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Old 6th November 2012, 02:04 AM   #541
LBHajdu is offline LBHajdu  United States
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Are you sure about that? I didn’t read Idss as the maximum current the jfet would pass, because the servo loop could pull the gate connection above ground making it conduct more then Idss. Or is that wrong?

I still support the BL grade though.
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Old 6th November 2012, 05:23 AM   #542
ndmrslp is offline ndmrslp  United States
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Originally Posted by LBHajdu View Post
LSK170 is also a good option because you are supporting a company who is actually committing to keep it in current production now (they have the same grades).
Do you know where can I buy them?
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Old 6th November 2012, 03:31 PM   #543
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Originally Posted by LBHajdu View Post
Are you sure about that? I didn’t read Idss as the maximum current the jfet would pass, because the servo loop could pull the gate connection above ground making it conduct more then Idss. Or is that wrong?

I still support the BL grade though.
I'm sure. The Toshiba data sheet specifies Idss at 10V. It will conduct slightly more, maybe 5%, at 15V and if the Gate-Source voltage goes slightly positive before gate conduction begins. There is also a "V" grade that should work if you can find them.

The operating point of Q4 depends on the threshold of Q1. I'm using an IRF640 which is supposed to use the same die as the IRFF240. I tried an IRF630 and the distortion was lower. I've found the threshold of IRF parts to be remarkably consistant, but Nelson has far more experience with them.
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Old 6th November 2012, 06:14 PM   #544
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I'm using an IRF640 which is supposed to use the same die as the IRFF240
That should be IRFP240.
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Old 6th November 2012, 08:06 PM   #545
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if this amp were to be built in a different enclosure how large of a heat sink is necessary?
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Old 6th November 2012, 09:59 PM   #546
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Originally Posted by kyuzo View Post
if this amp were to be built in a different enclosure how large of a heat sink is necessary?
no change needed. The heatsink is sized for the amps heat output, not the case.
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Old 6th November 2012, 10:56 PM   #547
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In the ACA article Nelson suggests a heat sink of 1 degree C per Watt for each channel. That keeps the temperature rise to about 25 degrees C.
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Old 8th November 2012, 06:25 PM   #548
tinitus is offline tinitus  Europe
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..... how large of a heat sink is necessary?
just dont make it smaller
and notice, its one channel
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Old 8th November 2012, 08:13 PM   #549
nr12 is offline nr12  Germany
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Just measured: A 0.7 C/W heatsink per channel gives you 47 °C heatsink temperature.
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Old 8th November 2012, 08:24 PM   #550
Variac is offline Variac  United States
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That original Amp Camp chassis as in the photo above, stays very cool since it radiates from lots of thick extrusions in addition to having a quite large heatsink. NP had no hand in its design.

We supply smaller extrusions for the kits (215mm x 75 x 35) those work fine too, in temperate climates and if the amp isn't in an area that's enclosed in any way. My experience is that the micro-climate (is it under a low shelf above?) and actual climate (do you live in southern India? ) are the big determiners. A hot interior room temperature could easily double the amount of cooling needed.

Last edited by Variac; 10th November 2012 at 08:05 PM.
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