Circlotron:
Super-Symmetry is the name Nelson Pass has ascribed to his U.S. Patent # 5376899. If you have a look at the patent, in figure 9 you'll see an op-amp style rendering of the invention. There's also an article on Super-Symmetry posted on the Pass Labs website. If you have an interest in balanced topologies, there was an excellent article in TAA 1/91 by Erno Borbely titled "Balanced Audio Amplifiers". I haven't yet tested the experimental design. I've made a number of changes to the schematic and I will post my changes and findings as soon as I can. I just completed the floating power supplies, so I'm hoping to test this weekend. I have tested the circlotron output stage on its own, however, and it appears to perform well, in fact it might make an interesting unity-gain power buffer albeit AC-coupled at the inputs.
Thanks
Mike
Super-Symmetry is the name Nelson Pass has ascribed to his U.S. Patent # 5376899. If you have a look at the patent, in figure 9 you'll see an op-amp style rendering of the invention. There's also an article on Super-Symmetry posted on the Pass Labs website. If you have an interest in balanced topologies, there was an excellent article in TAA 1/91 by Erno Borbely titled "Balanced Audio Amplifiers". I haven't yet tested the experimental design. I've made a number of changes to the schematic and I will post my changes and findings as soon as I can. I just completed the floating power supplies, so I'm hoping to test this weekend. I have tested the circlotron output stage on its own, however, and it appears to perform well, in fact it might make an interesting unity-gain power buffer albeit AC-coupled at the inputs.
Thanks
Mike
Looking forward to seeing your schematic. This is the extent of my thinking at this point. I'm sure there are a few things with this cct that others would do differently 
. The first set of opamps have their supply rails referenced to the same rails as the second so they too are bootstrapped. I have put rc's allover the place to damp things down a bit; I'm quite sure it would oscillate all over the place if I didn't. The idea is to fire it up and then reduce these rc's to a useable point. The bias is set by the 100mV forcing 100mA therefore 100mV across the source resistors. When the fet is on it's non-working cycle it will not turn fully off but instead have the constant 100mA through it. I think I will have to fiddle some of the biasing rc time constants so it won't try and follow the audio too much on the working half cycle of each fet.
GP.
. The first set of opamps have their supply rails referenced to the same rails as the second so they too are bootstrapped. I have put rc's allover the place to damp things down a bit; I'm quite sure it would oscillate all over the place if I didn't. The idea is to fire it up and then reduce these rc's to a useable point. The bias is set by the 100mV forcing 100mA therefore 100mV across the source resistors. When the fet is on it's non-working cycle it will not turn fully off but instead have the constant 100mA through it. I think I will have to fiddle some of the biasing rc time constants so it won't try and follow the audio too much on the working half cycle of each fet.GP.
Attachments
circlotron
To MROTHACHER:
On the patent side, the earliest reference to this type of circuit I found is from Philips. They had an early 60's implementation with tubes, and called it PPP for parallel push-pull, as the output tubes are paralleled for AC (and in series for DC). I built it at the time with 2 x 807's.
I noticed you used separate supplies for the output stage and the pre-stage. I think you can use the output stage (floating) supply to power the input stage as well. Note that the output supplies are offset from ground by half the output voltage. So, powering the input stage from this will give it a supply that tracks the output voltage to some extend if you chose the polarity correctly. That may even be beneficial for the linearity of the thing, since it decreases common mode swing from the input stage's point of view. Are you simulating this circuit? I would be interested how that would work out.
janneman
To MROTHACHER:
On the patent side, the earliest reference to this type of circuit I found is from Philips. They had an early 60's implementation with tubes, and called it PPP for parallel push-pull, as the output tubes are paralleled for AC (and in series for DC). I built it at the time with 2 x 807's.
I noticed you used separate supplies for the output stage and the pre-stage. I think you can use the output stage (floating) supply to power the input stage as well. Note that the output supplies are offset from ground by half the output voltage. So, powering the input stage from this will give it a supply that tracks the output voltage to some extend if you chose the polarity correctly. That may even be beneficial for the linearity of the thing, since it decreases common mode swing from the input stage's point of view. Are you simulating this circuit? I would be interested how that would work out.
janneman
Circlotron:
I should have mentioned that according to the X series manuals, Super-Symmetry cannot be done with op amps. Figure 9 simply illustrates the feedback and cross-coupling of the gain blocks.
janneman:
Using the floating supplies to power the input differential would be very interesting, but exactly how to carry it out eludes me at the moment.
Thanks
Mike
I should have mentioned that according to the X series manuals, Super-Symmetry cannot be done with op amps. Figure 9 simply illustrates the feedback and cross-coupling of the gain blocks.
janneman:
Using the floating supplies to power the input differential would be very interesting, but exactly how to carry it out eludes me at the moment.
Thanks
Mike
The prototype is complete. I've made a number of changes to the schematic, which I'll post later tonight. The bias is (reasonably?) stable once the heatsink is warm. A condition which may take 20-30 minutes to reach. I'll also post my initial findings and several questions (of course questions!) as well.
As currently implemented, the circuit has increased gain at LF which I can't explain and I'm quickly exhausting my brief electronics education!
Thanks
Mike
As currently implemented, the circuit has increased gain at LF which I can't explain and I'm quickly exhausting my brief electronics education!
Thanks
Mike
Conserver Zen power
So, when building an XZen -- basically the X1000 input stage with low voltages and high current -- it would probably be opportune to use op-amps to control the current sources to reduce power consumption (each volt dropped across sense counts at multiple amps quiescent current). Such a scheme would also enhance stability, but if I know NP right, he would do it with a small Zetex BJT ...
I suspect these sources can be lifted from the Aleph low side.
Petter
So, when building an XZen -- basically the X1000 input stage with low voltages and high current -- it would probably be opportune to use op-amps to control the current sources to reduce power consumption (each volt dropped across sense counts at multiple amps quiescent current). Such a scheme would also enhance stability, but if I know NP right, he would do it with a small Zetex BJT ...
I suspect these sources can be lifted from the Aleph low side.
Petter
I've tried perhaps 10 or 20 different versions of this circuit now with different resistor values and in all cases it suffers from excessive LF gain.
In some cases it has HF gain problems as well, but lowering R4 and R5 to 10 Ohms or so seems to flatten out the HF response.
The only way I've been able to reduce the LF gain is by reducing the gain to near unity, which takes the fun out of "amplification."
A couple of details about the prototype:
1. All fets are mounted to the same heatsink.
2. The measurements posted are unbalanced (- input connected to ground
3. It is built on a breadboard.
A couple of additional questions for the group:
1. How do you derive balanced outputs from your signal generators? Would a low-distortion op amp unity gain converter be the way to go? My generator doesn't have balanced outputs.
2. Anyone know how to make a balanced distortion measurement with an HP8903B?
Thanks again for your help!
Mike
In some cases it has HF gain problems as well, but lowering R4 and R5 to 10 Ohms or so seems to flatten out the HF response.
The only way I've been able to reduce the LF gain is by reducing the gain to near unity, which takes the fun out of "amplification."
A couple of details about the prototype:
1. All fets are mounted to the same heatsink.
2. The measurements posted are unbalanced (- input connected to ground
3. It is built on a breadboard.
A couple of additional questions for the group:
1. How do you derive balanced outputs from your signal generators? Would a low-distortion op amp unity gain converter be the way to go? My generator doesn't have balanced outputs.
2. Anyone know how to make a balanced distortion measurement with an HP8903B?
Thanks again for your help!
Mike
Hi Joe:
My thought was that R9 and R10 would allow the input differential to swing a higher voltage without disturbing the voltage referenced by R11 and R12. But my scheme must be reaking havoc on the frequency response. I've added .2 Ohm resistors to the output sources, which helps the bias, but if I eliminate R9 and R10 The output clips at 10 Vp instead of 20Vp. I wonder if I should flip the circlotron around and drive the output across the drains of Q1 and Q2. Hmmm. I hope to post some more info this weekend.
Thanks again for your help.
Mike
My thought was that R9 and R10 would allow the input differential to swing a higher voltage without disturbing the voltage referenced by R11 and R12. But my scheme must be reaking havoc on the frequency response. I've added .2 Ohm resistors to the output sources, which helps the bias, but if I eliminate R9 and R10 The output clips at 10 Vp instead of 20Vp. I wonder if I should flip the circlotron around and drive the output across the drains of Q1 and Q2. Hmmm. I hope to post some more info this weekend.
Thanks again for your help.
Mike
Mike, if you don't mind another suggestion, you could also try leaving R9 and R10 where they are and referencing the output stage sources to a common bias voltage source. The attached zip file shows a (very crude -- sorry!) picture of this. For the actual bias source, you could just use an adjustable voltage divider across the -20V supply, i.e. a pot and a resistor. If you get a lot of bias drift with temperature, you could then go with something more elaborate.
ADDENDUM -- You could also derive a bias voltage from the drains of the input differential pair, and that might work better once you have solved the frequency response anomaly and know how to avoid it.
ADDENDUM -- You could also derive a bias voltage from the drains of the input differential pair, and that might work better once you have solved the frequency response anomaly and know how to avoid it.
Joe: Thanks for the suggestion. It helped to determine that the frequency response problem is not related to the output section. I built a copy of the front-end only and I'm getting the same results. I'm beginning to suspect something more sinister. I think the change you suggested may ultimately be the way to go, but for now I've got to troubleshoot this front-end. I'm perplexed, there's nothing to it... I'll let you know what I find out. If I can figure it out I'm certain this thing will work quite well.
Thanks
Mike
Thanks
Mike
Just curious, Mike: if you are actually driving the inputs balanced, are you using a floated signal generator to do it? I could imagine this being a potential source of problems if the signal source isn't designed for balanced or floated operation. In any case, you could check for problems on the source end by measuring the signal appearing at the inputs of the diff pair.
Joe: I'm driving the circuit unbalanced - negative input connected to ground. My audio generator is single ended - negative input connected to earth through the chassis. I was wondering if the oscilloscope might be a problem, it is a battery operated PC scope, so I assumed (perhaps falsely) that the input was floating. As a crude double check, I used my DVM to test and swept through a few frequecies. The result was the same - a long downward taper from 100Hz to 20KHz. This is my first attempt at a balanced circuit, so I'm not familiar with the proper measurement techniques. I've found very little info on the Internet or elsewhere with respect to floating scope measurements. And still, I'm not even completely sure the measurement is the problem...
Thanks for your help. When it's perfected, I'll owe you a couple of PCB's!
NP: No word as of yet. You're just going to let me wrestle with this one for awhile aren't you...! No pain, no gain. (no pun intended)
Thanks for your help. When it's perfected, I'll owe you a couple of PCB's!
NP: No word as of yet. You're just going to let me wrestle with this one for awhile aren't you...! No pain, no gain. (no pun intended)
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