D
Deleted member 148505
D
Deleted member 148505
Yes those are needed because the virtual ground of analog input_A to D of TPA3255 is at AVDD/2 or 3.5VDC.
Input caps for the buffer opamp are needed because single supply is used so the ground of opamps is VCC/2 or 6VDC.
D
Deleted member 148505
My hunch is correct that the sound is being limited by the opamp feedback caps. Might be the group delay between the opamps are different.
In slaa788A, they increased it for stability from 22pF to 330pF (corner freq is ~48kHz in PFFB).
I think as long as the module is stable, the value can be decreased. In my module, the whole amp is stable even when the opamp feedback caps were removed (using LME49860).
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Further upgrade is matching the resistors of SE to Diff buffer, as well as PFFB resistors. It will lower down the distortion further.
In slaa788A, they increased it for stability from 22pF to 330pF (corner freq is ~48kHz in PFFB).
I think as long as the module is stable, the value can be decreased. In my module, the whole amp is stable even when the opamp feedback caps were removed (using LME49860).
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Further upgrade is matching the resistors of SE to Diff buffer, as well as PFFB resistors. It will lower down the distortion further.
I agree the sound is a bit more clearer & airy on the upper end after removal of the 4 feedback caps. Just took them off and listening now (PFFB removed). Will play the amp for the day and see what happens. I have WIMA 1uF caps pre-3255 right now but I’ll put back the Silmic IIs later today to hear the difference. I’m also running OPA1656s.
Cheers, Pete
D
Deleted member 148505
Can you enable the PFFB? or you enabled it but still don't like it?
In my test module, the PFFB is enabled.
I'm not sure for the stability if OPA1656 is used. Do you have LME49860?
Regards,
Lester
In my test module, the PFFB is enabled.
I'm not sure for the stability if OPA1656 is used. Do you have LME49860?
Regards,
Lester
Lester: I’ve changed so much at this point it would be a lot of work to re-enable PFFB. The LC is 6.8uH+0.68uF, pre-opamp caps are 4.7uF WIMA MKS, pre-3255 are 1uF WIMA MKS, RA&RB 3-6 have been changed to 50K to prevent low frequency cut off due to the 4.7uF WIMAs. I have the following caps by-passing on pins 4-to-8 of the OPA1656s (10uF Silmic II + 10uF X7R Murata’s + 0.1 uF C0Gs) as per ClaudeG’s work on the Aiyima 3251 amp.
I have 49860s to try .
Pete
D
Deleted member 148505
To re-iterate, PFFB is disabled on my board. Using 3e Audios 48V SMPS. I have no oscilloscope so I’d have to be able to hear any oscillations. My speakers are 8 ohms. Changing back to the original 10uH+1uF LC structure is easy to do (since I didn’t glue down the inductors
).Pete
Thanks for the explanation.
Pity! The less capacity there is in the signal path, the better.
I agree...and the less polar caps the better
Pete
D
Deleted member 148505
D
Deleted member 148505
I plan to have an external split supply for the opamp buffers on version 2, so that frontend caps will be removed.
Then LM317/337 for onboard opamp supply, if the external supply is ultra low noise, then a clean supply will still go to the opamps.
Lester, 49860 sounds fine...just not as "pretty" as 1656. Sounds seem to be a trifle more compressed into the speakers than 1656 (on my speakers & system). High frequency clarity is there but not emphasized as much as with 1656s. Just my take..
Really a personal choice of what one likes I guess.
Pete
Hello
Which diagram are you referring to?
I guess these are C_op from slaa788?
I only have the TI documentation.
Best regards
Roger
D
Deleted member 148505
I agree, OPA1656 is cleaner and clearer sounding.
Just need to tame it with ELNA Silmic II or other compatible caps, otherwise it will sound sharp.
I ordered small quantity on digikey, it is out of stock on Mouser!
Just need to tame it with ELNA Silmic II or other compatible caps, otherwise it will sound sharp.
I ordered small quantity on digikey, it is out of stock on Mouser!
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