Talk about put the pressure on. The ZCA has been a huge success for such a simple cct. Lately the ZCA has taken a renewed interest and foolishly I suggested a bipolar version with a 2N3055. I must confess I had thought about this for some time but have 6 other projects I would rather start. Anyhow I have come up with the bipolar prototype based on the ZCA. Not only that many pro-builders claim Fets will never sound as good as bipolars. Of course this argument has no conclusion but here is your big chance to build a bipolar version of the the ZCA though there is still a Fet in it (jFet).
Because this version has two active components I feel it should not be called a ZCA. So this amp falls under a general category called "Minimal Component Amplifier" or MCA! It worked first time and sounds OK. Here's the schematic.
Not hard to see that it shared a few areas with the ZCA. Because the hand drawn schematic is a bit rough I'll talk you through it. Firstly the B+ is about the same, 24 or 28 volts are fine. If you look at the images you will see my test PS uses a 9mh Xover choke. This works fine. The load is still 15 ohms at 20W in this case. I turn the bias up slowly until I have 15V across the load resistor which means 1A through the 2N3055. 4700uf snubbed by a 10uf poly and snubbed that with a 0.1uf poly is the same as the ZCA. I added a 1ohm 5W (one ohm) resistor in the emitter leg of the 2N3055.
Because the 2N3055 would have required a fair amount of bias current to operate I used a 2SK170 BL jFet to supply bias current for the 2N3055. These are connected as a darlington pair. This allowed me to keep the same bias cct. I used for the ZCA. I increased the bias stabilizer cap to 100uf 25V. A 1M (one meg ohm) resistor still feeds the bias current to the jFet. Also I used a 2.2uf input cap and a 200 ohm blocking resistor to feed the gate of the 2SK170. Output caps and input caps can be varied in value so experiment. I'm only dropping 15V across the load resistor but again experiment. I set the bias voltage once and it remained stead over 20 mins or so and the 2N3055 stayed pretty cool.