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Old 3rd September 2012, 11:52 PM   #281
richiem is offline richiem  United States
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Richard, the Active Twin-T schematic is here -- Active Twin-T notch filter. I'm using an OPA1641 for the input amp at the moment, but the OPA134 works just as well. I need to update the performance pix with current results, but the noise floor says a lot.

I can't see any distortion from the filter, but there has to be some there somewhere. With careful attention to capacitor size -- not too small, so as not to be confounded by stray C -- the T-T will work at 100kHz if you need to go that high. It also can be configured easily for spot frequencies and use a *lot* fewer parts -- and go in a smaller box.

As to the best sound module -- clearly having a full 192kHz Fs is important for both ADC and DAC. I believe that all the ADCs running at this sampling frequency are sigma-delta types, which can make higher frequency noise an issue due to the noise-shaping system. I would like to have an external USB ADC box that does 18 or more bits at 1MHz Fs. But I'm no good with digital design and building one is not in my set of cards. Maybe someone here is up to that -- I think the chips are out there.
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Old 4th September 2012, 03:16 AM   #282
1audio is offline 1audio  United States
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192 KHz Delta Sigma ADC's don't necessarily mean high noise at high frequencies.

This is a quick capture of the distortion spectra (of the ADC, the source is significantly better) of a 1 KHz low distortion source. The ADC is an AKM AK5394A, which is a reasonable choice and used in some current mid priced products. Adding a twin T would get better resolution on the distortion if you are really concerned about distortions below 120 dB. (The opamps on the ADC board are JRC 5534's, I have not tried others yet.)

Getting this performance from an internal card would be difficult but possible, however an external ADC will give better results usually.
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Old 4th September 2012, 03:58 AM   #283
RNMarsh is offline RNMarsh  United States
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Thank you, both.
before taking the plung -- is there anything better than AK5394A or E-MU suggested that may exist today?
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Old 4th September 2012, 04:42 AM   #284
richiem is offline richiem  United States
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@Demian -- Great plot. I'll look into the AK5394A -- did not know of it. Is the 60Hz from the ADC, the oscillator, or??? And is there a commercial product using this ADC or something comparable? I'd like lower noise between 30kHz and 90kHz.
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Old 4th September 2012, 04:55 AM   #285
1audio is offline 1audio  United States
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The 60 Hz was lazy setup on my part. I usually run a separate ground wire between the chassis and the demo board, but not this time.

I am not sure what uses this chip today. I believe its used in several pro ADC's and it might be in some of the better mid level ADC's.

I'm using the AKM demo board but laying out a board for it is not too difficult. If I weren't so busy I would do it. I may be able to get someone to attempt it. The chip is $22 for one at Digikey.
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Old 4th September 2012, 05:02 AM   #286
RNMarsh is offline RNMarsh  United States
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Default ADC with twin T -

Quote:
Originally Posted by 1audio View Post
The 60 Hz was lazy setup on my part. I usually run a separate ground wire between the chassis and the demo board, but not this time.

I am not sure what uses this chip today. I believe its used in several pro ADC's and it might be in some of the better mid level ADC's.

I'm using the AKM demo board but laying out a board for it is not too difficult. If I weren't so busy I would do it. I may be able to get someone to attempt it. The chip is $22 for one at Digikey.
I think I'll use the AKM ADC and add the twin-T to it. can you give source of demo board and approx price? Thx - RM

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Old 4th September 2012, 05:27 AM   #287
1audio is offline 1audio  United States
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I'm checking on the availability of the SDR-Widget, a special USB DAC that used the AK5394A. If its still an option I'll let you know. It would be the least expensive option and possibly the best execution (Open source).
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Old 4th September 2012, 05:41 AM   #288
1audio is offline 1audio  United States
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I cleaned up the connections and improved the grounding but the hum remains. It may be a limitation of the Heathkit bench supply I am using. It is -120 dB. . .

Warm up got the harmonics down some (who says warm up is a myth?).
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Old 4th September 2012, 06:06 AM   #289
richiem is offline richiem  United States
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@Demian -- OK on the 60Hz, that'll do.
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Old 4th September 2012, 05:56 PM   #290
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Quote:
Originally Posted by RNMarsh View Post
Thank you, both.
before taking the plung -- is there anything better than AK5394A or E-MU suggested that may exist today?
TI's PCM4222EVM evaluation board is claimed to be really outstanding. Check out the spectra on this:
DIY Distortion Analyzer/Analysis?

Pretty dang good I think.
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