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Old 16th June 2007, 02:18 PM   #1
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Default Mini Aleph rebuild

Here is the first picture of my rebuilt mini Aleph.

In the old case, the heatsinks were too big and never got very hot. In spite of oversized heatsinks, the power supply was anemic so upping the bias was not an option.

I am using a .68 ohm resistor to set the bias in the CCS's, so each heatsink is seeing about 2 amps at app. 23.5 volts.

I estimate it to get blimeycreaky hot in about an hour. I will post some more objective temperature measurements once I figure out where the thermometer has gone.

I plan on drilling some vertical holes through the heatsinks to get a little bit more cooling, but not too much.....

Thanks to Stefano and other 'over the top' DIY'ers (who was it planning on using 1KVA trafos for an amp intended to power tweeters in a bi-mped system?) for inspiring me to forget what is necessary and go for the gusto --- those are 100,000uF caps in the PS. Also many thanks to Grey and Nelson for the design and tutoring as to how the circuit works.

Oh, yeah, BTW, the inputs are Siliconix J174 jfets. These are big stupid fets, and the output is down 1 dB at 20 KHz.

JJ
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Old 17th June 2007, 04:35 AM   #2
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Nice work! What change\s did you make to use the J174 for the input?
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Old 17th June 2007, 06:00 AM   #3
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by MEGA-amp
Nice work! What change\s did you make to use the J174 for the input?

j174 have dreky xconductance
it's good for start
for conversion look at my pdf for Babelfishing somewhere buried in Babelfish thread

here it is :

http://www.diyaudio.com/forums/attac...amp=1169342523

just be sure to rename it to convert.pdf ........link is somewhat #$%^$#@!!
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Old 17th June 2007, 07:41 AM   #4
Manu is offline Manu  Europe
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Quote:
Originally posted by Zen Mod

----- for conversion look at my pdf for Babelfishing -------
Nice text, not bad for an ugly Ostrich (strauss) sitting at 8:00 AM on sunday morning at his Computer.
Planning some barbecue today?
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Old 17th June 2007, 09:36 PM   #5
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Quote:
Originally posted by Manu
Nice text
x2


Thanks Chocky
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Old 18th June 2007, 01:13 AM   #6
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Quote:
j174 have dreky xconductance
it's good for start

for conversion look at my pdf for Babelfishing somewhere buried in Babelfish thread

here it is :
Thanks Choky--actually, I already had your schematic by way of another member (Peter Daniel, perhaps?) who you helped do a conversion using J176's. So I owed you a thanks as well but forgot.....

Yes, the J174's are big dreky jfets, but it sounds amazing. And, the drain saturation current can be as high as 25 mA, so if you get the right devices, you can just plop them in the Mini Aleph circuit. That is what I did at first, and it worked just fine. However, my ps rails are about 24V, and 12mA of current puts it just a bit too close to the 350mW max power dissipation listed for the device. If I had 17V rails, I think I would have left it as is.

I just changed out r106 (see diagram in Choky's writeup) from 220ohms to app. 320 ohms--dropping the current source output to app. 15mA. Then I put a pot in place of r9 (the 392 ohm resistor) and found a value that gave lowest DC offset. I came up with a value of app. 590 ohms, but that was with IRF 634 output devices. I have since changed them out to IRFP 140's, and now DC offset is around -300 mV. I will be re-sizing r9 later in the week.

BTW--the amp. really starts to sing when the heatsinks hit about 55 C. The temp. levels out at 59 C.
It has been warming up with the JMC as I typed this....now its me and the G&T and the JMC......

JJ
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Old 22nd June 2007, 01:09 AM   #7
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Here is a picture of the back:

Heatsinks run a bit hot -- 60 C. It will be interesting to see how much some vertically drilled holes bring down the temp. I would be happy with the steady state temp. somewhere about 51 to 55C.

JJ
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Old 22nd June 2007, 02:29 AM   #8
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Very nice job jupiterjune - can you tell us how big your heatsinks are and have you had a chance to measure heatsink temps yet?

I think it was BillWW that modified an A3 or an A30 to use J176 with excellent results.

Looking forward to hearing your listening impressions.

Stephen
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Old 24th June 2007, 11:28 PM   #9
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I have some initial very basic listening impressions;

1) Bass is flabby driven single ended. It is much better driven balanced. This could be due to the DC offset of 300mV which I have yet to correct.

2) It is detailed enough to make to buzz output by my preamp un-listenable. I also just picked up a use X3, because of its higher gain, it makes the buzz really unlistenable.

I will be taking some time out from finishing the tweaks and measurements on the aleph-m to rebuild my BZLS into a temporary stereo version. It is perfectly silent.

JJ
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Old 25th June 2007, 03:46 AM   #10
jleaman is offline jleaman  Belgium
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Um, that is not a Mini A this is a Mini A.


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