Update 6/2/18: I completed the discrete op-amp modification after I got the replacement diodes. You need to replace Diodes 11, 12, 13, & 14 with 16V Zener diodes. If you fail to do this, your op-amps will be getting +18 and -18V (36V total). This is a pretty high voltage to use on op-amps, these days we usually see 15V to 16V (30-32V total). If you skip this step, you should at the very least contact the manufacturer of your op-amps to ask what the recommended max voltage is (or check the data sheet).
I have been running the Kenwood C-2 with the Burson Audio op-amps for about 2 weeks. Overall, I have been very impressed with an already excellent pre-amp. I noticed better high frequency extension (without being fatiguing on the ears), surprisingly the low frequencies also seemed to be clearer with less distortion during bass tests (I used DSD256 reference disks). I am very pleased with this small little mod.
Update 6/25/18: Now I have been running a different setup for about 2 weeks. I am running a dual V5-OPA-S op-amp setup using an adapter and flexible extensions from Burson Audio. I have been running this setup for a little over 3 weeks and the performance has been remarkable. I also added a Siemens 1uF (EPCOS) 450V film capacitor across the output instead of the stacked MKT I had in before also made by Siemens. The heat output from the op-amps have been much better since doing the 16V Zener diode mod detailed more below. This mod is essential to making this a safe mod longevity wise. You might get by running +- 18V for a while, but over time it will kill the components faster than with normal use in modern circuits. Keep this in mind when you are looking to change the C-2’s op-amps.
Update 3/27/19: I have gone through and replaced every single electrolytic capacitor in the C-2 pre-amp, aside from the dual 1000uF 35V capacitors (due to a mistake in ordering), that’ll get swapped out soon. This re-cap made as much or maybe even more of a difference than the op-amp + 16V diode mod! This also fixed an issue I was having where the tone controls didn’t seem to work except for when the treble was between 9-10. Now I can easily hear both the bass and treble adjustments. I went with a lot of ELNA SILMIC-II and Nichicon Muse ES (Bi-Polar) capacitors, as well as an assortment of what I had on hand. It’s pretty straight forward, just make sure to replace with equivalent capacitors. I also replaced the original DIP-8 sockets I added last time with some AMP branded all metal DIP-8 mounts, they have a much tighter grip on the op-amp pins.
Note: Op-amps are still doing great and running reasonably cool with the 16V diode mod.
One of the other projects on my workbench is this sweet combination of an excellent vintage pre-amp, using discrete op-amps from Burson Audio! The C-2 has an excellent flat amplifier and above average tone controls. Adding in discrete Burson op-amps really opened up the tonal signature, especially the higher frequencies. Then the re-cap gave me more headroom with a cleaner mid-to-low range.
Burson V6-OPA “Vivid”
Burson V6-OPA “Vivid” and “Classic” with cases removed (All green Muse-ES capacitors are Bi-Polar)
A shot of the Burson V5i op-amps in the tone stack/EQ/filter circuits (before re-cap)
C-2 Tonestack with re-cap finished
See one of the C-2 threads on DIYAudio here (16V diode mod): http://www.diyaudio.com/forums/solid-state/184318-kenwood-basic-c2-preamp-mods.html
More shots of the re-cap of the C-2 pre-amp: