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Old 13th February 2011, 06:46 PM   #1
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Default Fixing a Kenwood 700c preamplifier - help please!

Hi everyone! I am new to diyaudio but have been a lurker for a while. It is a fantastic site and I love spending time reading the threads here - having found many an answer I was looking for. I greatly respect some of the renowned members here and their knowledge.

I am a new owner of a Kenwood 700c preamplifier. I have some questions about improving the circuitry in this preamp and being a novice, only have a general idea about what can be done to bring it to 2011 - maybe at least half the way from 1974.

Here are the links to the schematics thanks to fotoalbumet.com:

http://www.fotoalbumet.com/u/00/24/4...del-700C_1.PNG

http://www.fotoalbumet.com/u/00/24/4...del-700C_2.PNG

I cannot seem to find any issues with the first buffer stage and the filter stage - except that the low pass filter stage has the 2.2K ohm resistor acting like a base stopper and has no resistor load at the output.

The tone control stage has no CCS. A good CCS would be an effective improvement. The Miller cap is ceramic, I will change it to a silvered mica. Feedback can be altered in this section as well, possibly eliminating global feedback. I discovered no local decoupling in the tone control board, so I will add a 100uF electrolytic and .1 uF MKP on the positive and negative rails.

The phono stage has a high working voltage, in part to give the CCS resistor a more ideal character. I cannot see anything that could be done except that the 47 pf cap on the LTP is a bit high; there is no capacitive loading of the cartridge as well.

The beauty of this preamp is that everything is modular with plenty of space to work, so I could even design PCB's with the same Amphenol connectors and swap them out without really making any disfiguring changes that would alter its original character.

I ask all members of this forum to give me advice, to confirm if the statements I made are right and possibly give me advice on how to proceed. Thank you all for your time and for helping me out
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Old 13th February 2011, 09:23 PM   #2
DF96 is offline DF96  England
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I can't help with your audio problem, but I can give you some advice. Use a thread name which actually says what you want to know about e.g. "advice on modding Kenwood 700c preamp". That might attract people who can help.
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Old 13th February 2011, 09:43 PM   #3
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Thanks DF96. I apologize. Do I have to let the moderators know to edit the thread name?

I re-read my previous post and realized that I was not very specific. When I wrote that there was no CCS I meant that the input LTP's in both the tone and phono stage have a simple resistor/resistors in series doing CCS duty - according to the KISS principle but my question was why not improve upon it with a discrete or IC two lead version in place of the resistor/resistors.

I'd love to hear any comments on this. Thanks in advance for your comments - I will figure out what I should do to change the name of the thread.
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Old 13th February 2011, 09:59 PM   #4
Pano is offline Pano  United States
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Done.
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Old 13th February 2011, 10:39 PM   #5
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Thanks a lot!
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Old 21st February 2011, 04:39 PM   #6
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Hi all

Obviously I asked some questions that were way off and made totally wrong assumptions in my first post. I am still learning - my background is not EE. I apologize for the dumb questions but the really dumb ones are the ones not asked.

I now have a specific issue with the 700c. I have replaced all electrolytics with Nichicon HE caps; signal caps have been replaced with film caps where there were electrolytics due to Kenwood's cost cutting decisions in the course of manufacture - there are holes in the PCB for larger film caps so it was just a matter of returning everything back to what the original engineers intended.

I have been using it for the past 2 weeks (roughly). I did not believe in burn in at first. I am a convert now. I was actually appalled at first. The treble has slowly revealed itself now. I can hear high hats and cymbals well. Thing is, there is still one "issue". Strong non-distorted treble information lack the attack and sharpness I find in my other preamp which is a current model from Parasound. Granted, I am comparing a 40 year old preamp to SOA. But I have not heard any other, even older preamps behave like this. The treble attacks in music including SS sibilants get distorted whereas I can hear them clean and sharp with the Parasound. This happens in both channels.

Now coming to the point - I used a Nichicon HE 100 uF cap for the emitter bypass caps C17 and C18. I read about the emitter bypass Ce cap and how to determine its value.

My question is why not a higher value Ce cap to bring the cutoff frequency lower than 106 Hz?

Please correct me if I got all this wrong: the Ce value is chosen so Xc is 10% of the resistor value.

Lastly, am I right in realizing now that these caps should have been a Cerafine or Silmic with a 1 uF FKP bypass?

I may be wrong in learning some of these electronic basics, please correct me if I am wrong. When I have the time I am going to get hold of a good book. Please give me some recommendations – all replies however critical and harsh will be much appreciated
Attached Images
File Type: jpg Untitled-1 copy.jpg (193.5 KB, 104 views)

Last edited by Novice62234; 21st February 2011 at 04:46 PM. Reason: Attached schematic
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Old 22nd February 2011, 04:06 AM   #7
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It was the emitter bypass caps! The first step I took was to take the Nichicon HE's out. The improvement was not subtle. Bass warmth suffered a little but the rest of the spectrum sounded so good I was tempted to leave it that way.

I've installed 100 uF SilmicII's now, currently burning in. It already sounds a lot better than before, but not quite like how it did without any cap. I think I am close to realizing why so many hold this preamp in such high regard.
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Old 1st August 2011, 10:42 AM   #8
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You might want try posting at audiokarma they have people who can help you out with the 700c.
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