In the original F5 with 24 volt rails, and in the F5 turbo with about 34 volt rails, a 4.7K ohm thermistor is in series with a 2.2K ohm resistor from the rail to the gate-stopper on the MOSFET (original F5 schematic attached). I understand the function and purpose of this with regard to thermal stability, and I have built F5s at both rail voltages using this circuit without problems.
But I now am ready to construct an F5 (JFETs cascoded and four output devices per channel) with 40 volt rails (large room, classical music). Do I need to adjust the values of the thermistor or of the 2.2K ohm resistor for 40 volt rails?
I have searched the forums for an answer to my query without success. Sincere apologies if this has been discussed previously. Thanks.
Steve
But I now am ready to construct an F5 (JFETs cascoded and four output devices per channel) with 40 volt rails (large room, classical music). Do I need to adjust the values of the thermistor or of the 2.2K ohm resistor for 40 volt rails?
I have searched the forums for an answer to my query without success. Sincere apologies if this has been discussed previously. Thanks.
Steve
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Why would you?
It's a Vgs control mechanism and the Vgs of the HEXFETs does not depend on the rail voltage - it's always about 4V.
Thanks juma. My inclination was to not touch it as the potential difference between the rail and the gate of the MOSFET should remain the same at bias in this amplifier, regardless of rail voltage. Just wanted to double check that I wasn't missing something.
You may however want to adjust the resistor network powering the Jfets. The optimal voltage for those should be between 12-16V.
The higher rail voltage may take that voltage above that optimum.
The higher rail voltage may take that voltage above that optimum.
Dazed2, yes I have changed my voltage dividers to 6.2K and 3.8K which should put me right in the ballpark. In fact, it was quickly doing those calculations that distracted me and caused me to wonder if I needed to adjust those in the thermistor circuit to the MOSFET gate stoppers. I was imagining that the gates of the MOSFETs were about 4 volts to ground, rather than 4 volts to source, but juma quickly brought me back to reality.
Woody, I have built with cascoded JFETs and four output devices previously without a problem. I have a bunch of matched 2SK170s and 2SJ74s with a range of Idss values (about 6 to 11.5 mA) to choose from. Since I am cascoding them and using the 5U chassis from the store, I shouldn't have a problem. It will be interesting to see what bias I can get away with, as I haven't built with that chassis before. Christian Viller designed me updated boards of his that now include P3 and a resistor to the base of the cascoding transistors that look very nice. I'm using FQA19N20s and FQA12P20s for MOSFETs since I still have some matched ones from a generous purchase a couple years ago. I have the dual mono power supply complete and am going to begin the amplifier boards this weekend.
Steve
Woody, I have built with cascoded JFETs and four output devices previously without a problem. I have a bunch of matched 2SK170s and 2SJ74s with a range of Idss values (about 6 to 11.5 mA) to choose from. Since I am cascoding them and using the 5U chassis from the store, I shouldn't have a problem. It will be interesting to see what bias I can get away with, as I haven't built with that chassis before. Christian Viller designed me updated boards of his that now include P3 and a resistor to the base of the cascoding transistors that look very nice. I'm using FQA19N20s and FQA12P20s for MOSFETs since I still have some matched ones from a generous purchase a couple years ago. I have the dual mono power supply complete and am going to begin the amplifier boards this weekend.
Steve
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