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Old 3rd September 2011, 07:29 PM   #21
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Quote:
Originally Posted by AndrewT View Post
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...

Last edited by kenpeter; 3rd September 2011 at 07:57 PM.
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Old 3rd September 2011, 08:04 PM   #22
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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..

Last edited by kenpeter; 3rd September 2011 at 08:34 PM.
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Old 3rd September 2011, 08:24 PM   #23
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C7 + C12 look a bit small to me and could cause oscilation.
I would be trying 100-220pf.
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Old 3rd September 2011, 08:37 PM   #24
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Calculating the base-emitter resistance
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Old 4th September 2011, 09:13 AM   #25
sbrads is offline sbrads  United Kingdom
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Q2 has collector and base swapped surely. I expect the board is OK though, just a drawing error.
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Old 15th September 2011, 07:06 PM   #26
sbrads is offline sbrads  United Kingdom
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Quote:
Originally Posted by sbrads View Post
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.
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Old 19th September 2011, 06:06 AM   #27
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Quote:
Originally Posted by sbrads View Post
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!
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Old 21st September 2011, 05:48 PM   #28
sbrads is offline sbrads  United Kingdom
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MX50x2 boards..........

Component side:-
Click the image to open in full size.
Solder side:-
Click the image to open in full size.

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.

Last edited by sbrads; 21st September 2011 at 05:52 PM.
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Old 21st September 2011, 06:02 PM   #29
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Excellent pics. Thanks!
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Old 22nd September 2011, 12:23 AM   #30
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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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Last edited by kenpeter; 22nd September 2011 at 12:42 AM.
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