Wildly expensive J-FET devices

[home_listening]

I'm working on a clone of the old Bozak DJ mixer which uses the 2N3822 Jfet in a few positions. Whilst researching a replacement I noticed that Mouser has started selling these again - at a whopping cost. Even InterFET is selling what I feel are very pricey transistors.

http://au.mouser.com/ProductDetail/Microsemi/2N3822/?qs=sGAEpiMZZMv4z0HnGdrLju%2fiT03Wm6h1UvhapzgqVz3nd02PUusXoA%3d%3d

http://au.mouser.com/interfet/Semiconductors/Transistors/_/N-ax1rp?P=1yvvuoi

Does anyone have any experience with these product lines? Just a bunch of hokey for rich Hi-Fi guys?

 

[RuudNL]

http://www.brokxelectronics.nl/index.php?main_page=product_info&cPath=123_287_104&products_id=1965

 

[home_listening]

Ask and you shall receive! Thanks v Much

 

[PRR]

> hokey for rich Hi-Fi guys?

That market is generally too small.

These prices are for maintaining ancient but essential industrial (or military) equipment. A lot of 1970s electronics is still in use for obscure jobs like paper-mill thickness monitoring. Monitor failure is thousands of dollars an hour  downtime cost. Re-engineering services to select a $3 JFET that will "work for sure" will also cost thousands. Paying $21 to get a JFET "just like" the one that cost $1 in 1973 is "cheap".

 

[JohnRoberts]

That is a pretty unusual JFET package with a 4th lead to ground the metal case, so expensive because it is so low volume. I would just try to cross that to a cheaper modern 3-legged JFET with similar pinch off voltage and Idss. May need to tweak part values around it depending on the actual circuit. 

I didn't recall the old 10-2DL(?)  using JFETs, I've seen schematics somewhere kicking around the WWW.

I did some consulting for Bozak  years later (70s), on a consumer electronic product. IIRC they were still making and selling the 10-2DL back then, but i had no involvement with it. 

JR

 

[analogguru]

.....
I didn't recall the old 10-2DL(?)  using JFETs, I've seen schematics somewhere kicking around the WWW.
.....

It's (only) used in the summing-amp as part of a µ-amp (SRPP) similar to that which can be seen on page 5 of this application note:
www.ti.com/lit/an/snoa620/snoa620.pdf

Here is a practical implementation of this circuit:
http://www.muzique.com/amz/mini.htm

 

[JohnRoberts]

.....
I didn't recall the old 10-2DL(?)  using JFETs, I've seen schematics somewhere kicking around the WWW.
.....

It's (only) used in the summing-amp as part of a µ-amp (SRPP) similar to that which can be seen on page 5 of this application note:
www.ti.com/lit/an/snoa620/snoa620.pdf

Here is a practical implementation of this circuit:
http://www.muzique.com/amz/mini.htm

I had to google SRPP (shunt regulated push pull ), apparently a tube thing.

didn't see one on page 5

I'll wait for a Bozak schematic.

JR

 

[home_listening]

Thanks for all the responses, Bozak Schematics are included (also used in a near identical implementation in the equalizer section)

I need 12 in total so obviously won't be ordering the original devices. I do have a roll of 100 2SK170 (minus a few) that I purchased years ago that will hopefully work ok.

 

[JohnRoberts]

Those JFETs are being used as current sources to load the emitter followers with constant current.

Modern low noise JFETs may have higher Idss (current for 0V gate voltage). You can adjust the circuit to bring the current sources back into the nominal design range, by tweaking the value of the resistors between the source and ground (increase R to reduce idle current.) 

The common connection to all the gates is probably some kind of soft-start circuit to reduce turn on thumps, common with single supply circuitry.

I suspect they are looking for low-mid (?) single digit mA class A bias current, but just speculation.  If class A current is too low for the load resistance they are driving, you will see distortion on negative half cycle  of sine waves. Too much class A current will increase thermal dissipation of devices needlessly and perhaps limit positive signal swing in the extreme.

JR

 

[PRR]

That circuit isn't the least critical.

Main constraint is that the total supply current (voltage drop on Source resistor) be "similar" to factory units (within 2:1). Looks like a little luck with J202 (or J201, or many-many other jellybean JFETs) will get you there. Fiddling source resistor is perfectly valid.

Gain varies with JFET except these modules are locked inside NFB loops. If the JFET current is near right, open gain will be near right, and closed gain will be spot-on.

I also think TL072 would do the whole job, better, but nobody cares what I think.