Class A BJT Powering on question. Will this send a jolt to my power amp?

garybdmd

Member
2016-01-03 5:13 am
Hi all,

This is a basic question about powering on a Class A amp. If I initially throw this switch on and the output capacitor is not yet charged, will the output be 9v for an instant while the capacitor charges? I was thinking of putting a volume pot as the collector resistor instead of at the emitter resistor. (This suits me a little better in a design I'm working on for a preamp).

Any experience appreciated,
Thanks all
 

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rayma

Member
2011-04-29 8:37 pm
If I initially throw this switch on and the output capacitor is not yet charged,
will the output be 9v for an instant while the capacitor charges?

There will certainly be startup and power-down transients while the output capacitor
charges and discharges, with a time constant of roughly 0.5 sec. An output muting circuit
can deal with this. Running direct current through a volume pot is not a good thing, though.
 
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Looking at the output of your AC coupled amplifier.
when power is OFF, both sides of C3 sit at zero volts DC.
At the instant of power ON, the left side of C3 will increase rapidly to roughly half Vsupply.

Being a capacitor, that rise in C3 left side voltage will be transferred instantly to the right side.
The voltage on the right side will get dragged up with the fast changing voltage on the left side.
Then the 50k load will start to discharge that right side voltage. That's the output pulse that the load sees.
You cannot avoid that in AC coupled circuits.
Similar occurs at the input capacitor. It too needs a resistor to audio ground to charge/discharge C2.

A time delayed relay that isolates the actual output from what happens on the 50k will solve the pulsing output problem.
The 50k allows the amp side of the relay to return to audio ground voltage, but you should also fit a grounding resistor to the output side of the relay. 1M0, or 2M2, would do.

You can also fit a muting relay to the input. It can use the same delay circuit as the output relay. Again both sides of the input relay should have a grounding resistor, both can be 1M0, or 2M2.
Even without an input muting relay C2 NEEDS a grounding resistor at it's left side.
 
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garybdmd

Member
2016-01-03 5:13 am
Actually, I'm not so worried about hearing a pulse. My main concern is about killing an amplifier with the signal. I really don't have any practical experience about this. I'm just trying to build a preamp and not fry the power amp section. If I hear a pulse or plop its no big deal to me. But I don't want to turn on a preamp and fry the amp section. Really, this is my main reason for posting.