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In Reply to: RE: SS pre hums with Mk IIIs posted by Dingojazz on November 22, 2014 at 16:50:45
Running a chassis ground amp to preamp stops the hum.
Thanks for the good input!
K.
PS: Anyone who feels inclined to offer why this is needed with the Mk IIIs but not for the ss amp would help further enlighten my ignorance...
Follow Ups:
I have measured grounds that should work just fine, but don't until you change ground points. Modern equipment makes a lot of assumptions.
Ok, so you're almost there. If the chassis wire solved the problem, then the issue is ground potential difference between the two units, which without the ground wire, causes the ground current to traverse the signal wire connecting the two units, thus making it amplified and audible.
This could easily point to a problem with the power supply of either unit, if there is significant ground current available. I'd do a safety check on the MkIIIs, make sure that there is no dangerous voltage present just to be safe.
Otherwise, it seems to indicate that on one of those units, chassis ground is not tied to signal ground. This could be either a fault, or be by design. It could also be that the shield/ground on your RCA connectors is no good.
So knowing that the ground wire solves it is really only part of the picture. It may wind up being the actual solution, but in this case I would want to know why there is such a potential difference between the two units, and I'd also want to know just how much ground current we're talking about here. This could be a symptom letting you know that you have a condition that might give rise to an electric shock if you were to say, touch the chassis and with the other hand, a grounded metal object. You don't want to find that out the hard way.
Thanks, yes some good issues to consider... No shocking is good.
Should note that the Mk IIIs are new (kit) construction. Assembled per manual, (AFAIK correctly..). All diagnostic voltage measurements are within spec., and neither amp exhibits any hum when connected to the tubed preamp, only with ss pre.
I would venture that the chassis and signal grounds are NOT tied, as initially I attempted to install un-insulated RCAs in the metal chassis, which would immediately cause fuse to blow. New insulated RCAs solved that problem, but from your comments, may reflect the chassis/signal grounding issue you raise.Thank again,
K.
Edits: 11/25/14
I've fixed ground loop hums by connecting mains safety grounds via contra parallel diodes ie a pair of say 1N5400s in parallel one arrow up the other down in series with the ground connection. They will blow any fuse in a fault condition but effectively isolate things so far as hum is concerned. I use this technique in my ham radio installation and it does cut PSU hum gremlins etc. This might not meet wiring codes etc but it does work.
Todd
The Mk III has a double insulated electrical lead (two wire)? The SS amp has a lead with a ground (three wire)? Just taking a shot in the dark here.
Petercapo has some wisdom too, kinda beat me to it.
Edits: 11/23/14
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