Passive Pre Amps

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  • 08-20-2007, 08:05 AM
    jim1961
    Passive Pre Amps
    Ive owned a Adcom SLC 505 and a Reference Line Preeminence One previously. While I enjoyed the super transparency of these units, I also experienced a rolloff at the lowend. Using both warble tone generators and measuring output voltage, and using spl meters, I was able to confirm what my ears told me. The reference line unit was a bit less prone to this than the adcom, but still displayed the basic tendency. My question is: Is this the experience all users have with these units or was it just my set up? I was using a CAL Aria CD player, KCAG 1/2 meter interconnect, and a Krell KSA-200 power amp. Could it have been the Aria output stage didnt like seeing a variable resister? Will my Rega Apollo act differently? I did discover that when I put a active unit like a EQ after the Aria, but before the passive unit, the problem was minmized considerably, which supports my theory that maybe this was just a effect caused by the Aria output stage. Anyway, if you have thoughts on this or recommend any particular passive device, please share it. My system is completely a line stage system, so no accommodations are needed for a turntable.
  • 08-20-2007, 09:42 AM
    Feanor
    I wonder ...
    Quote:
    Originally Posted by jim1961
    Ive owned a Adcom SLC 505 and a Reference Line Preeminence One previously. While I enjoyed the super transparency of these units, I also experienced a rolloff at the lowend. Using both warble tone generators and measuring output voltage, and using spl meters, I was able to confirm what my ears told me. The reference line unit was a bit less prone to this than the adcom, but still displayed the basic tendency. My question is: Is this the experience all users have with these units or was it just my set up? I was using a CAL Aria CD player, KCAG 1/2 meter interconnect, and a Krell KSA-200 power amp. Could it have been the Aria output stage didnt like seeing a variable resister? Will my Rega Apollo act differently? I did discover that when I put a active unit like a EQ after the Aria, but before the passive unit, the problem was minmized considerably, which supports my theory that maybe this was just a effect caused by the Aria output stage. Anyway, if you have thoughts on this or recommend any particular passive device, please share it. My system is completely a line stage system, so no accommodations are needed for a turntable.
    I wonder, because I don't really know. The rule of thumb is that input impedance of the target device should preferably be 100x the output impedance of the source device. Presuambly this applies CDP to passive preamp as well as passive preamp to power amp. In a purely passive preamp, doesn't the input impedance vary with the attenuator device, say a variable resistor? :idea:

    I really don't know if I'm talking nonsense, but I though I'd share the thought.
  • 08-20-2007, 03:03 PM
    E-Stat
    Quote:
    Originally Posted by jim1961
    Is this the experience all users have with these units or was it just my set up? I was using a CAL Aria CD player, KCAG 1/2 meter interconnect, and a Krell KSA-200 power amp.
    Such results are highly system specific. Your cables are of sufficiently low capacitance and the amp has a relatively high input impedance for SS, but I suspect it is your source that is the villain as you alluded to later. In the main system, I run a GamuT CD-1 (75 ohm output impedance / 4 volt output) through 10k DACT attenuators in a DIY cabinet with Cardas connectors and JPS Labs cable to VTL MB-450 amps (132 k ohm input impedance) using JPS Labs Ultraconductor cabling. In measurements for optimizing speaker and bass trap placement, I get flat 25 hz room response from the speakers (which is all they are really good for). No bass issues here.

    Some folks like Charles Hansen of Ayre suggest, however, that the macro dynamics are better when using an active pre. That is not the case when I run in through a quite decent Audio Research SP-9 MKIII preamp used solely as a phono preamp. He says you really need to spend about $10k on a preamp to keep the resolution and notice this difference with dynamics. Perhaps, but naturally he is motivated to make such a statement. :)

    rw
  • 08-20-2007, 03:46 PM
    jim1961
    What I failed to say, and should have, is now I have a Rega Apollo CD player and a Classe CA-100 power amp, and was wondering if I would experience the same low end roll off that I did with my previous system if I used a passive now????
  • 08-20-2007, 04:00 PM
    E-Stat
    Quote:
    Originally Posted by jim1961
    What I failed to say, and should have, is now I have a Rega Apollo CD player and a Classe CA-100 power amp, and was wondering if I would experience the same low end roll off that I did with my previous system if I used a passive now????
    I can only speculate.

    Good news: Classe amp has relatively high input impedance of 75 kohms
    Good news: Class amp has high sensitivity of 950 mV.
    Lukewarm news: Rega's output impedance is also relatively high at 600 ohms

    Looks good on paper, but you really need to try. If you can perform basic soldering, you can build a basic passive for $20 using Radio Shack parts. I did that originally for the office system and once I determined that worked, built a second unit using better parts.

    rw