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
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
Attachments
Salas said: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/FQPF2N80.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.
Sheldon said:
...
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.
Salas said: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
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
- Home
- Amplifiers
- Tubes / Valves
- MOSFET for power drive for low powered 845