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svokke 29th May 2010 06:36 PM

Inside a "no brand" PA Amp...
3 Attachment(s)
I just received a cheap (90€) "PA" amp that is supposed to have "480 Watt max achievement at 4 ohms (2 x 160 Watt at 8ohm)" I bought it for 15€ because it was broken, just to see what's inside.
This is what I found:
- A lightweight 2 units 19" rackmount enclosure.
- 2*26V torroidal transformer, weight between 1.2kg and 1.5kg. So I estimate it at no more than 120VA.
- 4000F of power supply capacitors :eek:
- Very loud fan that is always on full speed.
- a tiny heatsink.
- 8 blown mosfet's and 2 burnt resistors. Mosfet's read A940 on them, but I can't find any mosfet that matches this part number.
- A rare level of build quality.
As you can see, a "real" performer. :D
I traced the schematic just to check if this amplifier self-destructs by design. Anyone has some thoughts on the performance of this circuit?
edit: non mosfet transistors are 2SC1815 (npn) and 2SA1015 (pnp)

Stormrider 29th May 2010 06:48 PM

2SA940 is a PNP bjt, not mosfet.

HighTec 29th May 2010 06:51 PM

Datasheet for 2SA940. Btw, he seems cracked on that picture! :confused:

Edit: I think this would be the proper circuit?

richie00boy 29th May 2010 06:55 PM

Pretty rubbish circuit with resistive load for VAS. I think you may have the 0.22 ohm resistors the wrong way round - they should be in the sources. Maybe this is what caused failure.

Globulator 29th May 2010 07:03 PM


Originally Posted by svokke (
I just received a cheap (90) "PA" amp that is supposed to have "480 Watt max achievement at 4 ohms (2 x 160 Watt at 8ohm)"

If it is on 35V rails then peak power is 35^2 / 8 = 153W, so RMS power is 76.5W, allowing for loss in the resistors and mosfets you'd probably get 70wpc from it.

Of course my maths is a bit rusty - correct me if I'm wrong!

Andrew Eckhardt 29th May 2010 07:07 PM

never saw worse

HighTec 29th May 2010 07:37 PM

ooops, propering the proper :D

svokke 29th May 2010 08:10 PM

1 Attachment(s)
I checked and double checked the circuits around Q1, Q6 and Q7. They are as indicated on my schematic. All 0.22 resistors are on pin 3 of each "mosfet".
All 8 "mosfets" have the same markings: A940. I'm confused as marketing blurb said "modern MOSFET technology". As some other members mentioned, I can only find the PNP transistor with this name. All 8 are broken, so it's impossible to say if they measure as a fet, pnp or npn.
I found some people with the same broken amplifier and they also report that all "mosfets" have the same A940 marking.

HighTec 29th May 2010 08:29 PM

Well, if my eyes are still good, there is a Fairchild logo on those transistors. Fairchild doesn't manufacture 2SA940 but they do under a different name, KSA940 which is again BJT PNP. Maybe they are fakes from China, dunno. :confused:

Arnulf 29th May 2010 09:12 PM

KSA940, pin 3 is emitter which would make sense.

I'm curious though, how does 2x16V transformer provide enough voltage for +/- 35V rails using a simple rectifier bridge ?

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