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Kenwood Basic C1 Preamplifier Restoration/Upgrade

Karl vd Berg said:
I had to do the same with the previous Nippon Chemicon's KMF at PSU, turning the volume up high and there was no bass and weak sound. Now it's different, the sound is much more present, full, nicer. I think that may be room for further improvement, with C0G multi layer ceramic caps, soft recovery rectifiers and a couple of fresh Panasonic FC caps in the PSU...

I have the Basic C1 unit, the C45 and C46 follow the new 16V Zeners, before the opamps, and I'm using here PW's 390uF/35V (stock ones were Elna 330uF/25V).

I have to see the schematic of C2, and will help you for sure :)

Edit: do you have a thread for your C2 unit?
Click to expand...

I did not see the edit till now "do you have a thread for your C2 unit?" I am starting one now. Kenwood Basic C2 Preamplifier Restoration/Upgrade

Done :http://www.audiokarma.org/forums/in...c-c2-preamplifier-restoration-upgrade.767344/
 
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Hello guys,

I wanted to ask if the Basic C1 is close to the Yamaha CX 600U for specs...I just bought on eBay but the seller went and shoved a 'won't be back until May. 10th...then a few minutes later I check back and it is May.11th" unfair to us who paid right away and there were no indicators on being out on the yacht!

I missed out on the Yamaha today as I thought I could use a Yamaha AX 630 or 900 for a preamp but they did not get back to me so I just went and purchased something close (I hope)...

I just caught the end of your posts for this pre and may find it worth the check from page 1 to see how they fix these, the photos the guy took for eBay looked so fake like the amp just came off the line, but he said they are his so I will see...whenever he ships it now...looks like I will not see it for 1 month, just nice after I paid in seconds!

I thank you in advance to anyone who has owned these and what to watch out for...the amp that goes with this has 'unreal' spec, 1000 for a damping factor...really possible??
 
Leestereo said:
This Kenwood Basic C1 is the matching preamplifier to AK member Unican Eric's Kenwood M1. The M1 received a complete restoration and upgrade (http://www.audiokarma.org/forums/showthread.php?t=608332) and the same was planned for the C1.

The restoration/upgrade included the following:
1. Replacement all of the power supply electrolytic capacitors with higher spec. 105°C, low ESR types (increasing capacity where appropriate and possible).
2. Adjustment of the power supply rail voltages to ~16V for the line stage and ~18V for the phono stage (the original rail voltages exceed the maximum specified for the op amps).
3. Upgrading the op amps to NJM2068DD (line stage) and NJM4580DD (phono stage).
4. Replacement of the signal path capacitors, with either "audio grade" bipolar electrolytic capacitors, film types or C0G types, as appropriate.

Here are some pictures of the Kenwood Basic C1 preamplifier as first received:







Click to expand...
Thank you for this thread I am getting one in soon and I never owned any Kenwood before but I will be using this with a Nikko Alpha II amp and a DACmagic 100 in the link...music from my computer...
 
Broesel02 said:
Hello,

My OPA selection was:
IC1, Fono: OPA 2604
Flat Amp: OPA 2604
Tone Amp: LME 49720NA

Richard
Click to expand...
I did a few combinations, including an OPA2604 at flat amp and an LM4562 at tone amp. In the end, I found that OPA2134 at flat amp and stock 2043DD at tone amp suited best here. I don't have an LME49720 (yet).
 
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Karl vd Berg said:
I had to do the same with the previous Nippon Chemicon's KMF at PSU, turning the volume up high and there was no bass and weak sound. Now it's different, the sound is much more present, full, nicer. I think that may be room for further improvement, with C0G multi layer ceramic caps, soft recovery rectifiers and a couple of fresh Panasonic FC caps in the PSU...

I have the Basic C1 unit, the C45 and C46 follow the new 16V Zeners, before the opamps, and I'm using here PW's 390uF/35V (stock ones were Elna 330uF/25V).

I have to see the schematic of C2, and will help you for sure :)

Edit: do you have a thread for your C2 unit?
Click to expand...

Karl, you said "and I'm using here PW's 390uF/35V" what is a PW ? A brand name I am not aware of ???
 
Hi There. I hope everyone is doing good. I know this is an old thread but I need help on my C1. It only plays with 1 channel and other channel is totally out. If you crank up the max volume on the preamp and my power amp, you can hear it a little. I checked the voltage for a non working channel, there is no voltage. My question is: How can you tell if it's a balance control problem or it actually the preamp board issue? I test all of the inputs and same result. Should I spray the cleaner to input switch before anything else? Please help me. Thank you.
 
I would start by cleaning all of the switches with cleaner and see what happens. I would suspect the relay located toward back of the unit. Theres a link for a replacement above in this thread.
 
Thank you. I measured the voltage of output line before going to the relay and it has 0 volt (the other line has almost 4 volts). I suspect the IC chip that power the output signal but I am not sure anyone else having this problem?
 
Gonna be attempting this upgrade on my C1 after seeing the positive reviews here, but first I have a few questions if anyone would care to answer....

First, why were R41 and R42 replaced with a higher impedance?
And second, would there be any problems replacing the op-amps with 5532s providing I drop the voltage rails down to +/-15v?

Also, a thanks to Anti_Skate for the Mouser BOM!
 
rjsalvi said:
5532s are rated to +/-22V. Post #1 in this thread -- as well as the schematic -- has the details of the max voltages the op amps see, and why the (DC) resistance value of R41 & R42 requires changing. If you use 5532s, you might also consider dropping a small .01uF X7R ceramic cap between the op amp voltage rails for stability (Pins 4 & 8 of the op amp). You can mount them on the under side of the board.
Click to expand...

I'd ideally like to keep within the recommended voltage ratings for the 5532, I'm mostly just concerned about whether or not it'll be over driven at that voltage. I wouldn't think they would be, but it can't hurt to ask. Thanks for the tip regarding the power rail cap, I'll definitely do that! A little more power rail capacitance never hurts, 'eh? =)

I have a bad habit of mixing resistance and impedance, my bad! But I still don't see an explanation as to why the output resistance needs to be increased so much? R41 and 42 are the load resistors to ground on the output of the tone amp stage which goes directly to the output, so there should be no voltage across them except for the audio output signal, right? I fail to see the correlation between them and the op-amp's power supply unless it relates to how much voltage swing it has? I still don't have much practical experience with op-amps; I'm still learning!
 
rjsalvi said:
Edit: you might consider adopting Lee's mods with the NJM2068's. Those particular op amps are a major bang for the buck! I use them as the standard op amp in our EQC-1 active eq for the Cube loudspeaker systems. They're very quiet.
Click to expand...

After some consideration and research I've decided to do just that. The order from Mouser should be here Friday and I plan to install the mod over the weekend. I'll post here with the results when I'm done! Thanks for the advice.
 
R41 and R42 are indeed the output load resistors for the tone amp; the stock values are unnecessarily low and make higher current demands from the op amp. The replacement resistor values are a much easier load wrt current and are actually the same values used in the C2 preamp.
 
Lee don't kill me but I never have gotten to finishing the C1 and I sold the C2. I decided that I wanted a pre amp made in America to match the Polk SDA 2b's and the B&K EX 442. Got all excited over this Sumo Diana and bought it. IMO the Diana does not sound any better than the C2 and lacks many of the features. It is something quite special though and matches my other equipment very nicely ! No fear.....I have another C2 to build and the original C1 so I am smiling. Hope the winter weather is better than the summer for you !20180111_165401.jpg
 
Leestereo said:
R41 and R42 are indeed the output load resistors for the tone amp; the stock values are unnecessarily low and make higher current demands from the op amp. The replacement resistor values are a much easier load wrt current and are actually the same values used in the C2 preamp.
Click to expand...

That makes sense. And also answers how you arrived at what seems like an oddly specific value.

I've just completed the mod; it was a fun experience to be honest, I've never attempted such an in depth mod or repair like this and I'd like to think I learned a bit whilst staring at this things innards for several hours. I'm certainly quite familiar with the C1 now! As for the sound improvements, I could hear the difference within the first 3 seconds of music. It's so much more detailed and crisp (I actually thought mine sounded a bit muddy before). I'm also listening to it completely flat, when I used to have a decent bit of added bass and treble. Worth every penny to be sure! This preamp also offers everything I've been looking for in a preamp now; simplicity, 2 tape/processor loops, excellent sound, and easy serviceability! We'll see how it fairs in day to day use, but I have a feeling this preamp is gonna be with me for quite a while.

Only problem I ran into is my own fault. Somehow I neglected to order 100uf caps with sufficient voltage for C25 and 26. Oops! That means I also couldn't replace the cap under the board. I did pull the caps that were in there though and confirmed they were within tolerance with minimal ESR (<0.3 ohms). I didn't feel comfortable making substitutions based on my knowledge, but since they're just dealing with power supply I'm OK with it until my next order at Mouser.

2018-09-08 23.06.05.jpg

I also did one other mod in the phono section. One of the only things the C2 had that the C1 doesn't is an adjustment for the phono load; I found that 100k sounded better than 47k with my cart. But seeing as 47k is a bit more common, I sacrificed an extra opamp socket to allow swappable phono load resistors (I was inspired by the Hafler 915's phono section).

2018-09-08 22.24.41.jpg

Haven't tried the phono section yet, it's late and I'm ready for bed! It's been a long week. Thanks for the excellent guide!
 
That phono section MOD is slick as Owl grease! Really like that. I will bet you did have to add a bit more treble and a bit of bass (just not as much of it as the treble). That is how I have to run mine to this day. C1 still sounds good even though I have done nothing at all to it. Pulled down the other C2 today/night and took the selector switches out of the unit. I cleaned them all up and soldered them back in. Took a while but well worth the effort. Those Alps switches are damn well built but when they need cleaning it ain't no joke taking them all apart. There is no way to clean them otherwise
 
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