The maximum power rating specified in Toshiba datasheets refers to absolute maximum and not necessarily for reliable operation.
At 25 degC ambient temperature, running the device at maximum rating will mean the junction temperature goes up to 125 degC.
You can easily calculate Rthja from that, by dividing the Pmax by 100 degC.
So for 2SJ74, Rthja is 4mW/degC.
For 2SJ109, the 200mW Pmax is not specified whether it is for a single device or dual combined.
But datasheet of 2SC3381, which is based on the same package, suggests that 200mW is for a single device.
Thus Rthja for each JFET inside a 2SJ109 is 2mW/degC.
What would be an operating junction temperature that is reliable ?
Reference 3 of this article suggest an absolute maximum of 100degC :
https://linearaudio.net/sites/linearaudio.net/files/v3 euvl.pdf
You can now choose whatever safety factor you wish to apply on top.
And of course you also have to take into account the ambient temperature you expect you device to operate at.
Patrick
At 25 degC ambient temperature, running the device at maximum rating will mean the junction temperature goes up to 125 degC.
You can easily calculate Rthja from that, by dividing the Pmax by 100 degC.
So for 2SJ74, Rthja is 4mW/degC.
For 2SJ109, the 200mW Pmax is not specified whether it is for a single device or dual combined.
But datasheet of 2SC3381, which is based on the same package, suggests that 200mW is for a single device.
Thus Rthja for each JFET inside a 2SJ109 is 2mW/degC.
What would be an operating junction temperature that is reliable ?
Reference 3 of this article suggest an absolute maximum of 100degC :
https://linearaudio.net/sites/linearaudio.net/files/v3 euvl.pdf
You can now choose whatever safety factor you wish to apply on top.
And of course you also have to take into account the ambient temperature you expect you device to operate at.
Patrick
So if I will run the fets inside an enclosure where I have 50C as ambiental temperature and I assume 100C maximum junction temperature then I have the following:
for 2SJ74, Rthja is 4mW/degC so we have 4 x (100C-50C) which gives a max of 200mW of dissipation in a 50C ambient with a maximum junction temp of 100C
for 2sj109, we get 100mW for each jfet thus 200mW for both jfets in a package.
Very good info, saved me a lot.
Thanks.
for 2SJ74, Rthja is 4mW/degC so we have 4 x (100C-50C) which gives a max of 200mW of dissipation in a 50C ambient with a maximum junction temp of 100C
for 2sj109, we get 100mW for each jfet thus 200mW for both jfets in a package.
Very good info, saved me a lot.
Thanks.
I bought the J2 years ago from Reno Hi-Fi. I never had speakers that could work good enough with it. As shown in these measurements, the J2 needs speakers that are efficient but more importantly won't fall below about 6 ohms.
https://www.stereophile.com/content/first-watt-j2-power-amplifier-measurements
My new speakers will be at least 93db and between 7 and 8 ohms, so I should expect great performance. But I was wondering if a stock J2 is fine as is, or have their been proven modifications since then, but which are truly cost effective?
https://www.stereophile.com/content/first-watt-j2-power-amplifier-measurements
My new speakers will be at least 93db and between 7 and 8 ohms, so I should expect great performance. But I was wondering if a stock J2 is fine as is, or have their been proven modifications since then, but which are truly cost effective?
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The only difficult part of the build is a surface mount resistor and the surface mount opto. I used a through hole version and bent the legs at the ends to solder to the PCB pads. Gave the clearance needed. The resistor is tricky because of how small it is, Used tape to hold it in place and soldered one end, then removed the tape for the other end.