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Old 2nd May 2006, 02:07 PM   #11
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Errata:
In Post #2, I said that the peak output would be approximately one rail. Make that both rails added together.
No surprise: I'm waaay into sleep deprivation.
As an example, I've got the critter on the bench set for 25V rails on the output. It'll swing 50Vp-p or a little over 17Vrms. That's around 40W into 8 Ohms if you've got the current.
(I've already caught three typos in this post and am probably missing others. If I say something goofy, just nod and say that I warned you. Those who feel that everything I say is goofy will not be surprised no matter what I do.)
gl,
I am the cobbler who had no shoes. I never listen to things; I spend what little time I have available at the bench.
No pictures, sorry. I still have an old Cannon AE-1--the kind of camera that runs on (gasp!) film. However, I have no scanner or other way to convert from "real" pictures to digital. I have promised myself that as soon as I run out of film I will begin looking at digital cameras.
(Three more typos.)
Besides, no one wants to look at this thing...it's ugly.
metalman,
At first glance, you'd think so, but as always the devil is in the details. One, the bipolar would flip the polarity of the modulation signal, thus increasing distortion, not decreasing it. Two, it's a moot point because the RC filter at the base of the NPN takes out nearly all the signal, leaving only variations less than 1Hz.
Incidentally, the values of the RC filter can be fiddled around according to whatever you have in your junkbox. All that matters is that you get the cutoff frequency really, really low. I chose to use a relatively high value resistor expressly so that the base current would bump up the bias a bit. If you use a larger capacitor and smaller resistor, you'll get closer to the .65V/Rsource=Iq formula that you would expect.
lumanauw,
I've got more versions of this amp that you can shake a stick at, but I'll need to get a front end or two posted first. At this point, all I really want to do is fall on a pillow and snore, but my daughter needs feeding and tending to, so that's not going to happen.
(Another typo...and two others whilst typing this comment...man, I am toast.)

Grey

P.S.: Oh hell, just when I thought it was safe to go back in the water...I just thought of another way to bias the circuit. Blast and bother!
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Old 4th May 2006, 12:20 AM   #12
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I stole a few minutes this afternoon and scratched out the schematic for the simple front end I've used off and on during my fiddlings. This is not a be-all, end-all front end, just a decent one. In its favor: It's simple and relatively cheap.
In a pinch, it might make a fair-to-middlin' medium gain line stage.
The rail voltages are specified as +40V and -10V. A couple of thoughts:
--Use matched devices for Q2 and Q4.
--It would actually be better if the positive rail were closer to 45V, 40V was just a convenient voltage for me at the time. I didn't need that last few volts of swing on the output; you'll get more symmetrical clipping if you use a slightly higher rail.
--The negative rail is specified as -10V. Feel free to make it -40 V or -45V if it makes you happy, but be sure to heatsink Q3 accordingly. The device can take it, as long as you make arrangements for the heat.
--As always, the rails are important, but since both the positive and negative rails sum to DC, it will take some of the worry out of the power supply end of things. That doesn't mean you can be a slacker, just that you don't have to lie awake at night worrying about five decimal places of accuracy for the rail.
--It draws roughly 30mA of current, total. Call it 15mA per side. Q2 and Q4 will dissipate about .6-.7W each. I'm comfortable running TO-220 devices up to about half a watt without a heatsink. This is slightly over that, and although I'm sure things would have been just fine without, I went ahead and put heatsinks on them. Nelson would, no doubt, run them naked--he ain't happy until he smells burning flesh. I prefer my steak medium-rare rather than well-done.
--Q3--assuming a 10V rail--is okay without a heatsink. If you use a higher rail, use a heatsink.
Just hook this circuit directly to the output stage posted earlier. Note that the output stage is a follower, so it does not invert phase. With that in mind, the negative feedback comes back to the same side of the front end. It does not cross over to the other side like it did in the Aleph-X. If you do swap it over, you'll have positive feedback. This is only recommended if you like listening to the old Chicago track "Free Form Guitar" or live Jimi Hendrix recordings (e.g. "The Star Spangled Banner"). You will not actually need the recording--the circuit will provide all the sound effects you can stand.
Don't say you weren't warned.

Grey

P.S.: I've had complaints about the resolution of the schematics. I feel your pain, as the expression goes. DIY doesn't allow anything over 1000 pixels and I'm at wit's end trying to get a readable schematic in under their limits.
My schematic program exports BMP files. I am resizing and converting them to GIF in another program. There may be better ways of doing this, but &%$)!@# I've got better things to do with my time than sit around trying to convert files all day long. As it is, I was frantically pounding the keyboard this afternoon, running late getting to work, all because of all this file size/format/conversion nonsense.
Grumble.
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Old 4th May 2006, 03:25 AM   #13
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My head is too 2D to understand 3D idea.
Nevertheless, the post#2 circuit looks like a current amp. Right . . . ?
I think it could work as class AB and A at our will. Also right . . . ?

Regards
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Old 4th May 2006, 10:58 AM   #14
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Default balanced cascode modulation

Nice circuit, Grey - thanks for sharing!

If you'll indulge me, I'd like to propose adding a balanced form of Nelson Pass' cascode modulation technique. I believe that circlotrons benefit from some type of cross-coupling that would better enable each half of the circuit to sense the full voltage across the output.

The attached schematic shows how you could cross-couple two cascode bias networks using a simple resistor network. The 15k (nominal) resistor does the cross-coupling, and would be adjusted in situ for best performance. In the attached circuit, constant current sources feed current to the bias network from the output stage power supplies, avoiding the need for a separate bias supply at the cost of a few more parts. I think you could similarly cross-couple other preferred bias arrangements, including the one you show above.

I see the effects of balanced cascode modulation as similar to what happens in triode circlotron output stages such as the Atma-Sphere. Because triodes "see" the voltage at both their anodes and their cathodes, and because each circlotron anode follows the opposing cathode, each half of the stage is better able to independently control both output terminals. A benefit of the circuit shown here is that the anode (drain) signals reflect the output rather than the B+ supplies, so they are relatively free of power supply noise even with unregulated supplies.

Lacking a power FET model, I verified this circuit's basic operation in simulation by scaling the currents down by a factor of 100, in essence simulating a line-stage version of the design. Some component values in the schematic were then re-scaled for a power amplifier. Offered in good faith, but build at your own risk!
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Old 4th May 2006, 11:18 PM   #15
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All,
I realized that I had said that the biasing circuit was the only real difference between Nelson's basic building block and mine. There's actually another difference--I take the signal from the Source of the JFET, whereas he takes the signal from the Drain of the MOSFET.
jh6you,
Let's take a closer look at the biasing circuit in Isotope One.
--Assume the amp to be at idle, so there's no signal present. The NPN sets its Vbe across the Lovoltech's Source resistor. This results in a current through the load resistor (R5) that holds the Gate of the cascode device at the Lovoltech's Vds + the MOSFET's Vgs. So far, so good. In fact, it's a current source as long as there's no signal at the Lovoltech's Gate.
--Assume that the circuit is in use, but still within class A operating parameters, meaning that there's signal all the time; the stack never turns off. The RC network at the base of the NPN filters out the signal, keeping C1 charged via a small amount of DC drawn from the main bias current. Again, the cascode device's Gate is held at the proper voltage to bias the Lovoltech.
--Assume that the Lovoltech has been driven into shutoff during part of the amplification cycle. No conduction at all. The voltage across the JFET's Source resistor collapses and C1 begins to discharge. The '450 tries to adjust the bias by pushing the cascode's Gate higher, but it's not conducting. When the signal comes back, C1 begins to recover, and this brings down the cascode's Gate, but for an instant the Lovoltech is biased too hard.
In other words, you begin to modulate the cascode, but the wrong way. Distortion increases.
Granted, I'm overstating the case. The NPN's base current won't draw down C1's charge that much during a mere half cycle of music, but if it's more than a passing peak, you're likely to see some degradation of performance.
Yes, it's possible to use either a JFET or a MOSFET instead of a bipolar. But at that point, you've lost the simplicity and predictability of the NPN's Vbe and life gets much more complicated.
Why bother? I've got other biasing schemes that will do a better job. The entire point of Isotope One is to build an amp that doesn't need pots and fiddling. The more complicated mess-with-it circuits will come later.
Incidentally, I've only got one RC drawn. If you want, you can use an RCRC (or more) and increase the cutoff slope.
Another note: This is why I regulated Bias A and Bias B to 15V--so that if the NPN did, in fact, slap the bias rail, then it wouldn't blow the MOSFET. If you go for some variation of this circuit that actively sets the bias but runs from a rail higher than about 20V or so, you might want to throw in a Zener to protect the MOSFET.
Joe,
People are certainly welcome to post other front ends and variations on the output stage.
Could you elaborate on how you see the circuit working? I'm sure it's just because I'm sort on sleep, but I don't see the cascode modulation part of it.
Using a current source to drive a resistor in order to set the bias definitely works. I've got one of those in the queue, but if anyone else wants to post one, go ahead.
Note that either the CCS or the resistor will need to be variable in order to set the bias, unless you intend to add more circuitry to automate it (I may or may not do that later--I've got a couple of other things I want to do first).

Grey
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Old 5th May 2006, 09:48 AM   #16
djk is offline djk
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I'm missing something here.

We would seem to lose the 'triode-ness' of the J-fet by using it as a follower (no voltage gain), and then we are using a hex-fet for the voltage gain.

What about using a J-fet for the voltage gain?

http://www.supertex.com/pdf/datasheets/DN2535.pdf

Run it with about 40mA or so, smack in the triode region.

And a 400W big honker J-fet for the follower?

http://www.ixys.com/99192.pdf

Lot easier to mount a TO-247 than the TO-252 (Lovoltech) package.
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Old 5th May 2006, 10:38 AM   #17
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Well understood. Thanks for the kind explanation, Grey.
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Old 5th May 2006, 09:33 PM   #18
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Quote:
Originally posted by GRollins
Could you elaborate on how you see the [cascode modulation] circuit working?
Basically, it's a voltage divider. This may be easier to see if you get rid of the two top 620 ohm resistors, which have a purpose but are not essential here for basic operation. Then increase the remaining 620 ohm bias resistors to 1.2k or whatever value gives the correct bias voltage for the type of MOSFET you are using. Leave the cross-coupling resistor connected across the tops of the two bias resistors.

In actual operation, the swinging output signal produces a voltage imbalance across the cross-coupling resistor. This causes current to be subtracted from the bias resistor on the more positive side of the circuit and added to bias resistor on the more negative side. The result is an attenuated and inverted image of the output signal superimposed on the cascode bias at the gate of each MOSFET.

For a given output signal level, the magnitude of this modulation is a function of current, which in turn is a function of cross-coupling resistance. Higher resistance lead to smaller currents and hence smaller modulations, and vice versa. You would adjust this resistance experimentally for lowest distortion, then perhaps replace it with an equivalent fixed value.

FWIW, if I were prototyping this, I'd use fixed resistors for the bias and make the current sources adjustable over a small range, say 5-7mA. The LM317 CCS (see LM317 datasheet) has worked well for me where there's little or no signal voltage across it, which is the case here.

I hope this is clear - let me know.
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Old 5th May 2006, 10:12 PM   #19
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Quote:
Originally posted by djk
We would seem to lose the 'triode-ness' of the J-fet by using it as a follower (no voltage gain), and then we are using a hex-fet for the voltage gain.
No, the device does not know that you are using it as a
follower (CD) as opposed to voltage gain (CS). It has the
same characteristic in both cases.
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Old 6th May 2006, 12:27 AM   #20
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djk,
You mentioned JFETs, but the links you posted are both for MOSFETs. If you know of another source for power JFETs, please let us know.
I'm assuming that you mean for the DN2535 to be used in the front end. I don't see that it should be a problem to substitute it directly into the front end circuit in place of the IRF610s. If you try it, let us know how it works out.
I hope to get another front end posted fairly soon. I've got one resistor value I want to take another look at, then I'll have to go through all this file format foolishness again, which takes more time than I can easily justify. After that, I'll stick to back end topologies for a little bit. With a half dozen schematics in hand, people can mix and match according to their wants and needs.
Yes, the IXYS MOSFET could be used instead of the IRF644. I've got handfuls of '644s lying about, so I tend to use them just because they're handy. Once they're gone, I'll get something else.
Now, if your intent is to use the IXYS device to replace both the IRF644 and the LU1014D, that will work too. No one says that you have to cascode the ouput stages in a Circlotron, and as far as I know, no one has ever done so before, at least on a commercial basis. If you want to build a more conventional MOSFET Circlotron, that would be a good starting point. You'd have to scrap the biasing circuit I posted, but it won't be that hard to come up with another one.
There's no question that the TO-247 case is easier to mount than the TO-251, but unfortunately the TO-251 case is what Lovoltech chose to use. Given that Lovoltech is the only company making power JFETs at the moment, we're kinda stuck.
The output of a Circlotron doesn't have any voltage gain. It's a follower. The gain it provides is in the current realm. That's why the front end has to be able to swing all the voltage you will need at the output. In fact, followers lose a small amount of voltage swing, so the front end has to do a little more to make up for that loss.
In our favor is the fact that this topology effectively doubles the voltage swing, which gives us a 6dB or so head start. In a 'normal' single-ended amp, only one side of the front end differential's gain is used. The other side's gain is thrown away...literally. Frequently there's not even a load resistor on the 'back' side. In a design such as this, both sides of the front end differential are used, which gives us a "free" 6dB more gain, all for the price of a single resistor. The Circlotron output topology cheerfully accepts this balanced signal and we're set to go.
Joe,
Again, it may just be that I'm short on sleep (still/again)...but it looks as though you're using negative feedback at the cascode's Gate, whereas Nelson is using positive feedback. If true, then I would anticipate that distortion would actually rise a bit.
Perhaps Nelson will weigh in here and give his opinion.
Like you, I used fixed resistances and a variable CCS to bias the cascode stack. Works like a charm.

Grey
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