P channel follower

[hancock]

My speakers are about 116dBSPL from 2.8VRMS, so I don't really need any gain. And my D/A converter has pro and consumer outputs so I can just use the pro outputs for those rare occasions that I really want to crank it up. As I was reading Zen Variations Part 4, I was thinking, "why not just build a P channel follower to hook up between my D/A and my speakers?" And since I've got balanced out on my D/A, why not do it differentially, and since I will only be using the amp from 350Hz up, "why not use an air core inductor as a current source?"

So my question for you guys, since this will be my first solid state amp, is "what P channel mosfet to use at what operating point, given the various tradeoffs involved?"

Thanks for your feedback.

John

 

[uli]

Hi and happy new year,
when you look at www.irf.com there are some (not too many)
P-channel Powerfets. I use IRFP9140N

Uli

 

[hancock]

Thanks Uli,

What would you guys think of the IRF9510 in this situation at Vgs=Vds=5V, I=1.2A?

One other thing, do I need a resistor in series with the gate of the transistor if there is no feedback going back to the gate? I would think it would be best to leave this out and drive the gate with as low an impedance as possible. I know I wouldn't need a grid stopper in this case for a valve amp, but am I missing something for transistors?

Thanks alot, John

 

[hancock]

Hang on a second. I thought Rds was the analog of plate resistance for a tube. Rds is 1 Ohm for the IRF9510. "Plate resistance" for an IRF9510 is more like 150 Ohms--forget about that air core inductor idea. I've got a lot to learn about these newfangled transistors.

Still interested to hear opinions about what fet and what operating point for a 1/2 watt P channel follower.

Thanks again, John