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Understanding Op-Amp LPF in DAC
Understanding Op-Amp LPF in DAC
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Old 12th May 2020, 08:11 AM   #11
Rick PA Stadel is offline Rick PA Stadel  United States
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Am still puzzled why you want to fiddle the mylar LPF caps.

If phase is correct about your original op-amps being rebadged LT1057s, the change to OPA2132 may bring little joy: The 1057s are 4 - 6 dB quieter and more accurate, if you believe the PDF specs.

If you pick out something different to try, make sure the mfg doesn't recommend 'no-socket' mounting. Lots of high performance models hate sockets.

Cheers
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Old 12th May 2020, 11:26 PM   #12
itsikhefez is offline itsikhefez  United States
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Understanding Op-Amp LPF in DAC
Thanks for your thoughtful and informative reply.

Quote:
Originally Posted by Rick PA Stadel View Post
Am still puzzled why you want to fiddle the mylar LPF caps.
I may not end up replacing these. Since these caps are in the signal path I'm sure part quality can have an impact. If there are modern Polypropylene caps with same values (and tolerance), I think its worth a try.

Quote:
Originally Posted by Rick PA Stadel View Post
If phase is correct about your original op-amps being rebadged LT1057s, the change to OPA2132 may bring little joy: The 1057s are 4 - 6 dB quieter and more accurate, if you believe the PDF specs.

If you pick out something different to try, make sure the mfg doesn't recommend 'no-socket' mounting. Lots of high performance models hate sockets.
That's an interesting point. I have no idea if the Adcom 7AA is indeed LT1057 or not. There are many testimonials that replacing it to the OPA627 made a huge difference. These are all subjective so have to take them with a grain of salt but something I would want to try for myself.

Granted the OPA2132 is not the OPA627 but I dont have direct experience with either, and I'm not willing to pay ~$100 for quad OPA627's.

I chose OPA2132 after reading through a few threads here. They seem to be a good compromise between performance, compatibility to the original circuit and cost. The original PCM63 datasheet shows a OPA2604 for I/V & filtering and the OPA2132 looks to have lower noise and lower distortion.

edit: I also compared the OPA2132P datasheets with the LT1057. The OPA2132P has lower noise and better CMRR. The LT1057 doesn't publish THD spec so I have to assume the OPA2132P is superior here as well.

Last edited by itsikhefez; 12th May 2020 at 11:37 PM.
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Old 13th May 2020, 03:27 AM   #13
phase is offline phase  United States
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Join Date: Oct 2004
In other applications, I have had decent results using what are commonly sold as an opa627 on epay. They are really a pair of ad744on an adaptor.
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