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Rebuild of classic Krell KSA50 fan heater

Discussion in 'classic' started by Arkless Electronics, May 5, 2019.

  1. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Source was all digital so irrelevant as they all sound pretty similar. Spendor BC2 speakers, free interconnects that come with CD players etc of course. Music mainly the usual suspects of hi fi test quality which are also damn good music... "Jazz at the pawnshop", Joni Mitchell "Blue", Donald Fagen "The Nightfly", Bill Evens Trio "Waltz for Debbie" and "Sunday at the village vanguard" and all sorts of other stuff... Prefab Sprout "Protest songs" is another I recall... dodgy SQ on some tracks but very impressive on the delightful "Dublin" and "Talking Scarlet", and The Delines "Colfax" (much better than "The Imperial" IMHO). No classical was used. I do like classical but it forms a small part of my listening.
     
  2. herb

    herb music live

    Thanks Jez. I appreciate the quality of the Jazz recordings, though 90% of my listening is classical/real live classical. Did you once say that Naim CD players were not that bad? The liveliness of my CDX2.2 is important to me and I agree about cheap interconnects.
     
    Arkless Electronics likes this.
  3. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Yep I quite like Naim CD players.
     
  4. Jonathan

    Jonathan pfm Member

    i should imagine there are multiple paths to electrolyte evaporation - at least if the caps on the ends are porous or not perfectly sealed?
     
  5. Tony L

    Tony L Administrator

    Did the caps still measure anything close to spec? That might be a clue.
     
  6. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    I have no reason to suspect that they had leaked. There is not the slightest sign of leakage/staining/whatever so if there has been leakage it has to have been evaporation and without leaving any residue. My suspicion is that these caps are just light for their size.... as I said I've never before come across "Capacitor Technology" as a make.
    I don't have the equipment to measure the capacitance as even the originals are 40,000uF (new ones 51,000uF) and that's off scale for any of the capacitance meters I have. I could design and build something to do it but it just ain't important.
     
  7. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    More interesting is the variations that seemed to go on with these as time went by... If you google for pics there are 3 or 4 versions with the transformers and main caps mounted in different places, different makes of transformers etc. There are also different schematics... although variations on a theme... This early one has a buffer stage before the drivers missing compared to later versions and has more electrolytic caps present than later ones. Where I've seen a schematic definitely saying it's a MkII it's quite different and vastly more complicated.
     
  8. mandryka

    mandryka pfm Member

    I wonder what to make of this. Do you think it suggests that a suitable passive pre sounds better than a wire directly from the source? I think this is something along the lines of what Tony was saying, not that it’s sensitive about preamps, but that it’s better with a suitable preamp.
     
  9. Tony L

    Tony L Administrator

    As ever experiment and listen for yourself. It is a lovely power amp and I’d stick the very best stuff I could afford upstream of it. ‘Best’ being for you to define, not me or Jez Arkless!
     
  10. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    A 10K passive has a worst case output impedance of 2.5K and probably more like 1.8K in normal use, which is fairly active pre-amp-like. I was aware that the 50K one I was using was not optimum because the KSA50 has a 470pF capacitor at the input preceded by a 1K resistor and with the worst case 12.5K impedance before it this means worst case of -3dB @ 25KHz, probably more like 30KHz in normal use. This would mean probably 1dB down @ 18KHz or so, probably not too much of an issue for any of us over 25 or so! Not ideal though... (FM radio only goes up to 14.5KHz and Red Book CD to 21KHz ish)
    In practice though something "weird" was going on and you wouldn't have believed it was the same amplifier with the two different value passives.
    This gives some credence to TonyL's assertion that it's better with active pre amps.... or more likely that it likes low source impedance's such as those given by an active pre amp, or a 10K passive...

    I don't know the details of your Theta DAC but assume you are using it directly driving the Krell and using an on board remote vol control on the Theta? I would assume that the Theta has a low output impedance, probably similar to that of an active pre as in fact a variable output at low impedance from a buffer on anything is basically an active pre... just it's built into the DAC and has the DAC as its only source...

    There are a variety of reasons why it may not be ideal though. If the vol operates in the digital domain it may be restricting the amount of effective bits in use as vol is lowered, hence only giving its best at full volume and needing a pre of some sort after it to control vol.
    Another possible is that it has an unusually high output impedance for such a unit... or maybe has a motor driven passive pot without a buffer at output and so is like a passive.... maybe of 50K!
    The above are fairly unlikely though:)

    So.... at the end of the day try whatever you like with it and see how you like the sound (yep I could have just said that at the beginning instead of the long preamble:rolleyes:). For me the 10K passive driven by my modded DPA Little Bit DAC was excellent! YMMV

    Be sure to switch it off and leave it for a good couple of minutes before unplugging anything/plugging in anything new!

    Hope that helps:)

    Edit: As Tony just beat me to in far less words!
     
  11. sq225917

    sq225917 situation engineer

    My ksa 200 preferred my 10k goldpoint pot to a modded naim 72.
     
  12. mandryka

    mandryka pfm Member


    Thanks for mentioning that, I wouldn't have thought about leaving it for a few minutes after switching off.

    The sound in that setup is most like live music that I've ever experienced. So basically, I'm very pleased.
     
    Arkless Electronics likes this.
  13. mandryka

    mandryka pfm Member

  14. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    It has no similarity to your Krell. A few amps have very low input impedance like 2K due to their design and will need an active... it may be one... or it could just be the seller is one of those "only active pres are any good" types!
     
    mandryka likes this.
  15. Yank

    Yank Bulbous Also Tapered

    ...or it has very little, if any forward gain and needs to be fed by an active stage just to get any decent level out of the system.
     
  16. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Yes good point! Very unlikely to have no forward gain though... for 50W amp, pre amp would have to give 20V RMS! Not many other than valve ones can do that... and then at usually elevated THD...
     
  17. Yank

    Yank Bulbous Also Tapered

    The Pass Labs First Watt F4 amp was a no-gain buffer, so they are out there.
     
  18. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    And can you think of another one?
     
  19. Yank

    Yank Bulbous Also Tapered

    Well the amp in the eBait auction was described as a "Zen" amp, Nelson Pass has used that name for a series of published amp designs with minimal circuits, some with only one or two active devices. https://www.passdiy.com/project
     
  20. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Yes I know;) In fact some of them only have 20dB or so of gain and may need an active pre in some systems, depending on source level and speaker efficiency etc. I don't know of any other amps with zero voltage gain though... but I'm sure some will exist somewhere:)
     

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