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power amp hum and noise while starting up on variac

gogofast

gogofast

Addicted Member
i decided to restore my phase linear 400. while bringing it up on a variac, there was a very loud and annoying noise/hum/thumping from 50v to 100vac. after it was fully brought up and charged up for about 2 minutes, the noise went away and the amp sounded fine without much distortion.

what do you think is the problem here? i've ordered parts to recap the amp including the large 5900uf caps. does it sound like lytic caps having issues charging up? or possibly resistors or outputs?
 
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Hum when the mains are low is normal. If there's a regulator involved and the voltage is under regulation, all of the ripple gets through.

Odd behavior when not fully powered?
Sure, it can happen. The amplifier needs to be powered up for the components within the feedback loop the ability to do their jobs. If that doesn't happen, any number of odd things can happen.
 
If you're playing in Carver-Land, don't try to VariAc any of the TRIAC switched (Magnetic Field) supplies. You need to do certain things to do that.
 
ok, thanks for the advice. i'll recap it first and see what happens.

i've been reading some info on mods and upgrades for the 400, and was wondering if all those tweaks will make significant difference...like replacing insulators and etc.
 
Hmmmmm, I don't know of any accepted tweaks. There was a Carver bulletin about adding diodes around the diff amp for turn on pops, etc. I don't really recall the details so I won't lead you down a dangerous path.

Make sure the input grounds are tight to ground at the input jack area. Make sure that the high frequency load resistor (the series cap and resistor that connects output to ground) isn't fried.

In my opinion, though, if you can get away with having a boxer fan on each heatsink, that would be way better than any thermal improvement that a different insulator could offer. The 400's are on the underbiased side, with a little notch distortion. The problem is, the heatsinks aren't really large enough to bias it higher. With fans, the bias can be increased. With two fans, you can series wire them to make them turn slower (quieter).
 
Jon_Logan said:
In my opinion, though, if you can get away with having a boxer fan on each heatsink, that would be way better than any thermal improvement that a different insulator could offer. The 400's are on the underbiased side, with a little notch distortion. The problem is, the heatsinks aren't really large enough to bias it higher. With fans, the bias can be increased. With two fans, you can series wire them to make them turn slower (quieter).
Click to expand...

funny that you mentioned the box fan. the reason i'm trying to use the 400 is that my current SS power amp has an internal fan and it bothers the heck out of me even at moderately loud volume-between the notes. any recommendation on quiet box fans and how to hook them up?
 
Good, smooth, 120VAC fans can be wired in series. 60VAC per fan. Not all fans will turn with half the voltage across them. Pabst is a good brand. Find a model with roller bearings instead of the sintered bronze plain bearings and you should be set. When the fans are turning at this half voltage, they are super quiet, but keep the air moving enough. This should be good for even the occasional rock out session, however you do want to keep tabs on the heatsink temperature.

If you don't want to do the series fan thing, you can put a resistor in series with each fan. The Hafler DH-500 had a couple of speeds switched by thermal switches. I think they used a 330 ohm/10W for their low speed. The resistor will make it's own heat, though.

A sensing circuit might offer the best compromise. Low speed and quiet while the heatsink is cooler, but kick in to high gear when the temperature (and volume) is high.
 
i don't really play all that loud. so i think i'll try that series fan approach. maybe just plug it to a preamp and use it from rear of the amp?
 
Sure. IMO, you do want a fan on each heatsink.

For the 400, you might want to look into fusing the speaker line, or putting an after market DC protect circuit on there. The heatsinks aren't that big and there's very high rails (90V). When they "let go", it can be expensive in a hurry. Keep your speakers happy.

I don't want to scare you about the 400. I've repaired a lot of them back in the day. I will also say, they can really move the mail. They were used extensively at a chain of clubs in the SF Bay Area called the Keystone. Keystone Berkeley, Keystone Palo Alto, and the Stone in SF. They used fans and very few problems with theirs.
 
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