B&K ST-140 schematic

This one needs a lot of tweaking to perform high end

A very big and heavy empty box.

good luck!

Carlos
 

Attachments

  • st140.jpg
    st140.jpg
    46.6 KB · Views: 1,258
  • DSC01832.JPG
    DSC01832.JPG
    127.6 KB · Views: 1,183
Hi Guys

The schematic they showed me and that I verified by tracing the circuit back in 1981 was a bit more complex. At that time, they used six dual-transistor cans - 2N2920npn and 2N3811pnp. The circuit is essentially as above: a PNP input loaded by a four-BJT Widlar current mirror; feeding a Darlington VAS that had a current-mirror to load the input transistor and quickly discharge the base of the output; constant current sources for the input comprised of dual BJTs with a cross-collector degeneration resistor; current source load for VAS; mosfet output as above.

A friend and I showed them an alternate way to lay the amp out and an alternate circuit using all-discrete BJTs. Maybe they saw the cost savings of dumping the duals?

Their original schemo was on a folded piece of paper with a pencil-drawn circuit on it. The circuit follows standard lines but I suspect they borrowed the details from someone else. A few years later the original designer left and was replaced by a young Asian guy who could also design tuners and suchlike allowing them to expand their product range.

Have fun
Kevin O'Connor
 
For anyone needing a schematic of roughly the first several production runs of the ST-140 (pre-1990). I went ahead and upscaled the text and image contrast for easier viewing.

I also have the post-1990 schematics and write up as well if anyone is looking & interested. Message me if so 👍🏻
 

Attachments

  • 1666597425237-01~6-01~2.png
    1666597425237-01~6-01~2.png
    1.5 MB · Views: 340
I dare say from Kevin O'Connor's description from above that the original B&K ST140 was a copy of the Van Alstine mosfet amplifier, for which a patent was granted. I seem to remember a bit of a stouch between the parties over intellectual property, which may have resulted in B&K changing there design to a simpler front end (as shown) when Frank threatened to sue. I have included the patent schematics which pretty much align with Kevin's description. It is interesting to note that B&K went back to a similar complex front end in their later models such as the ST202+ and the EX442. Also at the time Trevor Lee's imported or used (perhaps without permission, Trevor was always cagey on not showing the schematic, his tech Dallas did though) Frank's boards and built up amps here in Australia, calling them the Transcedence Series. I had two mono amps built using the boards with a 625VA transformer, 3 pairs of outputs and 120000uF of PS caps. Drive anything but it still didn't have that special something that some tube amps convey, so I sold them.

Screen Shot 2022-10-25 at 10.21.14 am.png
Screen Shot 2022-10-25 at 10.21.38 am.png