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So why do some amps offer low input impedance?

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  • So why do some amps offer low input impedance?

    I have frequently seen caustic criticism for some amps with low input impedance, like the Spectral. Here's one example as forwarded to me by a friend, taken from the Lampizator manuals and as posted elsewhere on the internet:

    The DAC with volume control should sound audibly cleaner and more direct without any preamp between the DAC and the amp. The preamp, however good, will veil a lot of the DAC’s natural clarity, speed and directness.

    The load presented by the preamp or amp or simply the next analog component that the DAC sees, should be as high as possible. It is measured in kilo-Ohms and 100Kilo Ohms is a perfect ballpark value. More is very rarely seen. 47 K is next common value, and it is great too. 20K is kind of on a low side, but we can handle that.

    Lower than 10k is bad news. The bass extension may suffer a few hertz of the lowest octave. The DAC will not be damaged in any way, but at around 6K of load the dynamics of the dac will start to fade away.
    Having said that - every properly designed amp or preamp keeps the load value above 40k. And if it doesn’t - we simply don’t choose such amp because it was not designed with audiophiles in mind.

    So the simple reason that some amps offer low input impedance is because they are primarily current-driven and consequently the low impedance commands higher input current for proper operation. As such, these designs require preamps with high output current, like Spectral's own (1A max, balanced-out). So all the previous highlighted text says is that, said DACs simply do not offer a lot of output current.

    It truly aches me when folks - especially manufacturers - write all this stuff, which I will not characterize...
    Analog: VPI Aries 3, custom suspension + platter interface || Magnetically stabilized JMW 10.5i || Ortofon A90 || Modded & fully shielded Pass XP-25 || Last gen VPI SDS speed controller || Magnum Dynalab MD-90SE/105 Digital: Spectral 4000SV Amplification: Spectral DMC-30SV || DMA-500AR Speakers: Heavily modded MartinLogan (custom Mundorf dual xover, cabling, woofers, xformer; structural mods) || Modded REL sub Cabling: Fully shielded MIT Oracle 50ic,MA-X,90.1 || Shunyata Alpha v2 Power: Shunyata Denali 6000/S v2, Venom XC and NR cords, SR-Z1 outlets, PS10, Typhon. Tweaks: Isodamp, mumetal, 3M AB5100, Dynamat, Copper foil; Vishay diodes, resistors, Mundorf coils & capacitors

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  • #2
    It goes both ways. Back in Boulders early days they lambasted vacuum tubes as if they were evil. They even wrote a white paper on why those "fire bottles" were keeping your system from performing at SOTA potential.

    I looked at Lampi's Golden Gate output impedance, its 1600 Ω. no wonder they admonish connecting to anything less than 100k Ω input. Lampis probably don't use a cathode follower which would drop the output impedance. Depends on who you ask, some designers of tube gear would consider it dubious engineering to not achieve a 500 Ω output impedance. Ralph's Amta-shere preamps are a scant 30 Ω, Lamm by contrast are a whopping 50k Ω.

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    • #3
      And so the eternal pursuit of "system matching".

      Comment


      • #4
        Originally posted by ack View Post
        I have frequently seen caustic criticism for some amps with low input impedance, like the Spectral. Here's one example as forwarded to me by a friend, taken from the Lampizator manuals and as posted elsewhere on the internet:

        The DAC with volume control should sound audibly cleaner and more direct without any preamp between the DAC and the amp. The preamp, however good, will veil a lot of the DAC’s natural clarity, speed and directness.

        The load presented by the preamp or amp or simply the next analog component that the DAC sees, should be as high as possible. It is measured in kilo-Ohms and 100Kilo Ohms is a perfect ballpark value. More is very rarely seen. 47 K is next common value, and it is great too. 20K is kind of on a low side, but we can handle that.

        Lower than 10k is bad news. The bass extension may suffer a few hertz of the lowest octave. The DAC will not be damaged in any way, but at around 6K of load the dynamics of the dac will start to fade away.
        Having said that - every properly designed amp or preamp keeps the load value above 40k. And if it doesn’t - we simply don’t choose such amp because it was not designed with audiophiles in mind.

        So the simple reason that some amps offer low input impedance is because they are primarily current-driven and consequently the low impedance commands higher input current for proper operation. As such, these designs require preamps with high output current, like Spectral's own (1A max, balanced-out). So all the previous highlighted text says is that, said DACs simply do not offer a lot of output current.

        It truly aches me when folks - especially manufacturers - write all this stuff, which I will not characterize...
        Did a quick search. Rowland, Gryphon, Dartzeel (their proprietary connection) and Soulution 40 and under. Pass at 50 (SE) and 100 kohl’s (balanced).Goldmund not listed and Vitus website under construction. Krell and D’Agastino 100kohms.

        Not the first time the low impedance of solid-state amps has been raised. I think Dave Wilson talked about impedance mismatches between preamps and amps in TAS in issue 21/22 (?) and resultant sonic effects. Exactly as described in your post.

        I also thought that input impedance also had an effect upon noise levels. Well at least in tube amps.

        God forbid that tube design passes DC. Kiss your amp bye bye.
        Myles B. Astor, PhD, Administrator
        Senior Editor, Positive-Feedback.com
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        • #5
          Originally posted by MylesBAstor View Post


          God forbid that tube design passes DC. Kiss your amp bye bye.
          that's exactly what happened to my KSA-50. I had an SP-8 connected to it that emitted spurious DC, the Krell was DC coupled. In my case a resistor opened up and a small flame came up through the vent holes. The first thing out of the Krell service rep: "was it connected to a tube preamp?"

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          • #6
            I have a Chesky CD that causes both of my DACs to output DC in the same exact spot every time, and thankfully the preamp's protection circuit cuts off the signal. otherwise, bye bye woofers, if not the amps
            Analog: VPI Aries 3, custom suspension + platter interface || Magnetically stabilized JMW 10.5i || Ortofon A90 || Modded & fully shielded Pass XP-25 || Last gen VPI SDS speed controller || Magnum Dynalab MD-90SE/105 Digital: Spectral 4000SV Amplification: Spectral DMC-30SV || DMA-500AR Speakers: Heavily modded MartinLogan (custom Mundorf dual xover, cabling, woofers, xformer; structural mods) || Modded REL sub Cabling: Fully shielded MIT Oracle 50ic,MA-X,90.1 || Shunyata Alpha v2 Power: Shunyata Denali 6000/S v2, Venom XC and NR cords, SR-Z1 outlets, PS10, Typhon. Tweaks: Isodamp, mumetal, 3M AB5100, Dynamat, Copper foil; Vishay diodes, resistors, Mundorf coils & capacitors

            Main System Link
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            Comment


            • mep
              mep commented
              Editing a comment
              I would take a hammer to that CD.

            • jonathanb
              jonathanb commented
              Editing a comment
              a shredder would also work nicely.

          • #7
            Originally posted by MylesBAstor View Post

            Did a quick search. Rowland, Gryphon, Dartzeel (their proprietary connection) and Soulution 40 and under. Pass at 50 (SE) and 100 kohl’s (balanced).Goldmund not listed and Vitus website under construction. Krell and D’Agastino 100kohms.

            Not the first time the low impedance of solid-state amps has been raised. I think Dave Wilson talked about impedance mismatches between preamps and amps in TAS in issue 21/22 (?) and resultant sonic effects. Exactly as described in your post.

            I also thought that input impedance also had an effect upon noise levels. Well at least in tube amps.

            God forbid that tube design passes DC. Kiss your amp bye bye.
            600 Ohms used to be the standard input impedance in the old days in the studio. Most modern studio equipment is still built to drive 600 Ohms (for example a Studer tape machine).

            Tube preamps are less likely to pass DC than a solid state preamp is. If you think 'a solid state preamp won't do that' then you know how hard it is for a tube preamp to do that too.

            What the real problem is the preamp is supposed to be turned on and stabilized prior to the amplifier being turned on... This is true even if you have a solid state preamp; a lot of older solid state gear had turn on thumps that could take a speaker out with ease if the amp were on at the time!

            Krell started that rumor about tube preamps and its had legs solely on that account. But it is nonsense plain and simple. IMO Krell must have had a poor protection circuit; I've seen many amps that would trigger the protection circuit if DC were present at the input. Since most solid state amps are direct-coupled from input to output, this would cause DC to be present at the output, preventing the amplifier from coming out of 'protect' mode. The bigger question in my mind is how come they let an amp like that out on the market when any Kenwood or Pioneer had that problem licked back in the 1970s?

            It would take a damaged or shorted film cap to cause a tube preamp to make DC at its output. Film caps are pretty stable devices; you can get old Dyna PAS-3 preamps off of ebay that need their power supplies completely rebuilt, yet whether that was done or not the unit won't pass DC. This is true of ARC as well.

            We have a direct-coupled tube output on our preamps and even though we've been making that preamp since 1989, I've yet to hear of a solid state amp damaged by it.

            This is simply a rumor they started and needs to be treated as such.

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            • atmasphere
              atmasphere commented
              Editing a comment
              Honestly if it was catching fire it likely had nothing to do with the preamp at all!

            • MylesBAstor
              MylesBAstor commented
              Editing a comment
              But you’re essentially confirming what Dan said. And that problem occurred primarily with one tube preamp at the time.

            • atmasphere
              atmasphere commented
              Editing a comment
              How?
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