Upgrade Kits for Older K&K Phono Preamp Kits
The general topology of the K&K Audio Phono Kits has not changed over the years that they have been in production. However, as we have learned more about the circuit and tested improvements successfully, we have been able to offer them as upgrade kits to bring your K&K Audio phono preamp up to (or nearly, as physical constraints dictate) the level of the current product. Your original SE kit or Maxxed-Out Kit (main board Revisions 1-5) can be improved to a level of sonic performance that greatly exceeds the original by applying the upgrade kit described below.
Since I started experimenting in earnest with cathode constant current sources I’ve been aware that JFET-based current sources were clearly sonically inferior to depletion mode MOSFET-based constant current sources. So it made some sense to consider using MOSFETs instead of JFETs as input devices in the phono preamp. However, depletion mode MOSFETs that have the characteristics necessary for an amplifier stage like the phono input aren’t plentiful. Finally, after looking around for a few years I identified a couple of likely candidates and recently got around to testing them. One of the devices looked promising on the bench as a replacement for the existing input JFET in the K&K Audio Phono Kits. I have a phono preamp with socketed input devices for this sort of listening test and so it was an easy matter to install a pair that were hastily matched and listen. I haven’t stopped smiling when I think of that experiment since! The improved clarity over the whole frequency range was very impressive. Vocals were much more intelligible, percussive instruments like piano had greater dynamic contrast and a much more lifelike overtone structure, and the spatial relationships of instruments in the “soundstage” were more clearly and naturally rendered. However, there is a trade-off, but it’s not one with sonic consequences. Due to the way this MOSFET biases, the source terminal of the transistor can no longer be connected to ground through a bias resistor, but must be biased by means of a negative supply. Further experiments with the actual bias current showed that the sonically optimum current range was relatively narrow, so I chose to provide the bias current with a current source that has a low temperature coefficient. That way, as the phono preamp warms up, the sound will not change as a result of temperature-induced input stage bias current shift. The circuitry for this current source is contained on a small preassembled and tested surface mount PC board that takes the place of the original source resistor. The raw negative bias current is provided by a simple choke input power supply that draws its current from the existing power transformer.
If you have a SE Phono Preamp Kit that already has the Cardas capacitor upgrade and the earlier (~2010) input stage upgrade or a Maxxed-Out Kit with PC board Revision levels 1, 2, or 3 and the earlier (~2010) input stage upgrade, you can select from the options below. If you assembled your phono preamp, this is a straightforward project to complete with detailed instructions and pictures for all previous kits. If you bought your phono preamp assembled, then we will complete the modifications for you, test it to verify performance, and audition it to ensure sonic quality. If you are unsure or need the earlier upgrades, please send a digital photo of the inside of the chassis and then call or e-mail to discuss.
K&K Audio Phono Preamp Upgrade Kit: $85
K&K Audio Phono Preamp Upgrade Kit, Installed: $175