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SPEAKER HUM

I have been playing with a new system. I was driving my speakers with a MingDa MC 34 AB tube amp (75 wt/ch ultralinear 40wt/ch triode) which is powered with 8 EL 34 tubes. It can be used as an integrated amp, and when I used it as such, it was DEAD quiet.
I decided to make a good thing better, and just got the matching MingDa MC 2A3X preamp to go with it. The speaker hum when both are powered up is awful...not that very low level stuff you sometimes get with an amp/preamp set up...but loud enough to hear during quiet passages of musical pieces.
I have been tormented in the past with trying to eliminate speaker hum...any help out there for this system??
Thanks so much
rsasso
rsasso OP
39 posts
 
To Millicurie

amp and preamp are plugged into separate outlets
rsasso OP
39 posts
 
To Millicurie

I substituted a small Tono preamp for the big Ming Da...result was LESS hum (and higher frequency)...but still some hum. I am not sure what that tells us.
swampwalker's avatar
swampwalker
5,132 posts
 
IIRC, plugging the amp and pre to separate lines is a very good (BAD) way to induce a ground loop. However, since my electrical skills/knowledge are slim, I could easily be wrong. However, its an easy experiment to try them on the same duplex.
newbee
4,093 posts
 
At the risk of being too obvious - have you tried a cheater plug yet at the amp/current source? If not you should. I would guess that it will reveal that your problem is an old fashion ground loop. Once you know its a ground loop you can take further steps to try to eliminate it, or just leave the cheater plug in place (that is what most folks do provided the pre-amp is grounded).
rsasso OP
39 posts
 
To Millicurie

OK: I substituted the interconnects between amp and preamp with heavier shielded cables and that had no effect. The hum is NOT there if the amp only is turned on.

The hum is there when the preamp is on, even with no components connected to it, as soon as you turn on the amp.

If you TAP the preamp when the amp/preamp is humming...there is a ringing sound out of the speakers. If you even gently tap the ceramic bases of those 6SN7 tubes, there is a louder ringing through the speakers...now THAT can't be right.
Does that speak to defective tubes, or, more likely, a defective preamp????
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