How to solve loud thump at power off?

[dougietamson]

I picked up an old 80's 75W solid state guitar combo for free, the guy said it worked but pots were scratchy and was way too loud.

I cleaned the scratchy pots and swapped out the volume pot for a log taper to give it a much more stable lower volume setting.

I also threw in a dual channel input Marshall pre-amp (2 x 12AX7 based) which feeds into the op-amp based pre-amp.

It sounds great but for the very loud power off thump, disconnecting the speaker before power off is an easy fix but not that elegant.

Any ideas to fix it electronically?

Doug.

[Phil__G]

Try a zero-voltage-crossing mains switch?

[dougietamson]

Originally Posted by Phil__G

Try a zero-voltage-crossing mains switch?

Interesting, the currently installed mains switch is an ON/OFF/ON DPDT wired to reduce hum by reversing polarity, I could try a normal switch 1st.

Doug

[G.Castle]

Is it instability as the supply is collapsing?

May be worth hooking up the oscilloscope and watching the output as you switch off Dougie.

Another thing it could be if the amplifier output is DC coupled, is the PSU not collapsing symmetrically.

[Silicon]

Could you add a snubber circuit across the transformer primary winding?

[dougietamson]

Originally Posted by G.Castle

Is it instability as the supply is collapsing?

May be worth hooking up the oscilloscope and watching the output as you switch off Dougie.

Another thing it could be if the amplifier output is DC coupled, is the PSU not collapsing symmetrically.

It's got quite a simple PSU, 35-0-35 ac from the transformer into a 4 discrete diodes as a bridge rectifier then 4 capacitors for smoothing, I've not checked the capacitor arrangement yet.

[dougietamson]

One thing I did change to the PSU was to add an NTC for inrush current limiting.

There isn't an X2 cap (too old, before regs?) and no caps in parallel with the rectifier diodes.

[G.Castle]

Originally Posted by dougietamson

It's got quite a simple PSU, 35-0-35 ac from the transformer into a 4 discrete diodes as a bridge rectifier then 4 capacitors for smoothing, I've not checked the capacitor arrangement yet.

Sorry. I didn't mean instability in the PSU, though i would expect some regulation on the early stages, (circuit diagram?), my meaning was the amplifier going unstable as the voltage dropped. I've worked on some PA amplifiers that used multiple parallel pairs of 2N3055/MJ2955. Sometimes a rouge transistor in the output stage would cause HF instability and pull the output of the amplifier to one rail, (output was DC coupled). Often long leads to speakers on some marginal examples would tip them over the edge: unlikely to be the problem with a combo of course.

Does it only do it with your bottle pre-amp connected?

I have assumed that it was speaker thump (large cone excursion), rather than just a switch click?

[Aub]

I once, accidentally, wired an on/off switch in the neutral instead of the live in my little 3 watt stereo amp. At switch off there would be a slight hum which would gradually die away. Putting the switch in the live feed cured it.

Aub

[Craig Sawyers]

It is quite common for a solid state amp to be unhappy during power up and power down, with thumps and squeaks.

There are circuits of varying degrees of sophistication to deal with that.

There is this finished product from Canada https://neurochrome.com/collections/...ts/guardian-86 and this from the UK http://www.signaltransfer.freeuk.com/protect.htm which you buy the circuit board and build it yourself.

I also have the schematic for switch on delay for a monster professional amp (C-Audio 602; 840Wpc into 2 ohms) attached. Don't worry about connections vanishing up from the schematic - they go to other functions like the cooling fan control. RL1 is to do with a soft start; if your amp is not blowing fuses or circuit breakers you don't need that.

Good luck

Craig

[Craig Sawyers]

FWIW this is the soft start

Craig

[John10b]

Phil G in post #2 mentions a certain type of switch which I have not heard of, or completely forgotten, can you tell me what it is ? Thanks
John

[G.Castle]

Originally Posted by John10b

Phil G in post #2 mentions a certain type of switch which I have not heard of, or completely forgotten, can you tell me what it is ? Thanks
John

An electronic switch that connects and disconnect only at zero crossing of the mains waveform...

[Chrispy57]

I suspect a zero-crossing switch would have very little effect here due to the action of the smoothing capacitors?

Cheers
Chris

[dougietamson]

The amp has a built-in spring reverb so I checked power off with the reverb mix pot off and mid way, same thump.
I did notice the reverb sound tail off after the power off/thump, so the caps were discharging keeping the power amp supplied for a while after.

The thump has been present since I was given the amp and before adding the valve preamp section. The PSU for the valves is a dedicated off-the-shelf transformer, though with rewound secondaries for the HT.

Having searched google for similar issues it seems a common issue even with brand name amps, Fender support replied to a customer saying the thump would not cause any harm, even if the speaker was disconnected before power off.

Output transistors are 2N3773.

[Philips210]

Hello.

There was an interesting little circuit in Elektor magagine which prevents speaker thump at both switch on and switch off. It was in the July/August 1977 issue.

Please see attached circuit:
Hope this is useful.

Regards,
Symon

[dougietamson]

Originally Posted by Philips210

Hello.

There was an interesting little circuit in Elektor magagine which prevents speaker thump at both switch on and switch off. It was in the July/August 1977 issue.

Please see attached circuit:
Hope this is useful.

Regards,
Symon

That does look a lot easier to build, good find.

Doug

[John10b]

G Castle thank you for explanation.
John

[Lucien Nunes]

The first question here, which most people have probabloy thought about but has not IMO been stated explicitly, is whether:
a) The amplifier is picking up and amplifying the arc at the switch, in which case that would benefit from suppression, or
b) The amplifier is creating the thump itself, either as a DC offset or burst of oscillation.

The lack of influence by the volume controls suggests b) is the culprit. The DC offset situation can perhaps be recognised by a uni-directional speaker excursion that slowly settles back to rest position.

[McMurdo]

I see alot of guitar amps at work and FWIW alot of the transistor ones do thump either at switch on, switch off, or both. They just don't seem to consider it a problem.

I can't remember if it was carlsbro, laney or HH but one of the common designs had a fairly heavy resistor across just one of the power supply capacitors to ensure asymetric discharge at switch off that counterintuitively made the thump less.