Hum Hum Hum

rutledj · Post by **rutledj** » Fri Oct 05, 2007 11:31 am

I'm fighting a hum problem. What it weird is that it will go away just by touching a scope or meter lead to one of the relay connections. I have 2 clean channels so I'm switching the inputs via a relay as well as the outputs.

Do you find it necessary to do any special wiring around the relays (ie. shielding cables, ground plane on relay board)?

Rut

mdroberts1243 · Post by **mdroberts1243** » Fri Oct 05, 2007 11:42 am

rutledj wrote:I'm fighting a hum problem. What it weird is that it will go away just by touching a scope or meter lead to one of the relay connections. I have 2 clean channels so I'm switching the inputs via a relay as well as the outputs.

Do you find it necessary to do any special wiring around the relays (ie. shielding cables, ground plane on relay board)?

Rut

Did you reference the relay supply to system ground at one point? I've had a similar problem in the past when I've attempted to run a 'floating' supply.

rutledj · Post by **rutledj** » Fri Oct 05, 2007 6:19 pm

The relay supply ground is connected to the other grounds. I'm switching the relays via the ground connection with hot on them all the time

heisthl · Post by **heisthl** » Fri Oct 05, 2007 7:40 pm

rutledj wrote:The relay supply ground is connected to the other grounds. I'm switching the relays via the ground connection with hot on them all the time

That is the correct way, switching by grounding. Try putting a 1000uf cap on the relay power. It that doesn't get rid of it you may have to redefine "Hum". Is it maybe the kind of hum like having an open ended patch cord plugged in?

BobW · Post by **BobW** » Fri Oct 05, 2007 7:45 pm

Knowing if it's 60 Hz. or 120 Hz. tells you where the issue is. 60 Hz hum is prior to any rectification, and 120 Hz. is after (assuming a full wave bridge).

When you touch a scope probe to the relay circuit, two things are happening:

1. you've provided approx. a 20 pF cap to ground.

2. you've chassis ground referenced the circuit.

rutledj · Post by **rutledj** » Fri Oct 05, 2007 10:58 pm

Well, is there an easy way to tell the diff between 60 and 120? My ears aren't quite that refined:)

It does sound kinda like a loose patch cord plugged into the input but even when nothing it plugged in.

I'm still questioning my grounding scheme. Using Funk's supply board which has separate grnds for B1, B2, and B345 (together as one) I have both B1 and B2 going straight to the chassis gnd where the PT centertap is connected. The other ground is run to the buss wire behind the pots with that. Another wire runs from the buss wire (near the input jack) to the chassis ground. All of my preamp grounds and pot grounds are tied to this buss wire. Is there a better way to do this?

I have shielded wire from input to relay, relay to tube input, v2b output to vol and vol back to relay before od. Wouldn't think I'd need any others.

Thanks,

Rut

Rut

heisthl · Post by **heisthl** » Fri Oct 05, 2007 11:10 pm

You didn't say - did you try clipleading a cap(1000uf is best) from the supply point right at the relay with the pot bus as a ground?

rutledj · Post by **rutledj** » Sat Oct 06, 2007 12:04 am

Actually I think I found it. Put 100 ohms across heater wires to grnd and it went away. I should have tried that first! Duh.

Thanks anyway.

Rut

The Amp Garage

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