Another kind of hybrid

[Michael Koster]

The scalable amplifier. I call it the "Variamp" because you can dial in whatever power multiplier you want. You do need to reduce the turns ratio for higher currents so maybe a variac-like OPT would be in order.

Actually you don't need to reduce the turns ratio. This technique can also be used to improve the damping factor of tube amps without resorting to NFB.

BTW, there is a true push pull version of this where you have 2 tubes in PP and 2 opposing current mirrors. I have been calling it Perfect Push Pull because you can ideally balance both AC and DC (no issues with current shift due to signal level).

The Variamp would have switch settings for SE and PP flavors at various power levels and damping factors.

Michael

PS I think any of these designs would be a great idea for type 45s because they're so rare and expensive (only need one and no matching problems) and you could drive a wider range of speakers with it.

[Wavebourn]

Originally posted by kenpeter
Remember...

Interesting...

What's for the transistor in parallel with the tube?

[smoking-amp]

Re: Kenpeter remember...

Oh Boy! Now we have triode emulation and anti-triode emulation.
Better be careful, those SET fanciers will be fit to be tied now. I'll throw in one more outrageous design, then I'm going to hide for a few days until the flames subside. (Even Broskie got purple- plexed about this one.) Probably have to change our names to get back on the forum if this keeps up.

http://www.diyaudio.com/forums/showt...542#post240542

Don

[Wavebourn]

Originally posted by smoking-amp
Re: Wavebourn

Ahh, yes. The voltage amplifier and complementarily modulated current source P-P scheme. A somewhat different approach to getting SET sound. Some additional flexibility there since the voltage controlled current can be set up to act like a complementary current or can be set higher to keep the triode current constant or any shade in-between. Looks like Michael's AC servo control could be set up to do similar things also.

When I conjured up the CCS'd LTP triode/Mosfet version, I was encountering heavy resistance to the idea that a P-P design could accurately mimic a SET design soundwise, so I was determined to get exactly the same harmonic spectrum by accurate complementary current.

Some resistance even arose to the idea that a cascode Mosfet stage could have the same current out from the drain as input to it thru the source. In any case, it is still important to use a low capacitance Mosfet (with at least 30V across it to minimize capacitance variation), which Fairchild solved nicely just in time for me with the FQP... series.

Nice to get twice as much power out too (compared to a single tube SET)

So, Anatolyi, does this "Alligator" amplifier have plans to devour SET amps for lunch?

Don

Yes, this is the idea! My "Tower" room heaters (schemo of one above) ate them already, but consumed 12A from 120V outlet constantly. Audiofools did not like them because they sounded too clean, "No signature". Despite they had no "Transistor Signature" as well! Also, they were insane (no massive output toobs), regarded as a shameless heresy!

An "Alligator" will sound less pristine, but it will have a nice triode SET signature with cheaper but better output transformers (less of iron and copper is needed).

I think of it as a commercial project, but schemo will be available in public domain...

[Wavebourn]

Originally posted by smoking-amp
Re: Kenpeter remember...

Oh Boy! Now we have triode emulation and anti-triode emulation.
Better be careful, those SET fanciers will be fit to be tied now. I'll throw in one more outrageous design, then I'm going to hide for a few days until the flames subside. (Even Broskie got purple- plexed about this one.) Probably have to change our names to get back on the forum if this keeps up.

http://www.diyaudio.com/forums/showt...542#post240542

Don

Don, what about this one?

[smoking-amp]

Re: Michael
"But where does the distortion current go? By this I mean the increase in average current (conductance, Rp...) at full signal vs. quiescent."

Good point, my setup was using a large 100 Watt Edcor output xfmr which was able to handle the current imbalance at large signal level. But it really should have something added to fix this.

Maybe a servo control that adjusts the tail current to keep the triode and Mosfet avg. "DC" currents balanced. (as long as the tube has monotonically increasing DC avg. current with signal level, but could be a problem when hitting triode saturation though since the servo loop polarity might reverse.)

Or a servo to add avg. "DC" current to the Mosfet side (another Mosfet current source in parallel) to balance up the xfmr.

Without the servo, one can also just adjust the idle bias current off center some too (if the xfmr can handle it). Make the offset at idle equal to the opposite offset at max signal, then you only need to deal with 1/2 the original max offset current imbalance in the xfmr. Not too good for a toroidal xfmr though.

Don

[smoking-amp]

Re; Wavebourn "Don, what about this one?"

Hmmm, are those diodes supposed to be thermionic ones? I think this will make for lots of distortion, since triodes are supposed to have constant Mu, not be diode functions. This looks like emulation of the problems, not the benefits, of triodes. Back to the drawing board.

Since we are on this diode kick, I have seen a patent for a SS amp where they put thermionic diodes in series with the output transistors bases (usual emitter follower output, complementary totem pole), so they would have transconductances similar to tubes. Only problem is, it is similar to pentodes, not triodes. But maybe it helps with smoother crossover.

Don

[smoking-amp]

Oh, here is another one for possible SET "emulation":
http://www.diyaudio.com/forums/showt...030#post871030

Put a real SET amp in for the top corrector amp, and a SS amp for the main power amp (bottom). It does require some feedback for the SET amp though, which is not likely to give the usual open loop SET sound unfortunately.

Maybe could move the feedback subtraction point forward to compare the final (total) amplifier output signal with the SET plate signal and feed back correction to the SET input to minimize the difference. Maybe some positive fdbk that way though. Must be some way.

Wait, I got it. Make the SET amp with partial cathode feedback (CFB). That way the SS amp is effectively supplying the plate voltage for the open loop SET amp (backward thru its OT), and the SET amp is effectively monitoring the final speaker output thru the CFB fdbk. Well, low feedback anyway.

Don

[Michael Koster]

Originally posted by smoking-amp
Re: Michael

Without the servo, one can also just adjust the idle bias current off center some too (if the xfmr can handle it). Make the offset at idle equal to the opposite offset at max signal, then you only need to deal with 1/2 the original max offset current imbalance in the xfmr. Not too good for a toroidal xfmr though.

Don

Sounds like you're suggesting splitting the difference so zero signal has "negative" offset and FS has positive offset, in the same amount.

Or maybe it would be better to put all of the negative offset in at idle, where you have more headroom in flux density. As signal increases to FS, the DC decreases to zero. Many PP transformers are built to handle 5mA or more offset so it shouldn't be a problem. Hmm, I can probably prototype this in my hybrid amp. The idle current is 34 mA and FS is 37 mA. I was originally going to do it this fixed way but both kenpeter and VoltSecond suggested this LPF servo would be more trouble free to start with ad I'm glad for that but I still wonder about the transient response.

I'm also working on a bias regulator for PP outputs that has no drift, offset, or distortion current issues, but I can't see how to combine that idea with this thing yet...

Thanks!

Michael

[smoking-amp]

Re: Michael

"Sounds like you're suggesting splitting the difference ..."

Yes.
Although, on thinking about this again, I think I may see an elegant solution (at least for the CCS'd LTP version). All one needs is an appropriate resistance added across the tail CCS to track the increased current needs with increasing signal (due to dist.).

This strikes me as being related to the WE harmonic equalizer (HE) scheme. Just put in a tail resistance that nulls the 3rd harmonic dist. So simple. Why didn't I think of that before.

The high Gm of the Mosfet will keep tail voltage variation small, so it will require a lower value tail resistance than for P-P tubes HE. Unfortunately, this will attenuate the AC complementary current too. So maybe use something more like a gyrator circuit (inductor emulation). Probably can do this with just modifying the CCS tail circuit. Or just use a real inductor for the tail.

Don