I tried the eBay MX50 DIY Kit, Class AB 100W+100W; Worked but problem

Ken,
the lower pair of output BJTs have no emitter resistors.

The same value emitter resistors (0r22) as fitted to the upper pair must be added to the lower pair.

Emitter current vs collector current, nearly same thing...
As long as 0.22R is sensing it back to the driver.

In the original drawing, you would have a valid point.
Them .22s in the collector weren't doin' jack...
 
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Remember base to emitter resistance is 26ohms.
0.22R is like a drop in the bucket to the outputs,
unless you got driver gain involved in amplifying
that feedback.

Nitpicking that 0.22R should be on output CFP
emitter rather than (or in addition to) collector
is not significant.

Even you consider 2n3055 beta 20~40, that 0.22
may behave like 8.8 ohms at the high end. Only if
you multiply again by driver beta, then it matters.

I dunno, what transistors are his outputs? Could
beta be high enough for 0.22R to become significant
compared to 26 ohms? If so, then maybe it matters.
If not, collector placements are fine, as long as the
drivers can see them...

Base to emitter resistance of the CFP base + 8.8R
still ain't a drop in the bucket to driver collector Z...
Its gonna drive whatever current it wants into the
output base, regardless your emitter resistor there...
If not sensed at the collector, where it feeds back
to the driver of the CFP, its useless..
 
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Q2 has collector and base swapped surely. I expect the board is OK though, just a drawing error.

I've just received the mx50x2 pcbs from HK, I've checked the track layout and there's a few things....

1. It's a high quality pcb.
2. The track is laid out wrong I reckon at Q2 which is the 5551 nearest to the MX50X2 logo, and will need the collector and base leads swapping to make a typical current mirror circuit.
3. It looks quite easy to modify the power resistor arrangement with a couple of cuts and links as per kenpeters suggestion if so desired.

I may not build these for quite a while as I fixed my sub.
 
2. The track is laid out wrong I reckon at Q2 which is the 5551 nearest to the MX50X2 logo, and will need the collector and base leads swapping to make a typical current mirror circuit.
3. It looks quite easy to modify the power resistor arrangement with a couple of cuts and links as per kenpeters suggestion if so desired.

Do post the pics of the PCB. Thanks!
 
MX50x2 boards..........

Component side:-
componentsidepcb1.jpg

Solder side:-
soldersidepcb1.jpg


These scans don't show how shiny everything is, and the boards are strong, very difficult to flex at all. The black numbers on a few transistors are my marks to identify where I wanted to check out the circuit in the current mirror area, and the T-shape mark between 2x100R is where I reckon a cut could be made to help modify it as discussed.
 
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Doublecheck against schematic before cutting anything.
Guesswork of a madman with no guarantee of any sort..
I don't own one, never tried it. You take your chances...

Does not include fix for current mirror with the miswired
transistor. Somebody else can draw that one.
 

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Doublecheck against schematic before cutting anything.
Guesswork of a madman with no guarantee of any sort..
I don't own one, never tried it. You take your chances...

Does not include fix for current mirror with the miswired
transistor. Somebody else can draw that one.

Actually, you may not have to cut anything to fix the
current mirror. 2n5551 default pinout of EBC is problem.
Fairchild spec sheet mentions 2n5551-C (note suffix -C!)
pinout of ECB would probably fix everything, no knife...

Were any other 2n5551 lay'd out for ECB? I'm still chasing
traces around to see what goes where...

-----

Other three 5551 strapped as diodes could not care less.
EBC vs ECB, same circuit either way.... Use whatever...

Its only Q2 where Fairchild -C suffix pinout might matter.
Q2 seems to be 2n5551 nearest the MX50 logo.
 
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If not confusing enough, Philips spec sheet shows CBE pinout!
I think its same as Fairchild EBC default, just disagree which end
is pin 1... If you go by the D shaped outline, its all same same...

But you need ECB, not the default pinout of either mfg.
Else you will be twisting legs or cutting traces for Q2...
 
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I've just tested one of the kit's 5551 NPN transistors, it checks out as BCE pinout, gain 150. That would make the Q2 pin nearest the MX50 logo the emitter. I think that makes the pcb OK, it's the schematic that's wrong at Q2. Brain hurts - I'll take another look at the weekend.

The 5401 PNP measures gain 200 and has pinout CBE.
 
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I posted that original schematic which is what I got from the seller. There were problems with the drawing in the bias adjustment circuit where I was having trouble. I traced out the PCB and corrected that schematic before posting it. Didn't do that for the current mirror, but wouldn't be surprised if there was a problem with the Q2 & Q4 part of the schematic.
 
OK, just looked at my boards and the transistor datasheets. My kit was shipped with NCC 5551 transistors. The datasheet is here:

NCC 5551* datasheets and application notes, data sheet, circuit, pdf, cross reference, pinout, datasheet | Datasheets.org.uk

Looking at the traces, pin 1 (emitter) on both Q2 & Q4 are tied together and go to the minus supply post the RC filter. Then pins 3 (collectors) are tied together, so it seems the schematic is wrong and the PCB is correct.

All of the traces of interest are on the back side of the PCB. In your picture, you placed a dot under Q2 on the back side of the PCB. Q4 is to the right. The pins on both transistors top to bottom are:

E
B
C

Your picture is repeated below for convenience...

soldersidepcb1.jpg