My implementation of the Cordell Distortion Analyser - Page 29 - diyAudio
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Old 27th February 2012, 08:26 AM   #281
magnu is offline magnu  Sweden
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Quote:
Originally Posted by davada View Post

For two adjacent ranges with overlapped Fr, two different RC pair have a different gain at resonance.

An example is 2k ohm and 10nF, 20k ohm and 1nF. Each pair have the same Fr but the 2k/10n pair has a Fr gain of 58dB and the 20k/1n pair has a gain of 46dB.
It was a while since I looked at the exact circuit.
But my intuitive ansver to your question is that your phenomen lies in the change of resistors. The resistors in the RC net will also set the gain of the OP-AMP circuit.
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Old 27th February 2012, 01:17 PM   #282
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The maximum Q achievable without tuning should not be a first-order function of feedback network impedance level (it shurely depends on opamp o/l gain though). I quickly simulated this and was not able to repeat your findings; peak gain is in both cases a touch below 70 dB. Are you sure that you're not seeing simulation artefacts? As the peak is extremely narrow, I suggest you run the AC simulation with 10k points/octave. If this does not clarify the case I think a schematic of the exact setup might help.

Samuel
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Old 27th February 2012, 02:26 PM   #283
davada is online now davada  Canada
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Quote:
Originally Posted by Samuel Groner View Post
The maximum Q achievable without tuning should not be a first-order function of feedback network impedance level (it shurely depends on opamp o/l gain though). I quickly simulated this and was not able to repeat your findings; peak gain is in both cases a touch below 70 dB. Are you sure that you're not seeing simulation artefacts? As the peak is extremely narrow, I suggest you run the AC simulation with 10k points/octave. If this does not clarify the case I think a schematic of the exact setup might help.

Samuel
Hi Samuel,

The effect doesn't show up in simulation as well as other effects. I thinking the op amp models are too close to ideal.

I did this with a real world circuit using to different methods. One method was using a Hp 339A by connecting the oscillator to the input of the filter and monitoring the output on the meter in level mode.The level has to kept quite low, 10-30mVrms as there is a lot of gain at Fr.

The other method is to use a sound card and ARTA fft software. Input a low level PN pink noise, (periodic pink noise), while running the software in single channel frequency response mode. Same setup.

The first is full proof while the second is subject to software maximum resolution.

I can post a schematic if necessary.

David.
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Old 27th February 2012, 03:32 PM   #284
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I don't have a distinct answer for this behaviour, but the most reasonable argument appears to be differences in capacitor losses. What capacitor types do you use?

Besides this I wonder why you're investigating this..?

Samuel
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Old 27th February 2012, 04:59 PM   #285
davada is online now davada  Canada
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Hi Samuel,

The capacitors are Polypropylene albeit not a very good tolerance and section mismatch might be the reason.

David.
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Old 27th February 2012, 06:01 PM   #286
davada is online now davada  Canada
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Hi Samuel,

I sent an email to the address provided on your web page.

David.
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Old 27th February 2013, 03:01 AM   #287
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Quote:
Originally Posted by EUVL View Post
This is a preview of the schematics as built.
Probably not bug free, so build at own risk.




Patrick
Distortion magnifier version 2 public

Is this circuit the same as Bob Cordell's DM? I dont find a detailed schematic of Bobs design. I can only find links to the whole vol 0 of linear audio for $30.

Alfredo
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Old 27th February 2013, 03:38 AM   #288
richiem is offline richiem  United States
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Note that the various "distortion magnifiers" do not work with oscillators, only with devices under test that have an input and an output, so that the output can be attenuated and summed with the input to remove common signals, leaving only the added distortion and noise form the DUT.
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Old 27th February 2013, 04:00 AM   #289
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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.
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Old 27th February 2013, 04:38 AM   #290
davada is online now davada  Canada
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I can do one up for you from the LA volume.
It would just be in spice capture though.
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