Kenwood Supreme 700M Subsonic question?

[Lameduck]

Kenwood Supreme 700M Subsonic Filter restore question?

I recently acquired a 700M and have restored most part of it myself. I like to do the physical (technicial work) myself and thanks for the available info from AK users and specially much info from Echowars.

Since its restoration and it has been playing good for a couple months driving my older Merlin VSM speakers. I mainly use them for Jazz and Vocal and Acoustic classical. With MF VDAC II and a Kenwood 700C preamp I feel kind a good about the muscial result.

My question is about restoring the 700M subsonic filter, which is mounted near teh amp's front panel. From read its schematic, A and B inputs are fed to their selector knob and then to the subsonic filter before to their left and right main amp section. Whether the subsonic is turn-on or off, its audio signal will pass thru its input and output Mylar caps (plus 2sa620s BJT) before reaching its main amp section. When the filter is turn-on, its audio signal will pass additional 2 Mylar caps for its subsonic filtering.

I want to find out if any Ak users had experience on this part of the amp circuit. I googled it and only found restoration on the main section. Any info will be greatly appreicated.

attached before and after restored photos

 

[EchoWars]

Two-pole, 12db/octave. Pretty basic. What is it you are wanting to know?

 

[Lameduck]

Hello EW,

I am trying to find out if any ak user had experience with re-capping that filtering circuit. I am kind of curious to know since much work had been posted on the main amp circuits but I have not find any posting about restoring the filter circuit and it sits on the signal path in front of the amp's gain control.

I try not to just restore it and later found out the change or improvement was not worthwhile. EW, what is you thinking about recapping this filter?

 

[mike d]

Every time I see a 700-m I resurrect my secret hope that mine can one day be restored.:thmbsp:

 

[EchoWars]

Hello EW,

I am trying to find out if any ak user had experience with re-capping that filtering circuit. I am kind of curious to know since much work had been posted on the main amp circuits but I have not find any posting about restoring the filter circuit and it sits on the signal path in front of the amp's gain control.

I try not to just restore it and later found out the change or improvement was not worthwhile. EW, what is you thinking about recapping this filter?

Nothing terribly wrong with the stock film caps on this board. Might be able to lower distortion by replacing the stock 2SA620 with a modern 2SA970BL. But while you can probably upgrade the stock film caps a bit (some Panasonic metallized polypropylene for Cs5,6,7 and 8), I doubt the gains would be worth the effort.

 

[Lameduck]

mike_d:

Restoring the 700M is fun to do but take abit time. I have spent around ~2 weeks to disassemble and reassemble with new part. the amp was down to its chassis, xformer and its chassis wiring. It's fun and I learned how it was made. I mean history has shown that it was a reliable design and its design was easy to maintain. Each module can reassemble without needing a soldering iron. (except a minor meter rework). in the process I can use a DMV to check if voltage are clean or noisy.

EW, thank you for your info and this is the info that I need. i have been comtemplating why the OEM Ce1 has a polyester cap and Cs5, Cs6 were just Mylar caps since they are on the same signal path. I guess it could be either the subsonic transistor base current is less or was the emitter follower vs diff common collector configuration between the filter and the main amp input. I guess I will hold off the cap update for now.

 

[EchoWars]

FYI, Mylar is simply a brand name for what is otherwise simply a polyester capacitor. The two main types of film caps are polyester (Mylar) and polypropylene, with polypo being considered superior, but larger.

 

[Lameduck]

FYI, Mylar is ..

Thanks for the info, LD