Tuesday, July 15, 2008

The Kenwood BAsic C2 Preamp: What Was I Thinking???

The Kenwood C2 is a well thought of late '80s preamp that retailed for $329. It is notable for better than average tone controls, and both moving coil and moving magnet cartridges with selectable impedance. It is very slim and seemingly very well built.

I was looking for an easy project and what could be easier than a preamp? My C2 had a channel out so why not? What was I thinking?

The C2 is a very nice sounding preamplifier with high quality components, but it is pan built. There is no bottom plate to remove to get access to the controls and boards, you have to disassemble the entire unit. This is not for the spatially challenged. Here is my C2, de-origamied:Yowza. This is how the C2 looked just before I started reassembly.

The main board at the right has to be lifted to replace the caps. It is beautifully made and the caps were high quality Elnas. I am not sure if they existed 20 years ago, but many of the original caps were the distinctive Silmic brown.

There are actually 3 boards at the left for the tone controls. The two larger ones are stacked on reassembly. The smallest is still attached and contains the headphone jack and control. This board has 4 caps and was very difficult to work with. (Fateful words.)

There is a very small board at the front that contains the source selector LEDs. The selector switch itself is still attached to the front panel and has the long blue ribbon that connects in the back right corner to the angled switch.

Next to the selector is the small board with the phono switch.

At the far right is the very lovely blue Alps volume pot.

Here is the C2 partially assembled:
The boards at the left have been positioned and the front and back tacked on. Now the story gets ugly. After connecting enough of the unit to test it I discovered that it had a noisy headphone level pot. Noisy to the point that it seemed damaged. This was very possible since the small knob is exposed at the far left of the unit plus it was the source of some handling when replacing some caps. When not being adjusted the unit was quiet and sounded very good, but when adjusting it was subject to hum and drop outs. I needed some consultation.

I took the preamp by Mike's and managed to lose the knob for the headphone level on the trip. Acck! Losing a knob is just awful beyond the loss of utility. Mike recommended resoldering the pot as a first step, but it might have to be replaced. So, a photo and final wrap up will have to wait a bit. Who'd a thunk a preamp would be so exasperating?


Guy said...

Just got one of these. Suspect there is mod to it. There is a small cap soldered across from the ground screw holding the output jack to the back plate to ground on the jack itself. From your pics I can't tell, but does yours have the same? Also suspect have a cracked solder joint on the output jack...moves quite a bit when pressed from behind (intermittent bad connection).

Anonymous said...

Actually, it is easy to disassemble if one keeps in mind that all three boards and the faceplate is one assembly and the pan is just another piece. Takes 5 minutes to take it apart and put it back together.

Major issue with C2 is that Opamps used in it are designed to operate at +/-18V max. The actual power applied is actually greater than this voltage. As a result they often go bad. What is worse - they take input JFETs down too. Therefore if you get your hands on C2 - replace opamps with +/-22V ones (NE5532 for instance). They will sound better too.

Overall great preamp.

BDude said...

Hi, I know this is long dead but I recently picked up a C2 from ebay and have a question. I re-capped with DRM's kit, replaced the main amp op-amps with discrete Burson Supreme Sound V4's (night and day difference), removed the bad balance pot and shorted with 180K resistors.

Mac Mini->USB Regen->Schiit Bifrost multibit->kenwood c2->Pair Mcintosh MC-30 tube amps->Klipsch Cornwall III's

This pre is so transparent I can see through to next year. The Kenwood replaces a Vincent SA-T8 balanced pre with all NOS Telefunken tubes. The Vincent was cold and beautiful as the fortress of solitude but much too expensive to keep. It wouldn't be amiss in Cameron Fry's house.

Recapping made a huge difference in the old C2. Straight from ebay it sounded as I expected a not-really-vintage mid-fi component should sound. Kind of dull and congested but a great starting point for the money! No surprise since the original caps were bulging and some were leaking.

The new caps (same values) brought this thing to life. Nice depth, solid centrally-locked stereo image. Next the op-amps. I added sockets and went through a couple opamps before stopping with the Burson V4's.

Sooo much more detail and more air, sound stage grew considerably. Absolutely worth the premium price! The V4's are rated at 20v max supply voltage while the newer V5's will only accept max 15v.

Since removing the balance pot, the intermittent/scratchy left channel issue persists. Additionally, the channels are very imbalanced and I have to turn the gain up on the left amp to compensate.

So the culprit appears to be the volume pot. I have a Goldpoint dual gang stepped attenuator in the mail. However, I just realized that the existing Alps Blue Velvet is 4 gang!

Does anyone know why it needs a 4 gang pot? The only reference I saw in the schematic is "Super Coupled Volume" but i can't find any info on it.

Any advice? Can I use a regular stereo stepped attenuator here?

Thank you!

joseph hertzberg said...

Damn Bdude, nice upgrades!

Andrew M said...

After reading your upgrade post I think I'm going to have to invest in the DRM kit. I hope you got that left channel sorted.

Hafky said...

Anonymous, Could you possibly amplify on your comment regarding the easy way to remove the board? i.e.exactly how you do this.