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.
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.
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:
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
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.
Any news about the mods of this preamp? Has any one a service manual?
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.
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.
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