Another kind of hybrid
 

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 thought it was my idea. :confused:

http://www.diyaudio.com/forums/showt...22#post1415022

Hi,

Very interesting! I thought it was my idea at first too, but someone pointed me to smoking-amp's earlier posts.

I would very much like to see your ideas on implementing this.

Any ideas on how the DC balance scheme might be done better. Do you think there may be a problem with the slow reaction time ?

Before we get a dozen posts about the MOSFET being backwards, here's a corrected schematic.

Michael

I would bypass a cathode by cap taking from it DC only that reflects current through triode using this voltage to bias an opamp,
resistor of the same value (un-bypassed!) would use in emitter of a transistor (in source in your case), taking feedback by current from it.
Also, I would feed both triode and opamp from a phase splitter balancing signals for less distortions.

The idea I got from my previous hybrid design where I used a voltage follower loaded on a voltage to current converter. It sounds pristine clean, while if to replace a voltage follower by a triode there will be a "SE triode sound" instead from a small push-pull transformer.


Here was the original:

http://wavebourn.com/images/audio/tower-II.gif

Here is the distortion spectra of a 1 KHz sine wave.

The output spectrum and the driver spectrum are similar, so I conclude that it is indeed a SEPP amplifier producing a SE characteristic using a PP transformer. f2 is not canceled in the OPT as in PP (never expected it to)

My measurement setup is using a HP339 with the residual monitor output going to a PC running RTA software with a 24/96 sound card. Some of the fundamental shows up in the residual because my 339 needs alignment.

Michael

Hmmm,
Well, my idea was a little different in that it used a CCS'd LTP configuration to drive the complementary current (versus the triode) thru the Mosfet/Pentode side (acting like a folded cascode stage effectively).
No Op Amp was needed that way. I see Michael is using the Op Amp to do the DC biasing and the AC complementing. Similar setup in Wavebourn's.

An Op amp could be used as a bias balancing servo to good effect in the CCS'd LTP approach by controlling the gate DC. But I like the clean current differencing for AC of the CCS'd LTP versus the Op Amp AC servos.

Obviously, everyone here is thinking the same overall idea of making a truly accurate complementary drive on the other side of the P-P xfmr.

http://www.diyaudio.com/forums/showt...668#post638668

or just a Mosfet instead of the Mosfet/pentode cascode:
http://www.diyaudio.com/forums/showt...80#post1323780

Don

Oh, I would be careful about using a splitter and driving both sides. A diffl LTP splitter probably will introduce some odd harmonic signature of it's own into the sound. A concertina should be safe though, I would think.

The amplifier I built using a triode configured pentode on one side and a Mosfet (FQP2N90) on the other side sounded like a SET to me, so no surprise that the measurements confirm this. Plenty of expensive SET owners/ builders out there that will be surprised (dis-believing maybe?) though.

No, my idea is to mimic a DC current through the main active element only providing a variable current by a device with high dynamic resistance so overall output curve will be determined by a lower resistance device (the main active element) that works in privileged conditions.

Quote:

Oh, I would be careful about using a splitter and driving both sides. A diffl LTP splitter probably will introduce some odd harmonic signature of it's own into the sound. A concertina should be safe though, I would think.

Absolutely agree.

Quote:

The amplifier I built using a triode configured pentode on one side and a Mosfet (FQP2N90) on the other side sounded like a SET to me, so no surprise that the measurements confirm this. Plenty of expensive SET owners/ builders out there that will be surprised (dis-believing maybe?) though.

When one 6P3S tube in my PP amp run away thermally I had similar SET signature.

I still have no time to breadboard my version of a triode SE with PP output transformer... I even found a name for it, "Alligator" :D

Actually, I think your "figure 2" is functionally more similar where you get the MOSFET drive from the cathode, only I use the op amp to produce an exact AC current mirror around the bias point.

Now that I think about it, the LTP approach does seem to be a refinement on the basic idea. But where does the distortion current go? By this I mean the increase in average current (conductance, Rp...) at full signal vs. quiescent.

hehe something like this would probably not appeal to any diehard SET fancier

Yes, this is exactly the way I think about it. I guess the "privileged conditions" are provided by the high dynamic resistance of the MOSFET shoulder which should give the triode a SET world view :-)
as opposed to the dual tube PP where the dynamic resistance change of the opposite is the mechanism which cancels f2 distortion (and also the mechanism which converts f2 to f3 in diffamps)

It will be great to compare notes.

Michael

Throwing in my hat too
 

Remember...

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