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Old 1st December 2014, 11:38 PM   #1
wicked1 is offline wicked1  United States
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Default Odd noises; guitar, environment, or amp?

I built an amp a couple threads ago, in my "help choose a guitar amp to build" thread.
Was just using an old, cheap woofer. I put better speakers on it, and all the fun is over.. Lots of noise.. (old speaker didn't go high enough in frequency, and was inefficient)
I cleaned up most of it and the amp is silent other than a slight hiss when the input is grounded. Also, there's a volume control between the input and LTP, and w/ that volume down, the amp is silent.
But, with nothing plugged into the input the amp makes a loud buzzing.

I'm not sure what's normal w/ a high gain instrument amp, so I'll just list some symptoms..

Moving the input jack changes the buzzing, and in some places it almost goes away. In other places it's deafeningly loud. (jack is connected via ~5inch shielded cable, and not mounted to a chassis yet).

Playing an MP3 player through the amp doesn't have any noise, as long as the player is powered on. When it's powered off, it's as if the cable is unplugged.

W/ a guitar plugged in, it's silent when the guitar volume is down (because it's shorted to ground), but I get a lot of noise through the guitar. It's even got EMG active pickups, which should be quiet.
I've got an old Gibson lap steel w/ P90's, and those, the plucked strings and the noise are about equal!

To me the symptoms seem odd.. If this were a hifi amp, I'd think it was silent, as mp3 and cd players hooked up work fine.
But, w/ the guitar... noise..

My other amp is an old Magnetone, which doesn't have a lot of gain.. The same type of noise is present, but well below the volume of the instrument.

Oh, and the noise.. W/ nothing plugged in, and the input not grounded, it's all sorts of random noise.. Changes frequency, volume, etc.

W/ the guitar plugged in it's consistent.. I guess I could call it a buzzing sound, increases evenly w/ the volume.

I completely rebuilt the input stage a few times today.. Replaced all the antique carbon comp resistors I was using, cleaned up messy solder blobs, and replaced everything again! no changes.


Any ideas? Is the amp likely the culprit, or is it ok since it's quiet when the input is shorted or a cd/mp3 player is connected?
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Old 1st December 2014, 11:42 PM   #2
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This might be a stupid question but are you using 1/4" "shorting" jacks for the input? They ground the grid when there is no cable plugged in and are mandatory in MI amplification.

As far as the noise floor when an instrument is plugged in, sounds like a ground loop issue.
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Old 1st December 2014, 11:54 PM   #3
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+1 for input jack extra connections shorted to short input when no jack is present.

Could be poor screening in the guitar. Or maybe bad connection in the guitar.
Those guitar pick up coils will pick up everything if not screened properly.

Guitars like the zero volt line grounded otherwise the guitar screen becomes an antenna.
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Old 2nd December 2014, 12:02 AM   #4
wicked1 is offline wicked1  United States
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I'm not using a shorting jack... Thanks! I feel a little better now, because shorted, it's almost silent (just a little hiss, which I think is normal).

I chased ground loops all day.. I was using an old chassis, and originally left things grounded to the chassis as they were.. But now everything is star grounded to the first cap.

Well actually, I have the ef86 cathode and 'rg' and the input socket (so the guitar) all connected in one spot, and that junction connects to the first cap.. It's probably wishful thinking this will make a difference, but I'm heating up the soldering iron now..
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Old 2nd December 2014, 01:30 PM   #5
JMFahey is offline JMFahey  Argentina
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Glad you sorted (shorted ) it .

Guitar pickups are noisy little buggers, because they pick up all electromagnetic fields present.

Saying "they are EMG" just by itself does mean that much.

a) there are passive EMG which behave like any other pickup.

b) even if active, a single coil type is a noise catching machine
Humbuckers do the same, but since both coils are out of phase, they (mostly) cancel it out.

Only "humless" guitars are those which have properly mounted and shielded piezo pickups which by necessity are also active.

And yet they hiss a little.
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Old 2nd December 2014, 01:51 PM   #6
wicked1 is offline wicked1  United States
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So, rewiring the input grounding on my amp made no difference.

The EMG's are active.. They(emg) suggest not grounding the bridge, and whoever installed them did not ground it.. So, I rewired the guitar and grounded the bridge and grounded the conductive paint in the cavities. It made a little difference, but not much.

I'm getting a pair of passive humbuckers for xmas, so will have some more experimentation to do then.

I need to see if I can find another guitar or amp to test with as a comparison.. Or maybe even just bring my guitar and amp to a different less urban location, and see if it's quiet.
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Old 2nd December 2014, 02:25 PM   #7
kevinkr is offline kevinkr  United States
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What sort of cable are you using between the guitar and the amplifier? Is it properly shielded? I have run into at least one instance where just replacing the cable between guitar and amplifier with a decent shielded cable made a world of difference.
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Old 2nd December 2014, 02:41 PM   #8
wicked1 is offline wicked1  United States
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It's coaxial.. I did try two cables, but both are the same coax style.
I'll pick up a shielded cable to try.

Hey, different topic real quick..
I just realized I don't need a negative supply to add a source follower after my ef86, do I? I've only used them to drive negatively biased power tubes, in the past.. But here, the signal is well above 0v.
So, just a mosfet, 100r gate stopper, and 220k on the source, right?

(My schematic is no longer complete.. The tone controls are between the ef86 plate, and .047uf coupling cap.. I'd insert the mosfet directly after the plate, and before the tone controls)
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Old 2nd December 2014, 03:51 PM   #9
kevinkr is offline kevinkr  United States
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Yeah, the proposed mosfet topopology will work fine, just make sure to chose a mosfet with low CRSS (capacitance gate to drain)

More here: http://www.irf.com/technical-info/appnotes/mosfet.pdf


Perhaps George (tubelab) or Anatoliy (wavebourne) will suggest suitable mosfets based on their experience. Mine is limited to DN2540 in cascode ccs and mu-followers and I use low rp tubes with them. (Their CRSS is way too high for use here IMO)

Perhaps you could even use a cascoded follower to reduce the effect of
CRSS, and then you could use commonly available DN2540.. I would run enough current through it to get relatively high transconductance, perhaps 3 - 4mA or more.
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Old 2nd December 2014, 05:26 PM   #10
RTF671 is offline RTF671  United States
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Grounding via the bridge helps with noise. EMGs are much quieter than passives so the bridge ground isn't needed as much. It can be removed which reduces the chance of electric shock should any surges happen.
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