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protos 14th September 2005 03:43 PM

OK this info might get lost in this big thread but it belongs mainly here.Moderators might choose to make it a separate thread if they think it deserves it.
Discussions about the sonic signature of the F1 and F2 (ie preferences for second or third harmonic) made by NP and others as well as recent comments on subjective effects of varying the balance of the push pull pair on the recent PLH suggest that it is possible to design or even ``tune`` an amplifier to have a certain sound more to ones preferences mainly by varying its operating parameters so that its distortion characteristics are altered(theoretical explanation).
Also some others such as Eduardo da Lima of Audiopax(with whom I have had a few discussions) have expounded the very interesting theory- to put it simply- that the aim for very low distortion amps is not the correct path since the speakers are contributing huge amounts of THD to the signal.He claims that the amplifier can be made so that its own THD can cancel some of the (mainly second) THD of the speakers.He also claims that one of the reasons that single ended amps can sound so good on some speakers is that there is some cancellation going on and NOT because we really like to hear huge amounts of the second harmonic.This is the basis of the so called timbre lock patent in his designs- basically a method of altering bias of the output stage and thereby tuning the amp to better fit particular speakers.I have seen this in practice in his factoryand I can attest to the effectiveness of such a system.
So much for the theory.
Now in practice what does that mean for the aleph x?
I have what I think is a pretty good valve dual mono SET amp based on the JElabs circuit but with premium components such as Plitron trafos,oil filled caps only in PS,KR842 VHD output tubes 18 W output,RCA NOS 5692 driver, NOS GZ 37 rectifiers etc etc.Now this amp really excells in the midrange which is where most of the music is anyway.I could never get the AX to sound quite like that in the mids despite the overall great sound it has.It would sound a bit too ``clean`` or slightly lean compared to the very good SET midrange.
But now I have almost managed to address this after a lot of experimentation.
The trick is to slightly unbalance the bias of the two halves of the AX.Let´s say you measure 500mv over the source resistor of the positive half.Try adjusting the negative side bias by about 20 to 40 mv less.
That´s it - now sit back and see how that affects the sound.I know I like it a lot and it has come amazingly close to the sound of my SET.
Some technical purists will say we are losing the benefits of the SUSY etc etc but that´s why I put in the theoretical section on top.They can chew on that for a while.Meanwhile the others can just try it and see if it works for them and if so can enjoy music a little bit more while the techs work up spice models to try and explain what is happening.

wuffwaff 14th September 2005 07:35 PM

Hi Protos,

what do you mean with negative half? The other side of the bridged amp or the lower (as in drawing) half of the amp wich is connected to the negative rail?


Nelson Pass 14th September 2005 08:52 PM

We spent a lot of time tweaking each Aleph with different
ratios of gain for the positive and negative, much as you
can for the PLH. In fact, there's no reason you can't put a similar
knob on the Aleph.

Referring to the Aleph 3 service manual, the ratio is largely
controlled by R115. You can replace its 1Kohm value with
about a 680 ohm resistor plus a pot ( 1 to 5K maybe) so that
you can adjust from 680 to as high as you like. As the resistance
goes higher, the lower stage starts to own the output. At 680
ohm, the top half will be doing most of the work.

An of course you can do this on an Aleph X.


still4given 15th September 2005 03:13 PM

Hi Protos,

How did you adjust it? Did you use a pot as Nelson suggests?

Thanks, Terry

wuffwaff 15th September 2005 03:25 PM


I´m still not shure what Protos means cause he speaks of (DC?) bias and not ac-current-gain. To change the bias between the positive and negative half you need some absolute offset at the output to divert some current through the resistors from output to ground.

Setting ac-current-gain in an X is not so simple with a pot cause you need to get the two halves the same. So you would need a good stereo pot. I just use resistors placed in ic-feet so they can be changed in seconds.


protos 15th September 2005 04:37 PM

Sorry , to simpify things I meant the side of the amp that supplies the negative speaker output.You could as well tweak the positive side to be less biased according to results.There are many settings and variations.

The bias I am talking about is in fact the dc bias you can tweak with the pots provided in the ax.I have not messed with the ac current gain but that is possible too.However the balanced nature of the ax means that there is very little second harmonic which may be something we might in fact want to cancel speaker distortion.The ac current gain will not do this if you have the same gain on both sides.

I did not see any problems with large dc offsets in my amp after adjustment.

Of course you may need to readjust the cental pot which governs the current of the first stage and with which you zero out dc offset to ground.I don´t worry too much about that if it is not in the volts range because the speakers do not see this but only the relative offset between the two outputs.That too might change but as long as it is not more than 100mv as np accepts I would not worry too much about that either.A few mw will absolutely not damage your speakers unless you are connecting earbud headphones to your power amp.
I don´t see any problem with going ahead and giving it a try - no soldering- no extra parts.
I just thought I would share this info because to my ears it did make a noticeable difference for the better.But your mileage may vary.And as I said before for some people it is counterintuitive they will be biased (good pun?) against it.

Vix 15th September 2005 05:14 PM



Originally posted by Nelson Pass
We spent a lot of time tweaking each Aleph with different
ratios of gain for the positive and negative, much as you
can for the PLH. In fact, there's no reason you can't put a similar
knob on the Aleph.

So there no reason this can’t be also be done on the ZEN V2-4?

I was varying the value of R19 (1.5 K) on ZEN V3. It seems that it is possible to connect a 1 k resistor (instead of R19) in series with 1 k pot, which should give a sufficient range of AC-current-gain?

Now I have a switch and R19 is disconnected most of the time (constant current) but sometimes I find that sound too “soft”. With R19 switched on, sound becomes subjectively more “clear” but with a dose of “hardness” added, which is welcomed for Rock Music…

I am curious to know why Nelson didn’t initially include a pot on Aleph current source (both on Alephs and Zens 2-4, so everybody could “adjust” ac-gain….

Now gotta read, there's a lot of homework on passdiy...



lumanauw 15th September 2005 06:16 PM

Hi, Mr.Pass,

What's the difference if the positive : negative is 2:3 or 3:2?
Why is that you (and JLH) found out that bigger portion is better to put on the lower side?

I asked this because in DSelf book, he said it is the same effect between 2:3 or 3:2, but you certainly have the "on-field" experience :D

Blues 15th September 2005 08:03 PM


From AX ckt by Grey, that would mean trimpot V1 for -out signal. Adjusting this trimmer would mean varying voltage across Source power resistors (R5/R6) and would mean higher or lower bias current. In your case you want lower voltage for lower bias on the neg side.

Nelson Pass 15th September 2005 08:14 PM


Originally posted by Vix
I am curious to know why Nelson didn’t initially include a pot on Aleph current source (both on Alephs and Zens 2-4, so everybody could “adjust” ac-gain….
You don\'t need me to think about putting in a pot....

In point of fact, I learned long ago not to give consumers such
adjustments. The typical response is confusion: "Don\'t YOU
know the best setting?". Also, unscrupulous dealers will set the
knob to disadvantage your product in a demo and sell the
customer something else.

This is less of an issue now that I don\'t care anyway, and the
unscrupulous dealers are selling Home Theater, and a lot of
the competition has gone away. If you guys want to tweak
the sound to your own taste, then have at it.


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