Once upon a time, there were such things as tubes. They were, for all intents and purposes, N-channel devices. There were no P-channel tubes available, so a great deal of time, effort, and ingenuity went into developing circuits that used only N-ch devices.
One of the many clever circuits that came out of the tube era was the Circlotron. Since the Circlotron topology has put in only sporadic appearances since transistors gained ascendancy, it is not as familiar as other topologies. Essentially, it is a push-pull circuit comprised of two single-ended amplifiers operated nose to tail. That doesn’t sound too unusual until you notice the power supplies. Plural. There are two of them. In a Circlotron, you have one amplifier circuit, followed in series by a power supply, then by the other amplifier circuit, then another power supply, which in turn leads back to the first amplifier circuit, all in a circle. In the old days the conceptual schematics were frequently drawn in a literal circle to emphasize the series nature of things. Given that modern electronics CAD programs aren't easily convinced to draw circular circuits, nowadays a Circlotron diagram usually ends up looking something like this:
Many tube topologies were adopted by solid state practitioners once transistors became widely available. Some were not. The Circlotron has not been totally forgotten, but there have been few commercial examples. Jim Bongiorno updated the circuit to use solid state components in the Eighties and received a patent for his efforts. The next commercial example came from Atma-Sphere, but Ralph Karsten used tubes once more. His innovation was to direct-couple the output stage, thus getting rid of the pesky output transformer. He also received a patent. (Atma-Sphere, incidentally, is still in business. They can be found on the web at www.atma-sphere.com.)
A separate historical line joins at this point; that of the power JFET. Small signal JFETs are common devices. They generally have modest gain and are—by our standards—rather limited in the amount of voltage that they can withstand. They do provide a really high input impedance, however, and the transfer curve is frequently useful for audio work. They are also quiet. Those are powerful arguments for using JFETs in audio equipment, but at some point it becomes necessary to use either MOSFETs or bipolar transistors if either high voltages or currents are needed.
Back during the Seventies, there were power JFETs for a short period of time, but they followed the Dodo bird and the dinosaur and became extinct. During their short lifespan, they garnered a lot of attention and people have been on the lookout ever since for similar devices.
Not so long ago a company by the name of Lovoltech began manufacturing power JFETs again. They designed their parts for computer power supplies. It never occurred to them that people in the audio branch of electronics might be interested in such a thing as a power JFET. In fact, they were rather flustered to find that people wanted to use their parts for linear circuits at all. From my conversations with them, I gather that they think we're a little odd, but that doesn't mean that they won't sell us parts. This is one of those situations where money talks.
The primary problem with the Lovoltech power JFETs is that they have very limited power dissipation capabilities. The TO-251 package is absurdly small—roughly the size of your little fingernail. They are also limited in the amount of voltage they can take. But they can take a reasonable amount of current and that's enough to be getting on with.
The trick to taking the voltage (and with it, the heat) load off of the Lovoltech JFET is to use a circuit called a cascode. A cascode is simply one gain device sitting on top of another one's shoulders. The cascode device need not be the same as the one it is sitting on.
Nelson Pass has published several variants on the cascoded power JFET. All of his circuits to date have been single-ended. Visit www.passdiy.com and have a look at the Zen versions 8 and 9. Note that there are a number of variants on the Zen #9, so it may take you a while to read through it all. I highly recommend that you read both papers.
Judging from the number of power JFETs in the field at the moment, I would say that there are quite a few Zen 8s and 9s being built.
But then there's me. I persist in being a dinosaur (not the extinct variety mentioned above…at least not yet). I have speakers that need more power than the Zen amplifiers can comfortably produce. Granted, they can be scaled up, but some perverse part of me wanted to go in a different direction. There aren't a lot of solid state topologies that will allow the use of two N-ch devices. I considered building a quasi-complimentary amp for a while, then thought about a bridged design such as the Aleph-X. But in the end, I decided to go a different route: I decided to resurrect the Circlotron.
On the plus side, the Circlotron offers a like-device topology. You can use it with all N-ch output devices. It also has the curious property of using the N-ch devices in opposition. As one conducts more, the other conducts less. This lowers distortion because the transfer curves of the devices bow in opposite directions and essentially cancel out.
On the negative side of the equation, the Circlotron requires a little more complicated power supply than most amplifiers. Those two power supplies mentioned earlier aren't the usual plus and minus with ground in the middle that you're used to seeing, they are two identical power supplies, completely separate from one another. Don't panic though, many power transformers these days have two separate secondaries that can be used independently. That sort of transformer will work admirably for this circuit.
The cascode device will need a voltage at the Gate in order to bias the power JFET. There are several ways to accomplish this, but I chose to use a separate power supply. Given that the voltage and current required are very modest, this need not be an expensive proposition.
If you build the circuit the way I've built mine, this means four separate power supplies per channel, two large ones and two small ones.
As I've posted elsewhere, I got fascinated with the process of biasing the JFET. There are any number of other aspects that I could have gotten interested in, but that's the one that grabbed my imagination. Why? I don't know. It just happened that way. The way I've chosen to use in this circuit is clearly a variation on the scheme that Nelson uses in the Zen circuits. Varying the voltage at the Gate of the cascode device (in this case, a MOSFET) raises or lowers the Source, which in turn raises or lowers the voltage seen by the Drain of the power JFET. The JFET's Gate is held at ground potential. Given the number of people who have read Nelson's papers for the Zen amps, this should be a readily understandable variation.
Note that this biasing system is not recommended for class AB or class B operation. This is a class A biasing circuit. There are other biasing circuits that will work better for AB and B. I'll try to get some of those posted later. The reason I went with this one first is that it requires no adjustment. None. You choose the bias by the parts you put into the circuit. After that, everything takes care of itself. I went with this circuit because there are still people who don't understand how to bias the Aleph-X. I figured that they would appreciate a circuit that would not require a lot of fiddling.
Each half of the amplifier circuit is much like Nelson's Zen circuit. That's to be expected, given that one cascode looks pretty much like another. The difference between my circuit and Nelson's is the way I set the voltage at the cascode's Gate. Simply put, I use a small NPN transistor to watch the Source resistor for the JFET. The NPN sets the voltage at the cascode's Gate such that it sees its Vbe across the JFET's Source resistor. You set the bias by choosing the value of the Source resistor. It's that simple.
After that, it's a question of putting two of these circuits together in a Circlotron arrangement.
Yes, you can take half of the circuit and run it single-ended, if you want. Be my guest.
As far as I know, no one has ever produced a commercial version of a cascoded Circlotron. It's a new circuit. With that in mind, I decided to call this circuit the New-tron. Hopefully, that name will have neither positive nor negative connotations.
For the moment, I am presenting this circuit as a “hot follower.” Built as per the schematic, each cascode stack runs at about Iq=1A. That’s a little more than you might expect to see if you divide the Vbe of the NPN by the value of the Source resistor, but the NPN’s base current drives the actual value up a bit.
At the moment I am using 25V for V1 and V2. Yes, you can run it at higher or lower voltages. To a first approximation, your Vout measured peak to peak will equal one rail. Vrms will be that divided by 2.828. Set your current by adjusting R1, R8, R13, and R19 or add more cascode stacks slaved off of the inside ones. Or both.
Bias A and Bias B are set to 15V. Nothing fancy there, just one of those little Tamura transformers with rectification and a TO-92 case 7815. Would the circuit benefit from more elaborate regulation? Probably. However, my intent was to get the circuit going and that was the first part that came to hand.
Yes, the circuit will tolerate the usual parts changes. I used the IRF644 because I still have some I’m trying to use up. Feel free to use IRFP044s or IRFP240s or whatever. Keep in mind that changing the ZTX450s to another part will alter the bias by slight amounts. In fact, I spent an entire week just playing with different ways to fine tune the bias just within this one circuit. I had to slap myself… “Grey, stop playing. The intent here is to post a circuit that doesn’t require fiddling.” Oh. Right. So I took out all the pots and went back to the circuit as you see it here.
How do you drive it? Given that it doesn’t have a front end, you’ll need a balanced signal that will swing all the voltage that you intend to see at the output. I will be posting an IRF610 differential circuit in the next day or two that will do for starters. Those of you who have a spare Aleph preamp lying about can use that as long as your speakers are fairly efficient. I have other front ends that I will post later. I also have other versions of this output stage and other output stages that are different.
It’s all a question of time, folks. I had hoped to get this thread going back in January. It didn’t happen. Now it’s May and I thought that it would be best to do it on the installment plan rather than try to get everything into one monolithic post.
As always, questions and comments are welcome, but my time is increasingly fragmented and I may not get back to this thread as often as I’d like.
thanks for another nifty contribution, grey. here's hoping the inevitable group buy posts don't swamp the thread.
Are there gonna be boa-- oh, right. *heh*
It looks like an interesting circuit that I'd like to try. Any word on how soon (or whether) these Lovoltech power JFETs will be widely available?
Anyone who needs parts may check the Lovoltech thread in the Marketplace/Group Buys forum.
I suppose it goes without saying that this circuit will take a lot of heatsinking if you intend to generate more than a few watts. Power supplies will need to be robust, also.
I don't have it drawn in the schematic, but you are also invited to add Nelson's "cascode modulation" to the circuit. You'll need separate modulation for each side.
Do you have any comments on the sound of your circuit? And how about a photo? Those are always popular!
you can always flip it upwards .......
I just keep it in some little brain drawer till some little gremlins do their job
anyway ,interesting circ....
time for thinkin',again ;)
Very interesting Grey.
One quick question: Isn't there a form of cascode modulation already present in your circuit? I've only had a chance to do a quick once over of your circuit, but when a signal is presented to Q4&5, they will modulate changing the voltage of the source resistors, and thus the voltages at the base of the ZTX450's. Won't this then somewhat modulate the bipolars, causing the current across the bias resistors to modulate, thereby modulating the voltage at the gates of the cascode MOSFET's?
As I said, this commentary is based only on a quick once over look, so I may be blowing smoke. Please correct me if I am wrong.
Grey, you got the classAB one too?
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