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widi 6th January 2009 11:38 AM

Possible Class A Amplifier
 
Hi all.
I have an old amplifier class A, or so says the electrical diagram, and I
want to update to the affection that I have, but first I would like to know
whether it is a class A.

and would like your opinion.

regards.
http://img393.imageshack.us/my.php?i...lifierszj1.jpg

analog_sa 6th January 2009 12:54 PM

Re: Possible Class A Amplifier
 
Quote:

Originally posted by widi
but first I would like to know
whether it is a class A.



It isn't.

juma 6th January 2009 03:22 PM

Re: Re: Possible Class A Amplifier
 
Quote:

Originally posted by widi
... I would like to know
whether it is a class A. ..


From Q1 to Q7 it is in class A. Working regime of output stage depends on bias current setting (set it with VR2). With 1,2A running through output stage you'll have 25W in cl. A (@ 8 Ohms load) and about 120W thermal dissipation per channel without signal - that's cl. A.

Zen Mod 6th January 2009 03:25 PM

Re: Re: Re: Possible Class A Amplifier
 
Quote:

Originally posted by juma



From Q1 to Q7 it is in class A. Working regime of output stage depends on bias current setting (set it with VR2). With 1,2A running through output stage you'll have 25W in cl. A (@ 8 Ohms load) and about 120W thermal dissipation per channel without signal - that's cl. A.


I know that you know ...... that before doing that ( cranking it up ) - is very wise lowering PSU voltage , and installing sufficient sum of capacitance in PSU filter .......

:clown:

also - without pics of actual amp , we can't say - is it needed to ensure better heatsinking for driver stage

analog_sa 6th January 2009 03:27 PM

It is not class A at the rated output of 100W/8R, thus it is not a class A amp. Or if it is, it will only last a very short time :)

jacco vermeulen 6th January 2009 03:30 PM

I seem to recall a Class A power amp with 2 pairs of MJ15003/4 devices, running on ~37Vdc rails.
Sounded like something Otto said in Fish called Wanda : K-k-k-k-Ken.

Nelson Pass 6th January 2009 06:10 PM

Re: Possible Class A Amplifier
 
Quote:

Originally posted by widi
and would like your opinion.
Looks like an early Krell.

:cool:

GRollins 6th January 2009 09:09 PM

Although the voltage readings shown at the bases for the output devices look pretty light, I don't see any reason you couldn't fiddle the bias up into class A territory if you're willing to invest enough in heat sinks and power supply.

Grey

widi 6th January 2009 10:27 PM

Thank you all for your opinions, whether GRollins, the PSU has a large toroidal transformer 600W and four capacitors 30.000uf rail in each of the PSU and eight ultra-fast diodes 50A.Lo I want is to replace MJ15003 / 4, by MJL3281A and MJL1302A so that I occupy less space.
Where can I find this change of transistors?

GRollins 6th January 2009 11:24 PM

Beware...changing output devices may lead to instability problems. It's not that it can't be done, but I would suggest checking the amp thoroughly on an oscilloscope to make sure everything is okay before putting it into your system.

Grey


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