Dual channel high gain preamp build hum and buzz issues.

[Captain chunkulus]

Okay, so I built a  2 channel high gainpreamp. It works but I have a low hum and when I turn up the gain it adds a buzzing to the hum. I based this build off of the hoffman stereo preamp except I used a marshall jvm410 as the preamp bassis and also used the 410 power supply plus I made it mono instead of stereo.  However, I grounded it based on  the stereo pre. The preamp filter caps and cathode resisters are tied to a buss at the stage they go to, then there is 1 wire grounding the preamp to the ground on the pots. The power supply, artificial heater Center tap and earth ground are all grounded near the power supply and power transformer. I've been back over the circuit  over and over including removing the relay switching which is powered by the  heater circuit and still hum and buzz. You guys got any ideas? Im including a video.  Thanks so much.

[sluckey]

The hum may be just a ground loop between the preamp and the main amp. The buzz sounds like guitar picking up nearby fluorescent or neon lights.

Looks like you are connecting your preamp into the instrument input of another amp? That's a recipe for buzzsaw. The preamp is meant to connect directly to a power amp input.

[Captain chunkulus]

Thanks I found 1 problem. V 1 was a bad tube. That's where the main  hum was comming from. Also, I tried it last night with just an art sla1 power amp and there is still a slight about of buzz. I redid the heater wires but it's still the same issue. I tried using a dc heater circuit on the v1 tube and it was way worse. The tube sockets are mounted to the board and all of my wiring is as short as possible. I'm wondering g if it is heater buzz.

[Captain chunkulus]

Okay here's another video.

[shooter]

if it is heater buzz.

measure it at the speaker, 60hz is fil, OR external interference.  120hz is  typically a PS AC component

Since this is just a pre, you need to look at it without a PA, then with. 
when you have 2 or more devices individually plugged in the wall, that WILL hum.  For testing, try a 2 pronger for the pre, and a physical ground chassis to chassis

[Captain chunkulus]

Can you elaborate on physical ground chassis to chassis? I get the 2 prong idea. The other I'm not so sure.  Do you mean running a ground wire from the preamp to the actual amplifier?

if it is heater buzz.

measure it at the speaker, 60hz is fil, OR external interference.  120hz is  typically a PS AC component

Since this is just a pre, you need to look at it without a PA, then with. 
when you have 2 or more devices individually plugged in the wall, that WILL hum.  For testing, try a 2 pronger for the pre, and a physical ground chassis to chassis

[shooter]

running a ground wire from the preamp to the actual amplifier?

Yup, that way you  don't light up when your pre is floating at 120vac 
This is just a test, only a test, since we have no clue where or what your hum is, just making changes and seeing what sticks

[Captain chunkulus]

I don't have a scope but could I use a sine wave from another audio source to see if it's the same frequency? Well, I suppose I could record it and run it into protools and see what the frequency is? Thoughts? Thanks again for all the help.

[sluckey]

Just temporarily disconnect the power cord green wire inside the preamp to see if the hum is caused by an earth ground loop.

[Captain chunkulus]

Hum is substantially louder with the earth ground disconnected.

[shooter]

so, you're back in the box 

[terminalgs]

a schematic would be helpful.


preamps are designed to take a small voltage signal at the grid of the 1st stage and make it a lot bigger.  What we hear is probably noise at the first stage getting amplified.  (without a schematic, we don't know where the volume pot is, so I'm assuming its after the first stage?).  So, perhaps a small ground loop buzz or noise coming in on the ground shield of the cable., nonetheless,  noise applied to the preamp, and the preamp is going what it designed to do:  amplify signal.  unfortunately, the signal is noise.


In this test,,  what we don't know is how relatively small or large that noise is compared to the instrument signal you plan to use the preamp for.  is this noise 1/10 or 1/100 of the audio signal that'll come from the instrument.  Afterall, this is a super high gain device.  How many gain stages did you say?  3 or 6?  maybe +30dB or more?  even 1mV of noise will be a garden hose coming out the other end...   




Is this preamp designed for (1) amplification of a low output microphone  or (2) just for an elaborate EQ or (3) for tube overdrive crunch (like a matchless hotbox)?  If its for tube overdrive crunch?  can you hear the noise when crushing the 1st stage with high output pickups and power chords?


I've build several preamps for different purposes.  Here's some things I've learned:

• filaments don't need to be DC,  but no matter what the ground reference (CT, hum-pot, or otherwise) location to the rest of the circuit is important.
• a ground lift (not a open/close switch) is important.  the circuit ground shouldn't be connected directly to chassis and to green wire of power input.  See Valvewizards ground chapter and check out the ground lift section.
• RF will come in on the ground shield of a cable (your vid doesn't sound like RF) so google aiken amps grounding write up and see what he says about grounding the instrument shield with a cap at the input jack.
• read valvewizard's grounding and power supply chapters over and over...

[Captain chunkulus]

It's a 5 stage pre for guitar. Last 2 stages are directly  coupled like a jcm800 and many other amps. I am attaching cropped parts of the schematic I used. The preamp is the same minus the clean channel. It's just two sets of eq switched in or out using the same 5 stages. Also, I omitted r129, r130 and changed r127 and r117 to 1 Meg. The power supply is exactly the same. The filter caps are 22uf 450v and are located right at their stages. The power supply is currently grounded to the earth ground lug. I tried it grounded to the artificial Center tap of the heaters and it made no difference. The noise I'm getting at this point doesn't seem to be effected by chop sticking any wires or anything else. On the other hand the buzz decreases when I touch the chassis or my guitar strings. Been going back over marlins grounding papers as well as Aikens grounding page. Not really seeing where I've gone wrong at this point. There is only 1 ground buzz fir the preamps 5 stages and 1 wire directly to the input jack which is metal and firmly bolted down. The sound is definitely  more of a buzz than a hum.

[shooter]

try removing power from your relay boards as a test

[Captain chunkulus]

Okay just tried that. No difference.

try removing power from your relay boards as a test

[Captain chunkulus]

Okay, just removed the relay circuits and did 1 straight wired channel. No difference  there either. I'm stumped. It's got to be something simple. Right? Lol

[terminalgs]

Does the preamp have a 3 wire power plug with the green wire tied directly to the circuit ground?    in the video, it looks like an instrument cable goes from the preamp to an amp.  Does the amp also have a 3 wire power plug?  if so, maybe ground loop.

[Captain chunkulus]

It has a 3 prong cord. Earth ground is made right on the chassis near where the chord comes into the chassis.  The power supply ground is grounded at the center of the heater artificial ground. The preamp is grounded right at the input jack as well as all the tone controls and volume controls.

[shooter]

we did a "ground lift/ isolating" in reply 8&9

On the other hand the buzz decreases when I touch the chassis or my guitar strings.

got a audio source like battery power CD player that you can jack in and listen?

[terminalgs]

we did a "ground lift/ isolating" in reply 8&9

Ah! sorry about that.

you can also ground the first grid to see if noise is coming in on the instrument cable.

I believe the following is true, so please correct me if I'm wrong, There is so much potential gain in this circuit, but really all those successive gain stages will only amplify the noise at full Mu potential.  If each gain stage has an Mu=40.  a 1mV noise signal introduced at the 1st gain stage would travel through each successive gain stage getting larger and larger but a guitar signal would begin to clip and square wave at the 3rd stage.  successive gain stages wouldn't really amplify the signal, only distort., but the signal isn't any bigger in terms of amplitude., but the noise is getting amplified...  at input, noise could be 5% of the amplitude of the signal but 50% of the amplitude at the output.

[Captain chunkulus]

Im.pretty certain that I grounded the first grid and the noise went away.

[Captain chunkulus]

Okay, if I ground the input to stage one the buzz gets lounder. If I ground grid 2 of the input tube the sound goes away completely.

[Captain chunkulus]

Here's another clip.

[terminalgs]

noise getting louder when grounding the grid of V6a:  maybe you introduce a ground loop?  I would use a jumper wire connected to ground via a big capacitor (like 10uf).  with the aligator clip from the (+) side of the cap, probe different points along the signal path between grid of V6b and plate of V6a (be sure clip is insulated or take other precautions to avoid shock!).  As you move from V6b toward V6a you might be honing in on a culprit.


regarding C41 220pf and C52 1uf:  that noise is low frequency, maybe 120Hz,  so I don't think it can be feeding back via C41, but I'd check solder joints all around V6a and maybe lift those caps, see if it makes any difference.

[Captain chunkulus]

So, I removed c41 entirely. No difference. Used the capicitor and clip to ground and probed. No difference. The only thing that makes any difference is when I move the shielded input wire. The hum gets louder. However, even when nothing is plugged in there is a buzz/hum if you turn up the gain and master volumes. It's just there in the signal all the time. When I plug a guitar in it get a lot louder like ground hum and if I let go of the guitar strings it gets even louder. I'm using a buss ground schem where all the componates that get grounded like (grid leaks, cathodes, filter caps) are all in a line soldered to the one buss. Then, the buss is soldered  directly to the METAL input jack. The pots all have a bare wire soldered accross them and then are soldered to the input as well. I used the hoffman stereo tube pre wiring. Just like the pictures show.

[Captain chunkulus]

All be it that my wiring technics are not as clean.

[Captain chunkulus]

Another picture.

[Captain chunkulus]

Okay first of all, thanks to all of you guys for offer your input and experience. You guys are great! So, I decided to try a new grounding scheme. Isolated input jack, removed wire that was soldered to back of pots, grounded gain pot at its ground point on the buss that is on the board, grounded mid range and master volume to their point in the ground buss, then grounded the ground buss to the power supply ground where also the heater artifical Center tap is grounded as well. Seperate chassis ground for Earth ground. Still buzz's. There is a very slight HUM with the gain pot all the way down and the master volume up really high but that's livable. When the gain pot is turned up that's when the really Sharp buzz starts.  no chop stick wire  moving produces any effect whatsoever anywhere on the preamp. I'm stumped. I have continuity tested all of the solder points and such. I just can't see the forest for the trees anymore. Maybe powr supply location? Transformer Location? Dc heaters?

[Captain chunkulus]

Okay so I made a ground lift, that made the hum/buzz worse. I also tried an ebtech hum-x, also made it worse. I'm keep comming back to a heater issue? Maybe run the first tube on regulated dc. Like I said before, no wires moved by chop sticking produce any difference in noise level. Hum/Buzz is effected  by the gain pot which comes in after the first stage and the master volume cuts it completely when turned down. Any of you guys know someone in Nashville TN that might be willing to maybe go over the preamp and see where I might have missed something? I would normally get my dad to do it but he passed away 2 months ago. He was my go to guy about all things tube. Thanks in advance for any help.

[Captain chunkulus]

So I analyzed the hum and it's 120hz.

[shooter]

120hz.

so filaments are out, and PS, or inductive coupling from the PS are in
Measure Volts AC at each power supply tap, I would expect ~~ 2-4vac 1st tap, <20mV AC at last tap

[Captain chunkulus]

So, read for ac after the bridge rectifier on the hv line and also on the heater tap?  Just trying to clarify.

120hz.

so filaments are out, and PS, or inductive coupling from the PS are in
Measure Volts AC at each power supply tap, I would expect ~~ 2-4vac 1st tap, <20mV AC at last tap

[Captain chunkulus]

Okay, reading no ac voltage at any of the 22uf caps or 10k resisters on the supply side. No ac voltage on the plate side where the resisters are soldered to the plates either.

[shooter]

no ac voltage at any of the 22uf caps

I'd find a new meter 
I have Never had 0, 2mV is close I know, but it's not 0

[Captain chunkulus]

Okay, at the main tap comming off the main power supply board. It's reading 1.40 acv and climbing. Leaky capacitor?

[Captain chunkulus]

After about 10 mins, it up over 2. 04 vac and still slowly rising.

[shooter]

I expect a leaky cap to discharge over time, slow charging, guessing it's getting paid by the hour 

2vac is probably good at the 1st tap of a standard pre/pa amp, a pre only 

what's the ripple at the tap feeding V1

also, your freq shot, can you re-do scaling the left side for volts instead of db, also take it out to 4Khz,  I'm using mcdee's wifi and my slide rule is at home 

[Captain chunkulus]

I was using a phone app to analyze  the frequency. I don't think it will do what you're asking. Btw. I built a new power supply using brand new f&t capacitors but the same buzz still exists. 

[shooter]

I built a new power supply

so it's down to tubes and wires and solder and R' and C's

the buzz is there with nothing jacked in correct?

[Captain chunkulus]

Hum is there even when a guitar is not plugged in. However, if I turn the gain pot diwn it's dead silent. Also, it gets more of a buzz sound when I plug a guitar in. It's more of a hum sound when not plugged in. I thought I would post the voltages just for info's sake. V-1 115vdc at the plate. V1b 208vdc at the plate. V2a 263vdc at the plate. V3a 105vdc at the plate, v3b 319vdc at the plate and 107vdc at the cathode where I feed the eq.  Transformer is putting out 319vdc.

[sluckey]

Have you turned off all room lights to see if they are inducing any noise?

I think you can benefit by adding some 22µF onboard filter caps as shown. 47µF would be even better.

[Captain chunkulus]

22uf on backside of the board. Sorry, didn't post great pics.

[Captain chunkulus]

Okay, made a discovery this morning. When I disconnect the relays power the terrible buzz goes away. When re-engaging it it comes back. There is still a very faint ground hum sound but I feel like that's livable. So, should I try a seperate small transformer just to power the realistic and also make sure to use shielded cable to and from the relays? Also, maybe move the relays farther from the circuity for. The preamp? Thoughts?

[PRR]

shooter> try removing power from your relay boards as a test
Okay just tried that. No difference.

Okay, made a discovery this morning. When I disconnect the relays power the terrible buzz goes away.... Thoughts?

What a difference 12 days makes?

[Captain chunkulus]

Well,  it didn't make a difference  until later after I rebuilt the power supply and redid the grounds.  at the time it made no difference. I just on a while tried it again  after I had redone all those other things. I still feel pretty silly though.