Aleph vs SonofZen

Hello everybody. I am relatively new to this forum and have noticed that quite a lot of Nelson Pass fans abound here. This query is directed to all you folks.

How does the sound of the Aleph range of amplifiers compare with the Son of Zen?

How do these two amplifiers compare with Andrea Ciufolli's Power Follower 99 amplifier? Someone who has built atleast any two of the above can do an A-B test on the sonics.

Is it not possible to bias the SOZ with a current source similar to the one used in the Aleph? In fact, such a move was proposed by Ian Hickman in Electronics World. The main advantage would be cancellation of even harmonics by the two balanced arms; since the SOZ primarily exhibits second harmonic distortion, this could further be reduced by the use of a current source and we end up having an amplifier perhaps better than an Aleph. Please bear with me if I am wrong. Just an idea. This topology could perhaps, be driver with a line stage designed by Anthony Holton, which is similar to Nelson Pass' design except that it exhibits a current source. The design could be found at

Now let's hear it from you guys.
Son of Zen vs. Aleph


You pose an interesting question. I have heard both amplifiers and they have different strengths and I can't honestly say I would choose one over the other. The Aleph certainly has the advantage in terms of efficiency and the ability to drive low impedence loads. I guess the final decision will depend on the speakers you are trying to drive.

Current sources and active loads will most certainly help the amp and improve efficiency not to mention lowering distortion. The follower would help, but now you are approaching an X-circuit which is a whole different animal. I will admit that the X-amps are, at least to these ears are some best sounding amps ever designed.


PS. Grey are you out there? You might have some thoughts on this subject, having built an Aleph yourself.

[Edited by jam on 06-19-2001 at 09:09 AM]
The Zen is probably cheaper when built using current sources, and you get more power to boot ....

If you use an Aleph type arrangement, I suspect you won't get quite as good sound (that is if you like Zen's).

There is no reason why you can't run the Aleph without an input stage, so it should be easy to hack the service manuals open and rip out the input stage.

I have both SOZ (8 watts) and A-75. I like my SOZ a bit more
simply because of it's simplicity and easy to built and it does sound a bit better. I think the 50 watts version ( it's heavy and expensive ) should sound great. I have to build the Ariel 6 speaker ( 93db ) to get the good out of 8 watts. I am planning to build the Aleph 2 ( 60 ) this fall, may be I can tell you more by then.
Vince (vdi_nenna) and I went at this hammer-and-tongs for a while. In his case, the current source was to be the basic one from the original Zen amp, not the driven one from the Aleph. I'm not sure what his status is on that project, as most of his postings recently have been about video. Go dredge around in the Solid State Forum and see if you can find the SOZ stuff.
What you're describing sounds more like a bridged Aleph pair, rather than a SOZ, as the Aleph current source is a non-trivial little critter that has other functions besides just being a strict current source. I'm trying desperately to find another few hours in my days so as to be able to build a second pair of Aleph 2s so as to investigate some questions I have about imaging. It would also be easy to bridge them. (Already have the parts on hand, just need to etch & populate the boards.) That would put in me a good position to answer questions about sound, but for some reason UPS & FedEx don't seem to deliver hours to my house. I suppose it's against regulations.
A current source would make the circuit more efficient, for sure. It arguably would not sound as good. I, for one, and I think there are others here who agree, have found that current sources do not sound as good as resistors. That said, there are times when the 'infinite resistance' of a current source is a needful thing. Given that the SOZ amp is such a beastly heat producer, any increase in efficiency would be welcome. How much it would harm the sound is open to debate. It will also likely change the gain and frequency response.
Just to rattle your cage, let me toss another idea out (one I've advanced before). Since the Aleph topology is primarily a back end concept, and the X topology (a crosswired differential, not unlike the SOZ) is a front end topology, why not combine them? I'm doing a bit of preliminary head scratching towards putting something like this together. I've also begun to accumulate parts. If/when UPS delivers some extra hours, I'll try to make a little progress along those lines.

The responses have been only too quick than one can imagine showing the keen interest that so many share in the interest of musicality and purity of sound. This is indeed commendable.

There are quite a few suggestions. So let's take them one by one.

Jam having heard both amps, says that they have different strengths. Asking him to spell out these differences is perhaps, asking for too much, since we are here talking about two 'greats'.

Petter makes a point about efficiency and sound quality, both being affected in different ways when using current sources. His idea of doing away with the input stage of the Aleph is indeed a great idea. So this should be one line of thought that we may follow. Continuing on the same note, I suspect that the current source in the input stage of the Aleph would be noisy due to the use of a zener. Hence, the first idea would be to improve this current source and make it less noisier. I can assure that this will certainly improve the sonics as some of the noise that adds up with the distortion to modulate the final music, will be removed making the amplifier more resolving of detail. The second idea would be to do away with the input stage all together, retain the second stage of the Aleph and drive two Alephs in bridged mode using the Balanced Line Stage with 40mA current sources as suggested by Mr.Pass himself. What we end up with is a kind of more efficient version of the Son of Zen - no feedback, lower distortion, higher damping factor - a fully scalable amp which should be a lot cheaper to build without compromising on the sonic quality.

The above two ideas just evolved as I pondered upon the responses thus far. However, I started off with the idea proposed by Ian Hickman in the letters column of ELECTRONICS WORLD, August 1999 issue. I have no idea as to how to post the schematic on the web, so that those who do not have access to the original schematics can actually have a look in. He writes about the semiconductor equivalent of the White Cathode Follower which was popular with thermionic valve circuitry. The following is a reproduction of the original write-up. "This arrangement (White Cathode Follower) was popular in the days of valves. It offers increased drive and lower output impedance compared with a simple cathode follower with a resistor as the 'long tail'...... This consists of modulating the drain current of the lower FET, in the direction of cutting off when the input goes positive, and vice versa. Thus on a positive output swing, the tail current reduces, the extra current being available for the load. On the negative swings, when the source follower cuts off, the tail current increases. The necessary gate drive voltage can be obtained from a small resistor in the drain lead of the source follower, AC coupled down to the erstwhile constant current tail FET." Obviously this would make sense when the schematic is right before your eyes.

In summing, we have three approaches: (1)improving the current source of the input stage of the Aleph; (2)remove the input stage of the Aleph, replace with improved Balanced Line Stage, bridge drive and virtually we have an improved Son of Zen; and (3)modify the Son of Zen for more efficiency as proposed by Ian Hickman. I have listed them out, so that we all know what the other is talking about.

I don't have any idea about the topology of the X-super symmetry design, so no comments on this.

Let's get this to really something wonderful.
The Alephœ current source of the input differential could be noisy. Using Circuitmaker I noticed that the voltage flows with difference about (10)-2 mA - too much on not? So I added the capacitor to the Zener which lowers the noise and fluctuation of the voltage. There are better solutions but more difficult to do. The sonic impact I don´t know so far.
I use current mirror instead of simple resistor on the long tail - for experiments is everything on the board - resistor and mirror. I can only choose with linking the pins. Mr. Pass reported not to be excited with current mirror. Again - I will be more experienced within the month.
Aleph vs. Zen


You did not make it clear what speakers you are trying to drive. If you are using higher efficiency speakers 90bB+.I would choose the Son of Zen but on the other hand if I was driving a low impedence load which was relatively inefficient I would strongly recomend the Aleph.

Opinions are only valid if we list the parameters or conditions that evaluation is given. For example single -ended triodes sound great on fairly efficient speakers but if you were to evaulate one on a ribbon speaker your opinion would be quite different.

On efficient speakers the son of Zen has a purity that is hard to beat but the Aleph has a more controlled bottom(possibly due to feedback) and a slightly more extended high end.

I hope this has been of some help.

Current Source


Zener noise can be reduced by placing a capacitor across the zener as you say.
Another other idea is to feed the zener with another current source which I believe also improves the current source's rejection of power supply induced noise by about another 20dB.

Another suggestion is to replace the zener with a string of LED's and scale the resistor for the correct current (20ma for the Aleph differential).

The current mirror might work in the current source (tail) of the differential, but definately hurts the sound when used as drains loads, despite lower distortion numbers, just as Nelson Pass said it would.
This is another case where better numbers do not mean better sound.


I am using two 5 1/4" drivers along with a horn tweeter in a D'Appolito (MTM) configuration. Its a kind of bookshelf design, built very well with integral sand-filled MDF stands. Their sensitivity might be about 87db or so. They detail very well. I have compared these home made boxes with B&W601, CDM1 and these matched the B&W CDM1 in almost every department. I use a subwoofer to extend the LF.

Notwithstanding the above, my query was in the interest of coming up with a design, fairly efficient (by Class A standards) but sounding excellent, not only subjectively but to some extent by figures too. Get the music as pure as you can, adding as less as possible. That is the motto. I have sent a query to Mr.Pass in this regard, ie., a more efficient Son Of Zen kind of thing. We may wait with great anticipation. Are there any conclusive ideas on this thread as yet?

You are really making things hard on me, but if you are going to use a subwoofer my choice would be the Son of Zen.
I would use current sourceing to improve the efficiency.

This is just my opinion but Nelson Pass could give us a more informed opinion. All said an done we are just trying to build a better mousetrap, he invented it.

Basically, you've got two options if you're trying to increase the efficiency of a class A amp--use current sources, or use Nelson's Aleph circuit. It's the nature of the beast to be inefficient.
If your speakers are in the upper 80's as far as efficiency goes, you're likely to need at least 50W or so to achieve reasonable listing volumes, and that's assuming that you don't like to listen particularly loud.
My proposal about using an X front end to drive Aleph back ends would enable you to have a more efficient differential class A amp, as the X (like the SOZ) is a differential circuit, and the use of the Aleph topology for the rear would bring the efficiency up. Nelson's patent on the X topology is online at Read might give you some ideas.
Speaking as a fellow who has a pair of (DIY) Aleph 2s in his listening room, I don't think you're going to achieve much sonically by trying to to reduce the noise in the current source. Yes, Zeners are noisy in theory. In practice, it's not that much of a problem. By all means, tilt at windmills if you must, but I think you'll find that the time and effort might be better spent elsewhere.
If you want a straightforward schematic that you can use as a cookbook to build a class A amp from, I agree with Jam, go for the SOZ, as it's scalable and you are likely to need the power.

Thanks everybody for your varied inputs. As Jam rightly pointed out, we are all trying to make a better mousetrap out of Mr.Pass' invention. The whole idea was to design a more efficient SOZ with a higher damping factor, PSRR etc., without audible sonic penalties.

Well, the good news is that I just received a reply from Mr.Pass with regard to a more efficient SOZ as follows: "One of these days, certainly."

Hence, we may all wait with abated breath for the Master Designer's unveiling.

Thanks again everybody.

However, GRollins proposal for an X front end to drive Aleph back ends could still be pursued by the forum, if it generates enough interest from talented people.