2. amps-preamps
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Ground Loop Issue 427

After talking with the manufacturer's of both the amps & preamp, I still have a 60hz hum. Here's the story:

Just changed amps to 845 SET monos, and now I hear the hum whenever the amps are on and RCA interconnects are connected to the amp inputs. Didn't have the problem with two prior pairs of amps. The pre could be turned off, and I still hear the hum. With shorting plugs in the amps, no hum.

The pre is battery powered, with no ac cable, and the trouble persists whether or not any sources are connected to the pre.

I've tried multiple types of interconnect, including the heavily-sheilded cheapos from a vcr, but no change.

One friend questions if adding a "hum potentiometer" to the amps would make sense. Others have suggested the $600 Granite Ground Zero, which is unaffordable for me.

I've already tried cheater plugs any/everywhere. I've added a grounding wire between the monoblocks metal bottom plate and floating one power cable while leaving the other grounded, all per the amp manuf's suggestion. I've tried HighWire LiveWires, which might be good sonically for RFI, but aren't helping the hum issue. This is a music only rig, no cable tv anywhere in the room. I am in a heavy RFI area, 1000ft from a radio tower, if that matters...

Anybody got any suggestions other than moving elsewhere? Thanks,

Spencer
sbank

Showing 4 responses by atmasphere

atmasphere's avatar
atmasphere
12,101 posts
 
Spencer, low level hum is a common problem if you are putting your ear to the speaker. SETs usually use a directly-heated cathode, which is to say that the filament of the tube and the cathode are the same thing. This means that you have to have a DC supply if the power tube is not going to hum; getting the DC supply to be quiet suddenly becomes an issue!

RF issues are often caused by lack of what is known as 'grid stop' resistors in the circuit or issues in the grounding scheme of the amplifier (I'm talking about how the grounds are laid out inside the amp, not outside). If the RF is rectified, you will hear the station, and it is well-known for also causing hum.

Have you noticed the RF with any other amplifiers?
atmasphere's avatar
atmasphere
12,101 posts
 
Hi Spencer, I'm in the camp of 'you have an RF problem'. I think, based on reading all of the above, that you have more RF susceptibility with with this amp, and that the long cables are somehow able to contribute where the other amps were not so susceptible. I don't think its a sensitivity issue.

There could also be a preamp issue that has not manifested before. If you can get your hands on a DVM, check and see if there is any continuity (less than 1 ohm) between the connectors and the chassis. Sometimes if the chassis is only grounded through the wall and the circuit isn't grounded to it you can run into trouble. That's not so bad in an amplifier but in a preamp it can result in hum, even if it worked fine with other amps!

Back to the RF issue: Have you tried different cables?
atmasphere's avatar
atmasphere
12,101 posts
 
Hi Spencer, your DVM can measure Ohms. Just put it on the Ohms scale; the reading you are looking for between the RCA grounds and the chassis ground should be the same as connecting the probes of the meter together.

Most SETs have less gain than amps of larger power; the reason for this is that SETs really do need higher efficiency speakers to be useful. To keep noise down, and otherwise on account of the efficiency of the speaker, SETs simply don't need the same gain as amps intended to drive speakers that are 20 db less efficient.

If the amps have the 3rd prong of the AC cord plugged into the wall, often this will result in a rather substantial hum or buzz, depending on how the amp is grounded. So until this issue is solved, I would run the amps with some ground cheaters- the only ground in the system (for best results) should be the preamp.
atmasphere's avatar
atmasphere
12,101 posts
 
Hi Spencer, everything you mentioned just now is consistent with RF problems, how the amp behaves with shorted inputs, interconnects, etc.

Some RF beads and grid stop resistors at the input of the amp might take care of it and improve the sound at the same time.
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