Loud Thump When Power Is Switched Of In SS Amplifiers

[Mickey]

Although only a new member of this forum, I have noticed several issues regarding a "loud noise" when an SS Amp is switched of either by the Amp power switch or by the mains wall switch. This is a common occurrence in all budget type SS amplifiers whether Hi-Fi or guitar amp.

It is a relatively easy modification if you're comfortable working with mains voltage.

Find yourself a defunct front load washing machine and if you follow the mains cord coming into the machine you will most likely find a filter block. This usually consists of two coils bypassed with some capacitors and is used to prevent washing machine electrical interference from feeding back into the mains supply. This filter block will be sealed with spade connectors for both the incoming and outgoing wires.
I fit these filters after the amplifier power switch so that any switch noise as well as power supply spikes are virtually bypassed.
In my Peavey Bandit 112 which suffered severally from a loud pop on switch of, I also fitted one of these filter blocks on the mains input lead to suppress any line noise (interference/spikes) from entering the amp.

THIS IS NOT A MOD FOR A NOVICE/AMATEUR SO SEEK HELP IF YOU DON'T HAVE SUFFICIENT EXPERIENCE!!!

[Jazz P Bass]

I may have to try that.

https://www.encompassparts.com/item/...ilter_Assembly

Although a well placed jfet circuit is all it takes.

[g1]

This will only help with switch noise, correct?
I find the thumps are often due to DC imbalances at power off.
So I think this would be more for "pops" than "thumps" ?

[Mickey]

This will only help with switch noise, correct?
I find the thumps are often due to DC imbalances at power off.
So I think this would be more for "pops" than "thumps" ?

Maybe my description of the noise is not quite correct. It is a very loud noise which could be described as a "pop" and makes one think that the noise could blow the speaker.
I've never tried to determine what causes the noise so I'll see if I can locate the source with a CRO. It makes me wonder why if it is an imbalance somewhere/anywhere in the circuit how does a simple RC filter eliminate it?
The noise was also present if the mains was switched off at the wall so it must be the sudden collapse of voltage somewhere in the circuitry or as you suggest a DC imbalance.
The filter also assists in reducing noise which comes from effects cables, stage lighting etc and on my Peavey Bandit I fitted the filter on the underside of the chassis with the wires passing through a small hole drilled in the chassis, then to the appropriate locations.

[Enzo]

Switch pops may be annoying, but are unlikely to damage a speaker. First, they are limited to the amplitude the power rails allow. So that pop can be no louder than any peak that clips. So if the speakers will handle the full output of the amp, they will then be well able to handle the pop.

[DrGonz78]

I have noticed on a Vox Viscount amp and other solid state Vox amps from the 60's that they put standby switches on the rotary power knob. So you have Off|Stby|Operate, like on a tube amp but controlled by one switch. RG of course has some info about it here>>>
Thomas Vox Solid State Amps

Last paragraph on that page he says it is probably used to reduce the thump of on and off during operation. This is done by disconnecting the speaker while the power supply is first connected, while in Stby. Then turn knob to operate connects the speaker. I really like idea of disconnecting the speaker while powering on and off the amp. As Enzo states no real damage can be done if it is just a healthy amp thump. Still this idea seems most interesting to me and is very simple to implement. Just need to install a standby switch for the speaker and no pops on turn on or off. Just have to remember to turn on your solid state amp in the same way you would with a tube amp that has a standby switch.

[J M Fahey]

Yes.
I do exactly that, I add a "Standby" switch which just opens speaker out "hot" , and the instructions: "turn power ON, wait 10 seconds, turn Standby ON .... reverse for turn off" .... works like a charm.
The great VOX innovation was to add a specially built rotary switch so there's no way to do it in the wrong sequence
Somebody should make those switches today for *all* SS power amps.

[R.G.]

Does it count if I did a PCB that implements the Vox solution with PCB-mount relays rated for the speaker currents?

The switches Vox used are no longer available. But I did a PCB that used a low-current 30-degree index rotary switch to power two DIP sized 120VAC relays on the PCB. One set of contacts powers the "AC to the amp power" in both standby and on positions, another set of contacts powers the speaker relay only in the "on" position. Since it was cheap, I used a 4P/3T rotary, and used a third set of contacts to power two neon indicators, one for "standby" and one for "on".

The description is up at geofex.

[Mickey]

Switch pops may be annoying, but are unlikely to damage a speaker. First, they are limited to the amplitude the power rails allow. So that pop can be no louder than any peak that clips. So if the speakers will handle the full output of the amp, they will then be well able to handle the pop.

I did put up with the noise for quite a while without blowing any speakers but I wonder what the instantaneous voltages and currents could be? Had there been a transformer in the output circuit I'd be very concerned that a sudden collapse of voltage could induce a huge spike in a transformer winding with the potential of blowing a speaker or indeed inducing a substantial back EMF into the output circuit.

[J M Fahey]

The description is up at geofex.

A link or it didn't happen

[R.G.]

A link or it didn't happen

Sometimes it's buried a little deeply. It's here: How to fix the blasted switch

Originally the article only contained the rebuild info, which is now at the end. You have to plow through the epoxy-on-the-wafers to get to the new PCB stuff.

[Gingertube]

The switch Off thump may be eliminated or greatly reduced simply in some SS Amps.

If it is dual rail (that is +/- supplies) find the supply which runs to the tail of the front end diffamp, then add a series diode in that rail feed to the front end (with its own filter cap). This will allow the diffamp to hold up and continue to control output DC offset till after the supplies to the power section have colapsed.

Cheers,
Ian

[olddawg]

Many large SS power amps in the late 60s through the 80s (and a few still through today I guess) had speaker protection relays. They did three things: 1) They delayed connecting the speakers until the amp stabilized (usually 5 seconds) 2) They disconnected the speaker load before the amp powered down eliminating any powering down noise, and 3) They were also connected to a DC offset sensor circuit that would not allow the relays to come out of "protection" if any DC was present on the speaker lines.

[Mickey]

The switch Off thump may be eliminated or greatly reduced simply in some SS Amps.

If it is dual rail (that is +/- supplies) find the supply which runs to the tail of the front end diffamp, then add a series diode in that rail feed to the front end (with its own filter cap). This will allow the diffamp to hold up and continue to control output DC offset till after the supplies to the power section have colapsed.

Cheers,
Ian

Thanks Ian,

Your answer makes a lot of sense but I then wonder why it can also be eliminated by fitting a filter network as I posted?

[J M Fahey]

There's many different factors which can cause pop/thump; your idea addresses one, Ian's addresses other.
Both are useful.
The traditional high voltage cap across switch legs also helps.