Amplifier Buzz

[Alonzo]

My recently completed amp has a low buzz thru both speakers, not audible from my listening position, about 7 feet or so away from the speakers.  But right up close I can hear the buzz (about 2" away from the speakers).  This is buzz that's not affected by the hum pot and is much louder if no source is plugged into the amp and powered, once my quickie is turned on the buzz recedes, to barely auidible.  Also if you power off the source, the buzz isn't immediate, the amp will be quiet then the buzz will build up to an audible level.  Can someone recommend a quick checklist to try the next time I have the amp on the table?  I'm going to isolate the grid  chokes incase the buzz is mechanical, and add a ground from the grid chokes back to the ground buss (should've done this during the build, guess I got in a hurry).  Any other ideas on what to check?  It's not bad, only because I know it's there is there a problem.  And the fact that I don't know what's causing it so I can avoid it in the future.

Oh yeah, the amp is the one in my other thread "weekend project"  a 45 amp with MQ outputs and chokes, hammond power transformer.
Thanks

[Jim R.]

Alonzo,

I think you're on the right track with isolating the grid chokes from the chassis plate and grounding them to the chassis central ground point.

Of course I can't see the layout, but as I recall, you had things pretty well crammed in there so it could also possibly be an interaction with another magnetic field.

HTH,

Jim

[Alonzo]

Hey Jim,
It is pretty crowded.     I moved the chokes around a bit to see if any hum would be induced but didn't get much increase, but I wasnt that close to the speakers when I tested either.  I didnt know that buzz could come from an induced field or from interacting fields.  In my head they were always seperate, buzz would be  mechanical, some flaw in manufacture or install, hum magnetic by poor placement and interactions.  I guess anything that causes an increased resonance and vibration can come out of the speaker as many different sounds.

[Paul Joppa]

When you turn on the preamp, it provides a much lower impedance at its output (the power amp input). That suggests the power amp is  picking up an electric field by capacitive coupling. One test is the proverbial shorting plug at the power amp input, which provides a (very!) low impedance.

If it's in the power supply of the power amp - rectifier spikes, fro instance - then the buzz would stop instantly when the power is switched off, even though the tubes and capacitors take a while to cool and discharge. But if it's coming from outside (fluorescent lights with old-style 60Hz ballasts, for instance) it would continue for a bit.

[Alonzo]

It does die off immediately when the power amp is turned off.  I'll look at my grounding also, see what I can improve.  I didn't do a star ground, made a big U ground buss that everything is connected to.

[Paul Joppa]

Could be reverse recovery spikes, if you are using silicon rectifiers.

[Alonzo]

Well I am using UF4007's in a full wave rectifier.  I'll have to do some reading on RRSF's and snubbers.

[Alonzo]

For anyone working on kits or diving back into an old build, I highly recommend something of this sort:  http://www.staples.com/Ledu-Professional-Fluorescent-Magnifying-Clamp-On-Lamps/product_SS945334.  I've been chasing down a buzz I had in both channels of my amp and today when putting in some new caps I broke out my magnifying glass to check connections.  Well after moving components and putting in the new caps I still had the buzz.  I traced all my ground connections, lo and behold, non of my heaters were actually connected to chassis ground.  I had run all the CT's to one common chassis ground and crimped the wires but the shiny solder blob wasn't actually connecting anything.  I could move the wires around and watch the solder connection move.
I re-did this connection and everything sounds great.  Now to move in my horns and give it a proper listen...
Pic of the new (still crowded) interior.  It's now housed in a Blumenstein'd wood case.

[Grainger49]

MMmmmmm.... lots of iron and capacitors.  It has got to sound good! 

[Doc B.]

That's the same magnifier we have in the lab. I use it all the time.

[Alonzo]

I think I'm finally done with this amp (for now),  just shoved nickel BH-5's under the hood.  I can't imagine it sounding any better.  Got Gary Moore's "Bad for you Baby" playing to get my 1/2 hour warm up.  Maybe next winter it'll get a soft start upgrade.

[Grainger49]

I linked a similar lamp in the Tech Tools folder last week.  It was under $40.  It may be off sale now, I don't know.