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Best low-ohm power resistors?

Posted by andy evans (A) on June 16, 2010 at 23:56:06

I'm working on a preamp using the 26 DHT with filament bias - the filament supply also runs through the cathode resistor. So the part I need here is 10 ohms, with 1 amp going through it = 10 watts. The quality of this part is crucial and differences are audible. So I've been testing different low-ohm power resistors and this is what I've found so far:

Multicomp vitreous enamel 10R 50W
This is a big resistor, 100mm long. Heat dissipation is fine. This was the best sound - very clean with lots of life and subtle tone qualities to instruments. Vocals were lively and general sound smooth but vivid.

NEOHM Cement WW - 3 of 27R in parallel rated 10w each, total 30W
Heat dissipation is also OK here. Sound is neutral, good bass and treble. Easy to listen to. Not as lively or detailed as the above, but nothing objectionable.

Alu Clad Vishay 10R 50W
Very clear and detailed but a rather hard sound which gets tiring. This may be better with more damping - I had it in free air. Not an option unless it can be made smoother.

Vishay thick film TO-220 case 10R 50W on heatsink 3.9 C/W
I expected a lot from this, and was disappointed. Sound was dead and lifeless, lacked bass extension, and vocals were shut in. It was smoother than the alu clad but overall inferior to even the cement WW. Could do with a bigger heatsink, though 3.9 C/W was OK for testing. Does this resistor need burning in or something? Why so disappointing?

My conclusion so far is to use vitreous enamel wirewounds in some form. For a 10 ohm cathode resistor which suite the 26 in filament bias, 50W is a good rating, and 30W possible for mounting on top of the case.

I've had numerous suggestions to use Mills 12W resistors in parallel. Three of the 30, four of the 39 or five of the 47. Not cheap!! Also not available at good prices in the UK - they are £4 here.

Where do I go from here? Do your experiences support or contradict mine? What else should I now try? What has worked for you?

Andy

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