Bad capacitor?
- Thread starter simonsez
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Bo Deadly
Well-known member
If the capacitor(s) lose capacity, ripple will increase. If it dips low enough, it will pass through the regulator and you can get hum. If the capacitor(s) are leaking, then the voltage may drop to the point where again the regulator will simply not regulate and the voltage will be low, you'll get hum and eventually horrible clipping. If one of the caps fails badly to the point where one rail is much lower than the other, a DC offset could actually cause an op amp to latch up one way or the other and maybe burn out something downstream. But that's theory to me personally. I've never seen caps fail like that. I recapped some stuff that was maybe 40 years old and the capacitors removed were still mostly fine.
simonsez
Well-known member
squarewave said:
thank you, so is it possible C1 or C2 suddenly fail? assume that cap is good quality, running cold, more than double voltage rating, and all surrounding parts was fine.
Yes, fail is statistically distributed - sudden fail is rare, but not impossible.
Jakob E.
Jakob E.
moamps
Well-known member
squarewave said:
This phenomenon starts with raising of distortion in LF part of the spectrum. There is the biggest energy which opamps can't handle anymore because the rails are dropping in 100 or 120Hz envelope. It's most obvious in male vocals. The hum isn't so noticeable because opamps have good PSRR.
I didn't noticed this. Opamps can work very well under unsymmetrical rails.
I have this type of fail very often in, for example, DBX 166X, some Samson and Behringer compressors and distributions. The problem is in sub-optimal design of power supplies where capacitors are too close to the heatsinks of the regulators. After some time of overheating, capacitors leak and lose the capacity. Additional problem is the high primary voltage which is over here 230V +/-10% what's about 250V sometimes, what causes overheating of the regulators.
This type of PS design flaw I see here in DIY also very often.
moamps said:
Well the regulators only 'care' about what voltage they are seeing at input relative to output and the current draw.
So I guess you are saying that a high line voltage gives a secondary voltage significantly higher than the ;design voltage' - here 18Vac and that the increased voltage drop leads to voltage regulator overheating ?
Yes - you need to incorporate that headroom into your voltage regulator specification and thermal management.
Should also give a mention to the ripple current rating of the capacitors. This is often marginal in cost conscious power supplies leading to failure. Particularly known to happen in flat screen TV (or as we now say TV
products.
caps can be bad different ways.simonsez said:
A failed short circuit will generally make a mess with no output voltage.
A failure from electrolyte loss will generally show up as low capacitance, so deeper ripple troughs. If the bottom of the ripple is below regulation drop out you will get hum noise on the regulated rail, and likely in the audio.
JR
simonsez
Well-known member
Thank you all,
In my situation, it's look that the problem is not bad capacitor, but bad diagnosis. :-X
In my situation, it's look that the problem is not bad capacitor, but bad diagnosis. :-X
