#26 pre amp

[Curly Woods]

Originally Posted by ikoflexer

Blankets are nice, they keep me warm

(Curly, no disrespect meant, I'm just sayin'!)

None taken Iko

[coolzero]

One of the other thread which Andy posted in 2007, he mentioned about hum due to unbalanced filament supply when 26 tubes put in series. Can someone explain this? I use series filament and sometimes experiencing slight high frequency hum which I believe due to the same reason. I experimented with different filament voltage and current variations where I noticed that the 26 inducing hum when the current goes below certain limit.

[andyjevans]

Hi,

I've stopped putting tubes in series - separate filament supplies for each tube. Don't know if hum was the problem, but I just made the decision to use one supply per tube. I just use a big cap and a LM1084 so the filament supply is pretty simple, apart from the sheer bulk of it including the heatsink.

andy

[coolzero]

Originally Posted by andyjevans

Hi,

I've stopped putting tubes in series - separate filament supplies for each tube. Don't know if hum was the problem, but I just made the decision to use one supply per tube. I just use a big cap and a LM1084 so the filament supply is pretty simple, apart from the sheer bulk of it including the heatsink.

andy

I tried simple voltage sources following simple current source using LM317 and 10000uf caps but still there was a loud buzzing.

[disco]

Originally Posted by coolzero

I tried simple voltage sources following simple current source using LM317 and 10000uf caps but still there was a loud buzzing.

Do you use seperate transformer secundaries for LV?
Further, connecting the filaments in series makes a beautiful antenna for strayfields.

[andyjevans]

Hi coolzero,

you say you have "voltage sources following simple current source". I'm not quite sure I understand you here. If you mean "I have a voltage source following a current source" that makes no sense - the Ronan Reg type circuit has voltage reg first then current source as last in the chain before the filaments. Current source should be last, even a common mode choke seems to make no improvement after the current source, though curiously it improves things after a voltage reg. But you may mean "first I tried a current source, then I tried a voltage reg and they both hummed".

I can't diagnose your problem, but I can only say my setup doesn't hum. I use a LM1084. And I use a seperate power supply - the transformer is in another box, and so is the HT including the choke - only DC goes to the signal chassis. And each filament has a separate transformer. I have loads of small 6v transformers which I use for filaments. And another thing - these filament transformers can pick up hum if they are close to chokes and mains transformers even in the remote PSU box. The whole filament supply from beginning to end has to be kept away from any magnetic fields.

andy

[coolzero]

Originally Posted by disco

Do you use seperate transformer secundaries for LV?
Further, connecting the filaments in series makes a beautiful antenna for strayfields.

I used a single transformer with 2 separate secondary windings.

[coolzero]

Originally Posted by andyjevans

Hi coolzero,

you say you have "voltage sources following simple current source". I'm not quite sure I understand you here. If you mean "I have a voltage source following a current source" that makes no sense - the Ronan Reg type circuit has voltage reg first then current source as last in the chain before the filaments. Current source should be last, even a common mode choke seems to make no improvement after the current source, though curiously it improves things after a voltage reg. But you may mean "first I tried a current source, then I tried a voltage reg and they both hummed".

I can't diagnose your problem, but I can only say my setup doesn't hum. I use a LM1084. And I use a seperate power supply - the transformer is in another box, and so is the HT including the choke - only DC goes to the signal chassis. And each filament has a separate transformer. I have loads of small 6v transformers which I use for filaments. And another thing - these filament transformers can pick up hum if they are close to chokes and mains transformers even in the remote PSU box. The whole filament supply from beginning to end has to be kept away from any magnetic fields.

andy

Andy,

Sorry for the confusion in my explanation. It is somewhat similar to Ronan's with voltage regulator first and current regulator after that. It was not two of those separably I have used. I used a transformer with dual secondary windings of each 5V/1A. Everywhere I read about this, people mentioned that this works like a charm but for me it was a hummingbird in side my 26 pre.

Later I found those SMPS and used one for the filament but then came this slight high frequency hum from one channel I believe becuase of not enough current floating across the series filaments. One other thing I noticed was that, when 26 filaments put on series one of the filament slightly brighter than other and there is a slight delay always when those are lighten up. One fires up a second or two later than the other. There was no change of this behaviour even the different tubes were used. Something funny.

[disco]

hi, without knowing your exact build it's guessing what went wrong. This artice holds all information you need for building a hum free amplifier. I'll upload some data in a couple of hours so you can compare yours.

[coolzero]

Originally Posted by disco

hi, without knowing your exact build it's guessing what went wrong. This artice holds all information you need for building a hum free amplifier. I'll upload some data in a couple of hours so you can compare yours.

Hi,

I started with Jim's 26 pre and modified it to the present level with help of Kevin and many other in this forum. I have posted the schematic and the details in this same thread before. Initially I followed all Jim's advices mentioned in his site but with less luck. Even Jim had mentioned that he had hum issue with his 26 preamp which he couldn't get rid of. What I have now is far better than the original design in terms of hum and sound quality. I checked and re-checked many time of grounding, filtering, shielding and all the other possible hum inducing mishaps hence pretty sure that the slight high frequency hum I am getting time to time in my audio path is not related to any of those usual suspects.