Re: Mackie CR4 amplifier no sound
You know, that actually brings a good question:
Should Q2 be "hard on" (i.e. operating in Saturation) in the above circuit?
The more I look at it, the more I think not. Perhaps a Mackie designer can answer why R91 is connected between ground and Emitter of Q2 instead of pin 5 of amp IC and Collector of Q2 (with Q2 Emitter tied directly to ground.) In that arrangement, Q2 can easily be used as a "switching device". Whereas right now, I think it looks more like a current amplifier... and the problem with this is that even small mV changes in the Base voltage of Q2 can induce large changes in the current flow through its Emitter. And while B-E voltage will always be about a diode drop apart, C-E doesn't have to be. In fact, C-E can be any voltage if Q2 is operating in its Active region... which is what I think may be happening here too.
So if C39 is determined to be OK... or at least the amplifier still acting goofy and muting by itself without C39, then I think there is also a possibility that we may be looking at a marginal design in the mute circuit.
**EDIT**
I looked at the circuit again, and indeed Q2 is used more like an amplifier than a switch so that it can drive different voltages on pin 5 in order to go between STBY/Mute/Play.
But IMHO, this still looks like an awfully complex way to do that.
Originally posted by petehall347
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Should Q2 be "hard on" (i.e. operating in Saturation) in the above circuit?
The more I look at it, the more I think not. Perhaps a Mackie designer can answer why R91 is connected between ground and Emitter of Q2 instead of pin 5 of amp IC and Collector of Q2 (with Q2 Emitter tied directly to ground.) In that arrangement, Q2 can easily be used as a "switching device". Whereas right now, I think it looks more like a current amplifier... and the problem with this is that even small mV changes in the Base voltage of Q2 can induce large changes in the current flow through its Emitter. And while B-E voltage will always be about a diode drop apart, C-E doesn't have to be. In fact, C-E can be any voltage if Q2 is operating in its Active region... which is what I think may be happening here too.
So if C39 is determined to be OK... or at least the amplifier still acting goofy and muting by itself without C39, then I think there is also a possibility that we may be looking at a marginal design in the mute circuit.
**EDIT**
I looked at the circuit again, and indeed Q2 is used more like an amplifier than a switch so that it can drive different voltages on pin 5 in order to go between STBY/Mute/Play.
But IMHO, this still looks like an awfully complex way to do that.
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