somewhere; I haven't checked their web site and app notes lately. Hmmmmm ....

from Grey:
.... Give the distortion analyzer project its own thread so people will be able to find it more easily.

I'm interested.

I'd like to reiterate--for those who haven't understood what I've said on these matters--that I'm not against low distortion, per se, just the use of massive quantities of negative feedback to achieve the distortion figures. Just don't pursue low distortion as an end in itself, as you'll usually find that once you reach a certain point sound quality suffers. But up to that point it can be a useful tool....
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Old 22nd December 2001, 08:29 PM   #2
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Michael,

To evaluate distortion, I currently use a PC-based test instrument from Pico Technologies (www.pico-tech.co.uk). Their ADC-216 has only a 166 kHz bandwidth, but its 16-bit dynamic range and Spectrum Analyzer function makes it useful for looking at distortion products. The software will also calculate THD, S/N, and other standard specs from captured waveforms. At around US $700, it doesn't look cheap until you compare with something like the AP System One.

My signal source is a simple version of the improved Wien/HP oscillator described by Jim Williams in his book "Analog Circuit Design: Art, Science, and Personalities." I plan to build a better version when I have time, but even this little single-IC unit is quite useful as it gives .003% distortion at 1kHz. This is better performance than you'll find in many commercial designs, and costs very little to make.
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Old 22nd December 2001, 09:07 PM   #3
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I know some find the objective approach to measure in audio
design anathema, but I've always been design-oriented and
I have learned that my hearing isn't completely trustworthy;
I need test equipment for a reality check. But I like to think that
listening is complementary to objective analysis.

My test setup is mostly geriatric solid state Heathkit; I can measure down to around .03% THD on a good day. The THD meter has an autonull feature but it is tempermental. It is most
useful in conjuction with a scope display to visually judge the
relative amounts of noise, ripple and distortion products; I always
set amplifier bias so the crossover notch just disappears.

The IM meter has excessively high residuals and needs troubleshooting; it lacks an oscilloscope output. I found a used
Crown IMA a few years ago in a guitar shop for $30, but alas,
both of the meters are shot and I've been quoted $200 for their repair or replacement. And I still think the unit has other problems; I suspect a voltage surge and aging took a considerable toll on the poor thing. But it has some useful features like the oscilloscope output and a tracking level control so a range of power levels can be quickly measured.

Going beyond this level of performance demands not just low residuals but spectrum analysis of the distortion products; an
Audio Precision system does all of this in software with a handy
graphical display and helps to make documentation a LOT easier.
But the price of a new system is a killer, and I can't even bid on
the occasional units that appear on Ebay: I'm poor.

I know there's a company that local to me in Seattle (or thereabouts) which may have an offering similar to the AP, using
commonly available computer sound cards such as the Sound Blaster Live! (the newer Audigy should be even better). I'm not
clear on cost or how capable their system is compared to AP.

I'm convinced that the AP is the route to take for measurement
and documentation to the point that even used Sound Technology
and the HP339 don't interest me as much as they might have ten
years ago.

I could, however, continue to work with a conventional THD meter
if the residuals were an order of magnitude lower than my present setup.

I'd like to do more IM measurements, perhaps with the CCIF standard of two closely spaces HF tones, possibly swept.
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Old 22nd December 2001, 10:12 PM   #4
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Damon's point about spectral analysis is a good one, and in my opinion could stand to be put in italics & bold print.
My take on this is that the harmonic structure of the distortion is <i>far</i> more important than the total percentage. It's widely acknowledged (even by specs-are-everything folks) that lower order harmonics are much less obnoxious than higher order. To the extent that I must, in the real world, have distortion, let it be 2nd and 3rd order rather than higher order products.
I'm also convinced that a lot of the problem is transient in nature, hence difficult to spot with the averaging techniques commonly used. Crossover distortion is a good example of this, as it's there...then it's gone. But while it's there, it's really, really ugly. Yes, it's easy enough to bias for class A and avoid the problem entirely, but then you get into heat dissipation and overall decreases in efficiency. I choose class A and AB in order to sidestep the problem, but that doesn't mean that I'm always overjoyed about the tradeoffs.
Michael, thanks for breaking this out as a separate thread. Rest assured that I'll be watching developments closely.

Grey
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Old 23rd December 2001, 12:49 AM   #5
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I'm currently using a Velleman PCS64i, which is a PC-based DSO. It's available for around 350.00 US$ from www.qkits.com. I've found it to be a pretty handy instrument. Velleman also has less expensive models (as low as $129) available as kits. The software includes a spectrum analyzer, which can be used for distortion measurements albeit with some post-processing. I've been thinking about writing a post processor to do THD calculations, Distortion vs. Frequency, Distortion vs. Power, etc. But, I'm not sure if there are many people using this instrument, so for now I just do the post in Microsoft Excel.

For those interested in quick and dirty distortion analysis, Nelson Pass published a great sidebar in his Mosfet Citation 12 article which explains how to use one channel of an amplifier to analyze the other.

A DSO is really a pretty indespensible instrument if you're a serious audio hobbyist. And at $129 and up the PC based models available are hard to beat and they can replace several instruments on your workbench.

Another option (if you don't need to measure THD to .001%!) is to build a shunt that could be connected to the line input of a PC sound card. There a several sound card based software FFT analyzers available as open source and freeware. Again, you're limited by the accuracy of the sound card and your function generator.

P.S.
I asked a sales guy at AP if they've ever considered building a hobbyist-level product. He laughed at me.
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Old 23rd December 2001, 01:48 AM   #6
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Default low cost analyzer

One can measure distortion down to about .0001% cheaply with the following circuitry:
http://www.angelfire.com/ab3/mjramp/.../golopid6.html
Can listen to the distortion signal with a headphone while tweaking the amp or the distortion output can be observed on a PC oscilloscope or PC spectrum analyzer with a sound card and the following software (around $35 I think, free demo download):
www.sumuller.de/audiotester/
The usual PC sound cards are good for about 80dB dynamic range I think, can do better (100 dB) with a 24 bit 96KHz sampling card like the following:
http://www.zzounds.com/love.music?p=...=1254398378432
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Old 23rd December 2001, 01:51 AM   #7
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Quote:
Originally posted by mrothacher

Another option (if you don't need to measure THD to .001%!) is to build a shunt that could be connected to the line input of a PC sound card. There a several sound card based software FFT analyzers available as open source and freeware. Again, you're limited by the accuracy of the sound card and your function generator.

I asked a sales guy at AP if they've ever considered building a hobbyist-level product. He laughed at me. [/B]
Ah, for the good old days when Heathkit was around and providing us with mid-level test equipment at affordable prices.
Velleman is almost the only kit company left out there. At least
you can get good oscilloscopes, but otherwise signal analysis
has been the forgotten stepchild.

I think the best hope for an Audio Precision equivalent is going
to be based on the better sound cards and someone clever at
writing the necessary support software. I'm >really< impaired
when it comes to the latter.
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Old 23rd December 2001, 03:38 AM   #8
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Default Test Equipment

Some of you may not be aware that there is a lot of used test equipment available and sold daily on Ebay. If you are satisfied with older equipment that works perfectly well then this is a good place to look.

I have sold quite a few items of my older equipment there and have shipped them all over the world.

Look under the Industrial section and then test equipment. Don't get side tracked in the electronics parts section.

John Fassotte
Alaskan Audio
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Old 23rd December 2001, 03:47 AM   #9
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The line input on most soundcards is limited to 2 or 3 volts, so you'll need to design some kind of attenuator to scale the voltage down 10 or 100 times for measuring signals at high power.

The neat thing about using a PC sound card as an FFT with a PC-based function generator is that you could sweep the generator and automate THD vs. Frequency for example. However, I'm not sure about the distortion performance of software based function generators.
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Old 23rd December 2001, 03:50 AM   #10
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Default soundcard protection

Forgot to mention, when using a PC sound card for amplifier testing, make sure you attenuate the amplifier output with a 50 to 1 (or so) resistive divider before connecting to a sound card input. I smoked my old soundblaster card once when I forgot to attenuate the amplifier!
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