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Old 3rd March 2011, 11:45 AM   #1
jason12 is offline jason12  United States
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Default Kenwood Basic C2 preamp mods

Got C2 in my possession. Appears to be great preamp to start from and also great for modding.

Strengths - excellent board layout, well designed circuit (although there are some issues), DIP8 ICs.

Weaknesses - mechanical design is not that great that makes working on it somewhat difficult, but nothing serious.

In few next posts I will try to explain the mods I did on it.
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Old 3rd March 2011, 12:06 PM   #2
jason12 is offline jason12  United States
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Default Phono Equalizer

The mods in the phono section:

1. Replace original op-amps (NJM4560) that have max supply voltage of +/-18V with higher voltage op-amps. I used NE5532. Another candidate might be OP275. Off course solder DIP8 socket to make op-amp replacement easier.

2. The 2200.0uF cap in the feedback has to be gone IMO. There are many reasons for it. For instance it can be biased in the opposite direction which I do not think is acceptable. What I did was - shorted the cap and added servo with it's output connected to one input of IC1 with 51K while another input connected with 51K to the ground.

3. Replace output cap with something else. Options here are:
- just short it (servo will keep output voltage at 0) - issue here is that JFETs are not dual-in-one-package. Therefore temperature drift will keep voltage moving up and down in the range of 50-70mV. To correct this issue the JFETs have to be bonded in one package and thermal cap has to be added to avoid excessive air flow over the JFETs. Since I am not there yet I did option #2.
- replace it with two electrolytic caps with middle point connected to -18V. Place another polypropylene cap across it.

4. Replace 2SK170BL with pairs of 2SK170GR in parallel. I had to do it anyway since the op-amp was fried and took down JFETs too. Seems to improve S/N ratio by 3db.
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Old 3rd March 2011, 06:25 PM   #3
mlloyd1 is offline mlloyd1  United States
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jason12:

thanks for sharing. any chance of posting a larger schematic so the values are easier to read?
also curious if you have values for the * marked components that are doing some frequency compensation.

btw, you may want to look at these:
http://focus.ti.com/docs/prod/folders/print/opa604.html
http://focus.ti.com/docs/prod/folder...t/opa2604.html
they are jfet input parts that can handle +/-24V rails. might be worth a try if you're so inclined.
some people like them, some don't ... YMMV

mlloyd1

Last edited by mlloyd1; 3rd March 2011 at 06:32 PM.
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Old 3rd March 2011, 07:36 PM   #4
jason12 is offline jason12  United States
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mlloyd1,

The schematic I have is from AK database. Apparently it was taken with the camera. Unfortunately there is no better quality.

OPA604 is a single op-amp unfortunately. OPA2604 probably a good replacement.

I was surprised to see that Kenwood fed higher supply voltage than the device could handle. The other issue - many places where uni-polar electrolytic caps can be biased wrong way. Other than that the design seems to be very good. The phono section has "floating" ground that is connected to the main one at the RCA jacks.

On the positive side it appears that there are quite a few broken C2s out there that can be had fairly cheap.

There are few other mods that can be done to the phono section. One - to replace a current sourcing resistor in the differential amp with a CCS on the JFET. It needs roughly 8mA. Another - add bypass caps at the op-amp power pins.

I will elaborate on other parts of the pre-amp later.
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Old 21st July 2011, 03:57 AM   #5
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Quote:
Originally Posted by jason12 View Post
mlloyd1,

The schematic I have is from AK database. Apparently it was taken with the camera. Unfortunately there is no better quality.

OPA604 is a single op-amp unfortunately. OPA2604 probably a good replacement.

I was surprised to see that Kenwood fed higher supply voltage than the device could handle. The other issue - many places where uni-polar electrolytic caps can be biased wrong way. Other than that the design seems to be very good. The phono section has "floating" ground that is connected to the main one at the RCA jacks.

On the positive side it appears that there are quite a few broken C2s out there that can be had fairly cheap.

There are few other mods that can be done to the phono section. One - to replace a current sourcing resistor in the differential amp with a CCS on the JFET. It needs roughly 8mA. Another - add bypass caps at the op-amp power pins.

I will elaborate on other parts of the pre-amp later.
I know this thread is old but I'd certainly be interested to hear you elaborate on this preamp if you're still around. I have a C2 coming my way soon.
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Old 25th April 2012, 07:51 PM   #6
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Hi
Any news about the mods of this preamp? Has any one a service manual?
Best regards
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Old 4th May 2012, 10:13 PM   #7
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I found the service manual for C2, now i am searching for the volume knob and for the all four eq knobs, if any one knows a site with spare parts for Kenwood products please send me a message.
Best regards
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Old 8th November 2012, 03:57 PM   #8
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Hi
I will recap my preamp with Rubycon ZLG capacitors and for the BP capacitors i will use Elna capacitors.
I found a link with all the capacitor for recaping the C2 preamplifier.
Online Store

Good luck
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Old 20th December 2016, 02:26 AM   #9
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Default 4 gang volume pot?

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!
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Old 20th December 2016, 03:15 AM   #10
rayma is online now rayma  United States
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Quote:
Originally Posted by brownaye View Post
Does anyone know why it needs a 4 gang pot? Can I use a regular stereo stepped attenuator here?
Here's the manual, if you make an account.
file:///home/chronos/u-ba0979a3ce3421c5ab65a854a2128c53400eedb3/Downloads/hfe_kenwood_c2_basic_service.pdf

There's a level control before the line amp, and also after the following tone amp.
You could take the output after the line amp's output coupling capacitor, and bypass all the rest
of the circuitry (including the other sections of the volume control). It could only sound better
that way. That said, there are also a number of 4 section stepped volume controls available on ebay.

Last edited by rayma; 20th December 2016 at 03:24 AM.
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