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FS: F5 with 2SK2013/2SJ313

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For sale is my Juma version of the F5 with 2SK2013/2SJ313 output fets.
It's been my main amp for the last few years, and I really love it. But about a year ago I bought some 85db speakers, and it's just not cutting it, power wise.

I am not putting an asking price. Just send me a reasonable offer via PM if interested.

A few specs:

  • I used a very high quality Vicor switcher. Read about them, they are very low noise and have been been elsewhere on this forum. They are about $500 new, but I had one second hand that was still in packaging.
  • I matched all the FETs at 4 points: .1, .2, .3., and .4 A. Currently biased at .4.
  • I had 20 of 2013s and the 313s, so matching was pretty good. I can dig up a spreadsheet somewhere.
  • I used a very simple speaker protection consisting of a simple relay, in the rare event that one of the rails failed. Relay actuated to speaker terminals only when BOTH rails are up to full voltage. In reality, this is unlikely, but better safe.
  • Small fan running at 8V to keep the compact case cool. Very very silent, and works well.
  • Voltage on rails is set to +-15V, exactly half the dissipation of the real F5
  • Power is about 10W RMS, 20W Peak
  • Ouput FETS bought from member Prakit, so verified and authentic
  • JFETS bought from Spencer at FET Audio.
  • I took some measurements with cheap QuantAnalyzer, I don't always trust it down below 110db. But looks pretty good overal.
  • Also, I am using 1/2 the feedback as NP offered as an option. As a result, there is twice the gain at around 20db total (10X), and a little more distortion. Nothing to worry about. It sounds a little more relaxed this way.
  • Bandwith slightly limited vs the original F5 (500 kHz). I added a few small capacitors across the feedback resistors as NP suggested. Still totally flat across the audio band. Down about 2db at 40kHz.
Overall, this is a really great amp. Hope someone with a little more efficient speakers than I currently have can enjoy it.


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