[Garbz]

A friend donated a "dead" 2x250watt poweramp to me the other day to "fix". I use the inverted commas simply because I undid the changes he made that caused the breakage in the first place. He really couldn't be bothered fixing it and just went out to buy a proper one.

Anyway I replace the components that were (under the false idea that it would fix a ground loop) removed, and it is working as intended except for a slight hum which I may consider debugging at some stage.

http://www.garbz.com/shem1.jpg this is the schematic for the amplifier. The part removed was R31 (10ohm 5W resistor from -ve input of the Q3/4 longtailed pair to ground).

Bear with me but since I am trying to teachmyself the transistor theory I really want to know exactly what removing R31 did to cause the 50V of DC that torched the poor guy's speakers, that is in terms of how it affected the current through Q3/Q4.

 

[nikongod]

r-31 is at least 1 BIG thing to screw up what you wanted.

its part of the ground reference for the input of the amp. without this resistor the input can "float" wherever it likes, even with the use of a cap. the output is tied to the amp's ground, the input is not in this case.

this of course screwed up the outputs via cascading failure. say the amp has a gain of 25 for example.
you have the ground connected, and a 1v p-p signal comes in, it goes throught eh amp, and it comes out as a 25vp-p signal. smile.

say you have the same 1vp-p signal, but WITH 2Vdc offset. the 1vp-p comes through as 50v p-p, and the 1Vdc comes through as 25VDC. hook an 8 ohm speaker upto a 50V, 10A wall-wart, same thing.....

 

[amb]

Basically, this amp was designed with two "grounds". One is the input ground, which is the "-" connection at the input. The other is the general ground, which is what everything else with a ground symbol is connected to. The input ground and general ground are linked together with R31. The point of doing this is to provide a small bit of isolation between the two grounds. This is a technique that is sometimes used to avoid a small ground loop within the amplifier (dependent on the intra-amp wiring scheme). If R31 is to be removed, the pads must be jumpered with a wire or else the amp will not function properly.

 

[nikongod]

oops, i realies that my numbers above were off... dont type and drink kiddies...

 

[Garbz]

lol. Thanks for the reply. Anyway it's working great except for some buzzing in one channel. I'm considering just ignoring it since i'll use it for a subwoofer and mostlikely won't be low pitched enough to be reproduced.