P3A based question ....

[east electronics]

Now i need some advise ....

Current set up in my house is a P3A dual mono at 48+48 rails with 1302-3281 outputs @50 ma bias ....

Ok rails are a bit on the risky side Load also is low about 5ohms so i kept the bias low
Listening demands are very low i think that the average power needed should be no more than 20 W
Obviously i know i am out of SOA but since i operate the system and no one else will abuse it i might get away with it ....

Now my actual problem is bias meaning that amplifier plays superb when warm ,,,So given the conditions above if i crank up the bias @100 ma i think i am pushing my transistors way too much ...Often i power up my amp in the morning to go home in the night to listen ....or i keep it always on the weekends

remember i live in Greece so in my home as we speak temp is 18-22 degrees but in the summer time it could easily go 35 .....

One solution will be to go for 1216 combo .... 15 MHZ more faster( 40 instead of 25 ), 17A instead of 15, and generally better SOA , also LAPT technology ....

any ideas ? Lowering rails or adding more transistors is out of the picture ...

[mjf]

.........afaik the p3a has a cfp output stage and i guess you have emitter res = 0,33 ohms.
d. self recommends for this combo an optimal bias of 8,54mA (with an 8 ohm load).
quoted from
d. self "audio power amplifier design handbook" fifth edition, page 183.........

[adason]

Originally Posted by east electronics

Now i need some advise ....

Current set up in my house is a P3A dual mono at 48+48 rails with 1302-3281 outputs @50 ma bias ....

Ok rails are a bit on the risky side Load also is low about 5ohms so i kept the bias low
Listening demands are very low i think that the average power needed should be no more than 20 W
Obviously i know i am out of SOA but since i operate the system and no one else will abuse it i might get away with it ....

Now my actual problem is bias meaning that amplifier plays superb when warm ,,,So given the conditions above if i crank up the bias @100 ma i think i am pushing my transistors way too much ...Often i power up my amp in the morning to go home in the night to listen ....or i keep it always on the weekends

remember i live in Greece so in my home as we speak temp is 18-22 degrees but in the summer time it could easily go 35 .....

One solution will be to go for 1216 combo .... 15 MHZ more faster( 40 instead of 25 ), 17A instead of 15, and generally better SOA , also LAPT technology ....

any ideas ? Lowering rails or adding more transistors is out of the picture ...

...use small computer fans
I use 12V fans with DC regulator, and set about half the voltage, so they rotate comfortably, start reliably, and produce no noise
have two dozens of classA amps, almost none without fans, I do not like things too hot
reliability is important to me

[Ian Finch]

Hi Sakis. It seems to me that since you have been operating your versions of P3A with a higher (up to 130Ma IIRC) bias current for some years, the only problem to deal with is heat dissipation, meaning bigger heatsinks and better airflow than in typical Japanese amplifiers up to about 1990. Assuming this is basically a modified commercial product, perhaps the original power transformer will be running warmer and the power supply components may be stretched a bit too.

Even if your load is only 4 ohms, you are still talking of 96V rail-to-rail at 50 or 100mA which is only 5W or 10W idling dissipation for each output stage. Surely that's not a problem for TO264 power transistors operating at low average output level.

I think we are aware of the bad effects of protection relay contacts so the only concession I would make, is to add a version (such as Bonsai's design) of a Mosfet output protection relay to eliminate further risks from accidental overloads, shorts, failures etc. but with no affect on the audio.