60hz hum

[Doc B.]

This falls into the realm of clutching at straws, but are those metal V-Cap cases just floating, i.e., not grounded to the chassis?

[RestoredSparda]

Beginning to think the amp is fine (sounds wonderful) and it's just my house power. I get a similar very low hum through my emotiva amp to my speakers when my ear is to the driver.

Maybe I'll bbing the amp to a friend's house to see if there's a change. I remember my crack with Speedball also had a very mild hum.

[RestoredSparda]

That's correct. I was actually wondering about that as they are metal enclosures.  Any quick grounding test I can try without eletrocuting myself?

[RestoredSparda]

Notes about these caps from the manufacturer...

The short lead indicates the outermost foil, and should be connected to the lowest impedance path to ground. Another way to identify outer foil is the writing on the label flows towards the outer foil.

The ODAM, TONE, and OIMP series capacitors have metal bodies that are electrically conductive. Ensure that the body doesn't make electrical contact with any other portion of the circuit other components or electrically LIVE circuit traces. Heatshrink, 3M foam tape, or silicone tape around the body may be used to help insulate, and protect from electrical short. The TONE Cap series with values from .01 uF to .047 uF have a clear insulating sleeve over the brass canister and provides some isolation, but you should still safeguard against any laceration or puncture of this 'skin' to prevent any short...

[RestoredSparda]

I'd be willing to reinstall the original caps to rule that out if you think it's worth while.

[Paul Birkeland]

Can you play a 60Hz AC tone into the amp and measure the AC voltage across the RCA jacks?  Then turn coarse and fine switches all the way up, then measure the AC voltage between the ground buss wire at the front of the amp and each of the lower lugs on the coarse attenuator?  (you can/should do this with the amp off)

[RestoredSparda]

I'll try that and get back to you. Thanks!

[RestoredSparda]

.005 vac on rca jacks playing 60hz 10db file

.400 vac on front lower lug on coarse

.398 vac on back lower lug on coarse

This was with both coarse and fine on max volume.

[Paul Birkeland]

The volume shouldn't go up between the RCA jacks and the output of the attenuator.  Are you sure it's not 0.5V at the RCA jacks? If anything you could play a 2V signal at -0dB to bring the levels up.  No harm will come to the Mainline feeding lots of voltage into it.  I wouldn't feed the 120V available at the wall into the RCA jacks, but 2-10V of signal is totally OK.

[Paul Birkeland]

I should also mention that *if* the AC voltage does go up, that kind of nonsense is what I would expect from a ground that's not solidly connected.  We see this sometimes in Crack kits where a builder will somehow measure 400V of B+ in a kit designed to put out around 200V, and that's generally resolved by tracking down a bad ground connection.

[RestoredSparda]

My mistake. I did not have the power cable plugged in.

Now I get a reading of 1.1vac at the jacks, and .50 at both lower lugs, with a 0db signal playing. So, about half the voltage at the lower lugs compared to at the jacks.  Does this seem about right?

Turned all the way down i read .001vac at the lower lugs.

[Paul Birkeland]

Does this change with no power cable plugged into the Mainline?  That in and of itself would be rather significant as it shouldn't make any difference.

[RestoredSparda]

Ok, using the same test signal the power cable now removed, the result is the same as my last post. About half reduction in signal at jack compared to lower lug.  1.1vac to .5vac with volume all the way up.

This is a 0db white noise signal if that matters.

My first readings must have had the multimeter auto adjusting.

[RestoredSparda]

Ok, stranger and stranger.  With the 60hz signal instead, I am indeed reading .005vac at the jacks, but 1.255vac at the lugs.

This is without a power cable into the amp.

With 0db white noise it was the opposite.

[RestoredSparda]

I may be testing incorrectly.

Placing the black probe on the ground lug of either of the jacks, and the other on the solder point where the signal wire goes, I read 1.99vac at the jack, and 1.25vac at the lower lug with volume max on course. This is with a 60hz tone.



Prior, I was testing jack voltage by placing red probe on RI-1 signal wire/solder point, and black probe on LI-1 signal wire/solder point.