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carlmart 6th January 2012 07:13 PM

Amplifier kit on eBay
I found this amplifier kit on eBay:

NJW0302G NJW0281 Stereo Power Amplifier Kit for diy Ref MA-9S2 MA-9 | eBay

Has someone built it?

I seem to recall that someone here did buy it and built it, but my search did not get me anywhere.

johnmik 6th January 2012 07:44 PM

you're going to need a nice heat sink for mounting the outputs on. I've seen alot of these on Ebay out of China, never tried any yet.
just wanted to make a point about heat sinking on this kind of stuff. It's not included.

My .02
john M

carlmart 6th January 2012 08:51 PM

Well, of course I will need a good heatsink for this amp or any power amps that pretends to output more than 30 watts. Cooling an amp heat is not something to be taken lightly, or parts will suffer, such as the output transistors or even the speakers.

These kits never include the large heatsinks, at least those from eBay. If they do they put it quite clearly.

In any case, a valid warning for anyone else who's reading this topic.

clm811 24th April 2013 01:12 AM

1 Attachment(s)
I purchased and assembled the following(I double checked components and their placement):

Stereo Power Amplifier Kit for diy Ref MA-9S2 MA-9 Class AB/MA9-S2 clone 150W/150W@8ohm PowerAmp kit-high end MJL4281A+MJL4302A
through eBay.

MA9 S2 Clone 150W 150W 8ohm Poweramp Kit High End MJL4281A MJL4302A | eBay

I have questions about biasing and test voltages that are not given by seller.

Should the biasing pot(0-1kohms)adjustment (W1) be turned fully clockwise, counter-clockwise, or centered for initial power up/operating voltage testing?

I will be using an incandescent light bulb in series with the variac, then the transformer, rectifier, caps, then F1A fuses in series with the VCC and VSS rails; milliamp meters will be in series with each rail, and a voltmeter will probe across each 0.22 emitter resistor to determine the bias current(target is 50mA). Outputs and bias transistor are mounted on large heatsink.

I'm asking for your input before I power up(I'm wanting not to smoke the amp pcb nor the outputs).

Attached is the schematic given by the seller:


Superstrand 24th April 2013 01:20 AM

The Bennic cap looks nice, otherwise I am completely skeptical, will analyze the schematic and report back after a good night of sleep. Cheers.

Superstrand 24th April 2013 03:55 AM

I looked it over because I can't sleep. This looks like a DIY-gene style eBay deal to me (just search former user 777 for a full commentary) If I am not mistaken (which, forgive me if I am) you are attempting to replicate a Marantz design. Why not build your own, or share in any of the wonderful designs made available by the kind Mr. Pass? I apologize, but I would find it difficult to contribute on this one, Best wishes.

theologos 4th June 2013 12:25 AM

I was just looking for this amp and stumbled on this site:

Pair 150W 1NJW0302G NJW0281 Stereo Amplifier AMP DIY Kit Ref Marantz MA-9S2

Is also refers: "This amplifier kit is reference to the circuit of MA-9S2, We also add feature on it, Direct current servo added to the circuit,That allow no neccessory to adjust the neutral point, Only several mV(less than 10mV) and can be ignored. We only need to adjust the static current, Just rotate the variable resistors to make the voltage on any one of 0.22Ohm resistor is between 10mV to 40mV, If you want to set it at Class A mode, Pls adjust the current larger than 100mV, But larger heatsink should be needed."

Maybe it helps if you haven't find anything better yet...

alpefe 4th June 2013 06:50 PM

hi all, in the end someone mounted this kit?, he intended to buy one to see how quality has.
I want to put a preamp Denon Pra 1500 with this stage, believe that sound better than my current Denon amplifier PMA2000R?, greetings to all.

dangus 4th June 2013 10:35 PM

If you have to pay new prices for a transformer and filter caps and case, it starts to add up to real money. Might be wiser to look for an off-the-shelf Chinese amp that can be upgraded, like the Behringer A500. Or a used, non-Chinese "studio amp" like the discontinued Yorkville SR300.

Fast Eddie D 4th June 2013 11:14 PM

I have a whole power supply, case, heatsink, etc in an old power amp chassis. It even has LED VU meters. It was a Sony rated 135 watts RMS per channel into 8 ohms, so this kit might be a good deal for me, if I knew whether it was any good in the first place.

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