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mashaffer 9th July 2004 02:08 PM

Choke loading BJT output stage
I was thinking about the class A amplifier that I am toying with (My Class A Ideas and began wondering about choke loading the output stage (i.e. using an inductor instead of a load resistor or CCS). I had pretty much decided on a CCS load but when this Idea struck me I figured that I ought to check it out further. I searched and came across this interesting thread (Monster Source Choke , but I was wondering if anyone else has ever played with the idea of choke loading SS output stages. Especially using BJT and more normal sized chokes. One of the things that concerns me is the higher DC currents in SS amps v.s. the tube amps where this kind of thing is often used. I can measure the inductance of a choke but how does one determine the max DC before saturation? I am also concerned about the interaction of the choke and the output coupling capacitor.

All thoughts or experiences welcome.

millwood 9th July 2004 02:14 PM

I am thinking about something similar, except in my case, I am thinking about using the primary winding of a power transformer as the load and attach the speaker to the 2ndary.

The downside is the DC current going through the winding. Nelson's idea is to use a dual winding transformer in one of those aleph amps. Pretty neat too.

Fuling 9th July 2004 02:22 PM


I can´t see any reason why BJT´s wouldn´t work with choke loads. Tubes and Mosfets works, so why not BJTs?

Chokes with suitable values might be a bit hard to find, but Hammond has a few that should be fine. DIY chokes is another alternative but as you say the airgap is a bit of a problem.
Circlotron describes one way of calibrating it in his thread.

Interaction of chokes and caps? Well, never heard anyone mention any related problems.
I you´ve got the guts for it you could always aim for chokes with extremely (I repeat: extremely) low DCR and ditch the output caps.

Chokes are fun, but they do have some problems related to them.
Avaliability is one, the need of a controlled startup sequence is another. Read more about this in Circlotrons thread.

Good luck and have fun.

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