Aleph5. More bias-worse sound.

[masfe]

First of all, I am new in this forum and I would like to say hello to everyone.

Second, as you can see, my english is not too good. So I hope that you will forgive me if I am not able to explain the question clearly.

Recently I have build a pair of aleph 5 monoblocks with 1,5 amp bias (0,5 each mosfet). The sound with this level of bias is very good: open, very detailed, airy, maybe with a little excessive presence of highs and a little poor bass, but in general amazing.

But according wiht the experience of most of other owners of alephs and the affirmations of Mr. Pass, with more bias the sound of the amplifier is better. So I have changed R21 for a pot and raised the bias to 0,66 Amp each mosfet (my heatsinks doesn't admit much more). And that is the problem. The sound now is worse (in my opinion): More bass and less highs, but goodbye to details, more compressed, confused. It seem that the sound is inside the speakers.

The monoblocs are conected to an Audiolab 8000S working as preamp (no banlanced outputs) and to a pair of B&W CDM7.

So I would like to know if everyone else have the same experience or if this is a problem of my alephs or of my configuration or what...

Thank you.

 

[piewie]

I think you should replace R19 by a pot and not R21 if you only want to increase bias.
I left R19 out to get maximum bias current. I think R21 had something to do with current gain or so.

 

[masfe]

number error

I have make a mistake with the number of the resistor. I wanted to say R19. You're right.

Thank you.

 

[tomchaoda]

Refer to this thread. It might give you some hints:

http://www.diyaudio.com/forums/showthread.php?threadid=28552&highlight=

Thomas

 

[masfe]

What I want to say is that the changes in the sound increasing the bias of my alephs not only afect the ratio between highs and lows, but all the nature of the sound.

Maybe I have a problem with the amps.

Or maybe, it got to do with other parameters. Maybe there is a level of bias that match with a type of speaker or a type of room or a type of preamp and other bias doesn't match.

masfe

 

[stefanobilliani]

What I want to say is that the changes in the sound increasing the bias of my alephs not only afect the ratio between highs and lows, but all the nature of the sound.

Maybe I have a problem with the amps.

Or maybe, it got to do with other parameters. Maybe there is a level of bias that match with a type of speaker or a type of room or a type of preamp and other bias doesn't match.

masfe

Reading your description about the changes in sound increasing the bias I tend to think to the transformers and/or passive filtering . If the transformers power rating are not adeguate to the bias changing , the results in audio compressions and spectral unbalance are, from my point of view and experience ,quite audible as you report .


For the matter try download and read the article
Power Supplies by Nelson Pass .

 

[masfe]

Each monoblock has a 300VA, 25 V toroid and six 22.000 uf caps, bypassed with 0,47 uF polyester cap (no inductor).

I have removed the pot and placed again R19. Now the sound is great, as before, and there will be less heat.

I have readed the article about power supplies: very interesting. Maybe, as you suggest, passive filtering is not good enough.

thank you.

 

[macka]

You may wish to measure the Aleph current share of the ccs and negative side. Do a search for details.


Odds on its move of 50/50 % if the sound balance is weird.

macka

 

[jh6you]

msafe

I assume that you had too much current source gain when you tried the 0.66A. I would try the 0.66A with R19 of about 1K-1.2K.

JH

 

[masfe]

Really, I must tell you that my Knowledge in electronics is very poor. Recently I have understood how a transistor works. I'm still learnig, but in the last years my brain works slow and slow and I think that in a couple of years it will refuse to work at all.

So, I have built the aleph 5 with a little idea of what I was doing. I know more or less what are ccs and the components invoved. I will investigate the matter and search about it in the forum.

When I raised the bias to 0,66A the pot was setted at 1 Kohm, and with this configuration I didn't like the sound.

For the moment I'm not going to do any more until I learn more and know what I'm doing. The alephs are working and the sound now is very good to me. I only was wondering if the rule: more bias-better sound is correct in any case or if it can depend on other external factors.

I'm very grateful for your kindness and for sharing your knowledge with me.

masfe

 

[Netlist]

Hi masfe
Upper your confidence a bit.
Your English isn’t bad at all and the fact you build Alephs is worth a lot of respect.

R19 being 221k in the original schematic gives you about 580mA of current trough each mosfet.
Leaving R19 out of the circuit raises that current to something like 660mA
Trimming a pot at R19 to 1k lowers that same current to about 0.7mA.
So, the higher the value of R19, the more current and vice versa.
(Sorry JH, this is the opposite of what you say )
Are you sure you wired everything correctly?

Now, these values only affect the DC settings of the output stage.
There’s also something like AC current gain.
To set that gain, read the nice explanation from Nelson here:
http://www.passdiy.com/projects/zenv2-4.htm
The AlephX wiki gives some practical examples too:
http://www.diyaudio.com/wiki/index.php?page=Aleph-X
Look under section 4.7.1.
Keep in mind that the part numbers differ from those in the Aleph5.

To answer your last question: I can’t remember Nelson or anyone else saying that higher bias would degrade the sound. Of course there will be limits and sweet spots.
We aim to drive the mosfets in their most linear region.

Good luck

 

[jh6you]

(Sorry JH, this is the opposite of what you say )

Longtimenosee, Netlist

My simple study tells me that, if I increase R19 value, it decreases % AC current gain, and vise versa. If not, I would like to hear from you. Why not?

Best Regards

JH

PS. In the previous post, I was thinking about the % AC current gain. My personal taste of the sound was when the % AC current was at 30% through 50%.

 

[fcel]

Masfe,

I did some search on this forum and I think the following couple of links might help you to understand the bias adjustment procedure and in general what resistor does what.

http://www.diyaudio.com/forums/showthread.php?s=&threadid=12929&highlight=

http://www.diyaudio.com/forums/showthread.php?postid=159195#post159195

http://www.diyaudio.com/forums/showthread.php?postid=77253#post77253

 

[Netlist]

Longtimenosee, Netlist

I'm here all the time. Good to have a little chat with you.

Hmm..first to be absolutely clear I must say I referred to the Aleph5 schematic at Passlabs.

From my understandings it is R19 that sets the bias of the Aleph current source. We are talking DC here.
From simulation it doesn’t affect the AC current gain at all.
R21 sets the AC current gain. Then your statement: “if I increase R21 value, it decreases % AC current gain, and vise versa” would IMHO be correct.

 

[jh6you]

I MADE a mistake.
I do not know why, but I was looking at R21 as R19. My meaning was increasing R21 of about 450 to about 1-1.2K. Sure, changing of R19 (221k) changes the bias voltage across R40-42.

You are right, Netlist.

JH

 

[Netlist]

This for the many times you were right and I was wrong

 

[masfe]

When I raised the bias to 0,66A the pot was setted at 1 Kohm, and with this configuration I didn't like the sound.

Sorry, I wanted to say 1 Mohm instead 1 Kohm. JH has confused me.

Now you have given a lot of work. Many links to follow and many hours of study. Your interest has encouraged me to understand the insides of my amps. Right now I'm going over the schemes again trying to identify each component and his role in the circuit. Nothing easy for me. I think that I need to place a heatsink in the back of my head to disippate all the heat before it goes up in smoke.

Thanks again

Masfe.

 

[jh6you]

This for the many times you were right and I was wrong

No. I forgot writing "always" between " You are" and "right."

JH

 

[Luke]

What actually does current gain mean in regards to an amplifier and what effect does varying it have?

 

[masfe]

What actually does current gain mean in regards to an amplifier and what effect does varying it have?

I think that you must read this:

http://www.passdiy.com/projects/zenv2-4.htm

I've done it and now I´m in the right way to understand how aleph's ccs works.

I'll try to explain what I know till now.

It seems to be that the aleph's constant current source not only set the dc bias of the gain stage (with a constant dc current that in the case of aleph 5 is setted around 0,5 amps each mosfet) but also provides a variable amonut of current (AC current) to the output of the gainstages acording to its changes (that, in turn and as you now, are function of the changes of the input signal) . This way, the constant current source helps to increase the efficiency of the amplifier by adding more (an variable) current to the output (W=V*I or Watts= Volts*Amps) without increasing disipation or hardly increasing it.

The total amount of AC current that the CCS will provide can be setted at certaind level, expressed as a percentage (by the moment I don't know exactly what means this percentage) and if it can be setted then also can be changed. So, if you set the percentage to 0%, removing the parts associated, then you only have a tradicional ccs and the efficiency goes down. Surely this also afects other parameters such distortion. It seems to be that the normal level is 50%, but if you search in the forum you will find some people who has setted it at 60% or more. This is a cuestion of taste. Somebody prefers 0% and somebody more than 50%. I think that the effciency of your speakers is a question to bear in mind.

I hope that my little knwolegde about the matter has helped you. Perhaps someboy with more experience and more wisdom can explain this question better than me and correct the mistakes that, surely, I have made.

masfe