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    Impedance matching question - please don't shoot

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    deepee99

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    Impedance matching question - please don't shoot

    Post by deepee99 on Sat May 20, 2017 4:15 pm

    I probably could have answered this myself decades ago, but can't recall how to figger a work-around now.
    I'm trying to match my SP-13 here in the den with a Bedini solid-state amp.
    Not sure of the SP's output impedance, but the Bedini's inputs are only 20k ohm.
    Suffice to say, the bass is a tad flimsy. Other than a set of pre-amp output caps the size of a small dump-truck, is there another way to get them a bit more closely matched?
    Thanks,
    David

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    peterh

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    Re: Impedance matching question - please don't shoot

    Post by peterh on Sat May 20, 2017 4:44 pm

    deepee99 wrote:I probably could have answered this myself decades ago, but can't recall how to figger a work-around now.
    I'm trying to match my SP-13 here in the den with a Bedini solid-state amp.
    Not sure of the SP's output impedance, but the Bedini's inputs are only 20k ohm.
    Suffice to say, the bass is a tad flimsy. Other than a set of pre-amp output caps the size of a small dump-truck, is there another way to get them a bit more closely matched?
    Thanks,
    David

    Output caps of ANY size won't change this.
    The problem might be in the output impedance of sp-13, if it's higher then 20k you
    need a buffer between them. It could be realized with a single ecc82 ( 12au7 for overseas)
    some resistors and caps.

    If you have excess amplification you could try a 100k resistor in series with
    each channel, you loose 80% of volume but it should be good quality sound.
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    Dave_in_Va

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    Re: Impedance matching question - please don't shoot

    Post by Dave_in_Va on Sat May 20, 2017 9:35 pm

    I bought one of these to get my SP 14 working with my CD recorder......

    http://www.ebay.com/itm/YAQIN-SD-CD2-Stereo-Audio-Processor-Tube-Buffer-/142378289342?hash=item21266848be:g:uA8AAOSwrklVc26d


    It's been wotking flawlessly for over a year and NOS tubes for it are really cheap. I believe PeterH originally suggested it.
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    deepee99

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    Re: Impedance matching question - please don't shoot

    Post by deepee99 on Sun May 21, 2017 8:13 am

    Gentlemens,
    Thanks for the advice.
    Just to be clear, the consequence of the impedance mis-match (high-impedance outs from the tube SP, low impedance input to the s/s amp) is not an unacceptable loss or increase of gain. I don't think I need a buffer nor certainly not a signal amp. What happens is that the bass rolls off rather at a rather higher frequency than would be the case if both amp and pre-amp were solid state or both were tubes. The 20k input impedance of the s/s/ amp is not high enough to get even close to the output impedance of a tube pre-amp. That's what I'm looking to fix -- to recover that lost octave or two at the low end of the frequency spectrum.
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    Bob Latino
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    Re: Impedance matching question - please don't shoot

    Post by Bob Latino on Sun May 21, 2017 10:43 am

    The general rule is that the input impedance of the amp should be at LEAST 10X that of the output impedance of the preamp. I have also heard that some use the 50 to 100X rule of amp input impedance over preamp output impedance. A 20K input impedance of an amp is pretty low. You will have to find out from Roy what the output impedance of the SP13 is ? Yes - a bad mismatch can cause frequency response issues such as underwhelming bass response.

    As an aside, all the VTA amps have an input impedance of 270,000 ohms. This is plenty high enough to work with any preamp that I know of be it tube, SS or a passive preamp.

    Bob
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    tubes4hifi
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    Re: Impedance matching question - please don't shoot

    Post by tubes4hifi on Mon May 22, 2017 8:00 pm

    deepee, ask the source! The output impedance of the SP13 is about 700 ohms, should be good enough to drive any amp with an input impedance greater than 10K, including your Bedini.
    Now I'd have to ask, how long are your interconnects, and how much capacitance do they have?
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    deepee99

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    Re: Impedance matching question - please don't shoot

    Post by deepee99 on Tue May 23, 2017 10:23 am

    Thanks, Roy, yeah I shoulda come to the source.
    Cables are 15-foot 10-ga. stranded Belden (Blue Jeans).
    Actually, I'm gonna give this a little time before doing something drastic like adding a sand EQ. The tubes and speaks ain't broken in yet, prolly <50 hours all in now that the -13 is actually working right.
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    tubes4hifi
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    Re: Impedance matching question - please don't shoot

    Post by tubes4hifi on Tue May 23, 2017 11:27 am

    OK, so that was a good question, here's a quote from Blue Jeans cables . . .
    "Capacitance can be important, particularly in long cable runs, because it contributes to rolloff of higher frequencies"
    but their cables do have low capacitance, but still when you have longer cables like that, it can be a contributing factor . . .
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    deepee99

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    Re: Impedance matching question - please don't shoot

    Post by deepee99 on Tue May 23, 2017 11:35 am

    tubes4hifi wrote:OK, so that was a good question, here's a quote from Blue Jeans cables . . .
    "Capacitance can be important, particularly in long cable runs, because it contributes to rolloff of higher frequencies"
    but their cables do have low capacitance, but still when you have longer cables like that, it can be a contributing factor . . .
    I wouldn't think at 10 ga. it should be much of an issue, even at 15 feet. High end is fine; where I am just mildly deficient is on the bass end.
    Again, will just let things cook for awhile. The woofs on all of Tyler's speaks, even these little ones, have quite the break-in time. Mids and highs are already perfectomondo. Tubes are just a fistful of RCAs I grabbed out of the parts box. Maybe there's a better 6CG7 out there but as I mentioned in another thread, I'm in a 12-Step program for tube-rollers and have been loose-socket free for 362 days now - just a few days before I get my pin.
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    corndog71

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    Re: Impedance matching question - please don't shoot

    Post by corndog71 on Tue May 23, 2017 1:44 pm

    deepee99 wrote:Thanks, Roy, yeah I shoulda come to the source.
    Cables are 15-foot 10-ga. stranded Belden (Blue Jeans).
    Actually, I'm gonna give this a little time before doing something drastic like adding a sand EQ. The tubes and speaks ain't broken in yet, prolly <50 hours all in now that the -13 is actually working right.

    Those sound like speaker cables.  How long are the interconnects between the preamp and the amps? Do you know the capacitance of those interconnects?
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    peterh

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    Re: Impedance matching question - please don't shoot

    Post by peterh on Tue May 23, 2017 2:45 pm

    tubes4hifi wrote:OK, so that was a good question, here's a quote from Blue Jeans cables . . .
    "Capacitance can be important, particularly in long cable runs, because it contributes to rolloff of higher frequencies"
    but their cables do have low capacitance, but still when you have longer cables like that, it can be a contributing factor . . .
    The user complained about missing bass, not missing treble.
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    deepee99

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    Re: Impedance matching question - please don't shoot

    Post by deepee99 on Tue May 23, 2017 3:20 pm

    Yes, I was speaking of speaker cable runs. My interconnects are short as possible and none longer than 1 meter.
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    corndog71

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    Re: Impedance matching question - please don't shoot

    Post by corndog71 on Tue May 23, 2017 4:31 pm

    Your bass response could also be affected by room placement. Maybe try pulling the speakers toward your sitting position a bit more or spreading them out more... or both.
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    deepee99

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    Much improved

    Post by deepee99 on Sun May 28, 2017 8:42 am

    It appears my "problem" of thin bass has gone the way of the do-do bird. I had been worried about an impedance match but with the NOS RCAs having run in for about 20 hours now and the lower octaves already are blooming quite nicely. Then the highs got a bit brittle but that is now in retreat as well. It never ceases to amaze how many changes a new tube, signal or output, whether old-stock or straight out of current production, go through in their first 100 hours or so. The Audio Control (Carver) EQ will remain in the basement, gathering moss, unless somebody wants to buy it cheap.

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