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Old 24th May 2011, 02:04 PM   #1201
h_a is offline h_a  Europe
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Am I missing something?

The BLs run up to 12mA IDSS, do you really intend to forward bias them?

Or is that only a typo and you're going to use V-grade?

Hannes
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Old 24th May 2011, 09:05 PM   #1202
klopf is offline klopf  Denmark
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Oh, by 16,5mA-25mA I was referring to the current through the current source, so in those calculations the current through each JFET would be in the range 8,25mA to 12,5mA. Re-reading the datasheet, I can see the IDSS classification for the BLs (which I'm using) is 6-12mA, so I need to reduce the current.
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Old 25th May 2011, 04:23 AM   #1203
qusp is offline qusp  Australia
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yep and you'll have a hard time finding the 12ma devices, 11ma is achievable
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Old 25th May 2011, 11:20 AM   #1204
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You can also parallel devices and use a mosfet or BJT for the current source if you really need that much current.
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Old 25th May 2011, 01:05 PM   #1205
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Quote:
Oh, by 16,5mA-25mA I was referring to the current through the current source
Sure!

However, it takes not much of an signal to forward bias an 11mA IDSS jfet running at 9mA.

Hannes
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Old 25th May 2011, 01:13 PM   #1206
klopf is offline klopf  Denmark
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So what should I run them at instead? I guess it make sense it run them as low as possible to reduce the heat (and increase long-term stability). The datasheet suggests I shouldn't go under 6mA per JFET. Would 7mA be better in order stay a bit above the lower limit? Any disadvantages with this?
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Old 25th May 2011, 03:07 PM   #1207
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Quote:
The datasheet suggests I shouldn't go under 6mA per JFET.
Why?

Sure, you can run them lower than that; that's even the general case (just have a look at the original AlephJ).

You can see how close you can bias 'em:

- calculate how much input signal drives amp into clipping
- calculate how much standing current in input differential swings due to this voltage
- add some reserves to be happy camper
- bias input jfets by this amount below IDSS

Lots of additional complications imaginable, like wether you would like to trade current for degeneration (reduces THD) or you might really want to forward bias 'em (be aware of gate current).

It might be easiest to start with a known good value and work from this.

Enjoy! Hannes
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Old 2nd June 2011, 02:57 AM   #1208
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Quote:
Originally Posted by jacco vermeulen View Post
Nope.

Resulting thermal resistance number of a blown heatsink depends on the air flow and ; heatsink type, fan, synergy between the two.
A factor three reduction generally requires a substantial air speed, something in the order of >16ft/s.
Which is too high/noisy for audio purposes, a reduction of the natural convection number by half is more realistic for moderate air speed numbers.
OK, I built my finned forced-air duct and tested it. Using a somewhat crude heat source (a low power ski-waxing iron ) and barely audible fan speed I am getting 0.17 C/W from aluminum that should be about .33 C/W. So your prediction was extremely good, Jacco! No worries about fan noise at this airflow. Hopefully each stock Aleph J stereo channel will be happy with this much cooling...

Frank
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Old 10th June 2011, 04:28 PM   #1209
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I tested my air tunnel heatsink with the Aleph J board idling and believe it to be up to the job of cooling the MOSFETs. I don't have measurements but the '5 second test' was no problem. A couple photos are attached to solicit your comments. The reason I don't have measurements is the subject of a separate post, to follow.

Frank
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File Type: jpg IMG_4144.jpg (104.9 KB, 465 views)
File Type: jpg IMG_4146.jpg (122.2 KB, 459 views)
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Old 10th June 2011, 05:44 PM   #1210
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Default Smokin'

As a class A tenderfoot and non-ee, I need to ask advice from you generous people.

I made a dumb mistake and installed the wrong value resistors into R2 and R4 of the Aleph J. During heatsink testing, R4 (1/4 w) eventually failed and started smoking. (I should pay closer attention to the "Ks" on those parts! ) At the same time, the J74s were running hot. Can I check if they survived without removing them? ...and looking at the attached photo, do *genuine* J74s have the shoulder bend in the outer pins? If I'm also replacing the JFETs, I definitely want the right parts...

Any and all thoughts/suggestions much appreciated!!!

Frank
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