Tune your Aleph-x to sound like a good SET amp

[Nelson Pass]

Originally posted by lumanauw
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

Since we are discussing DSelf, I will set aside subjective
observation. It was JLH's and my experience that they
measure differently in the JLH type circuit, therefore it is
not the same effect. They might be the same in some other
circuit.

[wuffwaff]

Hi Protos,

so you have a 5-10% difference in bias between the two halves. I will try this in the near future (at the moment im still trying different ac-current-gain settings) and see what happens.

Do you have any explanation why setting the bias this way changes the sound?
Did you reduce the bias in one half or did you raise the bias in the other?
What is your ac-current-gain setting at the moment?

William

[protos]

Wuff waff ,
Have a look at this very interesting article


http://www.milbert.com/articles/DanCheever.pdf?milses

very long but well documented and researched work on psychoacoustics, thd and amp measurement

before that go to www.audiopax.com website and look at the white paper there which is concise and easy to read.

you may also want to look at various famous articles by Jean Hiraga of course which are in the same general vein

Now I don´t have a spectrum analyzer to see how changing the bias changes the distortion spectrum of the amp but I think there is plenty of documentation above to suggest that the type of this spectrum is more important than getting extremely low overall distortion.This may be either because of distortion cancelling at the speaker or because the spectrum better fits the psychoacoustic natural thd of the ear.
Of course theory is important to give you some type of compass bearing on your journey to better sound but in the end does it really matter what some theory says as long as you find something that satisfies your ears.
I also try to listen with my ´´body´´ because the ears per se can deceive when you are trying too hard to listen for something.After some time with an amp let´s say , I try and see how my body is reacting to the music.Is it grooving to the rhythm?Is it a little bit too tense?Am I swaying my head?Is my breathing relaxed?
These things can tell you a lot more if you are going to like a particular component than trying to decide if the cymbals sound more or less realistic or if the bass is too boomy because it is in the recording or because the amp is making it so etc etc.

My ac current gain is over 50% but I don´t remember now exactly how much.I have 4oohm speakers.

[wuffwaff]

Protos,

I just read the 87page article by Dan Cheever. Very interesting! The audiopax one will have to wait cause my eyes are a bit tired now.......

If your ac-current-gain is above 50% I would suggest lowering it a bit to around 47-50%. I think you will notice another step towards your SET amp.

William

[macka]

Interesting observations.

Ages ago I fiddled with the dc bias of the Aleph and also the current gain.

Its like a Red tasting ...what ever you like on the day.

However, siginificant changes in Dc bias will also effect the actual gain and you may need to revise the stability capacitor particularly on the Cs.

In general I found the Passlabs settings agreed with my taste Buds the most. I would suggest Nelson has a nose for a good red based on practical experience!

Heck that was fun..I recall we started off with the Zen...not sure what version, a delightful little amp (his daughters) and on an equally delightful diy speaker and it went on from there to the wee hours.

We should all do this more often.

Ian

[macka]

I am not sure about asymmetrical Dc bias of the X Aleph...kind of defeats the point of the design.

What maybe of interest is that I found that using a stock BOSOZ driving the X Aleph was not as appealing as the X-BOSOZ on the X Aleph.

The X Aleph really made the BOSOZ sound cloudy or woolly. So I figure that its true that certain types of distortions either compliment each other or are perhaps not complimentary..

From what I can gather the BOSOZ is dominated by 2nd harmonic distortion while the X Aleph is possibly dominated by 3rd harmonic distortion. Perhaps the Master can clarify this.

But I have found the X BOSOZ works well driving an Aleph and the stock BOSOZ even more so.

So there we are.....but its all fun.

Ian

[grataku]

Protos can maybe clarify a little more because I am not sure about that either. There seem to be two camps here talking about two different things: left and right balance for AX and up and down balance for the Aleph. It has been awhile since I played with the AX but I seem to recall it was all about balancing the two halves to get the 0 VDC at the output. I think in order to bring the DC back to 0 by changing the left and right balance bias one would need to adjust the what Nelson is talking about upper half vs lower half balance to move the DC point of the output. This is for basic operation alone. Or am I totally off base here as usual?

I wouldn't expect different sound from just having different bias in the two halves. Under normal current load the differential would still do it's job of canceling the second harmonic. Any differential circuit will cancel out the second harmonic whether it is an X or not X, I think.

Is there a "finer" circuit that cancels the bulk third harmonic besides the brute force of global NFB? Even the modulated cascode is about getting rid of second harmonic.

Any distortion canceling circuit including NFB, local feedback, longtail pair or whatever, is never going to be perfect, it's always going to leave or to generate some higher order distortion components that don't sound so good.

[wuffwaff]

Grataku,

two things were spoken of:

1. DC bias imbalance between the two halves of the amp

2. AC gain imbalance between the upper and lower half of the amp (ac-current-gain setting)

The latter has quite an influence on the sound, the first according to Protos also has an influence although I also can´t think of a reason at the moment.

As for getting near 0V relaitve DC at the output with an X you need well matched input pairs. A bit of imbalance in left/right bias is not so important for this (you never get both halve the same as they keep drifting a bit as the amp warms up and you change the settings for bias and abs. dc offset. I also have up to 40mA of difference between the two halves but never thought of this as critical.

William

[protos]

Originally posted by grataku

I wouldn't expect different sound from just having different bias in the two halves.

Why don´t you try it and see?
There is a huge difference IMHO.
About 10 times greater than those minute differences people hear between resistors,caps etc.

[grataku]

Protos,
I am still not clear about what you did and how you manage the DC.
I said I wouldn't expect different sound I didn't say it's impossible.
Again if you get into more detail I maybe be able to duplicate what you did.

Meanwhile enjoy you newly found sonic bliss.