Achievement Unlocked: 75 Watt Class A Zero Feedback SIT Circlotron


Soft clipping at 75W

Because of the high input capacitance of the 2SK180’s and the gain of this circuit, we have to pull out all the stops to get our 75W beast to make audio bandwidth. Here you can see that each SIT is getting its own transformer and buffer.

Here's the link to Nelson's presentation at BAF 2015:

Pass Presentation BAF 2015

Slides are available in pdf. See the video description for the link.

Video Blog #1 is posted here:

https://www.youtube.com/watch?v=mWTy...KhQ2ow&index=2

Ironamp project video posted here:

https://www.youtube.com/watch?v=FZymAeaVsAg

I hope you enjoy it.

I updated the captions if you want to try Youtube's translation option - should be funny

The topic of JFET and MOSFET matching comes up often in the forums, so I thought I'd show you the jigs I use.

First up, is my trusty old JFET IDSS matcher.



There's a 9V battery inside, and the meter probes are conneted across a 10 Ohm resistor which is connected to the JFET drain. I included a polarity switch for convenience when testing P channel parts.

It's pretty easy to connect the microclips, count to five, and take a reading.



And here's what I use for the big stuff. This is my MOSFET Vgs matcher.



First, I install a sample part and let it warm up for about an hour. There's a thermistor hot-glued to the heat sink,...

This is the artwork for the PCB in the pictures. It fits into the ExpressPCB miniboard profile so it's a little cheaper to have made.

It will be necessary to check the orientation of the bias pots (I can't remember). I wouldn't want you to blow-up your $30 JFETs.

I've been mostly out of the audio hobby for a while, but I've had a few inquiries about the SemiSouth version of the Amazing FET Circlotron as of late. So, I wanted to post the performance graphs here before I lose them.