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Old 22nd May 2012, 07:37 PM   #1
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Default CCS power amp bias

Hey there,

I was thinking of going for a basic CCS cathode bias on a 12au7 output, 22k-8 ohm output transformer, cathodes tied, but to be honest, I'm not sure where to start, or even if its a good idea. Ideally, I want to get the most power i can out of it at fairly low distortion.

First off, what transistor should i use? or MOSFET? What should it be rated for?
Secondly, I saw somewhere that a CCS won't allow class AB operation, would this set-up in class A give enough clean headroom?

Thanks for any help
Ricky
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Old 22nd May 2012, 08:20 PM   #2
Matt BH is offline Matt BH  United Kingdom
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Sounds interesting, what speakers are you using? Also you say cathodes tied together. For single ended fine but if its push pull I reckon a seperate CCS per triode is best with a cap between the cathodes.

I have been thinking about a tiny amp for some nice full rangers I have so I hope some of the more knowledgeable folk pick this thread up.

Cheers Matt.
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Old 22nd May 2012, 08:25 PM   #3
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The speakers are nothing speacial, fairly old 8 ohm. This is just for my workbench during the late nights but i seem to have a fair amount of distortion in the output, but the preamp is clean, just a standard 12ax7 pair with volume in the middle. a CCS came to mind, but i wouldnt know where to start, and if bias could be adjustable to find a sweet spot, that would be helpful too.
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Old 22nd May 2012, 08:35 PM   #4
Matt BH is offline Matt BH  United Kingdom
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I have been using the good old 10M45 for CCS in my stuff so far. You can get really fancy with these depletion mode FETs but to be honest they work well enough on their own.

Could you post a circuit diagram of your amp. Don't worry about software, just scribble it down and take a photo.

Last edited by Matt BH; 22nd May 2012 at 08:40 PM.
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Old 22nd May 2012, 08:48 PM   #5
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Still tweaking as necessary, but this is it right now, i cant remember the PSU off the top of my head but its as simple as the rest of the design
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Old 22nd May 2012, 09:03 PM   #6
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That's a guitar amp... I wouldn't bother with a CCS, as it is supposed to distort anyway, but... why not. A LM317 CCS would be easy enough to implement.
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Old 22nd May 2012, 09:05 PM   #7
Matt BH is offline Matt BH  United Kingdom
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Ok, looks a bit odd. Where did you get the design? Also are you at your bench right now?
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Old 22nd May 2012, 09:27 PM   #8
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Yes it did come from a guitar amp, but its not being used as one No im not at my bench now. i was hoping i could do something with a transistor. il go back to it tomorrow. by the way where did you get the 10M45s?
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Old 22nd May 2012, 09:39 PM   #9
DF96 is offline DF96  England
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A single CCS in the cathode for push-pull necessarily means Class A, but even then it can sometimes create problems because it stabilises the wrong thing: average current instead of quiescent current. For a power stage these are not the same thing.
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Old 22nd May 2012, 09:42 PM   #10
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I see, so this isn't a good idea? And what problems can it create?
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