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Transistor switching amp mute

J

JC

Jan 1, 1970
0
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,
http://www.national.com/ds.cgi/LM/LM3886.pdf ) 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?
 
J

JC

Jan 1, 1970
0
JC said:
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.

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).
 
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