Kenwood Basic C1 Preamplifier Restoration/Upgrade

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by Leestereo, Aug 14, 2014.

  1. Leestereo

    Leestereo Super Member

    Messages:
    2,059
    Location:
    Ottawa, ON
    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:

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
     
    Last edited: Aug 27, 2014
    Rodrigan, Tttank and 24Vboost like this.

     

    Please register to disable this ad.

  2. Lavane

    Lavane AK Subscriber Subscriber

    Messages:
    7,083
    Location:
    northern cal.
    It looks like the Kenwood tuner I had from about the same time period, kind of empty looking. It was the first piece of stereo equipment I bought with IC's. Kind of telling my age.:D I was shocked at how sparse it was on the inside. Sounded nice though. :thmbsp: not thinking about how much was in the chips I remember thinking I got ripped off. Now I think its really cool how much a small smart phone can do. :yes:
     
    doctor fuse likes this.
  3. Leestereo

    Leestereo Super Member

    Messages:
    2,059
    Location:
    Ottawa, ON
    I know what you mean about half-empty cases; I guess it all depends on one's expectations. The upside is that with less stuff in the box, the signal path is simpler...less is more, right?! :scratch2:
     
  4. Lavane

    Lavane AK Subscriber Subscriber

    Messages:
    7,083
    Location:
    northern cal.
    I agree. As a kid we had tube TV's, stereos ect. Lots -0- parts. I remember buying a new TV and it having one thin board and the tube. It had a better picture and stereo sound than the old one. But when I opened up that tuner and found one very small board and a small transformer I I felt ripped off. I'm over that now though. :D it was just "Future Shock" I guess. A friend bought the same pre with the amp your working when it first came out. I liked it a lot. It was the main reason I bought a Kenwood tuner. I liked how his sounded and looked.
     
  5. Leestereo

    Leestereo Super Member

    Messages:
    2,059
    Location:
    Ottawa, ON
    Power Supplies

    The main power supply capacitors (C43, C44) were originally 1000µF/35V and the replacements were 2200µF/35V low ESR types. The main rectifiers (D1 & D2) were replaced with UF4005 soft recovery types. The rectifiers D6 and D7 (obscured by R65 in picture) were originally 18V zeners, but since this voltage is at the maximum recommended for the line stage op amps, the replacements were precision (2%) 16V BZX85B zeners. The stock capacitors C45, C46 which follow the zeners were 330µF/25V, and these were replaced with 470µF/25V low ESR types.

    The stock capacitors in the regulator (C47, C48, C49), the LED indicator (C50) and relay drive circuits (C51) were replaced with low ESR types of the same capacity.

    [​IMG]
     
    Last edited: Aug 27, 2014
  6. Leestereo

    Leestereo Super Member

    Messages:
    2,059
    Location:
    Ottawa, ON
    Flat Amp Stage
    The stock flat amp stage op amp (IC1), a NJM2043, was replaced with a NJM2068DD op amp (a low-profile DIP socket was also installed). The stock capacitors in the input high-pass filter (C3, C4) were 1.0µF low leakage electrolytic capacitors and these were replaced with 1.0µF Panasonic stacked polyester film capacitors. The stock capacitors in the low-pass filter (C5, C6) were ordinary 100pF ceramic capacitors; the replacements were C0G capacitors of the same value. Similarly, the 68pF ceramic capacitors in the feedback loop (C11, C12) were replaced with C0G capacitors of the same value. The stock 220µF/6.3V polarized electrolytic capacitors in the feedback loop (C7, C8) were replaced with Nichicon Muse ES bi-polar electrolytic capacitors of the same capacity but higher voltage (16V) rating. The output capacitors (C13, C14) were 22µF bi-polar electrolytic types and these were replaced with Nichicon Muse ES bi-polar electrolytic capacitors of the same capacity.

    Tone Amp Stage
    The stock tone amp op amp (IC2), a NJM4560, was replaced with a NJM2068DD op amp (a low-profile DIP socket was also installed). The stock capacitors in the input high-pass filter (C19, C20) for IC2 were 1.0µF low leakage electrolytic capacitors and these were replaced with 1.0µF Panasonic stacked polyester film capacitors. Similarly, the stock capacitors in the tone control inputs (C21, C22), which were 1.0µF low leakage electrolytic capacitors, were replaced with 1.0µF Panasonic stacked polyester film capacitors. The 10pF ceramic capacitors in the feedback loop (C27, C28) were replaced with C0G capacitors of the same value. The ordinary ceramics at the output of the tone amp stage (C29, C30) were replaced with C0G capacitors of the same value. The final output high pass filter capacitors (C25, C26) were 47µF bi-polar electrolytic types and these were replaced with Nichicon Muse ES bi-polar electrolytic capacitors of the same capacity. Also, the stock 1.2kohm load resistors (R41 and R42) were replaced with 56.2kohm metal film resistors.

    [​IMG]
     
    Last edited: Aug 27, 2014
    Rodrigan likes this.

     

    Please register to disable this ad.

  7. Leestereo

    Leestereo Super Member

    Messages:
    2,059
    Location:
    Ottawa, ON
    Phono Stage

    The phono stage has its own regulated power supply and its rectifiers (D1, D2) were originally 20V zeners, but since this voltage exceeds the maximum recommended for the phono stage op amp, the replacements were precision (2%) 18V BZX85B18 zeners. The capacitors (C25, C26) which shunt the zeners were replaced with 100µF/35V low ESR types. And the stock 100µF capacitors (C27, C28) which filter the regulated output were replaced with 220µF/35V low ESR types.

    The stock op amp (IC1), a NJM4560, was replaced with a NJM4580DD (a low-profile DIP socket was also installed). The 100pF ceramic capacitors (C1, C2) at the input of the phono stage were replaced with C0G capacitors of the same value. The large electrolytic capacitors (C5, C6) in the RIAA feedback were 2200µF/6.3V and these were replaced with low ESR capacitors of the same capacity but higher voltage (16V) rating. A bi-polar electrolytic would have been the preferred replacement (as was done for C7, C8 in the flat amp stage) but the capacity of the originals limited replacement choices. The stock capacitors (C11, C12) in the output high pass filter were 3.3µF/50V bi-polar electrolytic types and these were replaced with WIMA 3.3µF film capacitors.

    [​IMG]
     
    Last edited: Aug 27, 2014
  8. Leestereo

    Leestereo Super Member

    Messages:
    2,059
    Location:
    Ottawa, ON
    Sound Impressions of Restoration/Upgrade

    Before I started the Kenwood C1 restoration/upgrade, I listened for a couple of days to it paired to the M1 or a restored harman/kardon hk770 or a Marantz 170DC. The listening sessions used CD and LP recordings that I'm very familiar with (some examples): Steely Dan/Aja; Miles Davis/Kind of Blue; Dire Straits/Love Over Gold; Beatles/Abbey Road; Fournier/Bach Solo Cello Suites; Duke Ellington Orchestra/Digital Duke; Karajan, 1962 Beethoven Symphony cycle. I even took a few notes of these initial listening impressions since I didn't want to rely on just my sonic memory since it would be a couple of months before I could get to the C1 restoration (a Marantz 2235B was currently on the bench). The C1 as received was only fair sounding; with the phono stage noticeably poorer than the line stage. The bass was overly full(bloated), mid-range and treble range were recessed, further back in the sound-stage, the treble had a brash, overly sibilant quality.

    The restoration/upgrade of the C1 significantly improved the sound of the preamplifier. It now sounds quite similar to a restored Hafler DH-110 now: overall, very clear and neutral, with a deep and "tight" sounding bass range. The treble range is very natural sounding with lots of detail (e.g., the different cymbal sounds are easily discerned) and the reproduction of the middle frequencies/vocals is now very realistic; singers sound "live" in the living room (the little vocal "ticks" and 'hiccups" are easily heard, and "mumbled" lyrics are easier to be understood, e.g., Morrison in L.A. Woman. Previously, Unican Eric used a tube preamp (http://www.audiokarma.org/forums/showthread.php?t=580444) with the M1 since it was so much better than the stock C1, he has since told me that is no longer the case: the restored/upgraded C1 is more than capable of excellent sound.

    [​IMG]
    Restored/upgraded Kenwood set sitting pretty

    [​IMG]
    Old parts removed from C1
     
    Last edited: Aug 27, 2014
    Rodrigan likes this.
  9. Lavane

    Lavane AK Subscriber Subscriber

    Messages:
    7,083
    Location:
    northern cal.
    Very nice work, as always! :thmbsp:
     
  10. Leestereo

    Leestereo Super Member

    Messages:
    2,059
    Location:
    Ottawa, ON
    Thanks for the compliment, Paul. How did your 3200 preamp re-do turn-out?
     
  11. Unican_Eric

    Unican_Eric Pioneerious Collecticus

    Messages:
    3,043
    Location:
    Cornwall,Ontario, Canada
    As I sit here and listen to Stevie Nicks (Edge Of Seventeen) on my restored M1 and C1, I am floored on the sound of these units and now is my main system.
    My Pioneer SX-1250 is been downgraded to TV and Movie listening, never thought I would say that.

    Can't thank you enough Ben for your awesome work. :thmbsp:
    My 1250 is calling your name!!
     

     

    Please register to disable this ad.

  12. Lavane

    Lavane AK Subscriber Subscriber

    Messages:
    7,083
    Location:
    northern cal.
    Its still a bright sounding preamp but not so harsh sounding now. Its part of my wife's system and she is very happy with how it turned out and I can listen to it now without frowning. Thanks again for your suggestions!
     
  13. Leestereo

    Leestereo Super Member

    Messages:
    2,059
    Location:
    Ottawa, ON
    You seem to have a penchant for using female vocalists as your reference test: I recall that you tested the Philips tube preamp with Linda Ronstadt. :tresbon:

    So that's what keeps waking me up in the middle of the night!:scratch2:
     
    Rodrigan likes this.
  14. Leestereo

    Leestereo Super Member

    Messages:
    2,059
    Location:
    Ottawa, ON
    Glad to hear that the changes worked out well; a very happy wife is always good!:D
     
  15. vsat88

    vsat88 Like my Bluetooth ?

    Messages:
    559
    Location:
    Phenix City, AL.
    I have a C1 and a C2. I would like to do these as you did this one. Did you save the BOM list ? Would the parts be the same (that need replacement) for the C2? Thanks.
     
  16. botrytis

    botrytis Trying not to be a Small Speaker Hoarder Subscriber

    Messages:
    29,343
    Location:
    PODUNC USA......
    LS - could you do a step by step? I have a C1 and I think it needs this....

    Just like VSAT said :D
     

     

    Please register to disable this ad.

  17. Leestereo

    Leestereo Super Member

    Messages:
    2,059
    Location:
    Ottawa, ON
    Unfortunately, I did not compile a BOM specifically for the Kenwood C1 restoration since I already had many of the needed parts in my own parts stash. Also, in most cases, there are several different capacitors that are appropriate (and I consider the type of capacitor more important than the specific brand/model). For low ESR types, I typically use capacitors from Panasonic (FM and FR series), Nichicon (PW series) and United Chemi-con (KZE, KYA, and KYB series). For signal path duties, I typically use Nichicon ES bi-polar types to replace larger electrolytic capacitors; for smaller sizes (e.g., <4.7µF), I will use Panasonic or WIMA stacked film types. For C0G capacitors, I will typically use AVX brand, rated at 100V. All of the components used in the C1 restoration/upgrade were sourced from Mouser.

    I do try to put enough detail in the thread for others to replicate the work, but if you have any specific question(s) I would be more than happy to try to answer them. Please post your question(s) in the thread so others may benefit from the answer.
     
  18. vsat88

    vsat88 Like my Bluetooth ?

    Messages:
    559
    Location:
    Phenix City, AL.
    Awesome thank you. Great work and I am sure I could catch on to what is going on by seeing what you have done as well as asking a few questions..I have a C1 and a C2 I would like to do after I finish (do not think I will ever truly finish it) my Fisher 500C that is on the bench now. I do appreciate your offer to help us thru these and I will post in this thread.
     
  19. mark9930

    mark9930 New Member

    Messages:
    19
    Location:
    United Kingdom
    Kenwood C1 Upgrade

    This is a great upgrade that really puts the C1 into a different class. I performed a similar upgrade on a C1 i have. The pots were all replaced on mine also. Bass is much tighter and treble less harsh than it was. All the instruments are now easily discernible where they were slightly muddy before. Brilliant write up by the way.
     
  20. vsat88

    vsat88 Like my Bluetooth ?

    Messages:
    559
    Location:
    Phenix City, AL.
    Post about C 2 restore

    The C2 I have does this. I turn it on and it sounds good, not great but good. After about five or ten seconds the volume drops lower on both channels evenly and the unit sounds not as good. I really do not know what would cause this so I intend to order all the electrolytics as I know they dry out over time and I would think it a good deal from what I have read to replace the Op amps. I don't know about replacing the rest of the capacitors (Ceramic and Mylar) as I don't want to bite off more than I can chew.

    So, here is where I started. I DL a copy of the service manual from HiFi Engine (Thanks y'all) . Listed all the values of the capacitors 21 in total listed In the manual. I noticed that the manual lists some capacitors as "ELECTRO" and some as "NP-ELEC" so I am guessing that the ones listed as "NP-ELEC" are electrolytic non polarized ??
    I did some research and the Op amp replacement that some recommend is the NE5532. I do not know if that is what I should use or not. I see four listed in the service manual so I guess I need all four unless some of these are not to be replaced?? It also puzzles me that two are the same part numbers and the other two are completely different. Maybe I just replace the two NJM4560D-N Op amps and leave the rest ?? The manual shows three different IC/Op amps.
    1: (Ref. IC1) NJM4560D-N
    2: ( Ref. IC2) NJM2041D-N
    3: ( Ref. IC3) M521BL
    4: ( Ref. IC1. No. ,2) .NJM4560D-N.
    I wonder if these are the same on both the C1 and the C2?

    I am still not sure what capacitors to order. I do have all the values however I would like to upgrade. I will order from Mouser as the thread says so hopefully I will get someone on the phone who will make good recommendations for the types I need. OK, now that I posted all this I hope I have not confounded you all as much as I have myself trying to figure on this ! Thanks all..
     

Share This Page