Preamp opamp instability

[egellings]

Hang a scope on it.

 

[rayma]

Hang a scope on it.

Sometimes that fixes an instability, instead of displaying it.

 

[Mark Tillotson]

Does the circuit have the appropriate decoupling for the new opamp?
Does the circuit avoid capacitively loading the new opamp beyond its stability limit?
Is the new opamp's input offset voltage and current going to cause output offsets larger than is desired?

These and other questions need asking and answering before any opamp substitution, they are not "rollable" in a naive sense.

 

[Mark Tillotson]

Sometimes that fixes an instability, instead of displaying it.

Hang the 'scope on the big amp's output, that won't have any effect on stability.

Also if you use x10 setting on a scope probe it provides a much smaller load and it less likely to affect the circuit being probed (ie never use the x1 setting without a good reason).

 

[justubes]

Thanks all, it seems possiblybultrasonics.

Cannthis cause or damage opamps downstream with ultrasonics being passed?

I will replace them back but concerned of any other issued caused downstream
Thanks again everyone for all the great advice and solutions!

 

[Mark Tillotson]

Opamps are not damaged by signals, its what they do!


Power amps and speakers can definitely be damaged by high amplitude ultrasonic signal.

 

[justubes]

Thanks, so best never to swap opamps without putting the outputs to a scope for checking for stability and not havr any ultrasonics.

Something which may not affect the sonics but can wreck havoc with amps and speakers.

 

[diyralf]

Your preamp is not designed for Muses. Put back Op627 and it works.

 

[kevinkr]

I suspect you do not have an oscilloscope and have not checked to see whether the new op-amps are oscillating continuously or under some specific signal/time/temperature related condition and that is what is possibly destroying transistors in your power amplifiers. I would worry about your tweeters too.

The OP627 is an excellent op-amp and your pre-amp was designed and tested using this part, changing parts without understanding the consequences and testing for them is how you end up at this end of the street.