Amp turn-off squeak

[Rhodesplyr]

I've been gradually doing repairs on a 1985 Dean Markley CD-120 amp I bought, and one of its quirks is that about one second after I switch the main power off, a squeak comes through the speakers. This happens consistently. It's not that loud, but I've never had an amp that did this.

I have had to fix a lot of bad factory solder joints, so I'm wondering--might this be the sound of a capacitor discharging through a poor quality joint?

I'll find the source eventually, but I just thought someone might have had a similar problem in the past.

David

[Enzo]

Turn off noises are common in solid state amps. Good luck "solving" the problem. I don't consider it a problem, it is just the nature of the beast. Some are silent, some pop, some moan on the way down. SOme squeak or chirp.

Capacitors discharge through the same joints they operate through. if that symptom was caused by a failed solder connection, it would make noises all the time.


Take a solid state amp and power it with a VAriac. The amps work fine on 120vac mains, of course. Turn the mains down to 100v. AMp will still work fine. Oh the power might be reduced some, but it will work. After all, the only real difference between amplifiers of various wattages is their power supplies. How about 90v. 80? 75? At some point, your preamp voltage regulators will run out of headroom. Hum will develop. And while the power amp can scale down, there is a limit. Circuits come up against voltages that won;t allow stability, too low. The amp can act funny.

Normally, when you flip off power, this trip through voltage reduction takes place over a couple seconds. With the variac, you can set a low voltage, but the power supply is being replenished at that level. SO the amp won;t respond the same as a simple turn-off. You won;t be able to set it at the exact squeak level and expect a steady squeak to track down. At least not likely.

Your amp as it turns off, has its power supply discharging. After about a second, the amp voltages drops through a region of instability, and the circuit goes through a few cycles of oscillation as the voltages fade away.

You could probably isolate this to the power amp or the preamp easily enough. You could follow the main voltages down and see if the instability corresponds with dropping below the headroom of the preamp regulators. You could see if turning down the reverb makes a difference - if it has one.

But then what will you do? You could probably create a speaker relay circuit that could drop out faster than your 1 second. or maybe a mute somewhere. Those would have to trigger off the AC, since the DC power supplies will be discharging slowly. But that seems like a lot of work to cure a brief noise that is not too loud and only happens at turn off. Of course that is only my opinion.

[Gtr_tech]

But then what will you do? You could probably create a speaker relay circuit that could drop out faster than your 1 second. or maybe a mute somewhere. Those would have to trigger off the AC, since the DC power supplies will be discharging slowly. But that seems like a lot of work to cure a brief noise that is not too loud and only happens at turn off. Of course that is only my opinion.

Thats a good time/excuse to implement a DC protection circuit as well to protect the speaker from DC in the event of an output device short.

I'm surprised more guitar amps don't have protect circuits....I do see them with shorted outputs often.

[52 Bill]

Is this a solid state amp?

Or is the CD-120 a tube amp, with a solid state reverb circuit?

[Rhodesplyr]

Is this a solid state amp?

Or is the CD-120 a tube amp, with a solid state reverb circuit?

I should have been more clear about this. The CD-120 is a hybrid tube/solid state amp. The inputs are tube, but it uses TL072CP JFET-input op-amps for reverb send/receive, master volume, and effects loop send/receive. The output stage is a standard long-tailed 12AX7 phase inverter driving 4 x 6L6GC, very much like a Twin Reverb. (Actually, I think an AT7 sounds better in it than the stock AX7.)

It also uses PN4393 JFETs for channel, reverb, and effects loop switching. So, it has both a standard B+ high voltage supply and a +18/-18 bipolar supply (with simple zener regulation) sharing the same ground reference.

The signal paths to and from the ICs are connected to diodes that run to the +/- supplies, but they are oriented such that no current flows under normal conditions. I assume this is a protection feature that would come into play if the voltage at the IC swung higher than +18 or lower than -18.

David

[Enzo]

Then ignore the power amp parts of my post.