I tried the ebay MX50 DIY Kit, Class AB 100W+100W; Worked but problem. - Page 2 - diyAudio
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Old 3rd September 2011, 04:39 AM   #11
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Got it, thanks.
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Old 3rd September 2011, 12:16 PM   #12
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If you install it inside a speaker cabinet where it might vibrate, gluing the filter/bypass assembly to a nearby component might be a good idea. Hot glue works well for that.
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Old 3rd September 2011, 12:47 PM   #13
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Buckeye....

you might not like the tone but please read the post ...

anyone the realizes that bias pot has no effect over the bias or the emmiter resistors and keep the amp playing makes a mistake

obviously either there is a problem caused by you or the designer arround the vbe multiplier area or simply you are not measuring a few millivolts and look for much higher voltage in the wrong resistor

you choise about amplifiers is dramatically poor a quasi complementary amplifier with 4 output transitors and each and everyone of them has its own driver will be a nightmare to stabilize if not designed properly

as about the choise of the cheap 150W amplifier all your results are may correct but not enough ....

your testing procedures are not even close to real life conditions and its expected your cheap 150W to only survive just a few minutes before blowing an expensive speaker ..

i will not bother to explain why once more ( Jhon fisher actually is a guy that i like very much ,exchange emails with ... he is a very cheerfull guy , quitar player and music composer ...he has not much to do with electronics and this a circuit given from a friend to a friend as a quick crappy solution . ) ..... obviously if the amplifier is not allready returned to u in a very smoky enclosure ....dont worrie ....it will any minute now ....


the idea is quickly get back the cheap 150w amplifier ....replace it with a working circuit

then since you dont like me any more ( cause of my tone ) ask other forum members to advice you on bias setups and real life resistive /inductive/capacitive tests on amplifiers


kind regards sakis
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Old 3rd September 2011, 01:00 PM   #14
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Buckeye View Post
......... notice there are no caps on the rails themselves. So I added a 100uF filter and a 0.1uF bypass cap to each rail. .................... There was no parasitic oscillation from zero signal to clipping. Looks like lack of caps was the problem..............don't underestimate the need for on-board filter and bypass capacitors on the rails.
called decoupling.

Absolutely mandatory to meet the fast changing current demands of devices on the PCB.

It might be even better to locate the decoupling caps right across the devices that have this fast changing current demand. 4 output devices = 4 decoupling caps.
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Old 3rd September 2011, 01:24 PM   #15
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The output transistor quiescent current is not monitored.
The bias knob is only having a direct effect upon drivers.
Who knows how hot the outputs will get???
0.22R in the collector circuit is useless.

How to fix:
Cut path from upper driver emitters and lower driver diode par 100R
direct to the load, and rewire these to the output collectors instead.

Now sum of each driver + output current is watched over by 0.22R
Your bias knob will now work properly, and maybe you can
tweak away the parasitic thing....

Last edited by kenpeter; 3rd September 2011 at 01:40 PM.
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Old 3rd September 2011, 01:33 PM   #16
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by kenpeter View Post
0.22R in the collector circuit is useless.
Quasi complementary output stage, with Baxandall diode and ?'s resistor.
Just needs the JLH capacitor to finish off that bit.

But, the lower pair do need the same value emitter resistors as fitted to the upper pair.
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Old 3rd September 2011, 02:14 PM   #17
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Something like this...
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File Type: gif MX50 hack.gif (35.4 KB, 910 views)
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Old 3rd September 2011, 05:56 PM   #18
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I don't know if I should ask this on a new thread, but any
recommendations for a nice, stable design without unobtanium parts,
for +/- 50V rails, around 70ish watts into 8 ohms?
Thanks!
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Old 3rd September 2011, 06:16 PM   #19
AndrewT is offline AndrewT  Scotland
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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.
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Old 3rd September 2011, 07:09 PM   #20
nrg2009 is offline nrg2009  United States
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Sounds like this design has issues...amazing that it still plays as well as it does.
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