amp hums on start then fades


2005-04-28 10:49 pm
I am working on one of my old amps, a Gibson GPA50(50 watt PA amp). I pulled it out and turned it on and it had quite a bit of hum(60Hz). So I replaced the power supply 2000uF/35V caps with new 2200uF/35V caps. Now when I turn it on you hear hum that lasts about 5 seconds then fades to a soft hum( you have to be 2ft or so close to the speaker) that never goes away. The hum is unaffect by the volume. Otherwise, the amp seems to work normally. I really want to clean up the hum as much as possible.
Any ideas? I have included the schematic.


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Defective bias circuit also produce that sound when switch on

Switching on, and having short time high current, will overcharge supply and the transformer hum can be heard during around half second...then bias goes reasonable and the amplifier goes silent.

I also think electrolitic condensers, the supply filters are the first and main suspect ones.


found a noise source

Looking at the schematic and checking some caps I wasn't able to reduce the hum. I just decided to experiment with a few things. I disconected the power lamp. When I did the noise level reduced. It now is softer and sounds "lower". The amp still hums at a audable level when first started. The hum fades as it did before but to an even softer level. It might be as good as I might be able to expect from this type of amp. I still wish I knew why the noise is louder when you first turn the amp on and stays that way for a full 5-6 seconds before fading. I've seen this from tube amps but never in a solid state amp. Has anyone ever see this behavior in a solid state amp.
I would disconnect the preamp from the power amp. If the hum still starts loud, then you know it's the power amp causing it. (add other stages until you get the effect).
Monitor output stage quiescent current and see if it starts at a high level then reduces.
Maybe you get HF instability, and the phase margin increases as the amp bias settles.
I haven't had this problem before on a solid state amp. It's a tricky one for sure:scratch1:
I think if it was a HF oscillating problem then you would have a constant sound that won't go away after 5 seconds.
It may be a circuit grounding issue. If the Gibsons are anything like the Peavey guitar amps then they may use the chassis for allot of the audio and power supply grounding points.Clean these up.

Disconnect at point A. If the sound diminishes then go looking
behind that. It could be around FET Q11?

If it is still there then look at the front end of the power amp supply. I have even seen some hifi amps exhibit turn on noises
and "thuds" if the input differential pair transistors are not close in hfe matching...