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Old 2nd February 2018, 10:13 PM   #1
Zen Mod is offline Zen Mod  Serbia
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Default Babelfish XJ , or JX …….. or whatever (Aleph X servo for Greedy Boyz)

Pretty mach pasting a text and just few pictures from my blog post ( still not fully finished) :

It started not by plan , but by chance ….. several (or even more than several) months ago , I tried to help one of our Greedy Boyz at Diyaudio.com/Pass Labs subforum , to improve things with his Aleph X monoblocks.

Problem was simple but not so simple ……. in first incarnation , amp was sounding divine , but it was unreliable and eventually it gave up in flames ……. or at least with some gray smoke puffing out .

As usually ……. ZM at work :
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File Type: jpg ZM-Wile-Genius.jpg (166.5 KB, 626 views)
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Last edited by Zen Mod; 3rd February 2018 at 07:05 PM.
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Old 2nd February 2018, 10:17 PM   #2
Zen Mod is offline Zen Mod  Serbia
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As always with these beasts , culprit was in badly matched output mosfets and generally somewhat lower quality of build execution , at least that's how I understood .

Later , when our friend Robert found another guy willing to make things better (Robert himself being much more versatile in other things than electronics) , amps were repaired/made better but magic was gone - they sounded thinner , lifeless in comparison to starting iteration .

Anyway , first I tried in thread to help Robert with some advices , later we proceeded with e-mail correspondence , made some progress (to my satisfaction , I had opportunity to show how small changes in strategically important positions are much more important than type of wire , Preputium-made of resistors and caps , thus making much more prominent changes sound wise )...... but all our efforts concluded somewhat badly , with amps again producing gray smoke puff ...... ya know - pretty common thing when playing Bad Telephone Chain game ......

Even before that , Robert wrote to me that he is already preparing to make another pair , this time even bigger , of Aleph X ; he's saying that Aleph X is his preferred type of amps , after being in opportunity to hear older and newer Pass Labs amplifiers , including .8 series.

He eventually asked me to make them for him , he providing already bought cases and whatnot .....

My comment was - first - I'm not making knockouts of PL amps (Aleph X was PL commercial model , even if disguised with XA200 title) and second - him living in Poland (EU) , Mighty Moi living in Serbia (non EU) , circulation of parts and later finished amps looks extremely complicated and expensive.

During next few weeks , we cross-checked our watches and came to satisfactory arrangement - I'm going to make Babelfish XJ (JX)- Jazz standard Aleph X iteration with few important twists , so hopefully worth trying and making. We also found least cumbersome solution for transporting goods between us.

Now ..... little history.

Almost 16 years ago , Gray Rolins came to clever idea (alone ? bugging Papa in private mail, as we all do more or less?) to bridge two regular Aleph channels ....... well , read by your self exact story HERE .


As far as I remember (origins), properly visible schematic is this one :
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File Type: jpg axschem.jpg (107.5 KB, 629 views)
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Old 2nd February 2018, 10:20 PM   #3
Zen Mod is offline Zen Mod  Serbia
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Several iterations of amp were made , in various sizes and complexity , several threads …… to my knowledge most comprehensive being this one , also extremely worth mentioning is Eric’s web page , dedicated to his Aleph X incarnation.

There were few major problems why , even if Aleph X was probably most massive project , considering number of pcbs made in Group Buys , there was even tinier number of finished amps …… let’s see:

-Human Nature Problem One -people are having troubles to break one whole in smaller logical modules , so circuit understanding seems unreachable;

-Human Nature Problem Two-ambition – project is scalable …… starting big is much more troublesome than starting small to gain experience and knowledge, then building from that; that way is , in the end , even cheaper …. then the other way (ending without any amp)

-Technical Problem One – too many mosfets , lousy matched;

-Technical Problem Two – not enough Aluminum …… or even – not enough (enameled) Copper



For ones , able to step or jump above (around ) mentioned obstacles, there was ( or they still are) few more down the road …

Amp being bridged , means both output posts floating (being and not being referenced to GND) so we are having two different DC Offsets to take care of;

First one (Boyz calling it Relative Offset, which I never remember) is most important, being DC Offset between Pos and Neg output of amp, exactly where we usually tie our speaker cable ends. Let’s just say that any Aleph X is pretty nicely behaving there, input LTP stage besides serving other important things, also serving as Servo for exactly that ….

Second one (DC Offset) is much more problematic to obtain at right value and to maintain- DC voltage between our floating speaker posts and GND (Boyz calling it Absolute Offset , which I also never remember)……. Simply put – if both outputs are at approx. zero (GND) potential , they can equally swing from zero to rails (positive and negative) thus our output signal is having max. amplitude equal in both directions ,so possible clipping is symmetrical.

That one , and few other problems (maybe smaller , maybe not), read at Eric’s page. I don’t have intention to cover even small part of things already covered by him …. why doing things twice , when you mustn’t.



So , I had intention to try/verify few small ideas I had/borrow/stole during years, applicable to Aleph X circuit, so being enough different and possible to call it Babelfish iteration.

-Trivial 1 – make it with JFet LTP input stage .

-Trivial 2 – cascode that JFet LTP input stage …. not so much because of Herr Mueller , but to keep them cooler, later possible to cover broader range of rail voltages.

-Little Less Trivial 1- put emiter follower in between input LTP and gargantuan bank of output mosfets (later about – why gargantuan).

-Little Less Trivial 2 – cascode that emiter follower ….. just for heck of it

-Whoopsie Daisy 1 – address issue of Absolute offset in simplest possible way

-Whoopsie Daisy 2 – address useful arrangement of easy way to influence THD spectra .

……… Wheel is turning ……. Robert sent cases to me , I was LTSpice-ing like mad , trying to implement all details ; I did tried them one by one in vivo , but never combined.

So , after countless hours of PC time (yes , I’m slow) , here is LTSpice schematic , intended for first prototype step ; Big Bada$$ mosfet banks omitted for clarity , each quadrant of 10 parallel mosfets shown as single one :
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File Type: jpg LTSpice-Babelfish-XJ shrinked.jpg (180.7 KB, 917 views)
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Old 2nd February 2018, 10:21 PM   #4
Zen Mod is offline Zen Mod  Serbia
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Then Ikebana prototype , measurements and some tweaking of values ; it passed , so it was time for pcb work.

Robert insisted on some fancy resistors , in most important positions ,so I made that – placement for both usual metal films or fancy Shinkoh/Kiwame.

Front End Eagle schematic and Front End Eagle pcb screenshot:
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File Type: jpg FE schm.jpg (352.3 KB, 616 views)
File Type: jpg FE pcb sshot.jpg (560.0 KB, 606 views)
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Old 2nd February 2018, 10:24 PM   #5
Zen Mod is offline Zen Mod  Serbia
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Output Stage Eagle schematic and Output Stage Eagle pcb screenshot:
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File Type: jpg OS schm.jpg (340.4 KB, 308 views)
File Type: jpg OS pcb sshot.jpg (324.5 KB, 300 views)
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Old 2nd February 2018, 10:30 PM   #6
Zen Mod is offline Zen Mod  Serbia
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few more lines from blog post :

Servo is doing its job , both relative and absolute offset practically rock steady from very first moment of powering up, same as Iq .

Changes from stone cold to temperature equilibrium are approx. in 5% range, so I didn’t even bother to write exact figures down.

.....

Will post tomorrow more tech data/details , same as explanation of arrangement used for relatively easy THD Spectra shaping ……. probably few more pics of amps with finally closed top lids .

Did I said that they’re Gorgeously sounding ?

===============================================

I'm lazy to post more pics here , so just go to my blog in case that you're having too much time on your hands .......


So , few more sentences about really most important details of entire build will follow ....

In a meantime , just observe red marked positions on schm in post #3
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Old 2nd February 2018, 11:32 PM   #7
mpmarino is offline mpmarino  United States
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Cute DAC


By my math, 10 MOSfets per QUADrant = 40 mosfets per channel.

Congrats on a worthy V/I converter
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Old 2nd February 2018, 11:58 PM   #8
Zen Mod is offline Zen Mod  Serbia
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yup

Mighty Moi practicing small ..... one day to make proper MC stage



hotlink test :

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Old 3rd February 2018, 12:11 AM   #9
Zen Mod is offline Zen Mod  Serbia
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hotlink test 2 (my blog did have hotlink block , now switched off)

- how to tighten mosfet without twisting on goo

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Old 3rd February 2018, 12:26 AM   #10
6L6 is offline 6L6  United States
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I think I found a photo from testing the thermal load....




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