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Old 13th June 2010, 11:29 PM   #471
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I'm thinking I may have to get hold of some filament chokes. Maybe Hammond 159ZA. I guess LCL is the ultimate for this. Four chokes - nuts but I guess I just have to hear this. It's not expensive - just HEAVY!!

Andy
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Old 14th June 2010, 08:24 AM   #472
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When you compare choke-fed CCS to solid state, please remember to take account of noise. The LT1086 is a better 3-pin regulator than most, but its data sheet still specifies

'Noise rms, 10Hz..10kHz 0.003% of Vout.' Sounds small, but at 5V that's 150uV, and it'll be worse than that because you're certainly not band-limited to 10kHz. And that's rms as well, so the peaks will be much larger.

If the output of this chip is injected across the cathode resistor, you're effectively putting the whole noise signal at the cathode-to-ground of the valve, which is the same as injecting it to the grid!

150uV is only 35dB or so down on line-level signals, so you'll hear the noise all the time, and it will be sure to degrade the sound.

When comparing solid-state CCS to choke-fed, a low-noise CCS will need to be built - but that's not very tough. The 2-transistor CCS shown in my old heater circuit can be adapted: just use a low-noise transistor for the feedback position (BC547C, etc), and use a low value of collector resistor in it, so that the BC547C Ic is at least 10mA. Most ZTX transistors are very low-noise if run at 10..40mA too (low enough for a moving-coil cartridge). I'll draw it when I get a minute.

Point noted about adding rectifiers! thanks Andy.
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Old 14th June 2010, 11:11 AM   #473
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Thanks Rod, look forward to the completion of your ps kit
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Old 14th June 2010, 12:03 PM   #474
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With filament bias and a 10 ohm cathode resistor with 10 watts through it, I need a big part like 30W or over - they get hot.

What would you guys suggest for the best sounding part here? I tried alu clad, which are very clear but rather hard, cheap ceramic WW which are nothing special but less hard at least.

what's the best sound here? I see some Vishay Sfernice 30W thick film parts for instance. Another option is those brown ceramic Ohmite WW resistors. I don't know if those dark green ceramic WW types (?make) go down as far as 10 ohms.

Ideas?

Andy
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Old 14th June 2010, 12:12 PM   #475
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In that position I like the TO-220 packaged film resistors. The 35W parts are ideal. I've used Meggit and BI branded ones. The great advantage is bolting them to a chassis or heatsink.

Or, for lower noise, old noninductive wirewounds in parallel (2x10W etc)
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Old 14th June 2010, 12:16 PM   #476
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The dark green WW are Welwyn Vitreous enamelled WW.

You can go 10R/14W -

http://uk.farnell.com/welwyn/w24-10r...10r/dp/9505580

Edit: watch out, these are inductive, and have magnetic endcaps! Try them though, they are low noise, and if they are fed from low noise source, and kept away from stray fields (trafos, rectifier wiring etc) they work well.

Last edited by Rod Coleman; 14th June 2010 at 12:18 PM.
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Old 14th June 2010, 12:45 PM   #477
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caddok TO-220?
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Old 16th June 2010, 11:42 AM   #478
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OK - here's some cathode resistor shootouts. Part is a 10 ohm resistor which needs 10 watts. So this must be over-rated for comfort.

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.

So the conclusion is clearly 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.

Andy
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Old 16th June 2010, 12:50 PM   #479
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Andy, I assume the Vitreous part was

MULTICOMP|MC14663|RESISTOR, 10 OHM 50W | Farnell United Kingdom

Thick Film parts do show higher current noise, and since you're running high current, they may fail due to noise. Don't let it put you off trying them in low-current high voltage jobs!

The Almi-clad types usually have steel terminals, which won't help there. WW does do best for current noise, so perhaps a noninductive (wound in a Ayrton-Perry stylee) will be best. I believe Ohmite make a good choice of these.
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Old 16th June 2010, 07:53 PM   #480
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The Vitreous resistor was indeed Farnell's Multicomp part. They had five, and I took four. Next delivery is in September, so I'll be looking around. A vitreous enamel WW rated 50w is the part to go for, so it seems at present. Will be seeking out some Mills 12W as and when I see them.

With such a simple preamp, every part is audible. I was surprised at the difference these resistors make.

Andy
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