The output stage . The cascode transistors completely isolate the lower transistor, eliminating variations in current due to changes in Vce and dramatically increasing output impedance of the sinker. I also connected the current sinkers from both half to the same reference, simplifying the hoopla and ensuring equal current.

The power supply is not symmetrical, the negative supplying a little more voltage for the cascoded sinker.

The pot marked 'I' is mechanically connected to the volume, so the idle current is adjusted from 1.4 to 2.2 amps(per half) in 6 steps, (stepped attenuator and switch), from low to max listening level.

The pot marked 'R' sets the current going thru the pnp transistors in the current mirror. It's mostly just for finetuning since different (but same type) jfets have such a varying drain current with a given source resistor. Even jfets from the same batch will vary, so it's nice to be able to finetune them. It is not necessary though, and a fixed resistor of 470 to 1k ohm will do fine.

When AC coupling the output stage to a VAS or preamp, gate resistors are needed, like 47kohms, to ground.

Be aware that balanced circuits have twice the output impedance compared to single-ended circuits where all else is equal. This amp doesn't use loop feedback to help lower the output impedance either. That means that no matter how high you turn the idle current, it will not do great into low impedance loads. You can of course parallel several power mosfets with high transconductance, and get a really low output impedance, but they'd better be well matched. I believe a single transistor sounds better than several in parallel, and use only one per output half. The mosfet I use happens to have low transconductance, like only 1-2 S, so the output impedance is about one ohm! That's high even for tube amps, but I'm constantly amazed at the bass slam and detail that shouldn't be there with such horrid dampeningfactor. It does not sound great into 4ohm speakers though.

And finally I include the circuit board layout I used: -See here

Update, January 2000:

Wouldn't you know it, I got another idea about improving the output stage. It's shown in a simple form, no cascode or anything fancy. But just to show the general idea:

Lower output impedance from a single-ended stage: -See here