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-   -   High power ALEPH J (6 output devices); any attempts yet? (http://www.diyaudio.com/forums/pass-labs/130542-high-power-aleph-j-6-output-devices-any-attempts-yet.html)

elac310 28th September 2008 10:45 PM

High power ALEPH J (6 output devices); any attempts yet?
 
I've seen the Aleph J schematic posted by Nelson Pass at http://www.diyaudio.com/forums/showt...197&highlight=

Has anybody tried to use it with 6 output mosfets and runing it at + and - 47 V (like an aleph2), instead of 2 mosfets and +/-24V? I have started to buy parts for an Aleph2, and found the AlephJ circuit even more attractive (less components, no limiting zeners in the input stage, higher input impedance etc.). I now intend to switch to a J instead.

Apart from increasing the OPS biasing resistors from 0,47 to 1ohm and adding a couple of 0,47 ohms power resistors in the output line, what would need to be done ? I presume R14 would need to be increased to 150ohms and some adjustments made to the active current regulator built around Q4 (decreasing the value of R27 to 56ohms??).

Many thanks for feedback and advice.

Zen Mod 28th September 2008 10:55 PM

besides that - some nice cascodes for input 2SJ109 ( to keep it in safesupply voltage ) ,
resistor in series with input CCS , to keep some dissipation on it , not just on tiny CCS transistor .

anyway - for proper picture of topology and scale - look at any aleph in that power range ; A5 maybe .

metalman 29th September 2008 05:30 AM

A concern I would have is whether the 2SJ109 can be safely biased high enough to provide enough drive current for the input capacitance of 6 power mosfets.

Terry

EUVL 29th September 2008 05:54 AM

There are many threads about people using a JFET frontend for the Aleph-X, which is essentially a doubled-up Aleph-J. So there are enough working examples.

http://www.diyaudio.com/forums/showt...763&highlight=

If you are really worried about MOSFET capacitances, use Fairchild's (FQA19N20C) or Toshiba's (2SK1529).

Also read Nelson's comments on choice of paralleling devices.

http://www.diyaudio.com/forums/showt...040&highlight=



Patrick

Zen Mod 29th September 2008 10:01 PM

2 Attachment(s)
something like this

elac310 30th September 2008 07:21 PM

Many thanks to all for your thoughts.
Zen mod was very kind to post an example of a suitable input circuit. However, what I found attractive in the Aleph J is precisely the simplicity of the input: no adjustments, few components, no zeners etc.. I have difficulties to accept zeners that are there to limit unexpected spikes, but bearing in mind that the music signal is essentially composed of spikes...I know, there is a big margin before the music signal would reach the limit set by the zeners but,but, but...(OK for pro and stage equipment, great if home FI could spare it...)
Also, I'm still uncertain about the necessary adjustments to the active current stage. In the early years when the aleph designs were discussed, some DIYers went so far as to suggest leaving out what would now be R27 in the Aleph J schematic I referred to earlier. Normally, the Aleph 2 with 6 devices had a constant current draw of 3A or so.

To EUVL: you said in another post (http://www.diyaudio.com/forums/showt...10#post1169010) that you've used 12 output Mosfets in total. Can you tell me more about the design? Is it an Aleph J or an X?

Zen Mod 30th September 2008 08:49 PM

zenners are there primary to prevent zaps ............ in other words - spikes possible in moment of plugging signal cables ;

Papa used to use them , in his commercial products ...... as he sez many times - "I don't wanna see those amps again , for repairs "

other moment - pot for adjusting current through CCS , above input LTP - it's convenient for offset adjust , and it's just another way for doing that , comparing to Papa's way ...... he use to just swap one of input mosfets , until offset is small enough .

choose what's more convenient to you ;

hehe - just a thought - if you use 2SJ109 ........ you can't swap half of it .........

:rofl: I didn't thought about that ...... 'till now .......

Luke 30th September 2008 09:23 PM

hi Choky,

would 2 X 2sj109 in parallel here be better?

Zen Mod 30th September 2008 09:48 PM

Quote:

Originally posted by Luke
hi Choky,

would 2 X 2sj109 in parallel here be better?


certainly .......... but only your ears will tell which way you like more with one or two ;

in case of two in parallel , you need to match them

EUVL 30th September 2008 09:50 PM

> Is it an Aleph J or an X?

An Aleph JX.

The frontend is 2SJ109, cascoded by 2x J174's (and not 2SJ174) directly without any gate voltage setting potential dividers (see how Borbely does it with 2SJ103s). The current source for the diff pair is 1x 2SJ74V with a 50R trimpot at the source. I don't think that is complicated. Drain resistors at the J174s are in my case 910R, but 1k will be just fine.

> would 2 X 2sj109 in parallel here be better?

Well it allows you to use the 392R in the diff pair as in the original, but you double the input capacitance. Not that it is a problem, just not necessarily, IMHO.


Patrick


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