Hi,
I've been playing with a tube/MOSFET hybrid of an unusual design and I thought I would share my experiences. This group seems to be receptive to wacky ideas.
The idea was described on this forum some time ago by forum member smoking amp. It would work with either a MOSFET or a good NPN darlington, like a horizontal output from a TV circuit.
Basically, the idea is to build a push-pull amplifier using a tube on one side and a MOSFET current mirror on the other.
I've implemented it using a cathode current bridge and an op amp driving the MOSFET to self-null the bridge for AC signals. The result is an exactly opposite signal current through the MOSFET, which is applied to the other end of the PP transformer primary.
The DC balance is done simply by LPF'd positive feedback to the op amp + input where it provides a DC reference for the bridge. This may not be the best scheme for transient response as the transformer gets unbalanced by the "distortion current" at high signal levels while the LPF settles. Maybe tune it but that's another story...
Anyway, I've tested it so far on a resistive load and it's very well behaved, with essentially SE harmonic profile. I used a 6BX7 because it's predictably non-linear ;-)
100mW .95% THD
250 mW 1.5% THD
1W 3.1% THD
4W 5.5% THD (Design max power)
with the residuals mostly 2nd harmonic (f2) with a "waterfall" series of f2, f3, and a little f5 until higher power, when the higher order harmonics show up at the onset of output stage grid current.
I think I'm done on the bench and ready to hook it up to a speaker to see what happens with a reactive load & through speaker resonance. If the concept is going to break down it should show up there.
I'll post the plots and other pics but it looks like I can only do done at a time.
Cheers,
Michael J Koster
http://redwoodcoastmusic.com
I've been playing with a tube/MOSFET hybrid of an unusual design and I thought I would share my experiences. This group seems to be receptive to wacky ideas.
The idea was described on this forum some time ago by forum member smoking amp. It would work with either a MOSFET or a good NPN darlington, like a horizontal output from a TV circuit.
Basically, the idea is to build a push-pull amplifier using a tube on one side and a MOSFET current mirror on the other.
I've implemented it using a cathode current bridge and an op amp driving the MOSFET to self-null the bridge for AC signals. The result is an exactly opposite signal current through the MOSFET, which is applied to the other end of the PP transformer primary.
The DC balance is done simply by LPF'd positive feedback to the op amp + input where it provides a DC reference for the bridge. This may not be the best scheme for transient response as the transformer gets unbalanced by the "distortion current" at high signal levels while the LPF settles. Maybe tune it but that's another story...
Anyway, I've tested it so far on a resistive load and it's very well behaved, with essentially SE harmonic profile. I used a 6BX7 because it's predictably non-linear ;-)
100mW .95% THD
250 mW 1.5% THD
1W 3.1% THD
4W 5.5% THD (Design max power)
with the residuals mostly 2nd harmonic (f2) with a "waterfall" series of f2, f3, and a little f5 until higher power, when the higher order harmonics show up at the onset of output stage grid current.
I think I'm done on the bench and ready to hook it up to a speaker to see what happens with a reactive load & through speaker resonance. If the concept is going to break down it should show up there.
I'll post the plots and other pics but it looks like I can only do done at a time.
Cheers,
Michael J Koster
http://redwoodcoastmusic.com