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Old 19th January 2014, 01:22 PM   #1
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Default Single to differential into amp - transformer or dual opamp

My amp has a single to differential input into a class D tas5613. I am not sure of the input impedance of the tas5613, probably somewhere around 10k-50k (datasheet has a 10uf input cap on the differential input)
The differential circuit uses a traditional bridge stage with a quad opamp
(refer to fig. 21 : http://www.tij.co.jp/jp/lit/an/sloa064/sloa064.pdf )

I wonder if there are better circuits using opamp (discrete will probably be too much for me) or will I get improvement if I go with a buffer and a reasonably good transformer (I have a Tamura 600:600 on hand).
Else I will just duplicate the existing circuit using 4 LME49710 metal can.

TIA.
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Old 20th January 2014, 10:14 AM   #2
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thatcorp.com will have good solutions.

Gajanan Phadte
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Old 20th January 2014, 11:37 AM   #3
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I would use a TL072, dual low noise op amp. Most manufacturers of quality analogue equipment use them, or equivalent. They will drive down to a 100R load.
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Old 21st January 2014, 06:24 AM   #4
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I don't think I can source anything from "That" here in small quantities. I can use the single to differential opa1632. Does anyone have opinion on the opa1632 vs 4 opamps?

From the many reviews, I believe the lme49710/49720 is better than the TL072.

So no opinions on the use of transformers?
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Old 21st January 2014, 06:30 AM   #5
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Transformers will probably beat opamps on subjective SQ. They have much better CMRR, have virtually no restriction on CM voltage (insulation breakdown isn't normally an issue) and no susceptibility to RF demodulation like active circuits.
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Old 21st January 2014, 07:20 AM   #6
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Originally Posted by JonSnell Electronic View Post
I would use a TL072, dual low noise op amp. Most manufacturers of quality analogue equipment use them, or equivalent. They will drive down to a 100R load.
A TL072 is probably what's already in there, it's hardly an upgrade.

Also, they will start distorting once they have heavier than 2k ohms to drive.
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Old 21st January 2014, 07:38 AM   #7
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You can measure TL07x output distortion rising with "heavy" load even over 2K - the output stage is really poor - especially the negative swing

I have seen it burst into oscillation on negative V excursions with the Cload of < 10 ft cable
for audio line out you can use few 100 Ohm series load isolation so at least the oscillation may not bite you


purpose designed Instrumentation Op Amps have quite adequate CMRR for home audio

a downside of the best high isolation doubly shielded audio input Xfmr is "poor" bandwidth - practically good enough but not going to impress the audio gurus who call for 200 kHz minimum in all the rest of their electronics
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Old 21st January 2014, 09:01 AM   #8
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Originally Posted by abraxalito View Post
Transformers will probably beat opamps on subjective SQ. They have much better CMRR, have virtually no restriction on CM voltage (insulation breakdown isn't normally an issue) and no susceptibility to RF demodulation like active circuits.
Ok, thanks. Guess I will try it. Currently I have the RCA input to a 25k pot, cap, and quad opamp.
For the transformer, after the pot, I assume a buffer is required?
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Old 21st January 2014, 09:07 AM   #9
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A TL072 is probably what's already in there, it's hardly an upgrade.

Also, they will start distorting once they have heavier than 2k ohms to drive.
No, it is a quad opamp opa4228, it is already one of TI premium opamp. And it actually sounds fairly good. I just want to know if I can further improvement.
From some of the opamp comparison (and my experience), the TL072 actually ranks fairly low in audio quality. It is one of those opamp I will pull out when I see it.
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Old 21st January 2014, 10:07 AM   #10
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Originally Posted by ChuckT View Post
For the transformer, after the pot, I assume a buffer is required?
I don't know how much voltage swing your trafo can handle (limited at the LF end usually by core saturation) but the TAS has only 21dB gain so probably you will want some more voltage gain after the pot.
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