Power on/off 'Pop'

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Hello, I am hoping you fine folk might be able to help me with what I suspect may be a reasonably common problem.

I have a Cambridge Audio C500 pre-amp connected to a pair of Cambridge Audio P500 power amps that are Bi-amping a pair of floor standing speakers.

Whenever the pre-amp is switched on or off whilst the power amps are powered up there is a horrendous pop/thump through the speakers.

For a while the pre-amp was replaced with a Yamaha DSP E800 AV pre-amp with which it did not suffer this. Therefore I have naturually come to the conclusion that the Cambridge Audio pre-amp is the cause.

I am now looking to re-instigate the Cambridge Audio pre-amp as I no longer require surround sound and preferred the musical performance of the Cambridge unit.

Is there anything I can do to the pre-amp to fix this problem?

I am handy with a soldering iron but not too clued up on electronics. I have used the search function on this forum and found some threads but there appear to be a number of ways of tackling this and to be honest I got a bit lost.

Any help will be most greatfully recieved.

Thanks for reading.
 
Thanks for the replies

Tracing the pop signal sounds fun, any pointers as to how to do this? SHould I be putting a multimeter across certain components and switching it on and off?
I believe I have a wiring diagram for the intergrated version of my amp combination somewhere, maybe that will give some clues.

Fenalaar - I ama aware of that, sadly the rest of the family (including one stropy teenage girl) doesn't treat it with the same respect.
 
Thanks for the replies

Tracing the pop signal sounds fun, any pointers as to how to do this? SHould I be putting a multimeter across certain components and switching it on and off?
I believe I have a wiring diagram for the intergrated version of my amp combination somewhere, maybe that will give some clues.

Fenalaar - I ama aware of that, sadly the rest of the family (including one stropy teenage girl) doesn't treat it with the same respect.

There could be two capacitors connected i.e. output of pre-amp and input of amp. You try a 4k7 to ground between them and see if that reduces pop.


Another source of pops is the mains switch. I had a disco turntable wit ha switch and every time it was turned on or off I got a huge pop through the speakers. I put 100nf 250VAC capacitor across the contacts and that reduced it massively.
 
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Any competently designed stand alone preamp should have fool proof muting on the audio outputs to ensure 100% silent switch on and switch off.

You would have to study the Cambridge circuit to see whether any such facility exists (look for output relay/s or possibly transistors used for shunting the final audio output to ground).

It would also be worthwhile as a test to switch the preamp on/off via another method (wall outlet switch) to ensure that the noise isn't something being generated by the preamp mains switch itself.

Building a small muting circuit (relay and transistor to give a delay) is fairly easy but you would need to be able to adapt to suit.
 
Any competently designed stand alone preamp should have fool proof muting on the audio outputs to ensure 100% silent switch on and switch off.

Agreed. And all you need is a shorting relay across the outputs that keeps the output shorted for about a second after turn on, and immediately shorts it on turn off. This can be accomplished with a simple 555 circuit.

Also capacitor across the on/off switch is required. I use a 47 nF/600 volt ceramic capacitor.

You can leave the power amp turned on and turn the preamp on and off and zero pops, clicks, or thumps.
 
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Even with a mute circuit in place, you can still have a small pop due to the charge injection of the switch or due to differences in DC offset between the source and the load. Those are obviously much, much lower than the typical rail-to-rail pops you get without the mute circuit, so the mute circuit does offer a significant improvement. I'm just saying that expecting power-up/down to be completely silent is a bit unrealistic. :)

Those interested in signal switching ought to read Doug Self's "Small Signal Audio Design". There's an entire chapter on signal switching and muting.

Tom
 
Even with a mute circuit in place, you can still have a small pop due to the charge injection of the switch or due to differences in DC offset between the source and the load. Those are obviously much, much lower than the typical rail-to-rail pops you get without the mute circuit, so the mute circuit does offer a significant improvement.

That's true. There is a tiny "tick" noise audible when you turn it off. It is only audible with no program.

I think that could be eliminated with a more elaborate control circuit. I also think it's not even worth it.

It's important to use an X capacitor which is rated for across the line.
http://www.kemet.com/Lists/Filestore...aRFIandSMD.pdf

I knew that and forgot it. Lame excuse. I do have some of those X capacitors because I always save them when I strip a chassis.
 
Thank you for the replies, your help is much appreciated. Sorry I haven't been back sooner but I have been busy with work.

Having taken the lid off the pre-amp and had a look around it already has a capacitor across the on/off switch.

Following the advice of an electrician at work I have try'd powering onand off with first the lower output part of the PSU for the control circuits and then the higher output for the pre-amp stage.

With the lower voltage side unplugged the pop was stll there but with the higher output unplugged there was no pop.

I will try to post up some photos when I dig them out.

Cheers.
 
Just a thought here on a possible source for this problem:

You mentioned that when you replaced the offending preamp with another (different MFG) unit the pop on power-down went away. One possible cause that I did not see mentioned is a single-ended, AC-coupled gain stage in the preamp. In that circuit, a coupling cap is used to remove/filter the DC bias voltage from the output. If the power supply rail collapses when the AC mains is shut off, you will reach a point in time where the bias voltage is rapidly falling back to zero. The output filter (just a cap, so first order) handles this like a step change and there IS a significant transient event that appears on the output, possibly a couple of volts at Vmax. This might be your "pop", since that is just passed on to the amps (which are still "on'). One solution would be to add a bunch of capacitance to the preamp power supply, so that you have some time (e.g. 10 sec or more) before the supply collapses to turn off the amps, or to trigger muting (speaker) relays after the amps that will disconnect the speakers from amps.
 
That is an idea but I would have to have some way of automatically decoupling the output such as the relays you mention as the family won't necessarily turn everything off.

I have dug out some photos but it appears that I need to host them somewhere and link to them for them to appear here. Unless I have missed something.
 
Aha, well in that case:

Please note that the manual is for the intergrated version of this amp so there may be some differences.
 

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