hum with 2 connected amps

[scotto]

I'm Running my Bumbox (1 watt tube amp) line out into another amp and it's humming very loudly. Any suggestions? the line out jack appears not to be directly grounded (loop jacks are).

[Structo]

You'll probably have to lift the ground on one of the amps.
Not the safest thing but it usually works.

[selloutrr]

assuming both are on the same power?

try lifting one amps ground if it doesn't improve try the lifting the other.

if it's going to be a constant setup consider a unified grounding schem.

if you have an isolation transformer you might have luck with that.

[paulster]

You shouldn't lift the ground at all. If you do that you are relying on the shield on your guitar cables and potentially the ground plane on the PCBs in pedals between the amps to be able to handle the full fault current until the time-delay fuse blows if you get a fault in your amp, whist protecting you from electrocution.

You need an isolation transformer, like a Lehle P-Split, or one built into some of the A-B boxes (I think Radial might have them in some of their models).

Ask yourself, would you use a guitar cable to connect between head and cabinet? Answer, no. Because you don't want your output transformer to die when the guitar cable gives up. So, why would you rely on a guitar cable to protect you from dying?!

[scotto]

Thanks guys. Priority one is obviously staying alive. I'll look into the transformer approach. Any schematics on this implementation available?

[Andy Le Blanc]

I've seen similar issues, even that crap behringer a/b is better than putting yourself at risk.

[paulster]

Thanks guys. Priority one is obviously staying alive. I'll look into the transformer approach. Any schematics on this implementation available?

I use Lundahl LL1531 transformers since I'm in Europe but the Jensen JT-11P-1 is the US rack builder's benchmark.

Application note:
http://www.jensen-transformers.com/as/as013.pdf

[M Fowler]

I tried the Radial A/B/Y box and you get plenty of hum with that pedal.

The solution is at the input of each amp not lifting grounds on the mains.

I still haven't tried Joe Bonamassa's stereo rig wiring yet for ground isolation. Maybe someday it will get done.

[paulster]

Joe lifts the ground on three of his four amps. Even in his European rig, running at 240V!

Great player but bad example to follow!

[Structo]

Yes but if you are not physically connected to the ungrounded amps, is it still dangerous?

[paulster]

You are physically connected to the ungrounded amps unless there is a transformer in between, though. That's the issue. B+ on the chassis would appear as B+ on your guitar strings if the guitar cable blew open between the splitter and the grounded amp before the fuse went.

[Structo]

What if you are going through FX pedals or a loop?

[scotto]

Just to clarify, I'm going from one amp into the other, so it's a series connection. Joe's rig and the device in the link are parallel unless I'm mistaken. Nate (Bumbox inventor/builder) just suggested that I attach the line out jack to ground. I'll try that tonight.

[paulster]

What if you are going through FX pedals or a loop?

Then, as well as the cables, you'd be relying on the ground connection in the pedals to carry the fault current as well.

Fortunately most pedals use Switchcraft open-frame jacks that are tied to the chassis so they'd carry the current across the pedal case okay, but those that used Cliff-style jacks would rely on the ground connections between the jacks and the PCB, and the PCB traces as well.

A series connection between amps presents the same issues, although there is less chance of electrocution if the amp at the end of the chain is the one that gets its ground lifted since the amp you're plugged into would be grounded properly. I still wouldn't chance it though.

[tubeswell]

If its a series connection (like say a standalone reverb unit in front of an amp), then use either a passive isolation 1:1 audio transformer with really high impedances (like 15k or more), which will be expensive, or make a buffered isolation box with some silicooties.

Another method would be to use an internal ground return lift (not a mains ground lift) in one of the amps, between the ground return path and the chassis using 2 x 6A diodes back to back in parallel with a 47R flame proof resistor and in parallel with something like a 1kV .01uF cap, whilst keeping all input and out sockets isolated from the chassis (like you see in many commercial amps these days, including the Fender RI standalone reverb unit)

2CW