1.2kw amp schematic to check

Could somebody who knows how amps work check this schematic, because author of that amp may put a little error(s) in it (cuz hes selling that amp too), and thing wont work.. Thank u:)
This one is for 800w, for 1.2kw 2 output transistors must be added on each side and voltage rised from +-95v to +-110v..


  • schematic.zip
    90.1 KB · Views: 1,722


2001-02-04 4:23 am
QSC, a well know manufacturer of commercial amplifiers in the USA, uses eight pair of MJ15024/25 (sixteen devices per channel) for +/- 95V supplies. If you intend to raise the rail voltage to +/- 110V you must use MJ21193/94, they have twice the SOA at 100V. The use of IRF 640/9640 as driver transistors is not to be recommended, they are very hard to stabilize over temperature. I would not build this project.
Can someone please just tell me what class is that amp, B or AB, because the bias is only few mA (writed in the article of that amp) and the fet powers the music until the voltage across R105 and R106 is big enough, then the power is accompanied by output transistors.. Schematic is in my first post in this thread. Please have a look.

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
If the bias is 40 mA or so, I would call it Class B, since
that is how much flows through the drivers before they
bias the output transistors.

Looking at the schematic, I don't see any errors offhand,
but I have to agree that using the Mosfet to drive the
outputs is perhaps not the best choice. Personally, I
have never made such a circuit work to my satisfaction.

Also, I would seriously question the reliability of the output
stage at the kind of power levels you are talking about.
I think something like 20 devices would be more like it,
maybe more.
Thank you for your reply Mr. Pass, then, ill finish it that it will work, then ill sell it. Author of that amp stated in his article that he used mosfets "because heat dissipation of normal design would be ~50w at idle (each of output transistors at ~30ma bias current), and with mosfet driver there is only few ma of bias current." <-- He wroted something like that.

Is Anthony Holton´s amp ok? I read in Nelsons article that class ab biased mosfets are not ok, but maybe theres a catch with this amp about that?
I think it will do his job.
Off course it,s a AB amplifier.
Th driver section is not that good i agree with the others it,s not stable and it will give a little noise.
Maybe you can change that thing into en set up with normal transistors it,s not that difficult.
Also you will never get 1,2 kw out of this amp not even close.
Maybe in some peaks and in distortion.
You need a lott of more power output transistors and a higher voltage level too get too this power.
I don,t even know this kind off transistors (live in the Netherlands)
I will tell you this that i can recomanded you an amp with the well known japanese transistors brand Sanken.
Ring emitter transistors which don,t need that much overall feedback with all the pro,s off it.
I hope you will understand this story my Englisch isn,t that good anymore :)
Maybe an 800 watt is what you can get with some more output transistor,s
250 watts each is NOT the power the transistor can handle and you will never find it back in efficincy in this case music.
A lot off the power goes off course into heat.
I mentioned heat because do you have an idea of the coolers?
Motorola is not my kind off transistor used in krell etc.
SANKEN ring emitter transistors are much more stable and with the spec,s off a sanken which in this case i mean the open loop circle you will see that it will not fall off bij example 800 hz and that,s what most normal transistor,s will do without feedback.
So how greather the feedback how les the distortion end the better the frequencie respons and that,s why i always used the fast ring emitter transistor,s becausse off the less feedback with better results and a much more better sound ONLY the thd will increase but not that abnormol much more important is the TIM as you will know
skaara said:
Of course it cant handle 250w of music.. in datasheet its written that it is 250w transistor, you run it at 100w + 150w goes to heat = 250w power dissipation..

Ok so sorry for the reply but i thougt because of the 800 watt,s you mentioned that you had the wrong number in mind.
And EVEN with a rendement of 100 watt in music power i guess it,s very high.
believe me but try the sanken ringemitter transistors it will give you sure more music.
skaara said:
I dont believe that I can buy them here in Slovenia..

Maybe you can order them in Germany.
I have an adress which i will find for you.
Do you have the spec,s off the motorola ones?
Maybe it is a good transistor although i don,t believe Motorola makes ringemitter transistors.
Very good sounding amplifiers with the RE are being publiced in elkektor.
Not 1,2 KW version,s which i think is a lott of power .
I myself used normal factory amps from SONY (TAN80) which bridged are good for at least 400 watt,s but there is a problem with those amps the coolers are too small so this evening (it,s very hot here now) i had a meltdown by one off them.
They used RE transistor,s which made them sounding better than a lott of other more expensive amplifiers.
In the near future i want to buy myself rotel amps which used them too but with enough cooling.
I build a lott of amps myself but most off the time you can,t get the right component,s like buffer elco,s with a high voltage range.
And specialy the good ones from ELNA and spraque are diffecult too get.
I wish you all the luck with your project which is sure now will sound good .
Bassicly it,s a very good amp only the output stage is not that which i say TOP.
Good luck Don