Humming on left side with SOHA

[xTr3Me.aka.Chris]

Hi head-fi forum,

I have a problem with my SOHA headamp. There is a humming on the left side. When I turn the amp on it is very quiet. After some hours it is getting louder and that it makes it quite impossible to enjoy music ( especially with quiet parts).

I have noticed the following things since I recognized the humming for the first time:

- It gets louder the longer I use the SOHA
- the first time I heard the humming was after I forgot to turn the AMP off for a night
- when I turn the volume knob louder the humming gets also louder and can be heard on right and left side
- the humming does not get that loud when case is open (maybe a heat-problem?)
- I already changed the tube (got the one from JJ now) but it did not change anything

Well maybe you can help me with that problem, that would be really nice and btw: im from Germany and I hope my english is not that bad

greetings,
xTr3Me

 

[tomb]

Well, my first guess would be the tube, but since you said you've changed that - I guess not.

I believe most of the regs in the SOHA have thermal shutdowns. So, if it's heat-related, you may have damaged a cap. If you have some 85 deg. caps, or some no-name caps, it's possible they got too hot in the case overnight and it boiled or dried out some electrolyte. They say that's why old tube radios are often noisy with a lot of hum - the caps have gone bad over the years.

The caps to look at are probably the B+ caps - those are the 100uf 100V ones or the cathode bypass caps - those are the ones next to the tubes. You might look to see if they've swelled or whether the tops of the cans look pushed out slightly.

Just a guess ...

 

[Eokboy]

The SOHA regs get really hot. I had to make vent holes on the rear panel.

 

[cfcubed]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif Well, my first guess would be the tube, but since you said you've changed that - I guess not. <snip> The caps to look at are probably the B+ caps - those are the 100uf 100V ones or the cathode bypass caps - those are the ones next to the tubes. You might look to see if they've swelled or whether the tops of the cans look pushed out slightly. Just a guess ...

I think this is a great guess

Looking all the caps over is probably not a bad idea... I've repaired hum a couple regular amps & both times it was PS caps.

I too drilled vent holes in my SOHA back panels & use larger heatsinks on the hotties... For the one in the pics on the muting/delay thread, I even installed C12 offset to get it away from VR3.

 

[xTr3Me.aka.Chris]

Hi,

thanks for your fast answers. I bought the level 2 kit with auri caps from glass jar audio, so I hope the regs are of good quality ?!

but looking into the inside of the amp I can see that there is an electrolytic capacitor that is only built for 85°C. It is the C11 (see here: http://www.mb3k.com/soha.html).

But this one is only for the "heat of the tube". Someone from a german forum said that it can be the C3 ,4 , 7, 8 or 9 that influences the audio signal.

Because of the fact that the C3 is the closest I should maybe change that one at first? Because the VR3 gets very hot... and maybe I also need another heatsink for VR3.. but the hammond case is not big enough for a bigger heatsink

greetings,
xTr3Me

 

[xTr3Me.aka.Chris]

@cfcubed
what do you mean with "PS caps" . u mean electrolytic capacitors from panasonic?

Should I i exchange them with caps from other manufactorers?

What kind of heatsinks did you use? the problem is that i do not have anymore space next to the VR3 - when I want to use bigger heatsinks I have to move the electrolytic capacitors next to it..

When I got time I will also drill some vent holes into the back panel of my hammond case!

@eokboy
a "reg" is for example the VR3?


greetings,
xTr3Me

 

[Eokboy]

^Yes. Especially VR3, when I say hot.

 

[cfcubed]

> what do you mean with "PS caps" . u mean electrolytic capacitors from panasonic?

No, I mean power supply caps - A shot cap there can let hum into the circuit.

> Should I i exchange them with caps from other manufactorers?

No, didn't mean a specific manufacturer... When I replace caps that fail I just try to go for ones w/higher temp rating + maybe higher voltage rating.
In the regular amps that I fixed the caps were just above the min ratings (e.g. PS voltage was like 55v & caps rated for 63v (?) & they were also the lower temp rating 85c (?)). Keep an eye on fit though.
Note that I'm having no trouble w/inexpensive Xicon caps in my SOHAs tho.

> What kind of heatsinks did you use? the problem is that i do not have anymore space next to the VR3 - when I want to use bigger heatsinks I have to move the electrolytic capacitors next to it..

Take a look at that muting & delays thread pics... I used a larger one (cut down) for VR3 regulator & did have to replace a cap that was in the way (needed longer leads to shift it). I think some have actually used the case itself as a heatsink (screw for VR3 through case).


It's a bummer to think of replacing caps until/hoping the hum goes away.
Maybe others have better suggestions... And in most cases it would be replacing the caps - they are hard to test for failure at operating temps & voltages (e.g. if not totally shot).
At least you could start w/left channel caps as that seems the worst for you.
One thing to try is to assure the case is grounded - I ran ground wire from board ground -> cover screw holes.

Good luck.

Edit: In the past one could use freon to look for bad parts when the problem was heat-related. E.g. maybe RadioShack dust-off used upside down & sprayed on various parts might help locate the bad one.

 

[dBel84]

I would start by having a long hard look at all the ground connections. It is not impossible for new caps etc to be bad but highly unlikely and I would most definitely not pull it apart to start troubleshooting the hum. Here is a link to perhaps guide your search http://www.geofex.com/ampdbug/hum.htm , I have isolated hum on a few of my builds and it can be something simple to more PIA , like having to reroute all the wiring. If the transformer is inside the amp, this would be something to consider too. The increased hum with temperature is lost on me, it would help to isolate source of the heat. The heater supply regulator gets a little warm but if it is roasting, something else may be wrong. ..dB

 

[tomb]

I agree with dBel - even if I was suggesting a bad cap somewhere, you should see a visible effect on the cap before considering replacement.

 

[xTr3Me.aka.Chris]

im so stupid guys.. after changing the rube i didnt adjust the right voltage.. well I read it is not required but next time I should check this for myself

now the hum disappeared and I hope I never have a problem like that again.. and i hope it does not appear after the amp is really hot ...

so the conclusion is: the old tube was broken after it was running for a too long time and the new tube made the same hum with the wrong voltage as the broken tube with the right voltage..

thank you for your nice help

greetings,
xTr3Me

 

[tomb]

Quote:

Originally Posted by xTr3Me.aka.Chris /img/forum/go_quote.gif im so stupid guys.. after changing the rube i didnt adjust the right voltage.. well I read it is not required but next time I should check this for myself now the hum disappeared and I hope I never have a problem like that again.. and i hope it does not appear after the amp is really hot ... so the conclusion is: the old tube was broken after it was running for a too long time and the new tube made the same hum with the wrong voltage as the broken tube with the right voltage.. thank you for your nice help greetings, xTr3Me

You might warn us where you read it was OK not to bias a different tube. That way we can properly spank whoever it was.

You should check the bias every time you change tubes - they're all different to some amount.

 

[xTr3Me.aka.Chris]

ye I am sure that I will check that next time.. but I am quite happy with the JJ tube.. the better telefunken tubes are very expensive and I do not think its worth it.

greetings,
xTr3Me

 

[xTr3Me.aka.Chris]

omg... its still there just more silent.. and I can only hear it after 1-2 hours warmup.. I go crazy with this crap

 

[tomb]

Quote:

Originally Posted by xTr3Me.aka.Chris /img/forum/go_quote.gif omg... its still there just more silent.. and I can only hear it after 1-2 hours warmup.. I go crazy with this crap

It could still be the caps. Rather than jump and replace them:

1. Check to see of there's any swelling or if the tops of the cans are slightly bulging. If so, then replace them.
2. Check your solder joints. It's possible to have a cold joint on a soldered lead or one that's been fatiqued to the point that good connection is lost. Ripple (hum) will leak through if some of the caps have bad connections.
3. This wasn't mentioned because you seem to think it was heat related, but where's your transformer? Is it possible that the leads from one channel are close enough to the transformer to pick up hum? Maybe you just never noticed it before and the episode of leaving it on overnight is just a coincidence.
4. Same as 3 - different issue - grounding issues. Do you have the center tap grounded to the case and also the power outlet ground? If so, that will cause hum. Isolate the ground connection to the transformer and don't connect it to the center tap. There may be other sources of hum as well - is the pot grounded, etc., etc.?