Power Off and popping

[J M Fahey]

Just checked the schematic and slightly correct myself.
I had assumed a single section switch only opening/closing the Hot wire, which is common in USA (and which classic fender used, and most others) but I see here Fender used the European way of opening *both* Hot and Neutral , doubtlessly so basic same amp serves both Markets.

In this case, you might use two caps, one across each switch section.
Switch is shown on Page 2 , section A-6

One cap across switch legs 2-3 , another across 5-6 *or* to use a single cap as Mick Bailey suggested place it across 3-6 , which puts it in parallel with transformer primary.

Just a cap alone absorbs , say, 80% of the pop, adding 100 ohms in series with cap will improve that even further.

[trem]

Great.....Thanks.
I suppose i can simply use some little jumper-wires to test this.....see if any of this Helps/Works....?
Thanks Again

[Gingertube]

you don't have to leave it alone if it bothers you, but it can be a PITB to design something to silence it automatically.

there are a million-six ways to deal with power-off thumps. you can design just about any kind of circuit to do this. a lot of SS audio amps use relays to connect/disconnect the speakers. my concerns are that an AC powered relay can couple hum, and a DC powered relay needs to react faster than the occurrence of the thump. you have to win a race, so the protection circuit can get complicated. I think the HiFi guys sell boards to do this sort of thing.

The simplest fix would be a switch on the speaker.

Actually it is not that hard to design it for silent power down but it is more difficult to retrofit to an existing design.
Basically you need to keep the power up to the diffamp/driver rails passed where the main rails to the output devices collapse.
So you put forward biased diodes in the power feeds to the diffamp/driver and add their own filter caps that hold up longer than the main caps. The diodes stop those diffamp rails discharging back through the main rails
That keeps the DC feedback running and the output close to 0V DC until the output devices run out of power.

Cheers,
Ian

PS for that Fender circuit in the 1st post I think forward biased diodes added in series with R63 and R67 would probably do the trick.

[bob p]

Actually it is not that hard to design it for silent power down but it is more difficult to retrofit to an existing design.
Basically you need to keep the power up to the diffamp/driver rails passed where the main rails to the output devices collapse.
So you put forward biased diodes in the power feeds to the diffamp/driver and add their own filter caps that hold up longer than the main caps. The diodes stop those diffamp rails discharging back through the main rails
That keeps the DC feedback running and the output close to 0V DC until the output devices run out of power.

Yes. That's basically the implementation that's used in the HiFi boards that I mentioned earlier.

Several years ago I did a restoration on a boutique HiFi amp that was basically derived from an old-school DC-300 circuit with a fancy LED meter display). It had the same problem with power off thumping. Because the plan was to use it on the bottom side of a biamped rig in my living room I wanted it to be thump-free. The solution was to establish a clamped power supply to the diffamp/driver that would outlast the discharge of the main filter caps (which after my resto-mods were big). I didn't build a separate daughter board to do this, I just added caps near the PSU and moved a few parts around on the amp's driver PCB to accommodate the hardware. I think I've still got a sketch of the circuit around here somewhere.

It's not a difficult task to design one of these circuits once you realize the nature of the problem, but the fix will require modification to the amp, and in most amps the hardest part is going to be determining how to fit the mods into the chassis.

I think de-thumping is more important in HiFi gear than in MI gear. If an amp is going to be used in my living room, I like it not to thump. For a guitar amp I really don't care.

[bob p]

I found the folder with my sketches. There was more than I remembered -- the mods that I did actually involved a two pronged approach.

1) A fault-tolerant vreg for the 12V bipolar rails
2) A power-off clamp for the opamp

I started off with the fault tolerant vreg for the opamp driver but that wasn't enough. I ended up adding the following power-off clamp. It doesn't list the time constants or the RC values, which I fine-tuned for the particular application. In choosing the JFET there was a trade-off between one that would had better controllability and wouldn't clamp as hard, and one that had bad timing control but hard-clamped well.

[soundmasterg]

I had a power off thumping on my 2 channel Vox AC100 build a couple years back. I solved it with a poly cap to ground after the standby switch I think, though I'd have to dig up the schematic again. I need to fix the layout and rebuild the amp one of these days so that problem might go away in the new layout. We'll see.

Greg