Shoog
Ciss is 100pf. Here you go:
http://ixapps.ixys.com/DataSheet/DS98809C(IXTP-U-Y01N100D).pdf
Ale
Many of these devices are simply not available anymore (they never caught on).
I am thinking that the IRF610 is great for the second stage as the biasing can be taken care of via the IT center tap. Its low capacitance and biases up nicely and is very easy to get.
However its not so ideal in the first stage as having an enhancement device there requires introducing caps at the input or an Input transformer to make it work. I have a balanced out on my preamp so it would be great to just drive the input stage directly. In this position a good depletion mode device or JFET would be great. It needs to have low capacitance and medium current (about 20mA) which is quite a tricky component to source. One of the Supertex FET's looks promising - the DN3145;
http://www.supertex.com/pdf/datasheets/DN3145.pdf
Ciss of just 120mA, could do service in both positions apart from the fact that its a surface mount device. Again though - not that easy to get hold of.
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In answer to the question, why transformer coupled. Two reasons, these FET's work best at high voltage or high current or both. I opt for the high voltage. Secondly, I have high sensitivity Vintage German fieldcoil drivers which would be very very vulnerable to transients and faults. i already fried a set a long time ago with my first experiments in valve buffered gainclones - its easily done. I like the security that transformers bring, I like the simplicity of design they afford, and I like the frequency response effects they bring to the mix. I personally think that the transformers are the thing that most valve enthusiasts like about the sound of the amps.
Shoog
I am thinking that the IRF610 is great for the second stage as the biasing can be taken care of via the IT center tap. Its low capacitance and biases up nicely and is very easy to get.
However its not so ideal in the first stage as having an enhancement device there requires introducing caps at the input or an Input transformer to make it work. I have a balanced out on my preamp so it would be great to just drive the input stage directly. In this position a good depletion mode device or JFET would be great. It needs to have low capacitance and medium current (about 20mA) which is quite a tricky component to source. One of the Supertex FET's looks promising - the DN3145;
http://www.supertex.com/pdf/datasheets/DN3145.pdf
Ciss of just 120mA, could do service in both positions apart from the fact that its a surface mount device. Again though - not that easy to get hold of.
-----
In answer to the question, why transformer coupled. Two reasons, these FET's work best at high voltage or high current or both. I opt for the high voltage. Secondly, I have high sensitivity Vintage German fieldcoil drivers which would be very very vulnerable to transients and faults. i already fried a set a long time ago with my first experiments in valve buffered gainclones - its easily done. I like the security that transformers bring, I like the simplicity of design they afford, and I like the frequency response effects they bring to the mix. I personally think that the transformers are the thing that most valve enthusiasts like about the sound of the amps.
Shoog
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