Transistor switching amp mute

Short - I'm trying to understand how to implement transistor(s), possibly a zener and resistors to switch a chip pin based on voltage change in another (sub)circuit with a common ground.

Long - I've a power amp and upon putting a preamp in front of it, find that the preamp capacitors draining at power-off results in instable preamp, opamp operation while the power amp rails are still up from much higher capacitance. Result is a very loud thump at power off, potentially damaging and needs resolved.

The power amp has bleeder resistors but not viable to increase bleed rate. Pre has had capacitance increased as much as viable but not enough. This power amp is LM3886 based ( datasheet,

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) and has a mute pin.

I would like to use a transistor(s) arrangement to mute the LM3886 when preamp rail voltage drops appreciably below "on" +15/-15. This preamp is running off the same supply as the power amp with common ground but rails reduced to +-15 by linear regulators.

To mute the LM3886, the pin (8) can be (left floating?) or pulled to Gnd. A test confirms it is muted while floating or grounded. Datasheet suggests 5mA current to V- rail unmutes it and this is how the power amp is currently configured.

The goal then is to use a rail collapse (to 0) on the preamp to trigger transistor switch to either: [connect the mute pin to ground]or [open circuit between mute pin and V- rail]. The former is highly preferable due to the original circuit board layout but either are possible. It appears the mute pin on the LM3886 is always at a higher potential (floating) than the preamp V+ rail. I'm suspecting I need at least 5mA at full "on" flowing from the mute pin and 0mA once the preamp rails start collapsing, at least by the time they drop to +-5V.

I only have a basic understanding of transistors and must be missing some key element of implementation. Just enough knowledge to get me into trouble as a beginner that has held a soldering iron for a few years and build simple(r) circuits. I have a handfull of 5V zeners, resistors, transistors, and would like to just do a basic discrete circuit (I know some of you probably have some elaborate IC that is beyond what I want to tackle and I am eager to learn-through-doing, where my transistor knowledge fell short.

What I am guessing (but might be wrong) is that I need at least a zener and resistor in series on the base of a transistor, but to what rail on the preamp and what else? Or is this wrong?

Reply to
JC
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Being the impatient sort that I am, I fiddled around and hacked out something. LM3886 spec sheet has a (10K??) resistor to unmute the chip which was removed from the circuit first. Turn-off thump elimination circuit then became;

Small signal NPN transistor (2N3904 was handy)

12V Zener 100K 1/4W resistor

LM3886 chipamp pin 8 (mute) to transistor collector Common Ground to 12V Zener to 100K resistor to transistor base Transistor emitter to wire lead to preamp V- rail

So I'm getting a couple dozen uA of drive and with typical gain, hFE of

100 or better, meeting the minimum 0.5mA UNmute current the LM3886 needs. Note I was wrong in my prior post, had thought the minimum unmute current was 5mA instead of 0.5mA. Good thing I reread the specs!

Ended up pulling out the soft-on cap, (those familiar with the circuit will know what I mean) as it's now always soft-on so long as it's been long enough after turn-off to drain the caps. IE- this particular amp less than 1 minute with the bleeders in the PSU(s).

Reply to
JC

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