MOSFET for power drive for low powered 845

[sganti]

Hi All
I am building a low powered 845 Amp and wanted to use power drive. I read thru the old posts and found that a mosfet with minimal reverse transfer capacitance is ideally suited.

I am looking for +/- 200V supply, and found the following MOSFET -- IRF820APBF-ND from Digikey. The reverse transfer capacitance is 2.7pF. Wanted to check with guru's here if this seems like a good choice, or are there alternate suggestions.

I am attaching a jpeg of the basic amplifier circuit.

These would be mono blocks. I just got the chassis from vt4c.com, and working on a layout right now.

thanks
Sridhar Ganti

[Salas]

Nice Mosfet find. Is this the IRF820A but pb free? Looks ace to me for the Crss thing. Doesn't the 845 need a grid resistor of a finite upper value so not to leak back towards the CCS before A2?

[coloradosound]

Keep us posted on results!
I'm interested in doing something similar for VT-4C/211.

[Sheldon]

Originally posted by Salas
Nice Mosfet find. Is this the IRF820A but pb free? Looks ace to me for the Crss thing. Doesn't the 845 need a grid resistor of a finite upper value so not to leak back towards the CCS before A2?

I've used this part with good results: http://www.fairchildsemi.com/ds/FQ%2FFQPF2N80.pdf
Comes in a insulated package too.

You don't have to worry about a grid resistor there. That node is low impedance, since the FET is a source follower.

Sheldon

BTW, I had intermittent oscillations problems with a CCS chip under the follower. After chasing it around a bit, I ended up with a 10K resistors instead. Couldn't detect any difference in sound.

[Salas]

Another very nice Mosfet for Crss. Thanks Sheldon.

This thing about no subjective difference between choosing a source resistor or a CCS its heartening towards simplicity by your finding but contrasts Ian's finding.

[Michael Koster]

Originally posted by Sheldon


...

You don't have to worry about a grid resistor there. That node is low impedance, since the FET is a source follower.

I'd say you don't even want a grid resistor here if you're planning A2 operation. I haven't seen a problem in my triode 4-65 A2 setup with similar Rp, mu, and gm to the 845. There is a 3" piece of wire from the MOSFET source terminal to the grid. I go over 30mA grid current in normal operation and any resistance here would probably increase third harmonic distortion.

[Sheldon]

Originally posted by Salas
This thing about no subjective difference between choosing a source resistor or a CCS its heartening towards simplicity by your finding but contrasts Ian's finding.

That I didn't notice a difference, doesn't mean there isn't one. Since I didn't do side by side tests, I won't make much of it. Other's may experience it differently. I would guess that the operating conditions will affect performance too. I'm running about 9mA through the follower, at +/- 110 volts, and a separate supply.

Sheldon