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Old 12th March 2004, 05:34 AM   #161
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I think you could still get it lower..... You're right on about blowing on the input pair and getting drifting readings! I lucked out and mine was very low at power up and was able to be brought down to a very low level. My input pair measuers 3.7900 and 3.7907. Q2 was 4.2464, and Q11 was 4.2469. Although Nelson says it only matters to two places mine being all the same out to three places may have made some difference. At any rate it was fun matching them up....
I stuck a 4" fan at one end of the sinks I'm using for testing so the temperature stays managable...the fins are face down right now with the board mounted to the flat back.

I haven't made any measurements except for DC offset at this point. My rails are plus and minus 15 volts. Will make more measurements tommrrow.

Now I need to get the second channel done!

Mark
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Old 13th March 2004, 12:39 AM   #162
Stabist is offline Stabist  Slovenia
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Hi!

I admit - probably it's a stupid question - but nevertheless - is it also important that the physical look af the amp is as symetrical as possible or not?

I mean - I've intended to build A5 in the first place - with a chassis similar with original Aleph series - 4 heatsinks as sides.

But now I just got lucky and some guy I know had additional pair of Chad's PCBs (btw - great work, Chad!!!) - but if I use 4 heatsinks as sides - it won't "look" simetrical ...

Well - I know - like I allready said - it's a bit stupid Q - but I just had to ask ..

Thanks in advance
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Old 13th March 2004, 12:44 AM   #163
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Default Heatsink Purchasing Thread has Moved

Titled Heatsink Sourcing (split from AXB thread)

http://www.diyaudio.com/forums/showt...702#post347702
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Old 13th March 2004, 05:03 AM   #164
grataku is offline grataku  United States
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Ok guys you got the DC offset done, time to check the AC current gain now.
I mentioned this time and time again. Be sure to box-up the amp in such a way as to be able to reach the various trimpots for final adjustment when the amp T is equilibrated. The process is iterative.
I'll add my YAHWN in here too.
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Old 13th March 2004, 06:03 AM   #165
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Huh!
You pups think you've got it rough, gettin' the DC offset to calm down, what with blowin' on the differential an' all...ain't ya got nuthin' better to blow on?
When I started this whole thing, the prototype had the two IRF9610s an inch or two apart, an' no case around 'em, an' the draft from the main heatsinks was drifting over 'em on acounta the fact that the majority of the circuit was sittin' on top o' the main heat sinks with the outputs right underneath...
And you guys think you've got it bad!
Harrumph!
Worse yet, when I was a sprout I had to walk five miles in the snow, uphill both ways...

Grey

P.S.: Although it won't really help with the thermal issue, don't forget the other two pots. They come in handy for fine tuning the DC offset.
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Old 13th March 2004, 06:20 AM   #166
moe29 is offline moe29  United States
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...i got the little critters settled down and acting right. Once
they're snug as bugs in their case they should be fine.

funny thing was, i was working on the amp during a warm spell
here in FL, and the air conditioner was on... i noticed the readings
were changing after the amp had been running for over an hour...
turns out the AC duct overhead was blowing on my table! That's
when i figured out the blowing on 'em thing... (i don't think i'm
ever going to type blow in a post again)

((yes we run the AC in early March here in Florida))

(((Global Warming??))) ((((must be the influx of DIY Alephs))))
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Old 13th March 2004, 02:06 PM   #167
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I haven't had any major DC offset problem with the first one. Will assemble the second this weekend and see how that one goes. I think 20mv is darn low! My input pair is fastened together with a thermal pad in between them.

Since I'm limited on heatsinks I will probably build a wind tunnel cooling setup for these two channels. I have a couple of 12VDC Pabst fans that ere really quiet running at about 8 volts.

Mark
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Old 13th March 2004, 07:46 PM   #168
Coulomb is offline Coulomb  England
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Quote:
Worse yet, when I was a sprout I had to walk five miles in the snow, uphill both ways...
Luxury... Bleeding Luxury.. I had to crawl to school with my sisters on my back in 3 feet of snow, uphill both ways...of course.



Actually I did have a question, the two IRF9610 trransistors used for DC Trim is it? Sgould they be back to back on a heatsink to thermally stabilize them to reduce drifting?

Anrthony
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Old 13th March 2004, 07:54 PM   #169
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No heatsink is necessary between them. Just a sil-pad between them with a nylon screw and nut holding them together.

Mark
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Old 13th March 2004, 08:15 PM   #170
Coulomb is offline Coulomb  England
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Quote:
Originally posted by Mark A. Gulbrandsen
No heatsink is necessary between them. Just a sil-pad between them with a nylon screw and nut holding them together.

Mark
Actually I was thinking of the heatsink as a thermal stablizer at room trmperature as opposed to dissipating heat. With a larger mass micro air currents would have much less effect on them, result less drift.

Regards

Anthony
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