Sunday, January 3, 2021

Krell KSA-5 Clone

When I saw, back in 2012, Kevin Gilmore publishing the blueprints for a clone of Krell KSA-5 headphone amplifier on, I thought it may be a nice use for the bag of 1000uF capacitors I had at the time. (Yes, using up the capacitors was my main motivation for putting my Krell KSA-5 clone together.)

I made every effort to make it look and perform well. I carefully matched the transistors, used the best parts available, and even asked the people at Modushop to CNC a custom machined and anodized front panel.

Although good looking and well within specifications of the original KSA-5, the clone was rather disappointing. It was fine working with headphones, but it could not compete with my Musical Fidelity X-CANv8. Connected to a pair of 8ohm speakers, the clone would become rather confused with anything but simplest music. Because of this, the amplifier fell into disuse and was gathering dust on my rack.

For the longest time, I wanted to make it work better but could not get to it until this weekend.

I decided to improve the performance of my KSA-5 clone without changing its topology, so as to keep the PCB, and to keep as many of the original parts as possible. Within a day, I was able to make quite some improvements:

  • The original KSA-5 was rated for 5W into 8ohm with THD < 0.5%. My unmodified clone gave 5W into 8ohm with THD @1kHz of 0.18%, well within the specs. The revised clone delivers 5W into 8ohm with 0.0015% THD, an improvement of more than two orders of magnitude
  • The original KSA-5 was advertised to deliver THD < 0.03% into 100ohm load, although the brochure did not give the signal level for this performance. Assuming the same output voltage as for 5W into 8ohm, about 6.3Vrms @ 1kHz, my unmodified clone would demonstrated THD of 0.02%, again well within the specs. The revised clone drives a 100ohm load to the same level with 0.0017% THD, an improvement of more than an order of magnitude
I made some measurements under different setting, including with a 32 ohm load and with 19+20kHz two-tone test signal. I will post them and the changes I made in the schematic in subsequent posts. Stay tuned!