My implementation of the Cordell Distortion Analyser - Page 30 - diyAudio
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Old 27th February 2013, 10:20 AM   #291
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Quote:
Originally Posted by avincenty View Post
Yes, understand the difference and i am looking for the Cordell Distortion Magnifier schematic. Posted on this thread as there has been previous mention of the circuit and a post by EUVL of a similar design.
Hi avincenty,

If you're thinking of building the DM, it is well worth it to pay for a copy of the LA in which it appeared, as there is a lot of associated information there that will help with understanding its operation, how to use it, and helpful if debug is necessary. Contact Jan Didden at www.linearaudio.net - home. There is also a kit for it available, well worth the money if your time can be better spent on something else (check with Jan re the kit). There is also some information about the DM on my website at CordellAudio.com - Home.

Cheers,
Bob
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Old 27th February 2013, 11:39 AM   #292
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Thanks Bob,

I am not interested in the kit, I like the therapy of building my own. So the only source for the schematic is via LA @ $30? I cant accept NAVADAS offer of a redrawn schematic? It is your intellectual property so i will respect your recommendation.

Alfredo
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Old 27th February 2013, 01:20 PM   #293
davada is offline davada  Canada
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I would have had to ask Bob and Jan first anyway before doing that but I think we have our answer.
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Old 2nd April 2013, 06:44 PM   #294
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Quote:
Originally Posted by Bob Cordell View Post
Hi avincenty,

If you're thinking of building the DM, it is well worth it to pay for a copy of the LA in which it appeared, as there is a lot of associated information there that will help with understanding its operation, how to use it, and helpful if debug is necessary. Contact Jan Didden at www.linearaudio.net - home. There is also a kit for it available, well worth the money if your time can be better spent on something else (check with Jan re the kit). There is also some information about the DM on my website at CordellAudio.com - Home.

Cheers,
Bob
I've been studying your article in LA and I think I'd like to do some things differently. I need some information though. For the phase adjustment, how many degrees (at which frequency) of range is required for most DUT's? I'm using an all pass filter for the phase adjustment, but I don't know exactly how much range I need to include. Any help is appreciated.
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Old 4th April 2013, 02:04 PM   #295
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For the all pass filter, I set it up to have a range of about 1 deg of phase lag @ 100kHz to about 12 deg of lag. When I look at the Bode plots for various amplifiers I've designed in simulation, the phase lag is not much, maybe 1 or 2 degrees @ 100kHz. I expect the real world to be similar.
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Old 4th April 2013, 03:30 PM   #296
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The need for phase adjustment is mainly a result from the DUT having low-pass characteristics, so it is probably more promising to model a low-pass rather than all-pass characteristics in the subtraction path.

Samuel
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Old 4th April 2013, 04:46 PM   #297
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Originally Posted by Samuel Groner View Post
The need for phase adjustment is mainly a result from the DUT having low-pass characteristics, so it is probably more promising to model a low-pass rather than all-pass characteristics in the subtraction path.

Samuel
I disagree because a low pass filter will attenuate the harmonics from the oscillator, thus changing the results of the residual distortion output from the DM. The change will result in a reading of more higher harmonics in the spectrum than actually exist from the distortion of the DUT.
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Old 4th April 2013, 07:23 PM   #298
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I disagree because a low pass filter will attenuate the harmonics from the oscillator, thus changing the results of the residual distortion output from the DM. The change will result in a reading of more higher harmonics in the spectrum than actually exist from the distortion of the DUT.
The low-pass function of the DUT will equally attenuate the harmonics of the oscillator...

Samuel
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Old 5th April 2013, 01:40 PM   #299
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Originally Posted by Samuel Groner View Post
The low-pass function of the DUT will equally attenuate the harmonics of the oscillator...

Samuel
Not "equally". Really? You believe that's true for all DUT's? In my limited experience, it's obvious that the LP function of a DUT can be almost any order (first, second, or something else) and it's pointless to try to emulate that in the source channel from the oscillator. It seems to me that it would create just as much error to assume that the DUT behaves as a first order LPF as it would to just ignore it by not using a LPF on the source channel. I do not have any evidence yet to support my theory however. It's just an assumption on my part.

Last edited by dirkwright; 5th April 2013 at 01:58 PM.
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Old 5th April 2013, 04:54 PM   #300
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Of course you're absolutely right that it is dangerous to assume anything about the exact low-pass characteristics of the DUT. But we have to assume something, or else we can't put anything to correct it in the subtraction path. That the DUT has all-pass characteristics seems pretty unlikely; for a casual power amp, a first-order model is probably reasonably accurate.

But we may just be splitting hairs here; most DUTs worth throwing a distortion magnifier at will have a bandwidth of at least several hundred kHz. So even slightly above the audio frequency range, the important 2nd and 3rd harmonics will not suffer much from modest errors in the low-pass characteristics. And higher harmonics should, with a decent source, be relatively low to begin with. Last but not least, measuring distortion is never a 1% measurement, more likely 10% or worse below -120 dB. Resolution counts, not accuracy (within limits, of course!).

A more practical reason to stay with the first-order low-pass is that it is easily implemented without adding another opamp, with its distortion contribution. Or did you find a way where the opamp is out of question?

Samuel
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