Weird Noise When Powering Off Amp After Op Amp Replacement in Transtube Bandit

[ScoopMyMidsDaddy]

Replaced all the caps in my transtube bandit, but still had noise issues. Decided to just replace the five 4560 op amps with some TL072IP op amps I had on hand that should be compatible. Noise went away completely but I'm having this ridiculous sound when turning off the amp. Only thing I could find on this noise was another guy having his Peavey Chorus make the same noise after an op amp switch out. If there's no risk of damage then I really don't care but I've never heard anything close to this before.

Any insight would be greatly appreciated.

https://streamable.com/pqt0yf

[TomCarlos]

Did the amp have the horrific turn off sound after you replaced the caps and before you replaced the op amps? Other than that, the amp is working ok?

[g1]

Don't have any ideas but can I sample the noise and your response for a song?

[Enzo]

Decided to just replace the five 4560 op amps with some TL072IP op amps I had on hand that should be compatible

That being why I always recommend actually finding the fault before any wholesale parts replacements. Some noise won;t hurt anything, but you likely would have found just one IC was sensitive to its position.

[Jon Snell]

The TL07 range of ICs are not happy when the rail voltages drop below +-9volts. The 4560 chips are happy down to and including +-3volts.
What is happening is when you power off the amplifier, the rails supplying the ICs are decaying alongside the amplifier rails. The ICs go unstable at a low rail voltage and the amplifier can still amplify because the main rails have not decayed enough to stop it.

That is why I always replace like with like but find the fault first. I seem to remember replying to your old post reference swopping out ICs. In an emergency a TL072 will work but as you have found out, differently to the original ICs.
If there is no fault, leave it alone it is only an amplifier that works.

As Enzo says, find the fault and fix that before playing with parts.

[ScoopMyMidsDaddy]

The TL07 range of ICs are not happy when the rail voltages drop below +-9volts. The 4560 chips are happy down to and including +-3volts.
What is happening is when you power off the amplifier, the rails supplying the ICs are decaying alongside the amplifier rails. The ICs go unstable at a low rail voltage and the amplifier can still amplify because the main rails have not decayed enough to stop it.

That is why I always replace like with like but find the fault first. I seem to remember replying to your old post reference swopping out ICs. In an emergency a TL072 will work but as you have found out, differently to the original ICs.
If there is no fault, leave it alone it is only an amplifier that works.

As Enzo says, find the fault and fix that before playing with parts.

Thank you for the insight.

I ended up replacing all the caps with the original op amps in place and the noise issue didn't go away, but the amp operated properly with the new caps.

At that point I just decided to throw in the op amps to see how it goes. The noise went away and it sounded perfect but this noise started.

Your explanation is exactly what I was looking for and clearly explains the issue. I'll explore other op amp options but I don't mind the current noise if there's no real risk to the amp.

[loudthud]

Anybody got a link to the schematic for this amp ?

The data sheet for the TLO7x chips has grown to 93 pages.

link: https://www.ti.com/lit/ds/symlink/tl...oogle.com%252F

I can't find anywhere in the data sheet that indicates a problem below +/- 9V, but I know the chips can go crazy when the inputs go outside the allowed common mode range. Output will suddenly slam the Rail. I think I remember the TransTube amps had some discrete transistor stages, these might apply Voltages outside the common mode range to the inputs of the opamps when power is going down.

[J M Fahey]

Quite common and what Jon Snell said.
My own explanation to Musicians:
"some Op Amps go crazy under low voltage in certain positions"
"no, itīs not a documented behaviour"

Like ground loops, hum, catching RF, etc. , builder one day gets rid of it and when itīs gone (in new layouts or projects)
"What did I do right?" .... who knows? ... but now itīs gone.

I had it at one time, now never ....

It used to be very common in GK amps, for contrast never saw it in Peaveys, I guess itīs WAY more common that whatīs visible BUT 90% of amps now incorporate some kind of MUTE, which sweeps it under the rug.

Your Peavey does not ... because it never needed it, so get it back to normal and all will be fine.

[DrGonz78]

This is the schematic for the Peavey Bandit Trans Tube from 95. Perhaps this is the correct schematic? There is another one that is the Bandit 2.
Peavey-Bandit-1-Transtube-112-4-95-Schematic.pdf

[loudthud]

Thanks for the schematic link DrGonz78.

Does inserting a plug into the Pwr-Amp In or Effects Return eliminate the noise ?

[ScoopMyMidsDaddy]

Thanks for the schematic link DrGonz78.

Does inserting a plug into the Pwr-Amp In or Effects Return eliminate the noise ?

I haven't tried that yet. I've been working all week, but I'm out early today so I will get a chance to play around with it. I did attempt to set knobs like pre/post gain and reverb to 0 before turning off the amp to see if that changed the noise at all, but it didn't. I'll try a patch cable in the effects loop today. Thanks

The schematic Dr. Gonz posted is the correct one for the silver stripe bandit I'm messing with.