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apoopoo999 7th June 2011 02:07 PM

Ebay KSA50 needs 2 voltages?
I found this ebay ksa50 clone, I built several amplifiers but this is the first time I seen one that needs 2 voltages. Can anyone explain this? The seller doesnt seem to helpful.

150W Class A KSA50 MONO amplifier board | eBay

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mullardel34 7th June 2011 03:06 PM

It's quite likely that the input/driver stages require a higher power-supply voltage level in order to allow the input/driver stages to drive the output stages rail-to-rail. See the Nelson Pass A75 design ( and, which has a similar requirement for the power-supply design.

apoopoo999 8th June 2011 12:40 AM

hmm, so is it safe to say this clone is nothing like a ksa50

stoc005 8th June 2011 01:05 AM

If this was a MOSFET output and not a lateral FET output, the higher voltage for the input and driver stage(s) makes sense. But the outputs are bipolar transistors. I guess they want the amp to clip in the output stage with an extra 20 volts (p-p) of headroom for the driver/input stage.
2 oz platinum???

I don't see a bias or offset pot. How is that so??

llwhtt 8th June 2011 01:17 AM

Not just platinum but "COPPER thickness of 2 oz. of PLATINUM" WTF?


john65b 8th June 2011 01:24 AM

A KSA50 at 150 watts?

Did the Original KSA50 go into class AB?

Original output devices were a pair of MJ15003 and MJ15004

I think it is a low bias Class AB KSA50...should still sound nice...

Price is awesome, but can't help but think the transistors could be fake at that price...

I have a KSA50 with a trio of the 2SC5200/2SA1943 and it is my favorite amp by far.

dihmels 14th January 2014 12:44 PM

I just built an amp built around two of these
I know, very old thread.

I have just finished connecting two of these and after much time on Google am starting to get somewhere.

I have a very large heat sink, am running +/- 40V to the output stage and +/- 50V to the input stage and have been creeping the quiescent current up. Currently at .5 A per for each output device (8).

One bit that none of the Chinese sellers mention is that the bias transistor needs removed from the board and mounted to the main heat sink.

I see that since I bought these boards that the many sellers are now recommending a 15v spread between input and output stages. I would imagine that that would be a better balance to push the output closer to its max pre-clipping.

Anyway, they are cheap for what you get. I'm not sure if the outputs are fake but they have yet to blow and sound nice.

I have two newbie questions:

1. Although these are symmetrical designed amps with 2 sets of output devices per channel, i cannot get each transistor to match in quiescent current. I have .22 ohm emitter resistors and a voltage drop of .11V so a current of about .5A. The voltage ranges from .08 to .13 depending on which transistor I check. Is that a sign of fake transistors or just standard without using a component matching process?

2. Is there a sweet spot for this type of design with the Toshiba outputs (1943 / 5200) with regards to quiescent current and trying to bais to class A as much as possible?

3. Am I calculating class A power correctly here?

.5 amp * 2 (NPN ans PNP) = 1 AMP per pair peak to peak.

p= I*E so 1A * 38V = 38 Watts??

I am assuming that having multiple pairs of transistors does not increase output if they are not in series (aka Darlington) so I assume that the main reason for multiple pairs is to split the workload and heat dissipation?

Finally, this is a new hobby of mine so please keep this in mind when pondering my questions.


richie00boy 14th January 2014 12:54 PM


Originally Posted by dihmels (
The voltage ranges from .08 to .13 depending on which transistor I check. Is that a sign of fake transistors or just standard without using a component matching process?

A combination of variation in transistors and variation in emitter resistors. The emitter resistors are most probably 5% tolerance parts. I would see how it goes, if they are fakes it will probably blow up once pushed a bit.

Make sure you have a DC protection for your speakers!

Multiple pairs increases output by increasing current capability, which in turn spreads the dissipation of load as you say. In series would simply add losses.

dihmels 14th January 2014 01:57 PM


Are my calculations correct regarding class A wattage then or do I add the pairs together as you suggested?

If you don't mind, and I'm sure it has been covered before but I can only allow for so much "hobby time" during a work day :) how would I go about calculating class A wattage with this set up?

Two pairs of 2Sc5200 / 2SC1943 each at .5 amps with 38V rails (under load) and with 8 ohm speakers.

My output power supply board has the rectifier and 6 15000 uf 80V capacitors to smooth the AC (big toroidal pulled from Arcam AVR 250) but also has circuits for speaker protection. Just waiting for my upc1237 chip that's in the mail.

richie00boy 14th January 2014 03:17 PM

That gives you 1A available current in each direction. Into an 8 ohm load that would be 8 watts peak, 4 watts RMS.

The idle power dissipated is 40 watts.

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