Tube supply question

[Rob Strand]

The most interesting thing is the fact that if I plug my preamp together with amp, and at the same time, hum is annoying. If I unplug preamp, then plug in again, hum gets reduced. HOW TO HELL IS THIS POSSIBLE?

I'm not sure if you are plugging in the signal lines or the power.

There's something weird going on for sure.  That and the weird click problem you were seeing before points to something is wrong.   Generally when you see this type of thing something *is* wrong.   It's always hard to find because it is often caused by something you are assuming is correct is not correct.  Something as simple as two wires connecting, like a ground.  At this point I can only suggest checking connections throughout the system.  It is a slow process proving what you think is true is actually true.

[rankot]

The most interesting thing is the fact that if I plug my preamp together with amp, and at the same time, hum is annoying. If I unplug preamp, then plug in again, hum gets reduced. HOW TO HELL IS THIS POSSIBLE?

I'm not sure if you are plugging in the signal lines or the power.

There's something weird going on for sure.  That and the weird click problem you were seeing before points to something is wrong.   Generally when you see this type of thing something *is* wrong.   It's always hard to find because it is often caused by something you are assuming is correct is not correct.  Something as simple as two wires connecting, like a ground.  At this point I can only suggest checking connections throughout the system.  It is a slow process proving what you think is true is actually true.

I am plugging in the power.

[Rob Strand]

I am plugging in the power.

You could  measure the current draw and see if there is a consistent difference in the humming and non-humming states.  (Sometimes oscillation can appears as a hum.)

[rankot]

OK, I will.

[rankot]

Well, I thought that it could be the problem with underpowering the preamp, so I changed some things (put bigger capacitors on voltage doubler, replaced LM317T with LT1083, but no progress yet. I have also tried to add few 100n capacitors between DC power and ground at some places to kill some oscillations, but it didn't help either.

Here is the clip (https://clyp.it/rcguc4b1)

So the first 4 seconds are just the preamp's own noise, after that few seconds when I touch the stings and the noise gets stronger (seems I behave like an antenna, so you can call me Radio Ranko), and after that few tones. I like the sound of it very much, but this noise is freaking me out completely! 

[Rob Strand]

Thanks for the clip.    Yes, it's annoying at higher volumes.

I had a look at the spectrum of the noise/hum.    There are harmonics of 50Hz across the entire spectrum (all the way up to the 8kHz limit of the mp3).  It is present in the good case and the bad case.  When you touch the strings it all rises up by about 8dB.  However at 100Hz and 200Hz (but not at 50Hz) it rises up quite a bit more say 20dB;  I can't explain that at this point.

From what I can see the problem is related to grounding.

Have you tried a different guitar, different cable, or shorter cable?

Does the noise/hum decrease if you touch the mains ground (say the amp chassis) with the palm of your hand while touching the strings?

Does the hum change when you set the guitar's volume to zero.  And with the guitar's volume set to zero, does touching the strings have any effect?

[Edit:  One thing you can try is increasing the caps from the circuit's 0V to the chassis.   Normally you wouldn't have to do this.]

[rankot]

Rob, preamp is still in modular/experimental phase:



So no chassis yet. It seems I will have to make one to continue with debugging. I tried to shorten DC cable from PS to preamp, but it didn't change a thing.

But I will certainly try all chassis unrelated suggestions before that!

[Rob Strand]

So no chassis yet.

Ah! The lack of chassis may actually be the cause of the hum!
It is quite common to get the problem you are seeing with unshielded circuits.

Try putting a square of aluminium foil under the boards.
Make it extend past the edges of the board.
Then connect the aluminium foil to either 0V or mains ground.

If you want to be safe put a piece of paper between the circuit and the foil.

[rankot]

I'll put a cardboard, just to make better isolation, but I have also tried what you suggested and here are the results (have in mind that when I mention pots here, I relate to pots on guitar/bass, not pots on my preamp):

- I work as an antenna regardless volume control on guitar/bass. Actually more on bass (seems it somehow knows that's MINE instrument) So when volume is at zero, I touching strings on both instruments increase hum.
- If I unplug guitar and left cable plugged into preamp, hum is HUGE and it seems to be clear 50Hz; touching cable tip with finger reduce that huge hum to previous level (as with bass/guitar plugged in).
- Volume control of my bass doesn't affect hum a lot, but if I plug my guitar, hum seems to be strongest with volume at middle, reducing or increasing volume decrease hum level.
- Tone control on bass/guitar affect hum level a lot, even with volume at zero - turning tone pot to high adds high noise to hum, turning to low it does reduce higher harmonics of hum.

[Rob Strand]

Is your bass active?

The guitar in the middle being worst is "normal" when crap is getting into the signal lines.

I just noticed on your picture that the input wires are not shielded - is that right?
You should use coax.  For the interim make them short as possible also twist them tightly together.

[Edit:

- If I unplug guitar and left cable plugged into preamp, hum is HUGE and it seems to be clear 50Hz;

I forgot to mention that is normal.

Normally what I do to get a baseline for hum/noise is short the input with a *very* short piece of wire.  The hum/noise usually doesn't get any better than that.    Next is to put in a lead and then short the instrument end of the lead.   In this configuration you should be able to touch the ground without the hum/noise increasing; the more junk you hear the poorer the grounding.
]

[Rob Strand]

Have a look at the pic in post #5 by Stallik,

http://www.diystompboxes.com/smfforum/index.php?topic=116423.0

That type of set-up avoids a lot of problems with hum and noise. 
You can try to approximate the same idea with foil and cardboard.

[rankot]

I have replaced simple input wires with shielded ones and hum is still the same.

This is how I built PS, I had only 6V transformer at hand, so I decided to build a voltage doubler based PS. Could it be the cause of 100 and 200 Hz hum?

[Rob Strand]

This is how I built PS, I had only 6V transformer at hand, so I decided to build a voltage doubler based PS. Could it be the cause of 100 and 200 Hz hum?

It may be contributing but at this point it is hard to know.

One way it can contribute is the transformer itself is too close to the circuit, or the guitar.   If you can move the guitar and the preamp circuit about 1m away from the transformer it would be a good test.  Really you need to move at least upto C1 away as well because you don't want the current pulses from the transformer on the long wires.  Best would be to move the whole PSU circuit upto the 9VDC part away.

Another way it can cause trouble is by putting current pulses through the ground.  However, from the previous discussions the way you connect the power looks OK and should not cause such issues.

Yet another way is junk is coupling through the transformer.   Normally this isn't a problem.  That would be reduced by connecting the transformer chassis to mains ground (and possibly adding small caps from the transformer outputs to mains ground.)
Is your transformer chassis connected to mains ground?

The reason why I think it is grounding/shielding related is the fact the problem gets worse when you touch ground.  This is a common characteristic of a grounding or shielding problem.  From your sound sample there is hum present before you touch it.  That can be PSU or grounding.

Fixing these types of problems takes a lot of patience and experimentation.  The aim is to narrow down the problem to a smaller zone.  You need remove all possible causes of noise, like shielding the wires, placing it over a ground plane, moving the PSU away, grounding the pot cases.   While that might fix the issue, there's chance it might not.   After that, through a sequence of experiments,  you start narrowing down where in the circuit the noise is coming from.   At any time you might discover the problem is one silly thing.    Sometimes you just have an idea try it and it fixes it but that relies on luck.  It's much like how we narrowed down the hum problem before to one tube, and then the grounding of heaters.   (There could still be a problem with the grounding of the heaters!)