DC heaters - leave floating or elevate?

[Lico]

I've been struggling with hum in my new preamp. I assumed it was a ground loop, but it's actually being caused by the heaters. They are DC fed with the 0 volt side tied to the star ground. If i disconnect that from the star ground and leave them floating, 90% of the hum goes away and I only hear it when the volume is much higher than I ever listen. Does it make sense to keep it floating like that? Or should I try elevating the ground? The circuit is the cornet octal: http://www.hagtech.com/pdf/cornetoctal.pdf

 

[rayma]

Try floating the filament supply at +60VDC above ground, with a decoupled resistive voltage divider.

 

[kodabmx]

What is the hum frequency? 60 or 120? Short the inputs. Hum gone?

 

[Lico]

60Hz. With the input shorted it's almost all gone.

 

[mdpaudio]

Sounds like the line from the input RCA is picking up the hum. Is it shielded? Is it new the AC?

 

[Lico]

That cable is new and shielded, the old unshielded one sounded the same. I don't think that cable is the problem, it still hums with it disconnected and nothing attached to the RCA jacks. The source seems to be the heaters. Or the heater rectification circuit.

 

[Diabolical Artificer]

Does your heater DC supply have ripple? If not it's more likely a grounding issue, try moving your grounds about, it sounds like there's some high AC current element getting into your IP/hi gain stage.

Have a probe around with your scope,have another look at your ground scheme; star ground isn't a panacea.

Andy.

 

[Chris Hornbeck]

rayma has already provided a working solution, which really should be integral to all designs, and should not be considered to be a "patch". Elevating heater standing voltage positive of cathode voltage is simply "best practice". This term means that serious engineers just do it, if for no other reason than they'd look foolish if failure to do so causes a well known issue. It's a "don't be that guy" thing, and a matter of professional pride.


Also, if the filament's DC voltage is not pure as the driven snow, it's better to assume that it's not perfect, and take signal from a resistive Y from both ends.


These elements won't cure issues elsewhere, but will still benefit the best constructions in all cases.


All good fortune,
Chris

 

[Koonw]

If the heater is floating and still a little hum, try to touch one side of the heater with DMM in volt range (10Meg) and ground see if the hum goes away if so then replaced with a 10Meg resistor. Also try to insert in series 10 ohms resistor between the screen cable and ground for each cable. The gives a hum lift to eliminate any ground loop introduced by 2 RCA interconnect screens/

 

[ruffrecords]

This kind of hum is often caused by heater cathode leakage in old tubes. Try changing them.

Cheers

Ian

 

[Parafeed813]

The schematic you link to is a phono stage with unregulated DC.
I would not do that. Use clean regulated DC, elevated a bit above cathode potential.
While you're at it, regulate the HT too.
Also important: keep the power transformer far away from the tubes and audio circuitry.