Class-A-Burn: an amp with capacitive feedback divider around MOSFET

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Wavebourn

Wavebourn

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Joined 2006
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#1
Class-a-burn.gif


+36V powered, 4 Ohm loaded class A amp.

All OPAMPs are with JFET inputs.

Anyone to simulate?

Should be very fast and linear.
 
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#3
atiq19 said:
is R21 connected right?

do you care to explain your schematic?
Click to expand...

R21 provides current for shunt-regulated power source for opamps (18V) and for bias reference (6.2V).

The explanation is, MOSFETS have big enough capacitances that are non-linear. In any case MOSFET stage will have a parasitic input capacitance. What I did, I shunted this capacitances intentionally, by a capacitive voltage divider, turning MOSFET into a kind of vacuum tube triode. However, it has big input capacitance, but it is well known, very linear, and under our control.

I loaded this "Vacuumless Triode" on a counter-modulated current source and got a single-ended triode amplifier, active-loaded (that is better than reactive-loaded in terms of distortions and frequency response).

Yes, I used OPAMPS. But all of them have JFET inputs, and are used in inverting mode, with no common mode errors that are one of weak points of opamps.
 
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#4
Wavebourn said:
R21 provides current for shunt-regulated power source for opamps (18V) and for bias reference (6.2V).

The explanation is, MOSFETS have big enough capacitances that are non-linear. In any case MOSFET stage will have a parasitic input capacitance. What I did, I shunted this capacitances intentionally, by a capacitive voltage divider, turning MOSFET into a kind of vacuum tube triode. However, it has big input capacitance, but it is well known, very linear, and under our control.

I loaded this "Vacuumless Triode" on a counter-modulated current source and got a single-ended triode amplifier, active-loaded (that is better that reactive-loaded in terms of distortions and frequency response).

Yes, I used OPAMPS. But all of them have JFET inputs, and are used in inverting mode, with no common mode errors that are one of weak points of opamps.
Click to expand...

My friend used some of these vacuumless-triodes. Until he turned the bias up too high... 😛

jd
 

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#7
It is a single ended vacuum-less triode amp. As drawn, you can get about 30W on 4 Ohm load, or 60W on 2 Ohm load. With no huge vacuum tubes, no B+ kilovolts, no filament heating. However, one of opamps must be capable of driving 160 Ohm load impedance, and a good heatsink will be needed. Source and emitter resistors mean you may parallel output devices spreading heat dissipation among them, so resistors must have higher resistance, number of devices times. Scott Wurcer gave me last year some samples of AD opamps that I may try in this amp.
 
#16
Wavebourn said:
It is a single ended vacuum-less triode amp. As drawn, you can get about 30W on 4 Ohm load, or 60W on 2 Ohm load. With no huge vacuum tubes, no B+ kilovolts, no filament heating. However, one of opamps must be capable of driving 160 Ohm load impedance, and a good heatsink will be needed. Source and emitter resistors mean you may parallel output devices spreading heat dissipation among them, so resistors must have higher resistance, number of devices times. Scott Wurcer gave me last year some samples of AD opamps that I may try in this amp.
Click to expand...

I have gone for a SRPP tube front end with MOSFET output transistors.
It gives a valve sound with lots of power at a very reasonable price.
 
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