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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
128x128sbank
jea48
4,425 posts
 
11-17-08: Atmasphere
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!"


11-13-08: Sbank
Abe, according to the preamp designer, "All of the rca's on the preamp are isolated from the chassis and connected vis the 8 ga. buss. The chassis is grounded thru the charger jack only."

Atmasphere,
Is it possible the preamp is acting like a big antenna?

I still think Amandarae is on to something about the SET inputs being more sensitive than the other Amps Sbank was using.

Sbank, did you ever connect a wire from the chassis of the preamp to one of the amps chassis?

http://www.audioasylum.com/audio/general/messages/52/525608.html
sbank's avatar
sbank OP
3,378 posts
 
Jim,

Until my new tri-wire speaker cables arrive in a week or so, I can't try anything(the old speakers were sold this week), but I will try running a grounding wire from preamp to power amp chassis.

I did already run a grounding wire from one amp chassis to the other amp chassis, per suggestion from the amp designer.

I'll post again after trying that(as well another preamp), and we'll see what progress is made. Thanks,
Spencer
atmasphere's avatar
atmasphere
11,807 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.
sbank's avatar
sbank OP
3,378 posts
 
Here's an update:

After having a number of other audionuts over to listen, they all tell me that they have more noise in their rigs than I do, and that "Your hum is nothing". Well, I guess we all have different levels of tolerance. Perhaps I should just live with it.

FWIW, I ran a grounding wire from preamp to amp, and it didn't change a thing. I have heard intermittent radio signal from one speaker, when placing my ear next to the tweeter, but it disappears most of the time, and can't be heard from even a couple of feet away. Other than transforming my house into a giant copper encrusted Faraday cage, I don't have any ideas worth pursuing. Thanks to all,
Spencer
atmasphere's avatar
atmasphere
11,807 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?
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