Amp hum

Jonathangus

A few months back I had a Metro zero-loss FX loop fitted to my JMP 2204 by a reputable amp tech.  The loop itself works brilliantly, but the amp has developed an annoying hum in some situations.  So far, I've managed to identify the following:

• It's nothing to do with the guitar as the hum is present with nothing plugged in.
• It's not affected by any of the controls, with the exception of the presence; it's greatly reduced with the presence full up.
• It occurs at home and at my Dad's house, but is much less noticeable at our rehearsal room; there I have to get right up close to the speaker to hear it.

That last point makes me think it's environmental - so is it:

1. A shielding problem?  Whoever built the head enclosure added a metal panel on the bottom, but it doesn't go all the way across and enclose all the circuitry.
2. A mains problem?  Would a mains conditioner improve it?  (I tried turning the fridge off, as I know they can cause issues, but to no avail.)
3. Unlikely, I know - but the cab?  As that's the only other variable between home and the rehearsal room.

TIA.

Jonathangus

And here's what it sounds like:

https://youtube.com/shorts/u6L9cIdeg58?feature=share

ICBM

Are the cabs at home and the rehearsal room different impedances?

Asking because the presence control is part of the power amp negative feedback loop which is derived from the speaker output - from the 4-ohm tap, although it affects all of them.

Jonathangus

Both 16-ohm.  Mine's a SV112; at the rehearsal room, it's a 1936.  (Unfortunately we've recently lost the use of the room, so I've no way of trying it again.)

ICBM

Jonathangus said:

Both 16-ohm.  Mine's a SV112; at the rehearsal room, it's a 1936.

A standard 1936 is 8-ohm. If you were using it with the amp set to 16 ohms, that could possibly reduce the hum level.

Jonathangus

I remember the guy saying it was wired funny - so I took a multimeter in and measured it.  It was 16, or thereabouts.  (I know resistance and impedance aren't the same thing.)

Possibly I need to try it with another cab or two.

ICBM

Jonathangus said:

I remember the guy saying it was wired funny - so I took a multimeter in and measured it.  It was 16, or thereabouts.  (I know resistance and impedance aren't the same thing.)

Possibly I need to try it with another cab or two.

Interesting - sounds like it had two 8-ohm speakers wired in series. If it was accidentally wired out of phase as as well, which I have seen done quite a few times with series wiring, that would definitely reduce the hum level. (But not change the impedance.) Or if it just had two rather bass-light speakers...

Jonathangus

OK, so I don't have another cab, but I hooked up a Captor X and listened through headphones.  Again, the hum is there, but it's much less noticeable than through the cab.  So it seems that, whatever it is, it's being picked up by the speaker and amplified.  So I guess the next step would be to shield the amp chassis properly, and see if it improves things.

maw4neu

Hum bug . . . ! . . . Its so frustrating, my old SJB Ant hummed a lot and it took a while to calm it down . . . Nigel, the Amp Tech and utter genius, is a patient guy and fortunately for me he's not expensive either . . . I think the extra screening helped enormously and I ended up fitting a full width aluminium plate to the bottom of my Amp as well as making an Aluminium Box / Guard to cover the Vol & Tone Controls as well as the Jack Inputs . . .  I also covered the Rear Panel, apart from the open mesh area, in 2 or 3 layers of the adhesive aluminium foil we use when lagging ductwork or pipework . . . All these small steps made a big overall difference . . . 

Here's a picture I took just before I fitted the Aluminium Plate along the bottom of the Amp . . 

https://i.imgur.com/eWStYFB.jpg

Jonathangus

maw4neu said:

Hum bug . . . ! . . . Its so frustrating, my old SJB Ant hummed a lot and it took a while to calm it down . . . Nigel, the Amp Tech and utter genius, is a patient guy and fortunately for me he's not expensive either . . . I think the extra screening helped enormously and I ended up fitting a full width aluminium plate to the bottom of my Amp as well as making an Aluminium Box / Guard to cover the Vol & Tone Controls as well as the Jack Inputs . . .  I also covered the Rear Panel, apart from the open mesh area, in 2 or 3 layers of the adhesive aluminium foil we use when lagging ductwork or pipework . . . All these small steps made a big overall difference . . . 

Here's a picture I took just before I fitted the Aluminium Plate along the bottom of the Amp . . 

https://i.imgur.com/eWStYFB.jpg

That's what I'm thinking of doing.  There's one there already, but it doesn't go all the way across - and I think the gap is under where the loop now sits.  So a trip to B&Q, I reckon.

Modulus_Amps

Have you got a photo of how the loop was installed, if you plug into the loop and turn the guitar down does the hum go away?
If the loop is grounded to the speaker ground then that might explain the hum.

Is the hum affected by the gain or master volume?

Adding shielding willy nilly might solve the problem, or might just make it bearable, if you can track down where the hum is coming from then it you eliminate it

maw4neu

Modulus_Amps said:

Adding shielding willy nilly might solve the problem, or might just make it bearable, if you can track down where the hum is coming from then it you eliminate it

100% agreed . . . 

That said, its absolutely no secret that at the same time I'd added the additional screening and made the Aluminium Box to fit around the Controls Area, as well as replacing all the Valves with new or nearly new, Nigel . . . . my good mate and Amp Tech had also replaced all the Filter Caps and fixed anything that looked remotely electrical :-)

Despite all the Amp Tech's painstaking work, I'd still like to think that it was "mostly" my efforts with the sticky backed silver Aluminium Foil that got the job done :-) and Nigel, if your reading this, the cheque is in the post  . . . 

Jonathangus

Modulus_Amps said:

Have you got a photo of how the loop was installed, if you plug into the loop and turn the guitar down does the hum go away?

Is the hum affected by the gain or master volume?

Here's a pic that I was sent; I haven't had it out of the head shell since I got it back.  AFAIK, it's grounded to the chassis as per the instructions.  The hum's independent of whether or not anything's plugged in, and not affected by any of the controls apart from Presence.

Modulus_Amps

That looks like it is mounted really close to the sockets which may have created some lead dress issues.

Have you asked the tech who installed it about the hum?
When I mount these types of loops I mount them closer to the top rather than forcing them onto the same C/L as the speaker sockets. 

These loops pick up noise from the sockets when mounted directly over them. So the more distance the better

Jonathangus

I haven't, no.  As it didn't occur in every location, I concluded it must be something environmental rather than an inherent problem with the amp.

I'll try shielding the base of the amp and see if it stops the speaker picking it up.  Can't hurt to try.

Jonathangus

OK, had the amp out of the headshell at the weekend.  Loop is grounded to the chassis as it should be.  Added an aluminium plate across the full width so that the chassis sits completely on it.  Still doing it.

What I don't get is - either the hum is reaching the speaker via the speaker cable, or through the air.  If it's the cable, why does turning the master right down not get rid of it?  And if it's EM interference, you'd expect the shielding to reduce it, at least.  I even made up a long speaker cable so I could separate amp and cab, and it's the same.

Stumped.

We've got another rehearsal room booked for tomorrow, so I'll take it along and try it.

ICBM

Jonathangus said:

What I don't get is - either the hum is reaching the speaker via the speaker cable, or through the air.

It's via the cable. I don't think electromagnetic coupling from the PT to the speaker would do it - but you've eliminated that, by moving the head away from the cabinet.

Jonathangus said:

If it's the cable, why does turning the master right down not get rid of it?

Because the hum is coming from the power section, and the MV is before the power section - but the presence control isn't, which is why it affects it.

Jonathangus said:

And if it's EM interference, you'd expect the shielding to reduce it, at least.

No, because even if it's electromagnetic coupling, shielding isn't effective against the magnetic component, only the electrostatic. (Unless it's Mu-Metal.)