Variation on the JC-2 preamplifier

[jonusgrumby]

I recently purchased the JC-2 preamp kit that tubeshunter is selling on eBay. I made some changes to the stock component values to improve performance and reliablility. I also increased the gain to a little over 20 dB.

The kit I purchased did not have idss matched jfets. I had several of the JFETs on hand and selected idss matched parts. The differential pairs were matched to 0.1 mA, and the n and p channels were as close as possible. It will work without matching the fets, but the offset and distortion will be higher.

It is stable into 1000 pF capacitive loads, and has a THD of .001% in the audio band.

If you assemble this kit, be sure to test the bipolar transistors. The ones I received had their pin out reversed from the silk screen shown on the board. The picture on the eBay listing shows them inserted reversed as well.

I hope someone finds this of some use.

[GRollins]

I'm having trouble reading the schematic--it comes up fuzzy on my screen--but it appears that the N-ch devices are 2SK170, but the P-ch devices are 2SJ49 instead of 2SJ74.
Maybe I just need new glasses.

Grey

[HKC]

My JC-2 pre-amp just finished last month. It is a simply but very good sounding pre-amp which I highly recommend for diyer.

[john curl]

Who is the jerk that is ripping off this design and calling it the JC-2?

[HKC]

I built my JC-2 based on the original JC schematic:

[HKC]

Here is the Feedback network:

[jonusgrumby]

Sorry the schematic is blurry. The P channel jfets are 2SJ74, not 2SJ49, my apologies.

Mr. Curl, the kit is available on eBay from tubeshunter. He calls it a STEREO FET LINE AMPLIFIER - BASED ON MARK LEVINSON JC-2.

I attached a revised schematic.

[john curl]

I am glad to see amateurs making their own. I gave it to the world in 1977, after Levinson would not pay me further royalties for it. However, I am concerned when someone is making money from the design, and calling it the JC-2, because then if something goes wrong, who do you think they call first?

[HKC]

Mr. Curl

What is the best idss for the differential JFets on the line stage? I am using 2SK170BL/2SJ74BL both are within 9.2~9.4ma matched.

Another picture of the pcb of my JC-2:

[john curl]

OK with Idss. Matching is important!