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Old 29th May 2010, 09:13 PM   #11
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It looks very much like the inside of cheap amp I bought off ebay.

I was shocked at the minmal heatsink and transformer for the wattage specified for the amp.

The amp worked but definitely did not give the wattage advertised.
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Old 29th May 2010, 09:18 PM   #12
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Quote:
Originally Posted by Arnulf View Post
I'm curious though, how does 2x16V transformer provide enough voltage for +/- 35V rails using a simple rectifier bridge ?
OP says 2x26v... 26 x 1.414 = 36.7vdc
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Old 29th May 2010, 09:43 PM   #13
Arnulf is offline Arnulf  Europe
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Oh, I'm sorry, I honestly misread that as 16V That makes sense now.
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Old 30th May 2010, 07:19 AM   #14
svokke is offline svokke  Belgium
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Well, the "mosfets" look exactly like the KSA940, a PNP transistor. I wonder why they would use an all PNP output stage. Looks like a design from the late 1950's. Also there is no temperature compensation at all, so it is possible that these amps self-destruct by thermal runaway.
I'll probably scrap the Amp PCB. It's no good at all.
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Old 30th May 2010, 07:31 AM   #15
h_a is offline h_a  Europe
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I can't imagine how a mosfet amp without manual bias adjustment should work in any reasonable way. VGS is usually all over the map. Matching is too expensive for cheap amps, anyway. VBE of bipolars is perfect in comparison.

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Old 30th May 2010, 07:54 AM   #16
Nrik is offline Nrik  Denmark
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Hmm - where does the bases/gates (most left pin) of the lower and the upper outputs go exactly? ( you have drawn them differently in your schematics so it is hard to gues) Double check that so that we can conclude what type it is.

Hereafter 2 options:
1) Replace all outputs with cheap new ones, and add a proper bias circuit.
2) Replace the PCB with DXamp or Symasym, and mount a decent heatsink.
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Old 30th May 2010, 08:17 AM   #17
HighTec is offline HighTec  Croatia
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Quote:
Originally Posted by svokke View Post
Also there is no temperature compensation at all, so it is possible that these amps self-destruct by thermal runaway.
I'll probably scrap the Amp PCB. It's no good at all.
Hm, that D2, 1N4148 should be mounted or at least touching the heatsink to ensure some thermal tracking. Anyways, like you said, that piece of junk isn't worth repairing. Replace it with some reliable design.
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Old 30th May 2010, 10:04 AM   #18
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Quote:
Originally Posted by Stormrider View Post
OP says 2x26v... 26 x 1.414 = 36.7vdc
So at full load that would probably give a rail of 26V then, RMS power = 26x26/(8x2) = 42.25W

If someone blew the transistors at that power they were probably fakes. I'd advise using the box for a nice DIY pre-amp, can't see any mileage in that amp..
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Old 30th May 2010, 12:25 PM   #19
Nrik is offline Nrik  Denmark
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Quote:
Originally Posted by HighTec View Post
Hm, that D2, 1N4148 should be mounted or at least touching the heatsink to ensure some thermal tracking. Anyways, like you said, that piece of junk isn't worth repairing. Replace it with some reliable design.
I just noticed the value of R7 - 33ohms - this is a pure class B design!

Svokke : If you want good sound you should skip the original pcb, and put in another amplifier.
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Old 30th May 2010, 01:03 PM   #20
gruni is offline gruni  Germany
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almost the same amp design can be found in those "hollywood" or "skytec" or "nightline" or whatever amps you can get off ebay here in germany. they are also rated 480 watts (although there is an even cheaper model rated 240 watts, which has only two output transistors per channel)... i have the schematic diagram of these amps somewhere, perhaps i'll find it, so i can upload it here.
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