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Old 7th December 2006, 12:28 AM   #1
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Location: Dallas, Texas
Default distortion analyzer recomendations?

Hi all,

I'm wondering if anyone has recommendations for distortion analyzing equipment.
I have an O-scope, signal generator, variac, bench power supplies, and voltmeters.
I have and use spice. I am a total novice, but I have built an amp or two. I would like
to test the amps I have built and see if they perform anywhere near what spice reports.

As an Audio Precision instrument (like what Nelson uses) is beyond my budget
(I think they start around $10,000 US), one of the solutions I was considering is
YMEC's DSSF3 software with (hopefully) a 192KHz/24 bit sound card. I'm thinking
this will cost around $500 US, or so. Has anyone used this software and have an
opinion?

I think I am mainly trying to measure THD and bandwidth. I'm open to suggestions
if anyone would care make recommendations.

thanks,
Robert
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Old 7th December 2006, 12:37 AM   #2
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AP is still the best, IMHO. We recently bought 3 more S1's used
at $1600 each.

You can easily build your own IM analyzer, and with somewhat
greater effort you can make a THD analyzer with a bootstrapped
twin T network and some trim pots. An op amp and a little
light bulb can make a quite low distortion oscillator.

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Old 7th December 2006, 03:03 AM   #3
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Quote:
Originally posted by Nelson Pass
AP is still the best, IMHO. We recently bought 3 more S1's used
at $1600 each.

You can easily build your own IM analyzer, and with somewhat
greater effort you can make a THD analyzer with a bootstrapped
twin T network and some trim pots. An op amp and a little
light bulb can make a quite low distortion oscillator.


Thank you for the input Nelson! I would LOVE to build some of this
stuff, but so far, I don't know what a "bootstrapped twin T network" is.
Again, I'm a CS guy, and a network has a completely different meaning
for me! Also, my plan is to use this equipment to check my work.
So we get into a kind of recursive problem here.

On the good side, I _think_ I could build some of the op amp based
oscillators mentioned in the Howowitz and Hill book. I have no idea
how to throw a light bulb into the mix, but the idea makes me smile.

Thank you again,
Robert
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Old 7th December 2006, 04:53 AM   #4
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Ebay can be your very good friend. Use "distortion meter" or "distortion analyzer" in your search.

I've been using the Heathkit IM-5248 intermodulation analyzer and IM-5258 THD analyzer for years; they're really about the minimum level of performance needed to do any serious work. Found a Crown IMA, but both meters were shot. A used Hewlett Packard 333 or 334 is a good starting point for a professional analyzer, and I wouldn't turn down Sound Technology equipment (I think ST still supports their old models).

Sound cards with appropriate software is another approach, but I really want a 'front end' with calibrated attenuator before I risk blowing a good card with too much voltage swing. A used HP 400 series AC voltmeter might be a good start in that direction.

Needless to say, Audio Precision is about the ultimate, but even used System Ones get bid out of my budget range every time. I'd also like some serious spectrum analzyer capability; not just audio range but RF as well.
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Old 7th December 2006, 03:31 PM   #5
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Quote:
Originally posted by Damon Hill
Ebay can be your very good friend. Use "distortion meter" or "distortion analyzer" in your search.
<snip>
Thanks for the reply Damon.

Do you (or anyone) happen to know if this type of equipment is easily damaged? Do people try to sell inoperative/damaged equipment on ebay? If so, is that a common problem? Who repairs this type of equipment?

thanks,
Robert
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Old 8th December 2006, 01:58 AM   #6
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Buying used equipment, especially from people or businesses you don't know, can be risky. I've taken chances on retired HP test gear from Boeing and have gotten some great deals; that IM analyzer I found at a music shop in Florida is probably a lost cause since replacing the meters will run over $200.

Most test equipment is not that hard to break, unless you peg the meters constantly or vastly exceed the recommended input voltages. Age is your enemy; parts can fail and some parts may require creative substitution, if it can be done at all.

Search Ebay a while and make notes of all the distortion analyzers that show up for sale; this will at least give you an idea of what's out there. Most of them are obsolete and the original manufacturer may no longer support them; there are companies that will give you quotes on repairs and calibration, usually at a considerable price (and may be worth it, too).

Pay close attention to the descriptions of any units that interest you; some people are honest enough to tell you that they have no idea if it's working properly or not, and are selling 'as is.'

I've taken a few chances and generally won, but I'm also prepared to troubleshoot and repair--or junk--the equipment I've got. And I do some modifications, too; researching improvements on my spare Heathkit IG-18 audio generator, hoping to lower THD down to the .001% level--current modifications have only gotten it to about .01%. I might tackle some improvements on my working Heath distortion analyzers, too. The Heath IM analyzer doesn't meet specs, so it needs work.
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Old 8th December 2006, 02:18 AM   #7
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Quote:
Originally posted by Nelson Pass


AP is still the best, IMHO. We recently bought 3 more S1's used
at $1600 each.


What could be the second best at similar price?
I'm eager to buy one

Thanks
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Old 8th December 2006, 02:31 AM   #8
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If you can't afford an AP here are a few to consider --
Tektronix DA4084 and AA501, Krohn-Hite 6900, Boonton 1120 and 1130, HP334, 339 -- you can also get very nice results with the heath IM and THD analyzers.

Of the above, you can hook up the Boonton to a computer via a data acquisition card and download the performance data to Excel.

For a high quality oscillator look to Linear Tech's PDF of the LT1115, or you can build the super-oscillator which was in one of their application notes (but you have to work with current feedback opamps which can be a bit tricky).

Low Distortion Signal Generator
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Old 14th December 2006, 06:03 AM   #9
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What about using this to put one together?

http://focus.ti.com/docs/toolsw/fold...71evm-pdk.html

It comes with some software to measure THD and costs $149.

I am not sure if there is an API that you can use for your applications (what I would prefer).

I hope this helps.
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Old 14th December 2006, 01:55 PM   #10
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Quote:
Originally posted by Henry Pootel
What about using this to put one together?

http://focus.ti.com/docs/toolsw/fold...71evm-pdk.html

It comes with some software to measure THD and costs $149.

I am not sure if there is an API that you can use for your applications (what I would prefer).

I hope this helps.
Just make sure to protect the opamp inputs -- the Boonton uses something akin to this setup (it is more complicated)-- with a relay for belt and suspenders. If you use diode clamps on the input the Vcc and Vee should be as quiet as possible:

Click the image to open in full size.
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