Hum Question

[alerich]

I live in an old farmhouse and the electric service here is a bit noisy when using higher gain amps. Every amp I have played here hums even with just guitar straight into amp. Below is a representation of my rig. All of the powered pedals are powered from a single One Spot supply that is plugged into the same power strip that the amp is plugged into. Nothing else is plugged into that power strip. The wah is battery powered. The Ebtech Hum Eliminator is passive - an isolation transformer.

In the beginning my rig hummed. I bought a BOSS Noise Suppressor (NS-2) and placed it at the amp's input. That helped. I bought a second NS-2 and placed it in my effects loop. That helped some more. At this point I was thinking that maybe I should look at filter caps and such in the amp. Then I bought the Hum Eliminator and placed it after the NS-2 in my effects loop. Dead silence. No hum when I am not playing and I play it pretty high gain and negligible impact (if any) on my tone other that to eliminate the hum.

This is my question. My instinct tells me that my noise suppression devices cleared up the stray noise and hum brought about by ground loops and noisy electric service. My instinct tells me that if the amplifier's power supply was causing the noise or hum and required service that the noise suppression devices would not mask that noise or hum. Am I correct in these assumptions?

[PRR]

That's a noise-gate. When "not playing" it mutes the signal.

Test: let a string decay ALL the way. When it gets faint, you may hear hum under it. When it gets even fainter, both string and hum are faded-out to total silence. Some old/crude noise gates were a little ugly when cutting out. This one seems well-regarded so it must be well tweaked.

How is the hum in your wired phone? Do you have cable TV and are there hum-bars in the picture?

[alerich]

That's a noise-gate. When "not playing" it mutes the signal.

Test: let a string decay ALL the way. When it gets faint, you may hear hum under it. When it gets even fainter, both string and hum are faded-out to total silence. Some old/crude noise gates were a little ugly when cutting out. This one seems well-regarded so it must be well tweaked.

How is the hum in your wired phone? Do you have cable TV and are there hum-bars in the picture?

I don't have a landline any more. I dropped it in lieu of my cell phone when I migrated to cable modem. My cable signal is clean. The NS-2 is supposed to quiet the noise underneath the guitar signal without affecting the signal. In practice it works much like the old school noise gates. While you are playing it isn't an issue.

It appears I may have lucked out through serendipity. I had the chassis out of the cabinet today. Just put it all back together. I was going to map out the mods that were done to it. It started life as a Sovtek Mig60 head that Splawn Amplifiers performed some mods on. Both the Russians and Splawn gooped it up a bit with RTV. I cleaned most of that away in the preamp and started mapping. Even the Russians didn't build it component for component per the schematic that is widely distributed.

Anyway, I checked the bias while I was in there and buttoned it up. When I fired it up in my rig it was noticeably quieter. Many of the mods were point to point and I move them around a little to get the component values. The stock amp itself is a mixture of point to point with a few smallish ckt boards that look like someone etched them in their garage. I must have moved something that made a positive difference. I'll take it.

Thanks for your input, PRR. You and your tech wisdom are truly an asset to this forum.

[HotBluePlates]

I live in an old farmhouse and the electric service here is a bit noisy when using higher gain amps. Every amp I have played here hums ... My instinct tells me that if the amplifier's power supply was causing the noise or hum and required service that the noise suppression devices would not mask that noise or hum. Am I correct in these assumptions?

Regardless of what's happening, the gates (noise suppressors) will mask the hum because the signal they let through is loud enough to mask the hum.

If you stand outside your house on a sunny day, you probably can't see anything inside because it looks dark. But inside the house, you see everything outside clearly. The situation is reversed after the sun goes down, you don't see anything outside when you're inside with the lights on. The intensity of the light where you're standing compared to that on the other side of the window impacts whether you see what's on the other side.

Same thing (basically) with the noise suppressors. Loud signal masks softer hum, and they kill everything when signal is soft. Net effect is reduced noise.

I don't know enough about power line disturbances to tell you if smallish inlet filters will nuke the line noise in your home electric service. I'd be thinking that since you say all amps suffer the same problem that it's related to line noise rather than power supply issues in the one amp.

[alerich]

Regardless of what's happening, the gates (noise suppressors) will mask the hum because the signal they let through is loud enough to mask the hum.
I'd be thinking that since you say all amps suffer the same problem that it's related to line noise rather than power supply issues in the one amp.

They're gating the input signal and gating the signal through the effects loop. The effects loop on this amp is passive and is injected between V1 and V2 of what is basically a Marshall 2204 JCM800 style circuit. My thinking was that if there was power supply ripple/hum from weak filter caps it would still get into the ckt beyond the loop (V2 stages, PI, power stage) and be heard regardless of any gates or devices before that point.

I really don't want to re-cap this thing unless it's really necessary. It's gonna be a chore. That's why I was trying to determine if a noise gates placed at the input and in the effects loop would silence power supply ripple in an amp. Now it's looking like my hum issues may be partially lead dress in this amp, anyway. It's noticeably quieter without the 2nd gate now since I reassembled it so whatever I did helped. A couple of the other amps were fairly new (Splawn Quick Rod and a Peavey 6505) so I kinda figured that that it was environmental versus the amp's power supply but since this Sovtek isn't built from top shelf parts to start with it had me wondering.

[HotBluePlates]

If there was power supply ripple, it could still be reduced by your gates.

Say 5v ripple is present throughout the amp (even though that is unlikely, as it would be reduce by successive filter stages). 5v hum in the face of hundreds of volts of signal swing at the output tube plates is nothing.

Assume your amp makes 50w output and has a 4k:8Ω OT. Turns ratio is 22.4:1, and this is the amount by which the plate output voltage is stepped down. 5v/22.4 = 223mV. 50w across 8Ω is 20v RMS, so hum is down about 20dB. Realistically, it is reduced further, because we haven't considered how the OT primary and tube plate resistance form a divider to reduce the hum even more.

BUT... that is mostly valid for a single-ended output stage; push-pull cancels power supply hum at the plate.

5v ripple at the screen is also basically nothing in the face of 400v+ of supply voltage. It will be amplified slightly by the tube, but is still low at the speaker, and similar to what we calculated above. Actual ripple input to the screen will have been reduced by the power supply caps and choke.

5v ripple at the output tube grid would be significant, but your amp will likely use fixed bias with its own filter and voltage divider, so the grid will not be relevant here. I did once fix an amp with a failed bias cap that had a nice smooth 60Hz hum injected directly to the grid (with peaks as big as the bias voltage). But you won't have that situation unless you yank the bias cap.

The probably long tail phase inverter will largely reject hum in the B+ supply, because it's a differential amp and the hum will be a common-mode signal. Cancellation is probably not perfect, but hum output will be well down from the 5v starting value.

That leaves us with the preamp, and I'm not certain where exactly your effects loop falls in the preamp. Regardless, hum will be somewhat reduced by the voltage divider made of a tube and its plate load (and following stage grid resistance). However, the gain following an early stage makes the amount of hum present more significant.

I'm thinking your amps' power supplies are probably fine and doing their job reducing noise, which is probably reducing the (possibly large) noise on the line. A power inlet filter consists of caps across the line and from each side of the line to ground, as well as chokes in many cases. It's designed to fail-safe if any part fails, and will try to reduce noise on the line itself.

But like I said, I don't know enough about ratings for these and what typical noisy electrical service stuff is like, so I can't really recommend anything. Maybe RicharD will see this and chime in.

Other than that, testing your amps somewhere else that you know to have clean electrical service is probably the best plan. I can't tell you how frustrating it is to try to fix someone's amp that only acts up in their house. I once worked on a guy's amp that hummed when I listened to it in his house, but was quiet when I got it back to my house. Then of course, it hummed when we were back at his house. I'm 95% sure he had wiring faults in his house's outlets.

[alerich]

Thank you very much for that explanation. I did not realize the interaction. Good stuff. I'm going to do some more location A/B tests although since my other amps hummed here I can predict the results. APC makes an affordable surge protector with noise suppression. I'll investigate that, also.

Love this forum. You guys rock.

[Platefire]

I didn't see this mentioned but I might have missed it. If your old house is like mine is on an out dated out of code two conductor AC. With the amps with a ground to chassis and house recepticals set up with one large/one small prongs/sockets there is no way to reverse polarity so your generally stuck with the hum that goes along with it---unless like you say---gate it. That sure interfers sometime with low volume response on your amp fighting the noise gate.

In my recording studio I run an external ground from equipment chassis to a railroad spike in the ground outside. On my guitar amps I just grin and bare it.   Platefire