IT coupled tube-SS hybrid amp suggestion- needs development - diyAudio
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Old 16th August 2012, 08:33 AM   #1
DeonC is offline DeonC  South Africa
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Default IT coupled tube-SS hybrid amp suggestion- needs development

Hi guys

In the thread Beyond the Ariel in post #7780 Lynn Olson suggested a topology for an interesting SS-tube hybrid. Read the posts above that for some background. The link for Susan's SS amp is at the bottom of this post.

Here is Lynn's post outlining his suggestion:
Along those lines, there's nothing to stop anyone - particularly in this intrepid forum - from combining the Susan final stage with the first stage of the Amity amplifier. In other words, 1:1 Input Transformer -> PP 5687, 7119, ECC99 -> 1:1 IT Transformer -> PP MOSFET followers -> Output Transformer.

The gain structure would avoid any need for step-up transformers - the input and IT could both be 1:1 - and distortion from the PP 5687, 7119, ECC99 drivers would be very low. The driver tubes would use a modest 120V to 150V B+ voltage, avoiding the more troublesome B+ supplies in the 450 to 550V range. And isolated supplies for the driver and output sections absolutely sounds better than shared supplies.

A more exotic version would use a 1:2 Input Transformer -> PP 45, 2A3 -> 1:2 IT Transformer -> PP MOSFET followers -> Output Transformer. This driver would have (much) lower distortion and more dynamic drive characteristics. The B+ would be raised to the 200~250V range for the best possible dynamic range and slew rate from the balanced pair of direct-heated triodes.

If the builder wants to go all the way with this design, I would recommend shunt regulation for the driver stage - fortunately, since the final stage is an ultrahigh-gain MOSFET follower, there's less need for regulation in the power stage (since the followers have inherently high power-supply rejection as well as inherent balance from using matched pairs). Gary Pimm's shunt regulators have extremely high PSRR, well under 1pF capacitance, as well as excellent reliability (they can withstand 1KV transients).

That's a complete hybrid amplifier; anyone interested?

Lynns suggestions are based on this amp:
Zero Feedback Transformer Audio Power Amplifier

Anyone here interested in developing this idea? Some of the other goals are wide bandwidth (something like DC - 200kHz +) and the use of Mosfet followers. I am not capable of designing such a beasty, but I am sure there are many on here that could do it. Any takers? I think it could be awesome!

Enjoy,
Deon

Beyond the Ariel: Beyond the Ariel
Lynn's original post: Beyond the Ariel
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Old 16th August 2012, 05:52 PM   #2
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What's to develop? I've built such a thing, except the output stage was mine which is a little different from the SP one (mine CCS loads the FETs and uses a servo to adjust the CCS eliminating DC across the transformer), and mine was sized for headphones. To do what Lynn suggests, all you need is to couple the two already designed stages together. The hard part is coming up with the output transformers.

At any rate, using a solid state buffer to push a stepup transformer (two of the above coupled together, basically) sounded (and measured) better than the tube stage, and it was much quieter. But I suppose it depends on how much gain you need.
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Old 17th August 2012, 08:18 AM   #3
DeonC is offline DeonC  South Africa
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The output transformer have already been developed. Susan sells OTs for her design, and those would surely suffice. My problem is that the circuit seems so simple. Deceptively simple I think. I think that to keep it simple, IT - 45pp - IT - SS - OT, may not be as simple as it seems.

I like the idea of a CCS for the output stage. Maybe one for the input stage too, but I'm not sure I like the idea of a SS CCS for the tube stage. Is there such a beast as a tube CSS? All in all, with a IT - 45pp - IT - SS - OT circuit with additional tube and SS CCSes, it could all turn out to be very complicated simplicity.

Enjoy,
Deon
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Old 17th August 2012, 05:15 PM   #4
kevinkr is offline kevinkr  United States
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It is just that simple. I've seen and heard one, (the Zeus I mean) worked just fine without any elaboration.
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