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CFP output stage adding more output transistors
CFP output stage adding more output transistors
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Old 2nd February 2013, 08:19 AM   #1
diy didi is offline diy didi  South Africa
diyAudio Member
Join Date: Sep 2009
Location: Pretoria
Default CFP output stage adding more output transistors

Hi all. I have recently built a basic amplifier using this topology. P3A amplifier to be exact.
I was wondering, just as a matter of interest, how does one add more output transistors in parallel with the current two- transistor setup in this CFP output stage???
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Old 2nd February 2013, 05:55 PM   #2
Ian Finch is offline Ian Finch  Australia
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Join Date: Apr 2010
Location: Coffs Harbour
I'd post that question on Rod Elliott's forum or at least search threads there on the P3a. It seems just about every DIY around the planet has played with trying to build their own version and the question would have arisen many times. It's likely been discussed here too if you care to search it.

Having said that, if you read real books and came across Douglas Self's "Audio Power Amplifier Design Manual" or "Self on Audio" in the later editions, you'd come across his "Load Invariant" design which is a "blameless" style CFP amp with 2 pairs of MJL3281/1302 driven by MJE340/350. In a nutshell, the output transistors are simply wired in parallel.

'Nothing complicated, bar the heavy emphasis on very good thermal bonding of the bias transistor to the driver transistor(s) - a lot more positively than in the the P3a, anyway. The issues with CFP are poorly understood thermal compensation schemes and stray inductance (long runs and messy routing) in the output stage, so top-notch PCB layout with increasing care as the number of pairs increases and then base stopper resistors, cap. and zobel bypasses may have to go in too.

CFP can be fine but it doesn't handle stray reactances too well. Keep it simple, short and neat and there should be no oscillation problems for only a few pairs.
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Old 2nd February 2013, 06:15 PM   #3
jaycee is offline jaycee  United Kingdom
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Join Date: Apr 2004
Location: Norwich, UK
Elliot shows such a configuration in his P27 guitar amp. small emitter resistors must be used so that the current is evenly shared by the transistors.
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