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Kenwood 700C unseats the Sansui CA-3000

xoaphexox

xoaphexox

Supernatural Anesthetist
I finished restoring my Kenwood 700C preamp tonight. Thanks to the EW post for some ideas.

Until tonight my favorite preamp was my restored Sansui CA-3000. The Kenwood 700C has just become my new favorite. It sounds so musical I could not turn off the music!

Some things I did:

Replaced the main filter caps with new screw terminal caps
Replaced the relay with a new one
Replaced the failure prone transistors on the buffer amp
Replaced the electrolytic caps with new Elna Silmic II or Panasonic FC
Replaced the signal path caps with new Panasonic or WIMA Polypropylene film caps (except for the mic amp - who uses that?! :) )
Replaced choice electrolytics in the signal path with Soniccap Gen I's (bypassed the 10uF in the tone controls with some .001uF Gen II's)
Replaced the diodes with ultrafast ones
Replaced the zener diodes on the power supply board
Replaced the resistors in the signal path with PRP 1% metal films
Resoldered all the joints
Cleaned the posts with a Q-Tip and metal polish
Replaced all the fuses
Cleaned and lubed the controls with Deoxit

I love this preamp!
 

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WOW that's some beautiful work :thmbsp:

Could you tell me more about the diode replacement: What did you use? From where? Do you have link(s) to the thread(s) where you learned about this mod?
 
Thanks.

The diode modification information is what interests me most. It that the thread you linked or another thread?
 
It might be the thread I linked, I am honestly not sure, I did a google search for "kenwood 700c" site:www.audiokarma.org and scoured the threads, taking notes before I started the restoration.

Here was the quote I copied:

"It is also not totally out if the realm of possibility that the rectifier diodes are at the root of the problem. This would be Dk1 ~ Dk4, and I'd certainly replace these four (UF4004 or some such). There is the small bridge labeled Dk6, but this supplies the voltage for the relays only. Dk9 ~ Dk12 are solely for the +/-5V supplies for the headphone amp. These latter two could be injecting noise onto the ground line, but I doubt it. "
 
I would not replace the original rectifier diodes unless one went bad. They are unusually quiet, and it's a lopsided coin toss as to whether a modern UF4004 is as quiet. But, there's good filtering anyway, so it probably doesn't matter much. I simply would not replace them expecting to see any improvement in noise level.

xoaphexox, you've got a burnt resistor in the power supply to replace. ;)

Those Sonicaps are butt-ugly, but probably sound OK. I don't like using big axial lead speaker caps simply because when you open the unit they scream "I've been modified!!". At this point, after doing 20+ 700C's, my parts choices have evolved to the point that, when you open the unit up, it looks as if the unit could have shipped from Kenwood that way. Stealth mods.
xoaphexox said:
It might be the thread I linked, I am honestly not sure, I did a google search for "kenwood 700c" site:www.audiokarma.org and scoured the threads, taking notes before I started the restoration.

Here was the quote I copied:

"It is also not totally out if the realm of possibility that the rectifier diodes are at the root of the problem. This would be Dk1 ~ Dk4, and I'd certainly replace these four (UF4004 or some such). There is the small bridge labeled Dk6, but this supplies the voltage for the relays only. Dk9 ~ Dk12 are solely for the +/-5V supplies for the headphone amp. These latter two could be injecting noise onto the ground line, but I doubt it. "
Click to expand...
This was on a unit that was having serious noise problems, and the issue was eventually addressed by replacing the zeners.
 
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OK thanks again.

I am just about to get started with my first "project" but I am thinking forward to some other ones down the road where I may try beefing up the power supply (in integrateds, not preamps) with larger PS caps as well as faster or better rectifiers.

I will read through the link and try the search as you suggest.

Really love the work you did there...all those big caps look awesome. And the worksmanship looks 1st rate.
 
EW: which resistor are you referring to? I think you may be mistaking a resistor that had a tiny ceramic cap soldered to it at the factory. The shadow of the cap on the resistor does kind of look like a burn mark in the photo. The other one that looks like it might have a burn mark is the black stripe on the 5.6K. The resistors on that board measure to spec.

I like the sound of Soniccaps. The size doesn't bother me and the price is right, too. I think they sound lovely. I suppose the generic white stickers on them does have a "DIY" look to it.

Thanks for posting information on the web for people like me to use, it's greatly appreciated, and as they say "imitation is the greatest form of flattery".

PS would something like a hexfred or stealth II lower noise, as opposed to the UF4004? Any benefit to be had by bypassing the filter caps with a .47uF poly cap? You know I can't leave "well enough" alone... :)
 
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There's a 5.6K 1/4W resistor (some units have a different value) right next to Q1 on the reg board that is commonly toasted. Simply looked as if yours was also in need of replacement.

I did not mean to belittle your work...it looks fine. I simply have another goal in mind besides changing to the film caps (and this goal is common to any piece I work on): 'If at all possible, try and make your work look as if it was what the original engineers intended'. The motivation to meet this additional requirement colors my part choices every day, and sometimes it does make the job of finding the proper part harder, but that's what I get paid for.

Didja polish all those male pins on the all the PC boards? You've got a lot of low-current paths and low-level signals traversing that wiring maze. Clean connector pins help more than a little to see that the signal makes it from one board to another unmolested. Yes, the PC boards behind the front panel are a bitch to do.
 
After having owned both preamps, I am not in the least bit surprised you came to the conclusion that you did. The 700C is the best solid state preamp I have ever owned, period. :yes:
 
EchoWars said:
Didja polish all those male pins on the all the PC boards? You've got a lot of low-current paths and low-level signals traversing that wiring maze. Clean connector pins help more than a little to see that the signal makes it from one board to another unmolested. Yes, the PC boards behind the front panel are a bitch to do.
Click to expand...

Yes, that was quite the chore. I had to step away for a break, I just couldn't do it all in one sitting. I used Q-Tips and Mother's Chrome Polish, then cleaned off the polish residue with some ammonia free Windex and a soft cloth. They looked shiny and new. I tried to find Brasso per your previous suggestion in another thread but they don't sell that around here.

Royal PITA but with as dark as those pins were I could tell it was an important step.

I can respect wanting to keep the amps looking original despite the amount of work you put in to them. I have never restored anything for anyone except myself though, so the pressure is not as high :D
 
hello echowars

i have followed your work on AK (it inspired me) and had 2 700c, currently only one at home now.
i have processed recaping, changing some of the signal caps for film good quality ones.
one modification at time to listen the effect.
for now i have processed some more modification.

first i changed the diodes (8 power diodes)for very fast ones in to220 package. i must confess that i expected nothing really in the sound. i was wrong , from quiet as the 700C can be (we all know about this) this preamp got extra quiet, unearthing some very faint information and getting sweeter at the same time.
i strongly suggest to other 700C owners to make this change. it is better than changing the signal caps.

on the other hand i considered the signal path....how can this unit be so transparent with a signal following meters of wire and never stop going from back to front from one selector to another one etc...
i decided to go to massive surgery. and directly wired the rca input to the buffer amp. in this i sacrify the input selector and the monitoring selector. i know this is rude and i have read what echowars told trying to follow the designer spirit when modyfying.
so i may be called silly by others but i tried.

did you think that you have already ear the real potential of this unit? if yes believe me you are far from the reality. doing this has opened this machine in a way that is simply amazing and a makes tremendous change with the classicly wired thing.

main change is in the bass that goes deeper than any preamp i have had before. it is like adding a very performant subwoofer in your system. more you get tons of vibratos in the voices and very subtle inflexions in the music that was not audible previously.

rude changes but huge benefits.
 
villajmarc said:
first i changed the diodes (8 power diodes)for very fast ones in to220 package...
i strongly suggest to other 700C owners to make this change. it is better than changing the signal caps.
Click to expand...

That is interesting. I used UF4004 for these diodes. I have some HEXFRED and STEALTH TO-220 rectifiers on hand - I wonder if there is really an audible lowering of the noise floor - I would imagine other components would contribute to the noise as well like transistors and resistors. I changed the signal path resistors in my 700C to the little red PRP audio resistors. I think it sounds pretty good.

Since you seem to be interested in taking this restoration to the next level, have you considered replacing all the transistors with modern low noise audio transistors like the KSA992 and KSC1845?

Why stop at just diodes, caps, and signal path wires? :D

PS I never even knew Snoop Dogg used a piano in the background on his Doggystyle album until I listened to it through a 700C. Really exemplary!
 
please echowars , see my post as no offense to your work and philosphy in restoration.
so far i've been, all of these mods regarding the signal path are completely reversible. it is quite simple when you see the overall construction made of boards and connectors.

the diode i used are from ST electronics type STTH8S06FP they have ultrafast recovery time of 12ns. found them on the bay. this is quite cheap considering the improvement.

as EW mentionned in the 700C nudies some cost cuts leads to parts selections. considering the overall improvement in passive components these 10 last years and the price of those exceptionnal components i am wondering wether the kenwood team could have used it if available at that time?

i believe that this preamp is a true hi fi gem, in terms of overall building quality, global layout and global performance. but the versatility of it and its capabilities may alter the final ability of the machine. as far as i do not record nor have any recording machine loop, and that i only use this preamp after my digital combo (another anachronism) i feel comfortable shunting the source selector and the tape monitor.


other identified bottlenecks are: the final use of resistor at the output rca, the attenuator itself (necessary giving the very high gain of this preamp)
the tone controls and loudness switches , the relay board just before the outputs.

on the other side i am not enough skilled to consider changing the transistors for new one. i am at stake when considering new transistors as being better in sound than older ones. in this case i believe that some sonics attributes of the 700c belongs to some active parts choices.

i am too conservative in some areas.:cool:
 
Those rectifiers are neat; 3ns faster on average than the Stealth II.

Let us know if you decide to do anything else to the unit. Always interested in hearing about it!
 
The 700C seems to polarise opinions somewhat.Put me in the fan category.I may never use my stock 700C anymore,nor my EW modded 700C.However I remain a fan of this preamp.

I upgraded from the 700C to the Lightspeed attenuator(a passive attenuator).For all it's complexity the 700C wasn't blown-away by the passive attenuator.It just sounded like the 700C virtues had been extended in the Lightspeed.This result may change in different systems however.Passive attenuators are prone to impedance interaction problems.

Enjoy your preamp xoaph. :thmbsp:
 
Question about the 700C. My service manual says the red wires coming off the transformer are putting 47 volts in to the power supply board. I am measuring over 70V here, though, with my DMM.

I also measure about 70V across the leads of the ripple filters when energized.

Does this sound right?

I am trying to fish out an issue with my power supply board and want to make sure I am getting the correct values coming in to the PS board.
 
xoaphexox said:
Question about the 700C. My service manual says the red wires coming off the transformer are putting 47 volts in to the power supply board. I am measuring over 70V here, though, with my DMM.

I also measure about 70V across the leads of the ripple filters when energized.

Does this sound right?

I am trying to fish out an issue with my power supply board and want to make sure I am getting the correct values coming in to the PS board.
Click to expand...


Try measuring after the regulators and see what the voltage is there. :scratch2:
 
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