Kenwood Basic C2 preamp mods

jason12

Member
2008-09-04 8:02 pm
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.
 

jason12

Member
2008-09-04 8:02 pm
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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mlloyd1

Member
Paid Member
2001-02-25 7:10 pm
Northern Iliinois
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/folders/print/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
 
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jason12

Member
2008-09-04 8:02 pm
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.
 

ficklecycler

Member
2009-02-17 11:00 pm
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.
 

brownaye

Member
Paid Member
2016-10-04 2:03 am
Chester NH
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!
 

rayma

Member
2011-04-29 8:37 pm
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.
 
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Here's the manual, if you make an account.
file:///home/chronos/u-ba0979a3ce3421c5ab65a854a2128c53400eedb3/Downloads/hfe_kenwood_c2_basic_service.pdf
You didn't mean to post that one, right? ;)

Kenwood Basic C2 - Manual - Stereo Control Amplifier - HiFi Engine

If the volume pot is knackered, You've Got A Problem. Measure wiper resistance to confirm it's the pot. It may need to be opened and cleaned properly, but this is not generally for the faint of heart.

Every self-respecting high-end preamp back in the day had one. A 2-stage volume control allows for a much better compromise between output noise and signal levels in the circuit, i.e. dynamic range management. (See: Gain staging.) You generally find that if you want to get output noise floor down with a single-stage volume control, the stage in front of it has to sustain very high levels all the time, which is not generally doing distortion any favors.

Going 2-stage permits very low noise levels even on high-sensitivity horn speakers if the power amp is up to it, while still keeping signal levels throughout the unit reasonable and hence distortion low.

Several manufacturers (e.g. Yamaha) limited the adjustment range of one section by including some series resistance, thereby improving channel imbalance at low volumes. The adjustment range of one decent pot tends to be quite adequate already, so nobody is seriously going to require twice that.

Incidentally, a lot of electronic volume controls are 2-stage as well. Otherwise their characteristics would be quite poor.
 

brownaye

Member
Paid Member
2016-10-04 2:03 am
Chester NH
Thanks to both of you for giving this some thought. I'm going to see if i can get goldpoint to sell me two more decks for the beautiful series stepped attenuator i received today.

I read some where that the existing alps volume pot attenuates 100K so thats what I ordered. Does this sound right? The part# on the alps is Kenwood's own and doesn't state a manufacturer model #.
 

brownaye

Member
Paid Member
2016-10-04 2:03 am
Chester NH
you should be able to measure the pot and confirm its value, unless it is open ckt or smoked.
I am sure the value is also in the manual, sch, bom. i would think if you are doing mods you would know these design details.


Strangely, the value is not in the service or owners manual nor the schematic. Only the Kenwood (not Alps) part# R11-9016-05. Multimeter shows gangs 1,2,3,4 (from the front) as 44.8K, 43.9K, 2.8K, 2.8K. Is this what it means to be 'tapered'? I haven't seen any pots on ebay or parts express that have these values stock

Boy was I wrong. :rolleyes:

I'm pretty new to diy electronics and I'm learning as I go. I disassembled the alps pot, deox'd and I'm going to go ahead and put it back in for now. I think the good folks at Goldpoint will make a custom for me.

Thanks for your help!

Thanks for
 

Andyman

Member
2008-10-10 7:59 pm
Any updates on this., as I have the same issue.

Here's the pic from the back of my pot and the ratings are close to what's reported above. I will give mine a good cleaning and probably desolder and disassemble it, but if there was a new part to solder in I'd be fine with that.

[IMGDEAD]http://audiokarma.org/forums/index.php?attachments/20170328_191146-1-jpg.899707/[/IMGDEAD]
 

Andyman

Member
2008-10-10 7:59 pm
It says right on the back, it is a quad pot, one dual section is 3k and the other dual section is 56k. It is a custom part

Marvelous, but as I expected....:(

Anyway to make a 2 gang or another value 4 gang pot work in there?

I'll try a teardown and clean first, but if there are other options besides harvesting from a parts unit, I'd like to know what they may be. I ws hoping for some feedback on how the 100K pot was massaged in, if at all...
 

brownaye

Member
Paid Member
2016-10-04 2:03 am
Chester NH
I ws hoping for some feedback on how the 100K pot was massaged in, if at all...

I never got around to finagling the 100k dual gang attenuator. The imbalance and intermittent right channel outage improved significantly with a good cleaning of the existing Alps. I sometimes get some noise/scratching when rotating but I'm content to leave it as-is for now.

Did you find a solution or an off the shelf part to replace the 4 gang pot?