Fender Champion 20 amp makes popping sound after shutdown

[alsetalokin4017]

Is it broken or not?

Patient: Fender Champion 20 small guitar amplifier, new out of the box, and all the others like it too.
Symptom: About 2 seconds _after_ shutting off the power switch, the amp emits a rather loud "POP" from the speaker. Every time. Even with volume and gain and all other controls turned all the way down. Regardless of whether there is anything plugged into the input or not. Turn it off, wait 2 seconds, POP!

I was in the guitar store today and I wanted to try out this amp. So I did, and loved it... until I turned it off. 2 seconds later: POP!
So I experimented, and found the above. I asked one of the sales techs about it and he said "yeah they do that." I looked on the internet and found lots of threads describing and discussing the problem, with the usual non-answers (Put it on standby! It's the power switch arcing! They all do that!) from people who didn't actually understand the problem.

But this cannot be right. Seriously, it's a loud POP, that surely must be stressing the speakers in these little amps, and it's enough to wake up somebody who has just been lulled to sleep by one's gentle serenade.... silence... then POP! There must be a solution that makes sense.

So I'd really like to buy one of these little, full-featured modeling amps, but not if I cannot find a fix for that damned POP!!!!

[alsetalokin4017]

It's kind of funny actually. So far the best solution I've seen on the forums is to plug something into the small headphone jack, even just a blank plug, because this disconnects the speaker. So you don't hear the POP. I thought of applying a DPST power switch with one side making the speaker connection and the other side for power, but these kludges are unsatisfactory somehow. All that energy making the POP has to go somewhere and if I prevent it from reaching the speaker... where does it go? What does it stress?

I emphasize that it does not make any noise at the instant the switch is flipped to OFF. The POP comes about 2 seconds later. So a power switch that disconnected the speaker would stop the POP, but I am concerned about the fate of the energy.

[grifftech]

what about this? the resistor is 8

[duak]

I download the schematic from Fender and see that it has a DSP and a switching supply.   http://www.fmicassets.com/Damroot/Original/10001/2330200_gamp_schematic.pdf

My guess is that the power supply voltages to DSP drop off a bit quicker than to the output stage and when the DSP stops running because its supply voltage is too low, it looses control of one or more of its outputs.  Since the output stage still has some of its supply voltage, it dutifully passes the anomalous signal on to the speaker.

I'd think about maybe adding some capacitance to the DSP power supply to extend its operation during power down.

An alternate is to look into the 'mute' function shown on the lower right of the schematic.  Fender may have encountered this problem and put the mute function in but it doesn't always work.

I've been building & repairing amplifiers since the 70s.  Some used to make a thump when first turned on.   Some used to make odd noises or pushed the woofer back and forth when shutting off.   Some would do different things depending on how warm they were.  These pretty much stopped when designs got rid of coupling capacitors and went to balanced DC coupled topologies or if protection relays were added.

Cheers,

[SilverSolder]

It seems a little dubious that products sold in 2019 have these kinds of blemishes.

[Renate]

"They all do that." is the usual lame excuse.
There is no reason that they have to.

Uncontrolled power down is chaotic and things go to the rail and/or oscillate when single or multiple power lines all crash.

In the old days of discrete transistors you might put in a protection relay.
Nowadays with single IC class D amplifiers there is no excuse not to correctly implement standby/mute.

I have a Fender Acoustasonic 40.
It uses a TDA7296 which has two inputs for standby and mute.
The main power is +/- 28V with +/- 15V for the low level.
Fender put in a poor design that would mute when the +28V crashed to a few volts, after the pop has occurred.

I put in a Atmel ATTiny85 to monitor the +28V and mute, then standby when it drops below 23V.
This is something that Fender should have done.
But as I say, "They all do it" sets a very low expectation for performance.

[echavez]

this element only appears in the diagram but not in the list of elements J301 must be a discharge capacitor. also does not exist physically in the champion 20

[Renate]

It looks to me like a power outlet for accessory stuff. It is a "J".

[echavez]

it's definitely a design issue fender was aware of and didn't care to fix. it does not affect its performance but the noise when turning it off is annoying. my 100% functional solution was to add a 120 v ac relay to the output of the general switch and interrupt a wire from the speaker connecting them to the normally open terminals. so when I turn on the amplifier the signal reaches the speaker and when I turn off the speaker is interrupted

[CaptDon]

The 120vac coil relay is the only real answer here. The amplifier design sucks and a lot of these Chinese designed and produced amplifiers with major brand labels are doing this at turn-off. Going all the way back to the mid 1980's even the Carver PM-2.0T's used by major touring companies has a turn off thump. Some were much worse than others even though the parts were identical. I fixed an Ampeg bass amp for a friend that had a habit of blowing up exactly when it was switched off. I had to add a speaker relay to it. It thumped so hard at times it would crash the voice coil into the structure. It also had a habit of going into thermal runaway and shutting down at outdoor gigs on hot days!!!

[SilverSolder]

The 120vac coil relay is the only real answer here. The amplifier design sucks and a lot of these Chinese designed and produced amplifiers with major brand labels are doing this at turn-off. Going all the way back to the mid 1980's even the Carver PM-2.0T's used by major touring companies has a turn off thump. Some were much worse than others even though the parts were identical. I fixed an Ampeg bass amp for a friend that had a habit of blowing up exactly when it was switched off. I had to add a speaker relay to it. It thumped so hard at times it would crash the voice coil into the structure. It also had a habit of going into thermal runaway and shutting down at outdoor gigs on hot days!!!

Maybe turn it off in step with the music, as a mega kick drum? 

[syntax333]

Can anyone share simple schematic of relay and power switch solution?