Can main amp caps cause distortion?

[Quakeos]

Hi guys ,

I put up a post earlier of this vintage MOSFET amp I’m trying to restore and long story short channel A is distorted and I cannot answer why . I can confirm , after bypassing all circuitry with a low level single straight to the power proceeding stage that it seems to be a problem on the actual power stage of the amp .

This amp is old and seen many nights, is it possible the main two caps (10000 uf) that one of them has gone haywire andpossibly be causing this distortion? It should also be noted that the amp Starts clear and becomes distorted when turned up. DC offset is also 100mv (yikes?)

Thanks guys

 

[Mooly]

I would doubt the main reservoir caps were responsible. I'll have a look at your other thread 🙂

 

[Quakeos]

I would doubt the main reservoir caps were responsible. I'll have a look at your other thread 🙂

Thank you Mooly Appreciate it 😀

 

[nigelwright7557]

Old amp smoothing capacitors can lose their capacitance with age.
Look for excessive ripple on the power rails.

 

[traderbam]

What does channel A sound like with no input? That is, when you put your ear to the speaker what do you hear. How does it compare with channel B?

Electrolytics can fail over the years. They tend to dry out and become more like resistors than capacitors and sometimes short. So bad caps should cause more psu ripple at the least.

Other common faults in old circuits are cracked pcb traces and component leads, some which may get worse with temperature.

 

[prasi]

may be a problem with bias circuitry?, as amp starts clear and then distorts?

 

[edbarx]

Calculation for ripple:

Definitions:
C: smoothing capacitance
f: mains frequency
I: ouput current

For any capacitor:
Q = CV {Q is charge, V is charge voltage}

Differentiating we get:
dQ/dt= C.dV/dt

We can drop the dt and get:
dQ = C.dV

But dQ = time*current = (1/(2f))*I = I/(2F)

Therefore:
I/(2f) = C.dV

dV is the ripple:

dV = I/(2fC)

Example:
For a 50Hz supply, a DC current of 10A and capacitance 6600uF, we get:
dV = 10/(2*50*6600x10^-6) = 15V approx.

 

[Mooly]

The other thread is here:
Perreaux 6000B Problems, Advice needed

 

[nauta]

Hi Guys

DO NOT test amps with speakers until after you've done proper bench tests.You can easily have a rail of DC across the speaker and then no more speaker.

You do not say if each channel has its own PSU?

If the supply is shared, then the distortion in ch-A is due to a failed component there.

Filter caps have a maximum service life of 14-years if they were good ones to begin with. Most caps will be "functional" for decades, but for hifi, the DA and DF go crazy and make them add more THD than the active circuitry.

Remember: the power supply filters are the other half of the signal path.