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Old 10th April 2012, 09:48 PM   #51
MiiB is online now MiiB  Denmark
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nope they are hard to find...sj74 could be an option.. but low idds makes them not so Suitable for CCS.. You need a longer channel higher IDSS part...

a depletion p channel mosfet would be nice.
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Old 11th April 2012, 12:43 AM   #52
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Originally Posted by buzzforb View Post
Who wants to minimize even order distortion Question. If paralleling a resistor with pot, you want the power resistor to be smaller than pot so that it takes the heat, correct
Exactly - the power resistor should be higher in value than what you would calculate for the current you want to achieve, then use a pot that is even higher value so that it draws less current than the power resistor and shunts the power resistor to increase the overall current - adjustably.

If you want to idiot proof it (just in case you're building it for somebody else ) instead of a putting a pot in parallel with the power resistor, put an extra resistor in series with the pot, then put the Pot + series resistor combination in parallel with the power resistor. This extra resistor protects you in the case that the pot is turned down to zero

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I could easily do TSSA BIGBT HP since it is much simpler than SSA and potentially even better performer.
yeah ! (I just like the elegance of 'simplest')


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hehe almost

Problem is to find and get some good p-jfet, any suggestions
don't do it, CCS's are evil, just say 'no'.... just kidding, although I've never used one (CCS or JFET for that matter)
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Last edited by Bigun; 11th April 2012 at 12:46 AM.
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Old 11th April 2012, 07:15 AM   #53
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Originally Posted by Bigun View Post
don't do it, CCS's are evil, just say 'no'.... just kidding, although I've never used one (CCS or JFET for that matter)
Hi Bigun

Over the weekend I measured few of them, not for the first time, I did it before also and was never realy satisfied. BJT as cascode in CCS had ten times higher Z100 kHz, noise and much more nonlinear impedance curve over audio band. But the one I posted with jfet cascode impressed me deeply. For rock solid DC current there's precision zener, for perfect AC performance there's jfet.

Jfet itself to form simple CCS is way too much temperature dependent (DC stability is bad) but here in cascode position it feels just perfect, made for this.
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Old 11th April 2012, 07:39 AM   #54
MiiB is online now MiiB  Denmark
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I think You saw that Jfet cascode trick in some other thread....
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Old 11th April 2012, 07:48 AM   #55
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I think You saw that Jfet cascode trick in some other thread....
Yes here. Thank you MiiB
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Old 11th April 2012, 05:05 PM   #56
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Hi guys

TSSA all BJT version needs to be thermally compensated, for that reason 1N4148 were replaced by BD139/140 mounted on the heatsink together with outputs, also input BJT pair flats are glued together. In that way amp should be thermally stable even at high output bias current.
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Old 11th April 2012, 05:18 PM   #57
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Or even less parts solution with thermal track output transistors, where tracking diodes have double function: bias input pair and compensates thermal drift.
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Old 11th April 2012, 05:30 PM   #58
MiiB is online now MiiB  Denmark
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think that it would work better with lateral Fets as they are much easier to drive..
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Old 11th April 2012, 05:37 PM   #59
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Agree, for 1 A load current BC's would have to provide extra 10 mA into bases of the outputs. Small power amp only.
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Old 11th April 2012, 05:43 PM   #60
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Quote:
Originally Posted by Lazy Cat View Post
Or even less parts solution with thermal track output transistors, where tracking diodes have double function: bias input pair and compensates thermal drift.
LC is it worth to work on a pcb for this, i think i have all parts in stock

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