Solid State Instrument Amp Hum -- How To Eliminate
I have a nice California Blonde amp with speaker. Recently I plugged in my violin to the amp, and heard a hum. I was fiddling with the various knobs to see what effect they had, and as I placed my hand on the amp cabinet top and touched a knob, the hum stopped. I repeated this a few times, but then touching the knobs did not remove the hum. I realized that what I was actually touching were the screws attaching the amp to the cabinet, which are accessible on the cabinet top. Sure enough, touching those screws stopped the hum.
Since those are at ground potential, as am I, I'm thinking that the power from the wall outlet must not be grounded?
Any other diagnoses / fixes come to mind?
------- Trying to simplify my life...
June 30, 2018, 09:40 AM
two-two-niner-romeo
Does it still hum when the instrument cord is unplugged from the amp? It could be the cord, or the instrument's pickup causing that hum.
Also, are there any electrical noise inducing devices running near the amp, like fluorescent lighting, air conditioning, computers, etc?
"Being miserable and treating other people like dirt is every New Yorker's God-given right!" - GhostBusters II
"You have all the tools you need. Don't blame them. Use them." - Dan Worrall
June 30, 2018, 09:42 AM
mcrimm
While I'm not an electronics expert, I had a Fender guitar amp that had a hum in one of the inputs. I did a bunch of internet searches and found the potential cause. It was a cold solder joint on one of the capacitors.
I disassembled the amp and re-soldered all of the capacitor connections on the circuit board and reassembled. That fixed it.
My guess is you have a similar situation on the circuit board somewhere...........good luck Mike
I'm sorry if I hurt you feelings when I called you stupid - I thought you already knew - Unknown ................................... When you have no future, you live in the past. " Sycamore Row" by John Grisham
June 30, 2018, 09:49 AM
Woodman
A grounding issue. You touch the screw, hum goes away, you become ground.
June 30, 2018, 09:50 AM
4MUL8R
It does not hum without an instrument cord.
It hums more with a violin plugged in.
It hums with both a violin and a guitar.
It hums with two different but identical instrument cables.
------- Trying to simplify my life...
June 30, 2018, 09:50 AM
P220 Smudge
Be it California or Strawberry flavor, those Blonde amps like to develop funky issues with hum and noise gating. I’ve seen it personally on two other occasions with one that went to three techs and back to SWR without ever being made right, and you’re reporting the same thing I witnessed. Get thee a Roland and you won’t have to post these threads. A Jazz Chorus is about as sweet as it gets in a solid state.
You posted about it before and I recall typing a reply, not sure if I actually posted it, because my reply doesn’t seem helpful. That’s a horse I would shoot and get another.
______________________________________________ Carthago delenda est
June 30, 2018, 11:57 AM
LS1 GTO
quote:
Originally posted by Woodman: A grounding issue. You touch the screw, hum goes away, you become ground.
This is where I'd begin my search also.
Life moves pretty fast. If you don't stop and look around once in a while, you could miss it.
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The definition of the words we used, carry a meaning of their own...
June 30, 2018, 12:07 PM
Krazeehorse
You need an outlet tester. If you don't have one hit the bigbox. They are inexpensive. Good tool to have around anyhow.
Be careful what you tolerate. You are teaching people how to treat you.
June 30, 2018, 04:30 PM
Some Shot
Does the amp have a grounded power cord? Or a polarized cord (one wide prong only allows plugging in one way)?
If no ground or polarized plug, try turning the plug over in the outlet.
July 02, 2018, 06:31 PM
Woodman
You're using an XLR x ¼" cable from the mic to the amp?
Maybe a direct box will work better? You set it on the amp with a short cord, and run an XLR (shielded) microphone cable to the DI. Then try the polarity switch on the DI to eliminate the buzz.
July 02, 2018, 08:15 PM
CoinRoller
DynaMo hum, DynaMo hum... Sorry.
All previous replies provide good advice. I do agree it’s likely a grounding issue.
However, since the hum occurs when the instrument cable is connected, as well as when both violin & cable are connected (and no hum when both are disconnected), there’s high likelihood the issue is with the violin/cord. But, since it has hum with both violin/guitar, I’d focus on the cable.
Pease make sure the cable(s) you are using are designed for instruments, and not for speaker connections. They share the same connectors, but instrument cables are shielded from radio/electrical interference.
Also, does the hum increase with volume, or stays relatively constant?
Steve
I Drink & I Know Things
July 02, 2018, 09:08 PM
4MUL8R
Cables 1/4 inch both ends, are Monster brand. I think they are instrument cables. The hum gets louder with gain or main knob turning.
------- Trying to simplify my life...
July 02, 2018, 09:12 PM
Oz_Shadow
What kind of pickup?
July 03, 2018, 05:32 AM
4MUL8R
The guitar pickup is a strip fitted below the plastic bridge. The violin has two pickups into an onboard amp.
------- Trying to simplify my life...
July 03, 2018, 08:15 AM
two-two-niner-romeo
Another thing you might try is shut off the amp, unplug the main power cord from the amplifier, and then clean the instrument input jacks with a .25 caliber bronze bore brush. Sometimes a little build-up or tarnish inside the jacks can cause weird problems.
I would avoid putting any sort of cleaning chemicals in those jacks because they could harm plastic or electronics inside the amp.
You can find the electrical schematics for that amp at:
"Being miserable and treating other people like dirt is every New Yorker's God-given right!" - GhostBusters II
"You have all the tools you need. Don't blame them. Use them." - Dan Worrall
July 03, 2018, 08:31 AM
Crom
Step one is that Ground Checker that Krazeehorse linked to.
Step two is a new instrument cable from a guitar store. Monster brand cables are usually speaker cables, but I am not sure if they make instrument cables.
If those don't fix it, you may also just be located close to a nasty noise source. Move the amp to a location away from any other appliance. Even under the best of conditions, guitar amps are not very well shielded and silent like stereo amps. THe reason it is silent when the cord is unplugged is because it may have a self-shorting plug. When you pull the plug out, the input shorts itself so there is truly no signal at all getting into the input of the amp. As soon as you open it up with a plug, all the world's noise jumps in, even though the cable may be "shielded".
"Crom is strong! If I die, I have to go before him, and he will ask me, 'What is the riddle of steel?' If I don't know it, he will cast me out of Valhalla and laugh at me."
July 03, 2018, 08:52 AM
TMats
quote:
Originally posted by Crom: Step one is that Ground Checker that Krazeehorse linked to.
Step two is a new instrument cable from a guitar store. Monster brand cables are usually speaker cables, but I am not sure if they make instrument cables. Yes, they do. Monster would be considered a moderately priced cable. My lovely wife gifted me with a Mogami Platinum cable. Spendy as shit, but the best. You can plug and unplug a guitar with the amp turned on—quiet. The cable it replaced happened to be a Monster. The issue wasn’t hum, just maintaining a solid connection. The signal from the guitar would be interrupted. If those don't fix it, you may also just be located close to a nasty noise source. Move the amp to a location away from any other appliance. Even under the best of conditions, guitar amps are not very well shielded and silent like stereo amps. THe reason it is silent when the cord is unplugged is because it may have a self-shorting plug. When you pull the plug out, the input shorts itself so there is truly no signal at all getting into the input of the amp. As soon as you open it up with a plug, all the world's noise jumps in, even though the cable may be "shielded". You never said if the hum was a 60-cycle hum. I know from experience that Sweetwater will let you try a noise suppression device like a “Hum-X” and allow you to return it if it doesn’t help.
_______________________________________________________ despite them
July 04, 2018, 06:26 PM
4MUL8R
Monster brand cables are labeled...
"Monster Pro Link - Monster Classic - Improved Performance Instrument Cable..." and all sorts of other words
The outlet is correctly grounded, based on the tester I plugged in.
I unplugged a LED lamp on the same APC surge protector outlet device, and the hum did not go away.
I also tried with a 3-prong-to-2-prong adapter (eliminating the ground) and the hum remained.
The hum increases as either the gain is increased, or the main volume is increased.
------- Trying to simplify my life...
July 04, 2018, 07:55 PM
Woodman
Sorry, I assumed it was a microphone, not a pickup.
Musicians Electronic Service in Ardmore might possibly be able to direct you to a like shop closer to you. They take out the amp, throw it on the bench, find a fried transistor or whatever, have it replaced in three minutes, reset the amp, test, charge you $77, and your done.
Or have one of your kids stand on stage, grounding the knob during performances ...
July 06, 2018, 03:58 PM
4MUL8R
The diagnostic saga continues. Today, using the violin with a small pickup attached to its bridge, the amp gave a noticeable hum. Until I touched the instrument cable jack exterior. Or the screws of the amp circuit board. But then, I set the violin down on its floor stand, and the hum returned. Only when I touched the wood of the violin AND the ground of the instrument cable did the hum go away. My best explanation is that when I touch the instrument and the far end of the cable, I am serving as an alternate path to ground. In essence, I am in parallel with the ground and signal conductors of the cable. Don't ask me how wood conducts electricity though. I have not figured that one out.
Testing with a Behringer XM8500 mike and a Shure Beta 57A mike, through a Mogami mike cable, there is NO hum at all. It sounds delightful.
With the mike in the XLR input, and the instrument cable in the 1/4 inch jack, the hum returned.
And, in a related test, moving the amp to a room without any electronic devices, downstairs, the hum is GONE. Like inaudible. Like not there.
So, something in this room is either RF interference on the instrument cable OR on the power side. Given that when I serve as a second path to ground, touching both the instrument body (wood) and the ground plane of the cable or amp, and it only occurs with the instrument cable in use, I'm betting on RF.This message has been edited. Last edited by: 4MUL8R,