Looking for help diagnosing buzz/hum/ground loop in my "HIFI" GUITAR rig!

[telecowboy]

I have been experiencing a bad, loud, and variable noise in my electric guitar rig. Here is the details of the rig (I will include as many details as possible since ground loops are tricky):
> Electric Guitar (active electric guitar with on-board buffer, 9V battery, and On-Board Effects Loop (two output cables, mono and TRS, which creates a loop for the guitar signal to bypass the effects chain from the guitar, not sure if this can contribute to the ground loop)
> Pedalboard (a few pedals run through the buffer box for OBEL, all powered by a high quality isolated power supply. The power supply uses a grounded, 3-prong power cable.)
> 1/4" mono cable to TUBE GUITAR PREAMPLIFIER (powered by grounded, 3-prong power cable, with a 1/4" preamp out jack to poweramp)
> 1/4" unbalanced to male RCA to poweramp
> McIntosh MC50 power amp (powered by original unpolarized 2-prong power cable, orientated at the correct polarity for least noise and chassis voltage)
> Banana Spade Adaptors on C and 8ohm terminals, running banana plugs to 8ohm JBL speaker
* All three power supplies (two grounded 3-prongs and one 2-prong unpolarized, are all plugged into the same Furman power strip that is plugged into one outlet. Therefore, I would assume I have a ground. I also think the Mcintosh uses the preamplifier's safety ground when I have them plugged into each other, correct? Is there any concern for ground loop due to this setup?)

A few points I've found in my troubleshooting:
- The noise gets louder and louder the more I turn up the gain on the Mcintosh power amp. Just past zero it isn't noticeable but as I approach 11 o'clock it gets very noticeable. I play it around 1-2 o'clock so it's an issue.
- The connections between preamp and poweramp don't see to change the noise when moved around
- The preamp gain and many of the knobs such as gain, output gain, treble, reverb, all add to the noise the more it is turned up.
- If I unplug the preamp and run the Mcintosh into the speaker, there isn't really any noise. This leads me to believe the noise is generated downstream from the poweramp.
- If I mute the guitar at the pedalboard, turn down the volume on the guitar, or unplug the guitar, the noise is still present. This leads me to believe the noise is generated upstream from the pedalboard and guitar.

I don't know much about vintage audio and the Mcintosh is my only two-prong piece of equipment. Does it sound like I've introduced a ground loop from connecting the different amps, or does this sound like an issue of a noisey tube amplifier that is sending noise upstream to the power amp and speaker? Thanks!

 

[Cbdb2]

Sounds like the amp is fine. Unplug everything except the Amp, turned on and volume up and then one by one add upstream components until the noise starts. Then check your cable from the last piece of gear you plugged in by swapping in a known good one. Start with that.

 

[telecowboy]

Sounds like the amp is fine. Unplug everything except the Amp, turned on and volume up and then one by one add upstream components until the noise starts. Then check your cable from the last piece of gear you plugged in by swapping in a known good one. Start with that.

By amp, you mean the Mcintosh? Is it fine to run the Mcintosh with no input signal into the speaker? Thanks!

 

[Cbdb2]

Should be fine. You said you already tried this? Just turn the Amp off when making any connections.

 

[telecowboy]

Should be fine. You said you already tried this? Just turn the Amp off when making any connections.

I did, but I thought it could be bad for it. Thanks for clarifying.

 

[Cbdb2]

Make sure all your 1/4" cables are good, this is a common problem. And try things with just the regular guitar output first and make sure your guitar battery is good.

 

[Cbdb2]

Hope you know the level of guitars (including actives) and pedals is a lot lower than hi-fi. They need a preamp with an instrument input to boost the level. What kind of preamp are you using?

 

[Speedskater]

Can you take your rig to another building?

 

[telecowboy]

Can you take your rig to another building?

it buzzes similarly at all venues we play. I only play it at home or gigging. Never been silent during a gig and never a major difference.

 

[AnalogSteph]

This sounds like a ground loop between your pedal board power supply and tube preamp. Please use a multimeter to verify that none of the power supply outputs are connected to mains PE. I suppose it's similar to these? They ought to be isolated from PE then...

Failing that, the tube preamp itself may be generating the hum internally, e.g. as a result of some tired electrolytic caps or poor design / construction practices. (Have you tried to run the guitar directly into it, with/without onboard effects? Or even better, an instrument cable with tip and sleeve shorted. The hum would remain at the same level if it's the amp.) Depending on what it is, this would be calling for an average to good amplifier technician.

If the hum increases with preamp gain, it is entirely possible that the issue is within the pedal board after all. Your tests have not excluded that as a potential culprit. Only plugging the guitar directly into the tube preamp would tell you for sure.

It is fairly easy to create ground loops in both pedal boards and electric guitars. You would notice by hum levels reacting to their proximity (and relative orientation) to mains transformers. Internal guitar wiring between pickup(s) and buffer must also be adequately shielded to keep electrical fields out.

BTW, "hi-fi" isn't necessarily the thing you want in a guitar amp... things like power supply sag are well-known to contribute to the "sound" of guitar amps, to the point that e.g. swapping a tube rectifier to solid state can make things sound worse. A mere tube preamp may not be getting you all that much over some good "sand" (and you might even be better off with an amp simulator thingy).

 

[telecowboy]

Hope you know the level of guitars (including actives) and pedals is a lot lower than hi-fi. They need a preamp with an instrument input to boost the level. What kind of preamp are you using?

Using a tube guitar amp with preamp out and power section bypassed as the preamp. Proven setup, just something wrong with my setup or gear...

 

[telecowboy]

This sounds like a ground loop between your pedal board power supply and tube preamp. Please use a multimeter to verify that none of the power supply outputs are connected to mains PE. I suppose it's similar to these? They ought to be isolated from PE then...

Failing that, the tube preamp itself may be generating the hum internally, e.g. as a result of some tired electrolytic caps or poor design / construction practices. (Have you tried to run the guitar directly into it, with/without onboard effects? Or even better, an instrument cable with tip and sleeve shorted. The hum would remain at the same level if it's the amp.) Depending on what it is, this would be calling for an average to good amplifier technician.

If the hum increases with preamp gain, it is entirely possible that the issue is within the pedal board after all. Your tests have not excluded that as a potential culprit. Only plugging the guitar directly into the tube preamp would tell you for sure.

It is fairly easy to create ground loops in both pedal boards and electric guitars. You would notice by hum levels reacting to their proximity (and relative orientation) to mains transformers. Internal guitar wiring between pickup(s) and buffer must also be adequately shielded to keep electrical fields out.

BTW, "hi-fi" isn't necessarily the thing you want in a guitar amp... things like power supply sag are well-known to contribute to the "sound" of guitar amps, to the point that e.g. swapping a tube rectifier to solid state can make things sound worse. A mere tube preamp may not be getting you all that much over some good "sand" (and you might even be better off with an amp simulator thingy).

The preamp is a custom guitar tube amp. It has a preamp out and bypasses the power section. There is some noise when using the amp alone but magnified when going through the Mcintosh and JBL speaker. Thanks for telling me it could still be the pedalboard. I'll do some more troubleshooting to confirm or deny if the pedalboard is involved.

The guitar preamp was serviced by my tech last year and will be going in again in May but checked out fine last year. Tubes have been replaced twice in diagnosing this and are not fixing things.

Can you tell me more about the DMM with the PSU to verify that none of the power supply outputs are connected to mains PE? What am I looking to test here? The pedalboard is powered by a CIOKS DC7 Power Supply.

I can try without the pedalboard and with other guitars that are not 9V battery powered and less complex.

 

[DVDdoug]

I'm not a guitar player but it seems like all electric guitars hum/buzz to some extent.

Usually it's the guitar itself. Guitar pickups are high impedance and the high impedance makes them more prone to noise pickup. Also, the fact that they are unbalanced. But I think the main problem is that the pickup works by picking-up electromagnetic signals so it picks-up electromagnetic noise. A humbucker is supposed to help cancel the noise, but again, I'm not a guitar player. And from what I understand it has a different "tone".

The active pickup (the preamp in the guitar) lowers the impedance so that should solve the impedance issue. Assuming it's boosting the signal, it will also boost any hum from the pickup. The amplification shouldn't hurt the signal-to-noise ratio but it makes the noise more noticeable during when the signal is silent or quiet.

Active pedals also have low-impedance outputs so there should be no "impedance problems" following the 1st pedal.

On the other hand, pedals can also pick-up noise, or get noise from their power supply (if not battery powered). And of course, pedals that amplify, including pedals that compress or "saturate" will amplify any noise coming into them.

Have you tried a noise gate? They make noise gate "pedals". A noise gate kills the sound completely (or sometimes just reduces it) when the signal falls below a preset threshold. The downside is that it will kill the last part of your sustain-fade-out (or it may have a delay/release time so you hear the fade-out & buzz together) but probably nobody will notice it if you are playing with a band, or in in a venue with natural reverb. I'd bet all mixing engineers use a noise gate plug-in on almost all commercial recordings.