Identifying source of Hum

[joelap]

I've got this custom preamp build cooking here, and its got a wee bit of hum.  It's not ridiculously loud but enough is there where I want to get rid of it.  When the volume knob is all the way off, the preamp is silent.  As I turn it up and turn up the treble/bass knobs, this higher frequency buzz starts to kick in and a low frequency hum also gets in there.  I changed all the wires going to the tube sockets to shielded cable, and there's no difference.  Does this sound like ground loop-style hum?  Heater hum?  I'm running the two 12AX7 tubes at 12.6VAC, transformer has no center tap and I haven't provided a reference as I dont have any 2W resistors below 820R (everywhere I read suggests 100R/2W) and turns out I don't have a humdinger pot lying around.  If the issue was heater hum, I'd expect the noise to be there even when the volume is all the way off so I'm inclined to say its not that.  Any wires that even go near the tubes/heater wires are shielded.  Heater wires are spun very tightly.  Power supply filtering off of the 230VAC secondary is 100uf - 1k drop - 100uf - 1k drop - 100uf - 27k in series with 2k2 to provide a reference voltage for some shielding and gives the power supply filters a channel to discharge. 

Any suggestions as to what it sounds like the cause is?  I've been careful to return grounds to the appropriate filter cap, maybe not careful enough?

Joe

[eleventeen]

Perhaps you should try the fabled "synthetic heater center tap" aka 100 ohm resistors from each leg of the filament winding to ground. I myself have not found this to make much of a difference, but I have heard enough stories of those who HAVE found it makes a diff that I'd be trying it early on in a hum-hunt.

[HotBluePlates]

... transformer has no center tap and I haven't provided a reference as I dont have any 2W resistors below 820R (everywhere I read suggests 100R/2W)...

Yeah, add the reference. You don't need 2w, and that recommendation is silly. 1/2w is more than enough, and is what is used in most old amps.

We could mathematically prove that a lower rating would work equally well, but 1/2w is probably the smallest value that's easy to handle physically.

[joelap]

Thanks.  Is there any reason for using 100Ohms, as opposed to another resistor value? 

[HotBluePlates]

I don't think I've ever read a good explanation. Maybe PRR knows.

Pulling a guess from my hindquarters, I think it keeps current flow through the resistors low, looks like a little higher resistance than a cold heater, and maintains a relatively low impedance to ground to shunt leakage currents.

But that is a complete guess. You could go a little higher or lower, but I wouldn't go very much higher or lower. 50 ohms for each leg should work, as many old amps (hi-fi, not guitar) used a 100 ohm pot with the wiper grounded as a hum balance pot.

[joelap]

Thanks, this is something thats always puzzled me and I havent seen much of an explanation on the internet.  I'm wondering if the 2W recommendation came from treating the AC with DC equations.  Say, "ok so 12.6V divided by 100R = .126A through the reference resistors.  .126^2 multiplied by 100R = 1.59W therefore use 2W" or some type of logic as such.  Either way, I'm happy I can solve this issue without having to order more parts.  I had a whole bunch of 82R 1/2W resistors I got that I likely will never have much of a use for.  They aren't 1%, but we'll see how the amp reacts in a few minutes when I fire it up to determine if I need to go with something else in there.


EDIT: Just fired her up, and the hum is GONE.  Amp is very very silent right now.  Heaters referenced to about 22V through a 27k -- 2k2 resistor combination from about 290V at the last filter cap (wasnt sure if 27k would be large enough per Merlin's website on referencing heaters but nothing seems outwardly wrong yet).  Interestingly, I used 82ohm 1/2W resistors that have a 10% tolerance.... I didn't measure to see how far off they were from each other.  Maybe I got lucky, or maybe extreme matching isn't that necessary for musical instrument applications.  Input shielded cable and output shielded cable (and the shielded cable I ran from my heater transformer to the 82ohm resistors) are referenced to ground.  All the shields on the wire running to the tubes are referenced to the 22V as well.  The heater wires are not shielded and just twisted tightly. 

I'm just glad my grounding scheme worked and I didn't have to start playing around with that.

Case closed!

[HotBluePlates]

... ok so 12.6V divided by 100R = .126A through the reference resistors.  .126^2 multiplied by 100R = 1.59W therefore use 2W ...

That is plausible, and may be were the idea came from. But it's based on wrong numbers.

Your 6.3vac heater circuit has 2 legs, each with 100 ohms to a reference. The 6.3vac is across the whole winding, so there's only 3.15vac from 1 leg to the reference. Instead of 12.6v across the 100 ohm, you really have 3.15v. Voltage is 1/4 the amount thought, so current is also 1/4 the amount thought, and dissipation is 1/16. I figure about 1/10th of a watt.

Interestingly, I used 82ohm 1/2W resistors that have a 10% tolerance.... I didn't measure to see how far off they were from each other.  Maybe I got lucky, or maybe extreme matching isn't that necessary...

It's not.

I have a pair of McIntosh MC-30's. They're hi-fi power amps with a 100 ohm hum balance pot. As long as the pot is not turned 100% to either end, I hear zero hum in the speakers. Granted, power amps by design are not high sensitivity, but you have to be really far off to have the artificial center-tap not kill hum (caused by no d.c. reference).

[joelap]

Thank HBP, very informative post.

Noise performance is very good on the amp right now, but I can't help but notice that the noise drops a bit more when I've got my right arm touching the strings and I touch the front faceplate with my left hand.  How can I complete this circuit internally to improve it that extra little bit?  This only is occuring with the faceplate and not any other part of the amp.  The rack is from Par-metal, one of their 10 series which has an aluminum front panel while the rest of the enclosure (top, bottom, and sides) are steel.  Is this as simple a fix as grounding the faceplate? 

[HotBluePlates]

What kind of guitar and pickups are you using? This sounds a lot more like string ground than anything.

The rack is from Par-metal, one of their 10 series which has an aluminum front panel while the rest of the enclosure (top, bottom, and sides) are steel.  Is this as simple a fix as grounding the faceplate?

I don't know how you built you amp, but I'd imaging the faceplate and chassis already have continuity, and that the ground wire on your power cord is attached to the chassis somehwere. So you should already have ground. You could prove it by checking resistance or continuity from the factplate to the 3rd prong of the power cord with the amp unplugged.

Is this only when the amp is out of the cabinet? Is it only with this 1 amp (guitar doesn't do it on any other amp)? Is is only with this guitar?

There are ways to shunt RF noise at the input jack, but that might not be necessary or effective depending on your other answers.

[joelap]

The guitar is a USA Custom Guitars Jazzmaster using Seymour Duncan Hot for Jazzmaster pickups.  Yes, the chassis is safety grounded but the chassis is powdercoated.  I think the powdercoat might be insulating, because using my Fluke MM on continuity test settings, I gotta really push into the chassis at two points to get a beep.  When I rest the negative probe in a screw socket, it also isnt providing an adequate reference to check potential from.

I have not yet put the amp in a cabinet, since this is a rack device and my rack is full.  If my bassist digs it, it'll probably go in his.  I havent tried it with another guitar yet.  The guitar is a -little- quieter in another preamp I have (not built by me).  I was thinking about putting a .047 cap from ground to the chassis at the input, but yesterday I soldered a wire to one of my star points to the back of a pot and that did the same thing as my touching the faceplate. 

[sluckey]

the chassis is safety grounded but the chassis is powdercoated.

You need to expose bare metal at every point on the chassis that will serve as a ground connection. A small rotary tool with a grinding stone or wire brush works well and may be nimble enough to work around a completed build. You may have to get creative to sand down to bare metal in some places, but it's really necessary for any ground point.