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I'm prodded to this post by the recent discussion of DSP amps and their virtues of low weight and low cost, relative to most tube amps.
I've been playing around with the idea of a hybrid tube/SS output stage and this is what I've got so far...
Now you probably have better things to do, but it's been a slow summer on this board, and maybe somebody has a few thoughts about this.
Basically, what I'm doing is using a PI stage (this is a power amp that any preamp can drive) to drive two follower stages. Only, rather than use an OT to drive a low impedance spkr, I have the two follower triodes use Sziklai pairs to drive the spkr load (that 8 ohm resistor in the center). Inversion, usually obtained by the OT windings, is provided by driving the spkr in a bridge mode so that each follower stage pushes current through the spkr in opposite directions. The Q1 and Q2 transistors (darlingtons) are essentially switches driven out of phase so that one side of the spkr is effectively grounded while the other is being driven.
This is pretty odd, I know. I had my doubts about the idea from the beginning, but after a few iterations, I have this thing working surprisingly well. In fact, it drives resistive loads below 1 ohm successfully. It does work and it's stable.
It's virtues include the output stage operating from a single-ended PS (low cost and low weight) that is independent of the tube B+ supply. There is no need for a big spkr coupling cap - in fact the whole output stage after the PI is DC coupled so there are no bass response or blocking issues.
So, too good to be true? Maybe it is... I'm having issues when driving actual speakers. The reactive load produces some spikes due to the darlingtons switching (I think that's the issue at this point). So, don't toss your 50 LB tube output stages yet...
I'm looking for some suggestions or ideas on how to proceed. How might I deal with the reactive load? Anyone have experience in switch circuits and inductive loads?
I'm also OK if you just want to tell me I'm crazy to even try this... I know I could use a standard SS amp, but I'm taken by the idea that a hybrid stage with the SS elements just following the tube cathodes may sound different (maybe good). And besides, why not try something new? I can't find any trace of this idea anywhere - although that may just confirm my initial doubts.
I've been playing around with the idea of a hybrid tube/SS output stage and this is what I've got so far...
Now you probably have better things to do, but it's been a slow summer on this board, and maybe somebody has a few thoughts about this.
Basically, what I'm doing is using a PI stage (this is a power amp that any preamp can drive) to drive two follower stages. Only, rather than use an OT to drive a low impedance spkr, I have the two follower triodes use Sziklai pairs to drive the spkr load (that 8 ohm resistor in the center). Inversion, usually obtained by the OT windings, is provided by driving the spkr in a bridge mode so that each follower stage pushes current through the spkr in opposite directions. The Q1 and Q2 transistors (darlingtons) are essentially switches driven out of phase so that one side of the spkr is effectively grounded while the other is being driven.
This is pretty odd, I know. I had my doubts about the idea from the beginning, but after a few iterations, I have this thing working surprisingly well. In fact, it drives resistive loads below 1 ohm successfully. It does work and it's stable.
It's virtues include the output stage operating from a single-ended PS (low cost and low weight) that is independent of the tube B+ supply. There is no need for a big spkr coupling cap - in fact the whole output stage after the PI is DC coupled so there are no bass response or blocking issues.
So, too good to be true? Maybe it is... I'm having issues when driving actual speakers. The reactive load produces some spikes due to the darlingtons switching (I think that's the issue at this point). So, don't toss your 50 LB tube output stages yet...
I'm looking for some suggestions or ideas on how to proceed. How might I deal with the reactive load? Anyone have experience in switch circuits and inductive loads?
I'm also OK if you just want to tell me I'm crazy to even try this... I know I could use a standard SS amp, but I'm taken by the idea that a hybrid stage with the SS elements just following the tube cathodes may sound different (maybe good). And besides, why not try something new? I can't find any trace of this idea anywhere - although that may just confirm my initial doubts.