Help with OpAmps frames & Sound Changes

[irgendjemand]

Hi everybody,

I am facing some unexpected phenomenon with my AVM (German) PreAmp:

Since I dismantle the frames which were lying under the OpAmps, the sound became too strong, punchy, not transparent, almost aggressive; the midrange is not natural and it certainly too dominant.

What quite disappeared are the lush, atmospheric, transparent, “human being” character of the sound!

Can anybody help with an explanation for this?

Regards, IY.

[unclejed613]

frames?.... you mean sockets?

[irgendjemand]

Sockets would be the right word, of course, and thank you for this correction.

The OpAmps are newly soldered directly to the PCB board, instead of using a socket. On the sockets it was sounding good! Now, without them, it sounds as described above...

Of course I could reverse the procedure, but this might well shows that there is a mistake somewhere around it.

What can it be?

[analog_sa]

Originally posted by irgendjemand

Of course I could reverse the procedure, but this might well shows that there is a mistake somewhere around it.

What can it be?

Pictures are always useful. Reversing the "procedure" may actually be a good idea as it will point out towards the sockets.

In the absence of pictures or a reversal there are just too many possibilities which will depend upon your ears and system.

Some examples:

- new solder joints done with a different solder

- new solder joints which are not really good

- overheating the opamps while soldering

- new solder joints, not yet broken in

- sockets provided a degree of mechanical decoupling from the board

- the new sound is better but you don't like it

Ime removing the sockets consistently yields small improvements.

[irgendjemand]

analog_sa,

Thank you very much for this valuable info!

Indeed, the new solder joints were done with a different solder, and an overheating of the opamps while soldering could have happened as well.

Mostly interesting is also your other remark, that sockets provided a degree of mechanical decoupling from the board - you are very right with this and this might be part of my actual problem.

My experience before always showed me that removing the sockets consistently yields small improvements! I 100% agree - and it has been like this until I removed the last 4 sockets pairs….

It might well be a “mix product” now, results of the above mentioned points. At least you gave me good starting point. Thanks once again.

[whoandcar]

I'm quite sure there is a solder bridge somewhere- between an input and ground, or between the plus and minus inputs or the like.

[irgendjemand]

Originally posted by whoandcar
I'm quite sure there is a solder bridge somewhere- between an input and groundm between plus and minus inputs or the like.

Thank you! I will check this carefully as well!!

[unclejed613]

you altered the pin-to-pin capacitances as well. this could have a large effect on the sound of an op amp in a noninverting configuration, especially if the impedance on the input side is approaching the reactance of inter-pin capacitance. for instance if the original capacitance were 10pf and it's now 5pf, the reactance has doubled. if the change in reactance is a significant chunk of input impedance (1 or more percent) then it could affect the sound. the change in the parasitic capacitance between the inverting input and output pins could also be enough to drive the opamp into oscillation, especially if the op amp isn't unity gain stable. the out/-in capacitance is an integrating capacitance, and slows the opamp down a bit. reduce that capacitance in a marginally stable circuit, and it will oscillate, usually above 20khz. although the oscillation itself is inaudible, it de-linearizes the chip and the audible effects can be anywhere from barely noticeable to very nasty.

[jan.didden]

Originally posted by irgendjemand
analog_sa,

Thank you very much for this valuable info!

Indeed, the new solder joints were done with a different solder, and an overheating of the opamps while soldering could have happened as well.

Mostly interesting is also your other remark, that sockets provided a degree of mechanical decoupling from the board - you are very right with this and this might be part of my actual problem.

My experience before always showed me that removing the sockets consistently yields small improvements! I 100% agree - and it has been like this until I removed the last 4 sockets pairs….

It might well be a “mix product” now, results of the above mentioned points. At least you gave me good starting point. Thanks once again.

Did you also replace the opamps with a different type?

Also, removing the socket will change the parasitic capacitances on the inputs and you *could* have a oscillating opamp. Do you have a scope?

Jan Didden

[irgendjemand]

Unclejed613,

I am quite sure that what you call as altering the pin-to-pin capacitances does have a large effect on the sound of my op amps. I have no other explanation.

Very strange thing is that the power supply of my AVM is actually very well done & is very stable! Still, we hear what we hear and your explanation of impedance changings + the changes in the parasitic capacitance make the audible effects indeed very nasty.


Jan,

I did not replace the OpAmps: Actually, I have done a long work through several months, until I was 100% sure that they are the best for my configuration (OPA627BP) and that they are the perfect matched pairs (!). I chose 8 of them (4 pairs) and only after they sounded just as I wanted for a quite long time period (3 month or so), I decided to take the sockets off, for ever (so I thought...).

You can imagine therefore how disappointed the situation is now for me.

By the way, basic measurements showed to be just fine!

I will have to put the socket back now, to see what is going on. Regrettably I will have to use new sockets as quite stupidly – I did not kept the old ones. I was so sure that it is going to be an improvement...

What of a mistake!!

And there is always the chance that some other thing went wrong without me noticing it.