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Old 6th April 2001, 06:55 PM   #1
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Can anyone tell me how the power resistors bias the FETs in the Son of Zen?

I've been trying to use Ohm's Law as best I can, but I get confused. What does the 1 OHM resistor do just before the Source connections of the FETS?
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Old 6th April 2001, 08:06 PM   #2
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Vince,
I haven't spent the time studying the SOZ that I have the Zen & Aleph, but my take on it is that the 1 ohm resistor is equivalent to the 'magic' resistor in the X circuit. Note also the dual legs on either side, just as in the X.
Trying to analyse the circuit with Ohm's Law will drive you to drink (tell me which bar, and I'll meet you there...). As long as the circuit is quiescent, you'll arrive at reasonable numbers, but things will change once you drive the MOSFETs.

Grey
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Old 6th April 2001, 09:35 PM   #3
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Thanks! I was starting to feel useless!

I have/had no idea how to apply it.

I'm going to mess around with the SOZ on PSpice for a bit. I have an idea or two I want to try out, before I try to burn my apartment down with a new/modified circuit.

Thanks!
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Old 8th April 2001, 03:32 AM   #4
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Vince,
What did you have in mind for the SOZ changes?
I was looking at the critter last fall, trying to figure out a way to reduce the waste heat so I could justify building one. Heat sinks are next to impossible to come by around here (this is why I'm sitting next to a water-cooled Aleph 2 cunningly disguised as an octopus--still working on it--early indications are good, although it won't be practical for most people--I'm not yet sure it'll be practical for me, for that matter), so I was trying to figure ways around the problem.
I offer the following:
1) Current source instead of the resistor array at the bottom. Current sources ain't that hard to do. No, this isn't exactly the most electrifying brainstorm, but it would help.
2) Here's one no one else has mentioned that I've seen--asymmetrical rails. Say you were going for a 15V rail on top. Try a 5 or 10V rail (with current source) on bottom. The gates are tied to ground, and the bias would adjust itself voltage-wise to produce the same offset at the source. The savings heat-wise would be in the reduced rail. Current would be the same, but you'd end up with less watts, since the VI product would be lower. This one will depend on what voltages you intend to swing.
3) All right, fasten your seatbelt, this one's sick...a differential Aleph. Not with a full Aleph up each side of the differential--that's way too easy. Attack it at the current source. Make the differential current source variable (ala Aleph) with a sense resistor in series with the load. Alternate hookup--sense network reads the incoming signal voltage. I prefer the output network idea, as it affords a degree of feedback as regards to what the speaker is doing. This is still in the thought experiment stage; it may take an absolute value (i.e. bridge rectifier) circuit to set the bias current properly, but that wouldn't be all that difficult to implement.
The objection: But, Grey, if the bias is bouncing around, won't that show up in the signal?
The answer: Who cares? It's common mode. It'll cancel out, in the same manner that distortion cancels in Nelson's X topology, or in any differential, for that matter.
If there's no signal, drop the quiescent bias current back to, say, half. If there's a signal, jack it up. Dissipation is cut, yet operation remains push-pull class A.
There are a few variations that I've come up with, but they're pretty obvious once you start thinking in terms of a variable current source for an output differential.
4) Okay, okay...but for the masochist, only. Variable current source at the bottom, as per 3, but with current sources up top, as well, also driven by the varying current scheme so that they wouldn't try to supply/draw (depending on whether you're thinking current goes from + to - or vice versa; also dependent on whether the output devices are N or P channel) more current than the 'downstairs' current source wants to supply/draw. While you're at it, make the top current sources do the Aleph thing, too; or call it a cascode...I don't care. This topology might be suitable only in a parallel output configuration, as I think the damping factor would be fairly low. Gang enough of these arrays, and you'd have plenty of power in addition to getting a decent damping factor.
5) For that matter, there's nothing to stop you from paralleling normal SOZ arrays, ditto with any of the above ideas. Individual device dissipation comes down. Power as high as you want. Distortion and noise lower without all that pesky feedback.
What do you say, Vince (& Petter, et. al.)?

Grey
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Old 8th April 2001, 09:55 AM   #5
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It seems to me that this deviating a little from the original design intention of minimising the number of active devices.
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Old 8th April 2001, 03:27 PM   #6
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Geoff,
So's the Aleph, compared to the Zen, and the X compared to the Son Of Zen...yes?
At least in regards to adding a current source, it's not (depending on your point of view) in the signal path. For me, whether a current source is in the signal path is something I can argue both ways--depends on the phase of the moon as to which side I take.
Does it effect tonality? Yes.
So if I were pinned against a wall and told to take sides, I'd go with the "it's in the signal path" side of things.
Regarding complexity as a philosophical vs. practical matter, I favor simplicity as long as I can get where I want to go. However, in the real world, I can't get enough heat sinks of sufficient thermal capability for a reasonable price to build something like a 50W SOZ circuit. It's in the fugetaboutit category. So, necessity being the mother that she is, I try to figure out ways to finess the situation. Nature may be an unforgiving wench, but she can sometimes be seduced, if you're clever enough...
I'm also curious to see what Vince is thinking. It will almost certainly be something different from my train of thought, whether it's aimed at the sonic side of things, or the efficiency of the circuit. Either way, it'll be interesting to hear, as I'm always open to new ideas. Endless iterations of the same ol' same ol' grow stale.

Grey
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Old 8th April 2001, 03:55 PM   #7
Geoff is offline Geoff  United Kingdom
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Grey

I agree. Personally, I am not concerned about the number of active devices in the signal path of a power amp, particularly when you consider the number of ics the signal has passed through in the recording studio, mixing console etc. What does matter is the enjoyment one gets from one's hobbies (amplifier building or whatever) and from listening to music. The amplifier topology is immaterial provided you can afford to build it and the results match those which you had hoped to achieve.

Geoff
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Old 8th April 2001, 10:47 PM   #8
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Geoff,
The number of critters in the signal path matters to me, as even a 'mere' current source has an audible effect. Unfortunately, I have to bow to reality once in a while. (Grumble, grumble...)

Grey
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Old 8th April 2001, 11:45 PM   #9
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Grey,

I didn't read you're whole post, as I am just looking in, but to answer your first question:

When the SOZ first came out, I liked it so much because it was simple, but balanced. The only real problem I had w/ it was the in-effecency. I don't care too much about matching speakers to it, because I like to design friendly speaker loads for them.

I emailed Nelson back in 97 when SOZ came out. I asked if it were possible to remove the power resistors in favor of constant-current sources. He said it was possible, achieving 30% effeciency, if a CCS was added to the neg. rail and about 2-3% if a CCS was added to the pos. rail. For a total of 33% effeciency, like the orginal ZEN. (I still have that email on hard copy.)

But, I'm no E.E. I had too many questions, and I didn't want to keep asking NP for help. So, I just scrapped it for the Hoffman amp. Then came the Aelph designs, and so I haven't built anything, because of the endless search for the "perfect SE amp".

I was thinking about building a couple of CCSs and attaching them to a modified SOZ. I just don't know what to keep in and what to take out. That where my question, 'what's the 1 ohm resistor do', came from?

If I could build a more effecient SOZ. I'd start tomorrow.

I have PSpice, but I'm having trouble using it to simulate a design.

Note: aside from what I asked, the only thing NP said to do was add some resistance from the source to ground, or feedback, or R? back to the Pos. rail. I was on the right track.

If anyone has any input on this, I'd love to hear it!!

Vince
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Old 9th April 2001, 02:28 AM   #10
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Pardon me whilst I cuss some more...this blasted PC ate my reply.
Vince,
Current sources are a 'can-do' thing--you can start tomorrow...
************************************************** **********
Folks, we need to hold a pow-wow and standardize on a common software base so as to be able to share schematics easily. I need to be able to E-mail Vince a schematic that he can see in an intelligible manner. The software I use is Ivex WinDraft. I *DO NOT* recommend this software. It's buggy, counter-intuitive, and unusually prone to those cute little error windows informing you that your program will be shut down, with or without your permission. I have PSpice on hand, but have not done the learning curve yet. There's also the difficulty that you can't just drop in something, you've got to have the model, and the model isn't alway available. Besides, I need tubes in addition to solid state, and I'm not sure that PSpice will recognize glass bottles. Will those of you who spend all your waking hours modelling things and whipping out schematics please nominate a candidate or two? (Even if it's PSpice, just tell me where I can find triodes and other sundry models.) I don't so much care about modelling, but I do like having a tidy schematic on hand. I've noticed several times in other threads where people needed to share scratch diagrams and couldn't...
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Anyway, Vince, starting from the bottom of the SOZ schematic, I'd recommend replacing the two 8 ohm resistors with current sources. you could do it with one, but it'd be asking an awful lot of one current source to carry that much load. Remove the tie across the top of the 8 ohm resistors. You can model the circuit with and without the vertical 1 ohm resistors, but my gut says leave them in. I'm betting that distortion will be a whisker lower with them, and they will also serve to equalize minor variations from the current sources.
Leave the horizontal 1 ohm resistor in.
The top 8 ohm resistors? Up to you. If you want to replace them, the easiest way out would be to start by pasting a fully functional Zen up each side of the differential, as Zens come pre-equipped with current sources up top. (Only one power supply for both Zens, though.)
If you want to go whole hog on the top, you could use a current mirror--essentially two current sources operating in tandem.
I'm willing to bet that the whole shebang will sound better with resistors up top, relative to any version of current source. Particularly in the purity of the higher frequencies.
Parts matching for the current sources will be a necessity.
If you do current sources up top, you'll need to match the current to the sources on the bottom.
Decide how much power you want from this thing. That will tell you where to start regarding rail voltage and how much current you'll need. Then look for output devices. Current sources and heat sinks will complete the picture. You'll get there.

Grey

P.S.: Someone's likely to get picky and point out that I've played hob with the nomenclature above. Yes, I know the difference between a current source and a current sink, but I, like many, tend to call them all current sources. I even drive myself crazy when talking about which way current flows (from + to -, or vice versa), as I tend to take the literalist view that it goes from - to + (i.e. the actual flow of electrons), in spite of the fact that this flies full in the face of historical convention. *Sigh* Can't win.
("You say tom-A-to, I say tom-AH-to, let's call the whole thing off...")
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