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Krell KRS-2

Discussion in 'd.i.y.' started by earthfield, Jun 27, 2014.

  1. earthfield

    earthfield earthfield

    Hi Guys,

    After three years of hunting and waiting, here are the before and after images of the Krell KRS-2 preamplifier refit, now beyond the standards of Krell's spec, updating Brian Eno's recomendations and now sonically sonorus.....


    [​IMG]


    [​IMG]


    [​IMG]
     
  2. S_J

    S_J pfm Member

    Top job! What's the volume pot you used? It looks familiar but I can't recall what they're called.
     
  3. YNWOAN

    YNWOAN 100% Analogue

    Very nice job - congrats and enjoy :).
     
  4. sq225917

    sq225917 situation engineer

    Penny and Giles. Nice return.
     
  5. earthfield

    earthfield earthfield

    This three chassis KRS-2/ krsp-2 was in full working order on arrival, it was clear after a full appraisal of this sonic legend, that it needed to be brought into the new millennium.

    The guys at Krell were a great help, however many of the masters of data for KRS series are lost, they made up for it with internal discussions and advised on aspects that would improve clarity and performance. This took over a year to collate.

    Naturally with a Class A preamp of this vintage, electrolytic caps need replaced, this was done with Elna Silmic II's, Nichicon FG's and for the last stage of smoothing, Vishay 021KO's (previously 021ASM) with their non resonant cans. These update the likes of Panasonic FM's and older Nichicons types with specific focus on keeping the ESR low as possible….and they are all matched. The elnas were superb with no need for buying more to match, the vishays required three times as many in order to match and the nichicons needed around six times the order quantity to get a match.

    The potentiometer was replaced with a Penny & Giles RF15A 10K Model D219992, what a difference from the Spectrol wire wound! Six month waiting time.

    inter PCB connections were re-done with 99% silver/ 1%Gold wire.

    I have yet to replace the internal signal wiring on the pre chassis. It looks like a balance patch type cable, and there is a lot of it.
    The smoothing chassis has yet to have poly film cap filters replaced due to proximity to heatsinks and Motorola equivalents for the discrete tracking regulators have been sourced.

    It has a total 212,000uF smoothing capacitance.

    The fuse for 240v use is a 1.5A slow blow.

    the sound?

    there is none…

    I'm hearing the music, the magic of studios and full depth of clarity and reveal.

    Eargasm!


    prior to refit:[​IMG][/IMG]

    any suggestions for replacement? these have been considered: Vandamme 268-026-000 starquad cable but the diameter is too large, so perhaps 268-000-000 reduced OD console cable.
     
    rontoolsie likes this.
  6. rontoolsie

    rontoolsie pfm Member

    ?HOW much capacitance???? This is enough for a 300 wpc amplifier. My NAP500 had 120,000 uF. Too much is never enough here. I would have suggested using some of the naked Texas Components resistors, but it looks like there are at least 100 (or more) resistors on the PCB.

     
  7. earthfield

    earthfield earthfield

    The Krell KRS mono was where the hunt began.....

    [​IMG][/IMG]

    273,920uF per channel....
     
    rontoolsie likes this.
  8. darrenyeats

    darrenyeats pfm Member

    I owned a Krell KSA-100 class A power amp. The capacitors were literally the size of fizzy drink cans! Ridiculous (-ly good sounding!)
     
  9. earthfield

    earthfield earthfield

    ...after a long chat with the main technical inovator at Mundorf, along with my own experiences, it became clear that for heavy current delivery in power amps, it's best chaining PSU cap's of 10,000uF or under to make up what ever you need, the lower esr dumps the current without delay, all important for that transitory bass response or slam.

    Would love to hear a fully refitted KAS mono.
     
  10. MLAS

    MLAS New Member

    Great pre-amp from Krell.
    Did this in february 2009:

    [​IMG]
     
  11. earthfield

    earthfield earthfield

    Finally! it's complete overhaul....

    The krsp-2 is the smoothing chassis; the regulators were all replaced with low noise equivalents, some looked a wee bit stressed, the voltage was a few volts off between channels. All balanced now. Some small capacitors replaced too.

    The KRS-2 pre chassis has the same regulator replacement, all the heatsinks now run at the same temperature.

    Intermittent problems with the in's and out's prompted their entire replacement with new Krell rca's.

    The internal pcb wiring was done with van damme 268-068-040 and took hours.

    All other wiring was replaced with silver with a touch of gold.

    In the initial refit of the above, the Pot by Penny and Giles D19992 was found to be unbalanced between L+R at very low volumes, which was incredibly frustrating having waited six months for it. It turns out that the silver oxide manufacture was changed in the middle of the recession. After long discussion with Penny & Giles, an older type with higher tolerances was resurrected and produced... another three months... however the results are world class, Its model/ type D460355/C and the best available, i'll upload the spec sheet soon, as this info needs to be out there.
     
  12. sq225917

    sq225917 situation engineer

    Nice, I've done two total rebuilds on ksa 100 amps. Nice to work on but the PCBs are like cardboard.
     
  13. James

    James Lord of the Erg\o/s

    Well, my little 30 wpc Pioneer M-22 has 4 x 33,000uF cans. That's pure class-A for you.
     
  14. earthfield

    earthfield earthfield

  15. earthfield

    earthfield earthfield

  16. earthfield

    earthfield earthfield

    The above is the penny and giles data sheet from the archive;
    Model No. D460355/C Notice how the max differential between left and right tracks is specified at a maximum of 0.5dB, that -as it turns out- was the critical figure! The D19992 was a max deviation of 2.5dB.
    The best potentiometer out there, that is unless you go to rotary fixed resistors....and some other exotica that may well out live my grandchildren!

    I would wax lyrical about how good the finished three chassis Pre sounds, or rather how much it's "not there", but language and images tends to fail on all accounts when the description of reproduced sound is concerned. This exceedingly lengthy, "cost no object" refit and the expensive information it has yielded is hoped to be use to those who venture for the best quality....
     
  17. earthfield

    earthfield earthfield

    Thanks goes to Krell for their extended patience and answers.
    Thanks goes to Penny & Giles for going beyond the call and digging into their archive...
    Thanks goes to Hifi Collective, Farnell, R.S in that order.

    Unthanks goes to the guy who packaged it wrong.
    Unthanks goes to HMCustoms for being corporate parasites.
    Unthanks goes to everyone who ever got in my way and the banksters.
     
  18. earthfield

    earthfield earthfield

    "situation engineer"....didn't find that here, very solid PCB's well machined and etched, on speaking with senior engineers at Krell he mentioned that these were made in 87, at the peak just before the recession, and very very few were made, none have been returned and that they took a long time to get out the door. Only the slightest machining error to an aluminum back panel was noticed by my eye. They're the kinda pre that you turn on and leave on, nothing goes bump, boom, clunk, pop, one can just jam it into the mains with everything else on and it just silently fades in, serious design!
    Can't wait to get my vinyl player connected.....
     
  19. earthfield

    earthfield earthfield

    Is there someone aware of the line voltage of these pre-amps outputs and input range, please let me know.

    Fuse for 240v operation is 1.5A slow blow.

    There is an internal switch in the transformer chassis, which after a lot of pissing about and wrong info, it turns out that Krell hadn't the time to get the correct idents for this 120/240 switch in order to get them out the door, so best to check it from numerous of perspectives when taking it to 240v.

    This is the full compliment, the manual:


    [​IMG]
     
  20. hifi239

    hifi239 New Member

    I was the chief engineer at Krell at this time and was responsible for all aspects of the Krell KRS-2 design except for the case (which Dan did). I laid out the PWBs by hand with drafting tape. The restoration photos shown above are beautiful work! These preamps are gems, even among the other Krells I did, and well worth the effort.

    Will
     
    orangeart and rontoolsie like this.

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