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!

[glentek]

The original author says he tried 98 different opamps but only 5 gave good results. I think it is his trade secret and he wants you to buy his product. No doubt the part number is obscured.

[pet_rushi]

I wish I had such an inventory!

[GigaJoe]

Would be an interesting to know minimum amplification coefficient   for TDA7293\4\5 .  So it would be minimum harmonics
second play a good mute standby circuit as it a bit affect harmonics
wonder about Pin-11,   can it be some additional correction, or loopback for more harmonics decreasing ...

composite as itself doesn't cure for better sound.

[pet_rushi]

Here is the data sheet - https://www.st.com/resource/en/datasheet/tda7293.pdf
All the info is there.
The composite architecture improves the sound significantly, you have to be deaf not to hear the difference.

[moffy]

Here is the data sheet - https://www.st.com/resource/en/datasheet/tda7293.pdf
All the info is there.
The composite architecture improves the sound significantly, you have to be deaf not to hear the difference.

Yes, the improvement in THD of a good composite implementation is impressive, but the struggle is to maintain stability with all that composite gain and phase shift.

[GigaJoe]

so you are saying THD in a composite connection can be lower than the same single opamp in the unity gain connection?

[moffy]

so you are saying THD in a composite connection can be lower than the same single opamp in the unity gain connection?

A power amp is almost always run at gain because it is raising the signal from normal levels to power levels. Look at the THD figures of the already mentioned page: https://www.kaltecs.com/tda7293-composite-amplifier/

[pet_rushi]

Yes, that's why I'm asking for help to find out which op amp Kaltecs is using.

I have his 60W kits - https://www.kaltecs.com/60w-ultra-low-distortion-kit/
and they are super, super stable and on top of that the sound is absolutely perfect!

I asked him a couple of times to disclose the op amp but he is unwilling to do this. I'm not going to start manufacturing his kits. I just want to know.

[GigaJoe]

interesting ...   
so typical PDF schematics K=33 , THD 0.005 at 5W,

from article base 5W   THD 0.003 and K unknown ,

composite noninverting K=11, divider 2 (from measurement) , so for TDA K = 6.6

That I'm curious of THD  of TDA when K=5, for example ,  and compare to composite.   
as for example for tda2050 K min stable 8 I have THD 0.0015 ( PDF - 0.02, K=30 ) , and for LM3886 K=3 was even 0.0002 (PDF - 0.004, K=20) and that was my limit to measure ...

so comparing apple to apple need to compare standalone and composite with the same K for TDA. And then it shows how much composite schematic dropping THD down vs standalone.
as significant decrease in K dropping THD to at least one order down.

[pet_rushi]

You better ask Kaltecs. He has a composite with LM1875 / TDA2050 as well - https://www.kaltecs.com/32w-ultra-low-distortion-kit/
He always replies, maybe not the same day but anyway he always replies.
My understanding is that the composite architecture increases the open loop gain and because of that decreases THD.
If the open loop gain is increased, lets say, by 40dB then the THD should go down by 40dB.

[moffy]

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

Because of the low division ratio set by R2/R5, I think the opamp will have a modest GBW like the NE5534 or OPA1637,OPA132. The higher GBW opamps from what I remember of my simulations need a higher division ratio to be stable. The OPAs have an extra 20db gain at DC over the NE5534 which could be an issue also.