This is the first post of an intended series relating to variations on Nelson Pass's Son of Zen amplifier circuit.
First off, credit where credit is due: I'd like to thank Nelson for sharing the circuit with the DIY community. For that matter, I'd like to thank him for sharing so many of his other ideas with us, as well. These circuits provide an interesting playground for the imagination, as well as good sound for the ears. They are (relatively speaking) kind to the wallet, and are, if approached correctly, educational.
The original circuit:
http://www.passlabs.com/pdf/sonofzen.pdf
Background:
The SOZ circuit is an amazingly simple circuit. A simple differential pair functions as a single gain stage. That's it. Finis.
(Those who wish to argue whether a differential is a single gain stage can exit through the door at the rear of the room. Bye.)
By definition, a differential is class A circuit, and therein lies the rub. The SOZ is a red-blooded, fire-breathing, carpet-singeing heat machine. It is the amplifier of choice for Eskimos, penguins, and polar bears who need a space heater that also happens to give off music. Others tend to look askance at amplifiers that require heatsinks the size of Volkswagens. It helps if you have a relative at the local power company who can get you kilowatts of AC for free.
Devising ways to get rid of the heat has caused many a sleepless night for would-be SOZ builders. The first thing most people think of is to buy standard passive heatsinks. Shortly thereafter, they come to the realization that life isn't simple; you can't just walk right down to the corner electronics shop and pick up a couple of large heatsinks for $1.98 each. First off, big heatsinks are hard to come by. Worse yet, they cost serious money.
What to do?
Well, one option is to rob a bank and go buy some fancy heatsinks.
Not recommended.
The local constabulary will be after you in no time, and I doubt that the judge will allow you to cart your shiny new amplifiers off to the jailhouse.
After careful consideration, I discarded this option. I chose instead to try water cooling the pair of Aleph 2s that I built. (Alephs suffer from the same tendency to melt inquisitive cats that the SOZ does. Depending on how you feel about your resident cat, this may be an attractive concept.) Not only did it work, it worked well. Better than I expected, in fact. See the Water Cooled Audio Circuitry thread elsewhere on this site for details.
But some people, mysteriously, just don't want 5/8" hoses snaking across their listening room floors. There's no accounting for taste, I suppose.
So that leaves us hunting for other options. Forced air cooling works well, of course, but still assumes that you can find medium-sized (as opposed to huge) heatsinks, and that you are willing to tolerate the noise of the fans.
Some have suggested Peltier modules, but to my knowledge no one has actually built a SOZ using them. Whether it's a practical option, I will leave to others to decide.
And, rather abruptly, we're scraping the bottom of the barrel. There just aren't that many ways to get rid of unwanted heat.
Back to first principles. Is there any way to reduce the heat produced by the SOZ amp? Well, yes. You could scale down the amp, but that's an unsatisfactory solution for those who were rather hoping to increase, not decrease, the power. A quick perusal of the schematic, however, suggests the possibility of substituting current sources for the lower 8 ohm resistors.
That brings us to our current discussion.
**********
Don't get your hopes up. I'm not posting a complete, paint-by-numbers schematic. Yet. I will do so as soon as possible. In the meantime, I thought I'd pass along a few points from the notes I've been taking as I've poked and prodded at the original design.
--As folks who've watched other threads here may have noticed, I'm not a big fan of current sources. Don't get me wrong...an *ideal* current source is a wonder to behold. They work well in simulations. They accomplish marvelous things.
In theory.
In practice, they're afflicted by capacitance and other annoying properties that keep them from realizing their potential. That said, they have their uses. In this case, we're going to try grafting one onto the SOZ in order to reduce the power dissipation. Those who feel inclined to debate the virtues (or lack thereof) of current sources are invited to do so elsewhere. There are already several threads on this site for this topic. Go resurrect one of them.
--The amplifier is not going to be a dial-a-wattage circuit such as Nelson suggested in his article. In fact, I'm not sure that the stock version would work well in that configuration. Why? Device matching. The output devices used in the SOZ need to be matched. To match a device, you pick a target idle current and match the output devices at that current. Unfortunately, in the real world, devices aren't linear enough that a pair matched at, say, 1 amp will stay matched when you ramp up to 2 amps. Yes, with a large enough box of MOSFETs, you could probably come up with a pair of devices that would track well at any given current. You could also grow old trying to find that magic pair. It's your money. It's your time. If you feel that it's worth the trouble and expense then go ahead. However, most people really only need an amplifier that has a certain fixed power output. This implies a fixed idle current, which in turn means that we can find a decent pair of semiconductors that will do the job. As a related point, if you're driving the circuit with current sources, you'll need a way to adjust the output of the current sources as you turn the power up or down. It wouldn't be that difficult to arrange, I suppose, but to me it seems like a lot of effort for little return.
--The current sources either need to be carefully matched, or you will need some way to adjust them. This will be your DC offset adjustment.
--The current sources will be dissipating heat. From what I'm seeing so far, it's somewhat less than three times the dissipation of the output devices. Plan on more heatsinks.
--I'm using parts I have on hand, so far. I have not (yet) gone to water-cooling for the SOZ. It's air-cooled; no fans. I'm using Motorola MTP7N20E MOSFETs for both the outputs and the current sources. Resistors vary. I've got two of the big Dale 8 ohm 1% 250W load resistors that I typically use for testing purposes. Here, they've been pressed into service as part of the circuit itself. The 1 ohm resistors are either NTE 5W ceramics or some 1 ohm Dale wirewounds. The power supply is primitive at present. Just a simple transformer/rectifier/cap job that I threw together. Nothing fancy. Yes, there's some ripple. I intend to toss some more caps (and whatever else I can find in the junk box) in there to smooth out the ripple. There are only so many hours in a day. I'll get it done eventually.
--The gain of the SOZ circuit is low. The stock version I whipped together had about 14dB gain (single-ended). The current source version is even lower, a little over 6 dB (also single-ended). If you intend to drive a SOZ directly, you'll need some voltage swing from your preamp.
--The amp is small...on the order of 2 or 3 watts output. It has only one channel. This keeps it from taking over my workbench. It also reduces finance problems. I'll consider building a big one if I like the results.
--I have several things planned after getting the kinks out of the amp as it stands now. The first one is to mate Nelson's Balanced Line Stage (aka Bride of Son of Zen/BOSOZ) to the SOZ in order to get some gain into the poor thing. It will also allow for better matching with a single-ended source, functioning as a phase-splitter. I also plan to parallel two SOZ stages (cutting back on current in each in order to lighten the load on the devices). This will require more matched devices, but assuming that you've got enough on hand, there's no reason you couldn't keep paralleling them to reach any arbitrary power level. What this does to your electric bill is your problem. I've got a few other ideas in mind, too, but this will be enough to use up more free time than I can reasonably expect to have over the next month or two.
--No, this is not the X/Aleph hybrid circuit I proposed in Petter's X thread. That's a separate project that I will approach later.
Grey
[Edited by Jason on 09-27-2001 at 01:26 AM]
First off, credit where credit is due: I'd like to thank Nelson for sharing the circuit with the DIY community. For that matter, I'd like to thank him for sharing so many of his other ideas with us, as well. These circuits provide an interesting playground for the imagination, as well as good sound for the ears. They are (relatively speaking) kind to the wallet, and are, if approached correctly, educational.
The original circuit:
http://www.passlabs.com/pdf/sonofzen.pdf
Background:
The SOZ circuit is an amazingly simple circuit. A simple differential pair functions as a single gain stage. That's it. Finis.
(Those who wish to argue whether a differential is a single gain stage can exit through the door at the rear of the room. Bye.)
By definition, a differential is class A circuit, and therein lies the rub. The SOZ is a red-blooded, fire-breathing, carpet-singeing heat machine. It is the amplifier of choice for Eskimos, penguins, and polar bears who need a space heater that also happens to give off music. Others tend to look askance at amplifiers that require heatsinks the size of Volkswagens. It helps if you have a relative at the local power company who can get you kilowatts of AC for free.
Devising ways to get rid of the heat has caused many a sleepless night for would-be SOZ builders. The first thing most people think of is to buy standard passive heatsinks. Shortly thereafter, they come to the realization that life isn't simple; you can't just walk right down to the corner electronics shop and pick up a couple of large heatsinks for $1.98 each. First off, big heatsinks are hard to come by. Worse yet, they cost serious money.
What to do?
Well, one option is to rob a bank and go buy some fancy heatsinks.
Not recommended.
The local constabulary will be after you in no time, and I doubt that the judge will allow you to cart your shiny new amplifiers off to the jailhouse.
After careful consideration, I discarded this option. I chose instead to try water cooling the pair of Aleph 2s that I built. (Alephs suffer from the same tendency to melt inquisitive cats that the SOZ does. Depending on how you feel about your resident cat, this may be an attractive concept.) Not only did it work, it worked well. Better than I expected, in fact. See the Water Cooled Audio Circuitry thread elsewhere on this site for details.
But some people, mysteriously, just don't want 5/8" hoses snaking across their listening room floors. There's no accounting for taste, I suppose.
So that leaves us hunting for other options. Forced air cooling works well, of course, but still assumes that you can find medium-sized (as opposed to huge) heatsinks, and that you are willing to tolerate the noise of the fans.
Some have suggested Peltier modules, but to my knowledge no one has actually built a SOZ using them. Whether it's a practical option, I will leave to others to decide.
And, rather abruptly, we're scraping the bottom of the barrel. There just aren't that many ways to get rid of unwanted heat.
Back to first principles. Is there any way to reduce the heat produced by the SOZ amp? Well, yes. You could scale down the amp, but that's an unsatisfactory solution for those who were rather hoping to increase, not decrease, the power. A quick perusal of the schematic, however, suggests the possibility of substituting current sources for the lower 8 ohm resistors.
That brings us to our current discussion.
**********
Don't get your hopes up. I'm not posting a complete, paint-by-numbers schematic. Yet. I will do so as soon as possible. In the meantime, I thought I'd pass along a few points from the notes I've been taking as I've poked and prodded at the original design.
--As folks who've watched other threads here may have noticed, I'm not a big fan of current sources. Don't get me wrong...an *ideal* current source is a wonder to behold. They work well in simulations. They accomplish marvelous things.
In theory.
In practice, they're afflicted by capacitance and other annoying properties that keep them from realizing their potential. That said, they have their uses. In this case, we're going to try grafting one onto the SOZ in order to reduce the power dissipation. Those who feel inclined to debate the virtues (or lack thereof) of current sources are invited to do so elsewhere. There are already several threads on this site for this topic. Go resurrect one of them.
--The amplifier is not going to be a dial-a-wattage circuit such as Nelson suggested in his article. In fact, I'm not sure that the stock version would work well in that configuration. Why? Device matching. The output devices used in the SOZ need to be matched. To match a device, you pick a target idle current and match the output devices at that current. Unfortunately, in the real world, devices aren't linear enough that a pair matched at, say, 1 amp will stay matched when you ramp up to 2 amps. Yes, with a large enough box of MOSFETs, you could probably come up with a pair of devices that would track well at any given current. You could also grow old trying to find that magic pair. It's your money. It's your time. If you feel that it's worth the trouble and expense then go ahead. However, most people really only need an amplifier that has a certain fixed power output. This implies a fixed idle current, which in turn means that we can find a decent pair of semiconductors that will do the job. As a related point, if you're driving the circuit with current sources, you'll need a way to adjust the output of the current sources as you turn the power up or down. It wouldn't be that difficult to arrange, I suppose, but to me it seems like a lot of effort for little return.
--The current sources either need to be carefully matched, or you will need some way to adjust them. This will be your DC offset adjustment.
--The current sources will be dissipating heat. From what I'm seeing so far, it's somewhat less than three times the dissipation of the output devices. Plan on more heatsinks.
--I'm using parts I have on hand, so far. I have not (yet) gone to water-cooling for the SOZ. It's air-cooled; no fans. I'm using Motorola MTP7N20E MOSFETs for both the outputs and the current sources. Resistors vary. I've got two of the big Dale 8 ohm 1% 250W load resistors that I typically use for testing purposes. Here, they've been pressed into service as part of the circuit itself. The 1 ohm resistors are either NTE 5W ceramics or some 1 ohm Dale wirewounds. The power supply is primitive at present. Just a simple transformer/rectifier/cap job that I threw together. Nothing fancy. Yes, there's some ripple. I intend to toss some more caps (and whatever else I can find in the junk box) in there to smooth out the ripple. There are only so many hours in a day. I'll get it done eventually.
--The gain of the SOZ circuit is low. The stock version I whipped together had about 14dB gain (single-ended). The current source version is even lower, a little over 6 dB (also single-ended). If you intend to drive a SOZ directly, you'll need some voltage swing from your preamp.
--The amp is small...on the order of 2 or 3 watts output. It has only one channel. This keeps it from taking over my workbench. It also reduces finance problems. I'll consider building a big one if I like the results.
--I have several things planned after getting the kinks out of the amp as it stands now. The first one is to mate Nelson's Balanced Line Stage (aka Bride of Son of Zen/BOSOZ) to the SOZ in order to get some gain into the poor thing. It will also allow for better matching with a single-ended source, functioning as a phase-splitter. I also plan to parallel two SOZ stages (cutting back on current in each in order to lighten the load on the devices). This will require more matched devices, but assuming that you've got enough on hand, there's no reason you couldn't keep paralleling them to reach any arbitrary power level. What this does to your electric bill is your problem. I've got a few other ideas in mind, too, but this will be enough to use up more free time than I can reasonably expect to have over the next month or two.
--No, this is not the X/Aleph hybrid circuit I proposed in Petter's X thread. That's a separate project that I will approach later.
Grey
[Edited by Jason on 09-27-2001 at 01:26 AM]
