The Aleph-X

[sonnya]

requires a bit of work but i can help you with SWCAD III get the models up and running.

Sonny

[Ian Macmillan]

I've briefly tried the above but wouldn't go as far as saying that I anything like evaluated it. I admit I didn't get along with it too well but then I didn't persevere either. Perhaps I'll give it another try sometime.

Sonny, thanks for the offer of help. I'll keep it in mind for when I have time to try SWCAD again.

I think we are getting a little off topic here - perhaps someone would like to start a new thread on Spice simulators? Come to that I seem to recall having seen something like this before on another of the forums.

Ian.

[Ian Macmillan]

Harry, just made some measurments of the above that seem to confirm your theory about the JFET front end reducing open loop gain. Using constant current sources (as opposed to Aleph) for the output stage, I measured the output impedance of the stock circuit (IRF9610 front end) as approx 0.4R. Substituting J109 increases this to around 2R. This always assuming I did my measuring right! Presumably distortion will also rise though I am unable to verify this by simulation.

Ian.

[HarryHaller]

Referenced to each drain resistor I get a voltage gain of 7 with the Jfets for 784 ohms load and 5 mA in each fet. For the mosfet pair with 392 ohm drain resistors I a gain of about 20. Not really theory but more based on experience. Nelson makes this amp design stuff look a little easier that it really is. I suspect that the mosfet front end for this amp was a well thought out choice on his part.

H.H.

[Ian Macmillan]

Harry, thanks for the information regarding voltage gain of JFET vs MOSTFET. Interesting - I had not appreciated the magnitude of the difference (haven't really thought about it to tell the truth).

I note you haven't passed comment on my previous question (see quote below). Is there some reason? Perhaps it has already been covered in another thread?

If I understand you correctly, you are using the same bias scheme for the front end tail current source as you posted for DC offset null purposes, i.e. same scheme – different application. Am I correct in assuming your primary focus here is DC (static) stability in the face of temperature and/or supply voltage variations? I ask since the obvious alternative is the typical two transistor/MOSFET technique in a feedback scheme. This would of course also affect the dynamic performance of the current source. Do you see the latter as a problem or is there another reason to prefer the single device current source?

Ian.

[HarryHaller]

"Am I correct in assuming your primary focus here is DC (static) stability in the face of temperature and/or supply voltage variations?"

Yes

Do you see the latter as a problem or is there another reason to prefer the single device current source?

No but the mosfet CCS with a solid voltage reference looked fine to me.

I believe the Aleph P 1.7 preamp uses a current source like what you have in mind . I guess one could compare this in simlation. I don't know how quiet or stable the supply for the collector resistor would need to be for the same performance, but I will include it as an option for the curious.

http://www.passlabs.com/pdf/aleph/apserv17.pdf

H.H.

[Ian Macmillan]

Thanks Harry. As usual you are correct about what I had in mind. I was really just wondering whether you had any opinion on the relative advantages and disadvantages of the two types (and I do appreciate that there are a great many other variations too).

Ian.

[HarryHaller]

I would probably prefer a mosfet CS with a fixed voltage reference
since it would have greater source degeneration and be free of the bipolar transistor and feedback loop. I think simpler circuits are better, even for current sources.

H.H.

[Ian Macmillan]

Just thought some of you might like to know that I did receive a prize from Nelson, just as promised. Thank you Mr. Pass!

Ian.

[grataku]

Nobody was doubting that you were going to get something.
Everybody want to know what you got!

[Evaas]

out with it!

[paulb]

Ian's fast asleep by now but hopefully will have responded by our morning.
Ian? Ian? Wake up, man!

[Ian Macmillan]

Oh, alright - I didn't think it would take long before folks started asking...

Well I am now awake and can report that I received a signed pin-up of Nelson Pass in his prime, standing next to some sort of edifice. The latter would be some sort of curious prototype panel type speaker without a diagphram (ion discharge) from the days of Threshold. I doubt somehow that it ever saw the light of day as a commercial product but then again, I could easily be wrong.

So, an interesting and collectable prize if not quite what I was expecting. To tell the truth I didn't know what to expect, I half wondered if it wouldn't be two resistors of precisely the right value for an XA.

Ian.

[cp642]

Ian... that sounds interesting..Now I believe that I've seen that pin-up before somewhere in the forum...but of course...minus the moniker....

[Samuel Jayaraj]

O.K. Has someone finished and listened to their Aleph-X as yet? Could someone post the final schematic of the Aleph-X with the Servo/DC offset correction circuit as finally accepted in this forum? Many might begin to try their own versions; I for one definitely. Thanks.

[Nelson Pass]

I finished mine, and it sounds great!

[sonnya]

Nelson :

Nothing better when it is inhouse parts!???

Sonny

[HarryHaller]

What, no pictures?

[Nelson Pass]

Sorry, Joe took them home and won't bring them back.

The very first proof-of-concept pieces were slightly rewired
Aleph 2's, and subsequently I built a pair from Aleph 60's.
They all worked just Jake. Currently I'm waiting production
PC boards, expected at the end of the month. When I
get them, I'll post a photo.

[gromanswe]

Hello Everybody and Nelson in particular!

I like Elliot with his projects, ESP, Elliot Sound Projects.
Although not the absolute HIGH END,
there are many things to learn and discover there,
as well as useful circuits in different areas of audio.
http://sound.westhost.com/
I have also enjoyed reading your, Nelson,
article no DIY OP-amplifier.
I read it with great interest.

Now when I first saw the name Death of Zener
I was surprised. That would for me be to cross the line,
to say that Another human beeing's work
is deserving to die.
Your amplifiers might have their drawbacks, but on the other hand
they have other qualities that are valuable.

I assume this was a provocative way of Elliot,
a trick to draw attention to what he has to offer.

Now my idea. Why not name some of your constructions

"Death of DOZ"!!!!!!

Would give us the smile of the day!!!!

gromamswe
not dead serious about audio
but it is a nice interest to give us new friends

[Nelson Pass]

I was tempted to rename Penultimate Zen as
"Resurrection of Zen", but I resisted.

It is bad enough that I named the Zen amplifier as a
tease to a reader who wrote into Audio Amateur
referring to an idea of mine as "goofy" and referring
to me sarcastically as "Zen Master Pass".

[FBJ]

Here I thought the ZEN amps were named after " Zen and the Art of Motorcycle Maintenance".
But (I think) since the ZEN articles in TAA, I have seen Zen tube amps, Zen cables, Zen capacitors, Zen speakers, and Zen headphone amps. What's next, An audio company name Zen?

[Nelson Pass]

Of course the other justification is the koan,
"What is the sound of one transistor clapping?"

[trigon]

Then Aleph must be this

Aleph 4

Yuuhuuu.

Trigon.

[rtirion]

Yesterday, I finished the first channel of my Aleph-X. Started testing and listening to this channel today. I am stunned.
I used the Grey's circuit as a starting point. Struggeld for weeks with this and the simulator. Did some very small modifications. Included Ian's resistors. DC stability is perfect. Details will follow soon.

Grey, Ian and of course Nelson, thanks again for sharing this.

And this is what the real prototype looks like. (Sorry for the quality of the picture).

[rtirion]

I am feeding this miracle with this:

[Peter Daniel]

So when is the 2nd channel coming and do you have an Aleph to compare it to?

[rtirion]

Second channel will be ready in two or three weeks.
Yes, I have an ALEPH 3. So I will be able to compare.

[Ian Macmillan]

Look forward to hearing the results of the comparison.

Ian.

[Nelson Pass]

MuuHaHaHaHa.....

[Peter Daniel]

I'm little surprised that nobody (besides Mr. Pass, of course) came out with working prototype yet.
If it wasn't for Aleph5 and CD project I would've been done by now. BTW, can it operate with a single ended source and what are the consequences?

[Nelson Pass]

For best operation with single-ended source, the input
diff pair must see high impedance on their Source pins,
which you achieve by active current sourcing them and
not having resistors wandering off to ground and such.

[Peter Daniel]

Originally posted by Nelson Pass
...and not having resistors wandering off to ground and such.

Does this also include DC offset trim resistors from sources to output?

[Ian Macmillan]

Not a problem (IMHO).

Ian.

[SteveG]

Did anyone ever determine what the best value is for the resistors from output to the node at the current source drain/differential pair sources connection? I somehow can't seem to find it by going back through the thread (could be that it's 23 pages long already!)
Rtirion,
Will you be providing the schematics your design was built with? I would like to see what actually worked, and how it was done.

Steve

[Nelson Pass]

I have used as low as 5K without things going to hell.

[SteveG]

Thanks Nelson

[rtirion]

I tried a couple of value's. 10k was the first and least effective.
Used 4,7k and now 1k which I like best.
Today I did a test with an unbalanced input. Works fine, but I like the balanced input better.
Full schematic will be posted end of this week.

[Peter Daniel]

Did Harry finished his Tex-X? The progress seemed to be pretty good.

[Nelson Pass]

The lower the resistance, the more effective the control
over absolute DC, but then you start depending more
on the common-mode output cancellation to get better
input common mode rejection. I feel it is probably better
to go to higher resistance and use output resistance to
ground to additionally stabilize the absolute DC.

Remember also, the absolute DC offset figure is not
particularly important until it gets so large that it cuts into
the maximum power rating, so a volt or so won't hurt
anything if your inputs are capacitively coupled.

I recommend capacitive input coupling for this circuit
unless you can be assured of equal DC impedance
sourcing both inputs, otherwise you get thumps and
higher differential DC offset.

Of course, I am happy to sit there all night with my
voltmeter and screwdriver, adjusting the offset between
tunes just to avoid a capacitor.

[Coulomb]

fanatic!!!

[Ian Macmillan]

Nelson, or anyone else for that matter, care to elaborate on why nomadic ground resistors degrade performance with SE sources?Are we talking about resistors from Sources to ground here (an obvious no, no) or others, e.g. from gates of input devices? I'm not clear why the latter should have any significant effect given that they are effectively connected to a virtual earth in this application.

Ian.

[Nelson Pass]

When you put resistance off the Sources of the
diff pair I recall that you start drawing the
cross-coupled error off to ground and you reduce
the effect, but I seem to recall that this does not
particularly apply to resistance going to the
output nodes, probably because, given the right
values, this error is also present there, and the
differential error current doesn't flow.

In any case, this is not a big effect. Originally the
resistance to ground was included to help stabilize
the circuit back when it had more gain stages.

[rtirion]

So, here is the schematic for the Aleph-X that I build.
Given the recent posts, I will probably give the 4,7k value
for the famous resistors another try.

[rljones]

rtirion,

Thanks for the post. Your schematic looks really good. (And thank-you GRollins and Mr. Pass). I look forward to building one up soon.

I have one question regarding bias. If I wish to run more class AB than A, what is the best method of adjusting this schematic to have the unit run cooler (I was thinking of having +/-25V rails)? I understand from previous posts, etc., that increasing R3,R10,R25,R29 from 0.22 to 0.39 will reduce current and the amp will run cooler, but this is at the expense of reduced power output. Can you maintain same power and run less into class A?

Thanks, Robert

[AudioFreak]

Aleph's cannot run Class AB as such because they are single ended. However you can increase the current source AC gain above the standard 50% which has a similar effect but will also effect the sound. No free lunch! In the schematic 2 posts prior, you decrease the values of R9 and R20 until you get what you want.

[The Saint]

Hi everyone

Great job on the schematic, But I can't help seeing that it has quite a lot of elements from the Aleph 3, 4, 5 designs.
In the patents for the Aleph X, it shows a folded cascode driver stage.
I don't see one here....
I can see the cross coupling as with the orginal patent, so I guess in essence this design would function pretty much the same. Anyway I was just curious as to why the folded cascode was not used in this design.

Great Job!

Anthony Holton
www.aussieamplifiers.com

[Nicwix]

rtirion

I see your diagram shows 12 volt rails - some questions:

1. What bias current are you using (per channel)?

2. What PSU design are you using?

3. What is the voltage and power ratings of your transformer(s)

4. What actual voltage to you get out under load?

5. What power output do you get into 4 and 8 ohm loads?

Thanks (in anticipation).

Nix

[rtirion]

At the moment I use a test supply with a variac. You can't live without one of these variac's when experimenting with these little amplifiers.


So I adjust the variac for 12 Volt per rail under load and that's about it.
The Aleph-X pulls aprox. 4,5 Amps out of this supply when idle.
Rating of the transformer is 500va.
I have not yet measured the outut power of the amp. Will do so in the coming weeks.

I am working on a solution for the final supply. This solves the problem of low voltage, high va rating. (try finding a tranformer of let's say 500va with 9 or 10v secondaries.)

[Nicwix]

rtirion

I can buy 250VA transformers with 2 x 9V secondaries for a good price. Thinking of using one transformer per channel with a pi filter with 50,000uF per rail. Seems like that would do fine.

With your 12 V rails, what power output do you get into 4 and 8 ohm loads?

BTW how did you decide on your output devices (IRFP150N)?


Nix