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Old 12th April 2018, 11:37 AM   #2651
Eric is offline Eric  United States
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Aleph-X builder's thread.
The original green boards bias the front end at 10mA per device, so the range of J74s that you have is ideal. You can also alter this bias by adjusting R23 & R25.
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Old 12th April 2018, 12:29 PM   #2652
Dennis Hui is offline Dennis Hui  Canada
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Aleph-X builder's thread.
Not sure what rail voltages you're considering but you should keep an eye on
the dissipation of the jfets if you're not cascoding them.
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Old 12th April 2018, 03:14 PM   #2653
Zen Mod is offline Zen Mod  Serbia
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Aleph-X builder's thread.
5mA per JFet in LTP

10mA if using mosfets
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Old 13th April 2018, 08:45 AM   #2654
yoke is offline yoke  Croatia
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Quote:
Originally Posted by Eric View Post
The original green boards bias the front end at 10mA per device, so the range of J74s that you have is ideal. You can also alter this bias by adjusting R23 & R25.
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Originally Posted by Zen Mod View Post
of course

they're biased via CCS , so no problem with broad range of Idss
Thanks
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Old 15th April 2018, 08:22 AM   #2655
Jason Hubbard is offline Jason Hubbard  United Kingdom
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Default Big thank you...

To Zen Mod...

I used the DC servo built from 2 x BC639 and injected the voltage back to the junction R24/R26.

I have not yet change the Macmillan resistors - can now be higher in value.

Absolute DC changed from 2.2v cold to less than 0.2v. Relative DC is 7mv.

Very simple change and for me it was well worth doing.

Thanks again!
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Old 15th April 2018, 08:59 AM   #2656
Zen Mod is offline Zen Mod  Serbia
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Aleph-X builder's thread.
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Old 16th May 2018, 09:19 PM   #2657
Jason Hubbard is offline Jason Hubbard  United Kingdom
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I made a small board (20 x 25mm) that just plugs into the R24 position with a short piece of ribbon cable to pick up the voltage rails and speaker outputs. I swapped out the Macmillan resistors (from 4k7 to 100k) and reduced the LTP bias a little.

Absolute DC offset is now +/- 5mv. I think I’ll get some PCB’s made for this next time I put my next PCB order through, should be a neat little modification.
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Old 16th May 2018, 09:29 PM   #2658
Zen Mod is offline Zen Mod  Serbia
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Aleph-X builder's thread.
swell!

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Old 15th June 2018, 01:55 AM   #2659
GeeVee is offline GeeVee  Australia
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Hello to all of you Aleph X experts
After several years in the waiting in the “to be built area of my workshop”, I am finally completing construction of my Aleph X.

I have configured the layout using 12 Fets (IRF140) per channel – I happened to have about 50 matched pairs of these!!.

For the most mundane of reasons, I happen to have a pair of 1200VA toroid’s, with dual 28V secondary’s. This will equate to a DC voltage of about 37 volts (after rectification / filtering/ losses)

By my calculations, and assuming a bias of 6 amps, this equates to a dissipation of about 37 watts per fet, and a total dissipation of about 440 watts.

Power out will be:
8 ohms - 144 watts
4 ohms - 72 watts.

I don’t want to increase the bias current any further, as the dissipation gets out of hand.
I have no issue with heatsinking – have some VERY big brutes on hand.

My question is, has anyone built this amp using a higher supply voltage? and am I asking for trouble. Is this workable solution?

Or should I stop being a cheap skate and go buy the proper transformers.

I look forward to your comments

Kind Regards
George.
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Old 15th June 2018, 02:14 AM   #2660
Eric is offline Eric  United States
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Aleph-X builder's thread.
At first glance, the MPSA18 devices and the 9610's can both handle the rail voltage that you are proposing. I don't recall anyone building an Aleph-X with rail voltage that high, though. As there are different parts lists for a low-voltage (15v) and high-voltage (22v) version of this amp, I suspect some of the resistor values will need to be changed for going as high as 37v rails. I don't have a good starting place for you - perhaps others can make a suggestion.

The power output that you are looking at will generate some serious heat. My Aleph-X monoblocks bake off about 400w each. You can find a link in my signature. If you are looking to drive 8-ohm speakers, higher rail voltage and lower current is to your advantage. If you are looking to drive 4-ohm speakers, lower rail voltage and higher bias current will work better.
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Last edited by Eric; 15th June 2018 at 02:16 AM.
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