Can anyone help to identify an op amp

[pet_rushi]

Hello everyone,

A few months ago I got a couple of composite amp kits - https://www.kaltecs.com/60w-ultra-low-distortion-kit/

I built an amp and the sound is absolutely fantastic. Much better then other much more expensive amps.

Now I want to replicate the circuit but the op amp is not marked / or the marking is obscured really well.

Here is the project page itself - https://www.kaltecs.com/tda7293-composite-amplifier/
but author does not specify the op amp.

Is there a way to find out exactly what op amp is he using?

Thanks

[coromonadalix]

could be similar to this

https://www.diyaudio.com/community/threads/xmas-amp-dibyas-tda7293-by-jhofland.365002/

check the pinouts if they match

[pet_rushi]

Thanks,
but that x-mas amp is not a composite amp and is very different

[magic]

Just use LM358, they are all functionally and pinout compatible anyway.

Seriously, you want people to tell you what chip is used based on circuit description and low resolution PCB images?

[pet_rushi]

Thanks for your reply!
I was hoping that someone has build that composite amp.

[Ian.M]

*DONT* use a LM358 - without an output bias resistor to the negative rail you will get highly objectionable crossover distortion.  See: https://www.sound-au.com/articles/lm358.htm

[pet_rushi]

Wow!
Thank you so much!

[pet_rushi]

If it would help I can take a very close  picture.

[moffy]

I was interested in the same composite amp, so I thought I would design my own board and experiment a bit. I've gone for the OPA1612, it's a dual because I have some, but also low noise. I haven't gotten around to building it yet but have included my current schematic. If you could provide values for your equivalent of my R2 and R5 and R1 and R3 that would be of interest.

[pet_rushi]

Happy to share:
R1 - 5.1k
R2 - 750
R3 - 510
R5 - 1.5k

[moffy]

Happy to share:
R1 - 5.1k
R2 - 750
R3 - 510
R5 - 1.5k

Awesome, thanks.

[moffy]

If a single opamp try the SA5534 or NE5534, for a dual the NE5532, inexpensive but good audio performance.

[magic]

*DONT* use a LM358 - without an output bias resistor to the negative rail you will get highly objectionable crossover distortion.

1. I once listened to some Burzum through a "CMoy" amp with LM358 (directly driving headphones) and it sounded perfectly fine, so objectionable my ass
2. It's only a problem when the opamp is loaded enough, hard to tell if this circuit is affected without knowing component values.
3. There are better reasons to avoid LM358 if you are a Hi-Fi snob, such as noise and limited high frequency performance.

I was hoping that someone has build that composite amp.

You may have more luck with that on the DIYAudio forum than here.

If it would help I can take a very close  picture.

Definitely take a close look at the chips you have, there should either be markings or at least some sign of them having been removed (scratches, maybe black paint). Sometimes markings are obscured by flux residue or other junk, so try cleaning the chips with alcohol or acetone.

Lastly, I had a look at the schematic and can tell this much:

U1 is a simple preamp, fairly noncritical for circuit operation. Any dual opamp will work here, the only difference being performance. The author says it's "high performance" so most likely at least NE5532 or LM4562, or one of the fashionable these days OPA16xx series from TI. Given a DMM, breadboard and some resistors, I could identify NE5532 or LM4562, but the TI parts could be more work because there is many of them.

U2 is part of a feedback loop around the output stage, so its high frequency performance may affect stability of this whole composite thing. This chip may be more sensitive to substitution. The pinout implies it's a single, so maybe NE5534 or one of the OPA16x1. National also had some LME49xxx single audio opamps, but I think they have all been obsoleted by TI.

For U2, start with checking for any resistance between pins 1,5,8 and either of the supply rails. And see what's the value of C71 and R8, because it could possibly be a JFET input opamp. Maybe OPA134, OPA627 (crazy but why not), OPA1641, or something from AD/LT.

[pet_rushi]

Thank you so much!
Great help!