Replacement JFET for ASM-1 VCO

Martin Czech martin.czech at intermetall.de
Wed Nov 3 16:38:52 CET 1999


:::> Leakage is not dependend on device type (bipolar, jfet), but on individual parameters. The jfet channel leakage can be considerable. At first sight the 
:::> gate leakage of jfets looks good, but wait til Ta=50C. See LM11 data sheet.
:::
:::   I don't know about any of you, but I do NOT plan to be messing with my 
:::synthesizers when it is either 1) 122 degrees Farenheit ambient temp or 
:::2) when my synth is _generating_ 122 degrees of heat!  ;)
:::
:::   Tony, who's not into extreme synthesis as a sport.  ;) lol

Tony, 
50C, 55C, even 70C is nothing unusual for consumer devices.
Look into the specs of the manufacturers.

Well, this depends of course on room temperature (I have measured 28C
this year, then on the case, the airflow inside the case, if the PSU is
inside or outside the case, any larger power consumers are near/inside
the case, any other power consumers outside.

Note that Ta is not to be measured on your sofa (;->), but -say- 1mm above
the device. Air is a very good insulator for heat, there can be large
gradients. So, 50C is nothing unusal. Consumer products are tested
with Tj=55C, normal consumer with Tj=70C, automotive even with Tj=120C
(at least here at Micronas, but other manufacturers spec the same).

After a few hours my Roland M480 mixer get's pretty warm OUTSIDE, I guess
40C or more. That means: inside is hell, the chips run perhaps as high
as 70C.

When I opened my Lexicon LXP1 to repair a knob, I noticed that all
the chips were pretty hot, I couln't touch them for a longer time.
60-70C ?

Have you ever touched a NE5532 dual audio op amp for a longer time
interval with your finger tip? (the finger cools down the package heat
capacitance in the first moment).

I know that these are rather extreme cases, but until I have not taken
a look at the temperature inside after a couple of hours I would not
be so sure about the thermal behaviour, there are surprises.

Why would people like Widlar or Pease write so many lines about leakage
and temperature?

m.c.





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