Quick trick to stop turn on/off pops.

As I've mentioned, I've been working on ways to stop switching transients in my 12v amps. Since a lot of 12v systems end up running some part of the audio circuitry from a virtual ground rail, which jumps up to ~6v at power-on and drops to true 0v at power-off, there is often the problem of a large and potentially damaging transient being generated. A 6v swing is very big in terms of line-level signals.

Dealing with the power-on transient is easy - you just use a time-delayed relay circuit to either disconnect the signal or speakers, or hold the amp in "mute" mode for a couple of seconds while all the rails stabilise. However, at power-off, the relay never releases quickly enough to stop the transient getting through, as the power amp usually stays powered-up for a couple of seconds, from its own rail capacitors.

So, I considered using some kind of microprocessor to enact a power-up/power down sequence, making sure the power amp is the last thing on and the first thing off. However, this has several disadvantages - complexity, cost, development time for the code, loss of circuit breaker switching (12v would remain live inside the amp at all times, instead of the circuit being broken by a main power switch. If the uP goes nuts, anything could happen....) 

After some lateral thought, I realised there was an analogue solution, using only two extra components. And am now kicking myself for not thinking of it sooner.

Basically, you want the relay to release as fast as possible on power-off, but there's not much you can do about the release time of a relay. What you can achieve, however, is getting the audio circuitry to stay on for a fraction of a second to allow the relay to operate. To do this, you put a diode in the positive 12v line feeding the audio circuitry - and only in this line, not the power feeding the relay coil. Then, you add significant capacitance to the rails *after* this diode. I used 1500uF. Now, when the power switch is opened, this capacitor can only discharge into the following circuitry, not backwards through the diode. Thus, the relay experiences an abrupt loss of power and switches off as fast as it can. The audio circuit, meanwhile, stays powered up for half a second or so from the new capacitor. 

Et voila...no more thumps :)

R1    ZD1

+50v---/\/\/\-----|<Z--------SHDN

  |       |

\ \

  R2   / /   R3

  \       \

  /      /

  |      |

0v------------------------------

where R1 and R2 are the voltage divider to drop the 50v to a reasonable value (3.3v? I think that's the internal logic voltage) and R3 is to pull SHDN low unless the Zener is conducting. Have I understood that right?