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Old 4th March 2014, 05:37 PM   #1
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Default Please help with opamp output stage

Hello all,

I need some help deciding what to do with the output stage of my pre-amp. Seems their are a lot of people in this forum who know about properly utilizing opamps, and that's really what this is about.

Below is the schematic, with some notes. This is a pre-built circuit that was supposed to have dual AD797 opamps, but the manufacturer pulled a fast one and installed dual L49710 opamps instead. Furthermore, caps C402 and C405 were not WIMA, but some generic junk. I have replaced those with WIMA FKP3 and the sound quality improved dramatically. However, I suspect that this circuit is still not what it could be.

Can anyone offer any guidance on a couple of points? I have spent some time with the datasheets of the various parts, but cannot wrap my brain around it all.

1) Since the schematic specifies AD797, is it safe to assume the circuit is designed well to work with that opamp? I do know that the AD797 requires special care to implement (esp. power supply bipassing) correctly.

2) Does a simple replacement of L49710 even make sense here? Sure, it is a good opamp, but different opamps require different circuits, no? (perhaps leads to the next question)

3) Why have they omitted C404 and C408? Could it be that they are not required, due to the different opamp being installed?

4) Even if you can't or don't want to answer the above questions, do you have any advice on what you would do to improve the circuit?

The simplest thing for me to do would be to replace the L49710 opamps with AD797 opamps and install 22pF caps at C404 and C408 (as per schematic), but I will only do this if I can have someone confirm that it is a good circuit.

Thanks in advance for any advice and opinions offered.

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Old 4th March 2014, 07:38 PM   #2
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You probably dont want to downgrade from a real diamond to a sapphire?
If you want to upgrade to something, consider lme49990.
C404/408 are to prevent hf oscillations. AD797 might need that...???

IMHO...
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Old 4th March 2014, 08:03 PM   #3
Mooly is offline Mooly  United Kingdom
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Isn't that the single version of the LM4562 ? Its probably as good as anything in my experience. As the stage is 100% DC coupled, have you checked for DC offset at the output. The 49710 has low input bias currents though, and so even though they may be unmatched (can't say for sure without knowing the circuitry to the left) it shouldn't be an issue. Just measure it for confirmation... and enjoy.
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Old 4th March 2014, 08:08 PM   #4
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Thank you for replying, palstanturhin. Given that the only Finnish person I know is a genius, and also given that Linus Torvalds created Linux, I must take your word for it.

Perhaps I am being silly, but certainly you know more about this than I do.

Any other opinions? ...
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Old 4th March 2014, 08:11 PM   #5
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Quote:
Originally Posted by Mooly View Post
Isn't that the single version of the LM4562 ? Its probably as good as anything in my experience. As the stage is 100% DC coupled, have you checked for DC offset at the output. The 49710 has low input bias currents though, and so even though they may be unmatched (can't say for sure without knowing the circuitry to the left) it shouldn't be an issue. Just measure it for confirmation... and enjoy.
Thanks. I will check DC offset.

The circuitry to the left is direct output from CS3310, pins 11 and 14, respectively.
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Old 5th March 2014, 06:59 PM   #6
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Have not had a chance to check DC offset yet, but I did spend some time listening more last night while doing other tasks. I have to say that I think the new caps (@ C402 and C405) have settled in nicely now and it really does sound quite good. Still not as good as direct source to amp, but certainly very good.

After some reading, I have ordered a PGA2311, which I will use to replace the CS3310. Perhaps that will give me the final increase in quality that I am looking for.

I will also try rolling various opamps in my DAC again, after the PGA2311 goes in the pre. I suspect I will find synergy there somehow.
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Old 5th March 2014, 07:21 PM   #7
Mooly is offline Mooly  United Kingdom
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The LM4562 is pretty good... in fact better than that, its excellent. If you are looking for something genuinely "different" then try an OPA2604.
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Old 5th March 2014, 07:41 PM   #8
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Quote:
Originally Posted by Mooly View Post
The LM4562 is pretty good... in fact better than that, its excellent. If you are looking for something genuinely "different" then try an OPA2604.
I've got a lot of options for the DAC. So far I have tried:


- OPA2132
- OPA2107
- OPA275
- LM6172
- AD8599 (SOIC on DIP8 adapter)
- AD8066 (SOIC on DIP8 adapter)

My 2 favourites from that list are LM6172 and AD8599.

I also have the following in various states of shipping/assembly:

- AD8620 (SOIC on DIP8 adapter)
- AD823 (SOIC on DIP8 adapter)
- dual OPA627 (in dual DIP8 to single DIP8 adapter)
- dual LME49710HA (cans, in dual-can to DIP8 adapter) (~ LM4562)
- LME49720HA (can, in can to DIP8 adapter) (~ LM4562)

Last edited by cogitech; 5th March 2014 at 07:48 PM.
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Old 5th March 2014, 07:53 PM   #9
Mooly is offline Mooly  United Kingdom
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You'll just have to try them and see what you think yourself. Some pretty serious listening tests on my part have confirmed the LM4562 really is very transparent. The OPA2604 remains a favourite of mine and one that really does have a slightly different sound to it... some love it and others don't.
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Old 5th March 2014, 08:44 PM   #10
Xoc1 is offline Xoc1  United Kingdom
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The circuit looks OK
I would look at the power supply, regulation, and decoupling circuits for ways to make improvements to the performance of the preamp.
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