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Can someone help me understand what this circuit does?
Can someone help me understand what this circuit does?
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Old 23rd May 2020, 02:24 AM   #11
roger2 is offline roger2  United States
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Join Date: Mar 2004
Location: indiana
OK, thanks again adason, rayma, and wg-ski

one last question about this circuit if I may...

Are any of these transistors in full saturation the whole time the amp is on? Or do they operate only at power on/off?

I ask this because I know that in relay circuits, one of the transistors that operates the relay is in full saturation continuously while the amp is on. And over a long period of time, this transistor's hFe becomes lower, and at some point will need to be replaced for the circuit to function.

Is something similar happening with any (or all) of the transistors in this muting circuit?



EDIT: thanks to sgrossklass also, who posted while I was typing

Last edited by roger2; 23rd May 2020 at 02:30 AM.
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Old 23rd May 2020, 02:29 AM   #12
rayma is offline rayma  United States
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Those transistors function as switches. For example, the two final transistors saturate,
and short the audio lines to ground, when muting. During normal operation all of the
transistors in that muting circuit are off (open). The timing capacitor is the weak point.
When it gets leaky, the muting time may become very long, or even eternal.

Last edited by rayma; 23rd May 2020 at 02:36 AM.
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Old 23rd May 2020, 02:32 AM   #13
roger2 is offline roger2  United States
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All right then! Learned a few things today....
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Old 23rd May 2020, 04:13 AM   #14
wg_ski is offline wg_ski  United States
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Saturation doesn’t do anything bad to a transistor. Reverse biasing the base-emitter into breakdown does degrade the Hfe (and make it noisy). The muting transistors usually have a high vebo spec for that reason - so that reverse breakdown doesn’t happen with large audio signals. The transistors won’t usually go bad, especially if not exposed to the outside world such as on an input jack.
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Old 23rd May 2020, 05:46 AM   #15
roger2 is offline roger2  United States
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Quote:
Originally Posted by sgrossklass View Post
It would, but would stay muted for longer.

wg_ski has a point, though I imagine that using a low leakage series electrolytic would be just fine as well... in fact I'd envision few problems even with some ordinary Panasonic FCs.

FC is what I will put in my next Mouser order. Likely better than that original cap that has worked fine for forty years.
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Old 23rd May 2020, 05:51 AM   #16
roger2 is offline roger2  United States
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Join Date: Mar 2004
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Quote:
Originally Posted by wg_ski View Post
Saturation doesn’t do anything bad to a transistor. Reverse biasing the base-emitter into breakdown does degrade the Hfe (and make it noisy). The muting transistors usually have a high vebo spec for that reason - so that reverse breakdown doesn’t happen with large audio signals. The transistors won’t usually go bad, especially if not exposed to the outside world such as on an input jack.
More good info. Thanks

Not sure if this is relevant, but I always turn down any amp before powering down, and stop the source signal as well. No music signal to make/break anywhere in the amp. I was thinking about power switches, but it seems that this would apply to the muting circuit as well.
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Old 23rd May 2020, 12:08 PM   #17
wg_ski is offline wg_ski  United States
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And you don’t want to get a loud obnoxious surprise the next time you turn it on, either. It might be too early in the morning for that.
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