Cheap Mosfet amps popping up on ebay for song. Any input???Please Comment

[spirod]

I found these amps popping up on ebay PCB is Labeled yuanjing,
they seem to have 2 channels with either 4 or 8 Power transistors. It mentions they are based on 2SC5200 2SA1943, But are called Mosfet power amps, lol.
I have ordered one of the 4 x mosfet version. I am not sure where the mosfets come in to it tho

Also they seem to recommend only 22 v ac x 2 supply. So i guess it may be a good tweaking project.

here is a link although it will probably expire inevitably.

http://cgi.ebay.com.au/MOSFET-POWER-...1%7C240%3A1318

In case it expires you should be able to find it by searching for (2SC5200 2SA1943)
currently there is a couple of sellers selling what seems to be very similar amps.

Any how has anyone tried one of these?
or have any comments good or bad are welcome , thanks.

I am thinking of getting the 8x mosfet version. if the 4x version sounds good.

[djk]

No bias transistor!
No bias control!
TO-92 Vas!

Typical buyer complaints include: Seller never responds, and no documentation.

Why not just buy a lottery ticket?

[sivan_and]

TWEAK!!!

[taj]

Looks like a typical Hong Kong eBay vendor flogging typical Chinese sourced counterfeit parts used in stolen circuit designs that are poorly implemented and not QC'ed. 15 negative reviews in the last month speaks volumes.

I'm pretty sure I can predict the outcome being discussed here in the next few weeks.

..Todd

[pacificblue]

I like the way, how they replaced the quad op amps with four transistors each.

22-0-22 V for 2x100 W? Must be bridged.

Could be an organ donor, if the parts are not fake.

[destroyer X]

The biggest world population....small costs there....will dominate the electronics (already have) and automobiles...and them will dominate even more.

Problem there is fuel only....i hope no one decide to stop them cutting the fuel to avoid competition.

Chocoholic, our forum friend, went there and will never come back....there's the place to work!...the point!.. beeing skilled and stranger..of course... naturally.

Native people earn not so good money....but this is changing too!

regards,

Carlos

[VictoriaGuy]

Originally posted by taj
Looks like a typical Hong Kong eBay vendor flogging typical Chinese sourced counterfeit parts used in stolen circuit designs that are poorly implemented and not QC'ed. 15 negative reviews in the last month speaks volumes.

It does pay to read the feedback carefully before bidding.
In the last month this seller has 15 negative and 10 'neutral' (ie not good) feedback and 794 positive from buyers. Keep in mind that quite often folks with a positive experience don't post feedback at all, but complaints usually result in feedback every time. (Especially now that sellers can't give buyers negative feedback- the seller has no 'defence' against unwarranted negative feedback.) (I say this as an eBay seller and buyer with 500+ transactions.)

So this seller has 97% (at least) positive transactions in the last month.
Now look at a few of the neg/neutral feedbacks. A number were about stepper motor controller boards which came without documentation- the ads didn't mention documentation and one shouldn't assume anything on eBay. Some of the other less than positive feedbacks were from folks who thought the item should have arrived more quickly- the ads clearly state 2-3 week delivery times or more.

Now look for the amps that the OP is considering buying- not many sold- I only went back far enough- a few weeks- to find 4 of those amps sold. All 4 purchasers were happy with the amps.

So the odds are pretty reasonable that you would get a satisfactory product from this seller.

After all, how many of us can report 100% satisfaction (over hundreds of transactions) with any mail-order firm? I had a recent experience with a well-known US seller (direct from his website, not eBay) where numerous emails, several phone calls -no tollfree number- and a couple of fax messages to place an order were never acknowledged. Finally I phoned, and got the answer, that the FAX had arrived.. After TWO months, an envelope with the items came through the door- no invoice, no 'Thanks!'. Checking my credit card records showed that I had been billed a few weeks before. But this seller apparently has a 'great reputation'.

So lets not be too unfair.

Cheers
John

[jaycee]

Look at the pictures. One of the small heatsinks has bent fins, and one of the driver transistors looks like it is rusty!!!

No evidence of thermal compensation, and it looks like transistors were bodged into where a quad opamp should be.

You can bet your bottom dollar those "Toshiba" 2SA1943/2SC5200's are fakes as well. You can't claim they are MOSFET amps if they dont use MOSFETs !

I second djk - you might as well just buy a lottery ticket, or throw your money down the drain. Better still - give it to me

[CBS240]

Hi

It's hard to judge something like this with only a few photos of one side. It appears those quad op-amps are actually quad transistors, 2222 & 2907 type. They just used singles instead. Same difference, those cheap monolithic quad transistor chips aren't matched anyway. You have to pay a lot extra for that!

The differences in PCB trace length between the drivers, outputs and fb loops of each channel indicate a poor understanding of layout parasitic effects on stability. The fact, as has been pointed out, that there is no bias thermal compensation indicates the lack of understanding of how the circuit is suppose to function by the designer. But then, it could be bias as a heavy class B in which case it's best use may be as a target for the ol' .30 .30.
Too bad you can't see the bottom of the PCB, trace widths and routes. As for mosfet's, they are where?

[spirod]

Hi All some great feed back here thanks.
The Amp arrived,
to my suprise it came complete with heatsink. It just needs a transformer and it just so happens that i have a few that will do the job nicely.

FYI, the earth trace is all over the place, a new type of earthing system similar to star earth , I call it snake earth. lol., and the heatsink is probably about the size of what I would probably use to mount an lm3886 chip amp to.
The toshiba ..cough.. mosfets are printed with different TOSHIBA fonts for the npn and pnp's

Hopefully i will get a chance to hook up and listen/see what is happening on the cro in the next few days, I will have to wait till i get my hands on a dummy load. probably wont be till the weekend tho.

just thought i would give you guys a quick update for now.