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Old 29th May 2010, 06:36 PM   #1
svokke is offline svokke  Belgium
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Default Inside a "no brand" PA Amp...

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
- 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.
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)
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File Type: gif schematic.gif (43.3 KB, 856 views)
File Type: jpg 1.jpg (90.4 KB, 836 views)
File Type: jpg 2.jpg (70.6 KB, 799 views)
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Last edited by svokke; 29th May 2010 at 06:38 PM. Reason: added transistor types
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Old 29th May 2010, 06:48 PM   #2
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2SA940 is a PNP bjt, not mosfet.
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Old 29th May 2010, 06:51 PM   #3
HighTec is offline HighTec  Croatia
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Datasheet for 2SA940. Btw, he seems cracked on that picture!

http://www.datasheetcatalog.org/data...pec/2SA940.pdf

Edit: I think this would be the proper circuit?

Click the image to open in full size.

Last edited by HighTec; 29th May 2010 at 07:20 PM.
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Old 29th May 2010, 06:55 PM   #4
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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.
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Old 29th May 2010, 07:03 PM   #5
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Quote:
Originally Posted by svokke View Post
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!
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Old 29th May 2010, 07:07 PM   #6
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never saw worse
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Old 29th May 2010, 07:37 PM   #7
HighTec is offline HighTec  Croatia
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ooops, propering the proper

Click the image to open in full size.
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Old 29th May 2010, 08:10 PM   #8
svokke is offline svokke  Belgium
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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.
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Old 29th May 2010, 08:29 PM   #9
HighTec is offline HighTec  Croatia
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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.

http://www.fairchildsemi.com/ds/KS%2FKSA940.pdf
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Old 29th May 2010, 09:12 PM   #10
Arnulf is offline Arnulf  Europe
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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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