Impedance matching between phono stage and poweramp

Discussion in 'Audio Hardware' started by Sean Sandoval, Nov 26, 2014.

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  1. Sean Sandoval

    Sean Sandoval Senior Member Thread Starter

    Location:
    Sweden
    Hi audio aficionados,

    I'm still learning about hifi equipment and all things tubular, and I would appreciate some input on impedance matching. I bought a Budgie tube phonostage about six months ago, which sounds lovely by the way, and have it connected to a SS amp which has 10k input impedance. The tubestage has an output impedance of 6k. I gather online that the input impedance of a poweramp should exceed a factor 6 to 10 of the output impedance of the preamp. This leads to my first question.. Does the lower ratio input/output impedance I have impact frequency response? If so, in what way? Thanks!
     
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  2. Mr Bass

    Mr Bass Chevelle Ma Belle

    Location:
    Mid Atlantic
    A high output impedance preamp driving a low impedance input will affect signal to noise ratios and have a complex effect on frequency response; usually less bass is the most noticeable effect. Whether this is audible or objectionable depends on the kind of music you listen to, the volume, the design of the two components etc. I assume you have an integrated amp with the phono stage going to the preamp inputs. If you are connecting the phono directly to the power amp, then I would get a high input impedance preamp which can resolve the impedance mismatch.
     
    Sean Sandoval likes this.
  3. chervokas

    chervokas Senior Member

    Keeping the load impedance 10X or greater than the source impedance will minimize insertion loss. So the nearer the load impedance gets to the source impedance, the more signal loss there will be at the input of the loading device. Along with that signal voltage loss you get a worse S/N ratio. In addition the output impedance of the driving device plus the capacitance of the cable between the phono stage and the amp, loaded by the relatively low load impedance, I think, could be giving you a pretty meaningful roll off of high frequencies, but as Mr. Bass notes, because the impedances are not constant but vary according to frequency, predicting the exact frequency response effects is difficult. My guess is that with a 10K amp loading a 6K preamp you'll have lower level, softer overall tone with rolled frequency extremes and worse S/N performance than if you had a 60K ohm or better load.
     
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  4. Warren Jarrett

    Warren Jarrett Audio Note (UK) dealer in SoCal/LA-OC In Memoriam

    Location:
    Fullerton, CA
    You have an unusual and incompatible combination of equipment there. The 7K output impedance is much higher than normal for a source component, and the 10k input impedance is much lower than normal for an amplifier. For proper performance together, you will need to insert a preamp or buffer stage with a much higher input impedance (than the amp) AND a much lower output impedance (than the phono stage).

    I am curious what amp has such a low input impedance? I had a McCormack amp with 10K, but that is the only one I remember seeing so low. If you were using a tube amp, most likely its input impedance would be about 100k.
     
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  5. DaleH

    DaleH Forum Resident

    Location:
    Southeast
    All good responses. 10x is the magic number for insertion loss ( 60K on the amp input). The output coupling cap on the preamp in your setup would need to be 16uF or so to pass the low frequencies without attenuation (roll-off).
     
    Sean Sandoval likes this.
  6. Rick58

    Rick58 Forum Resident

    Location:
    Eagle, ID, USA
    Are you sure of this spec? sounds quite oddly high, as others have pointed out. Maybe that's the minimum recommended input impedance of the amplifier input that follows it? That would make more sense (600 ohms output impedance -> 6000 ohm load (input to amplifier).

    I know some McIntosh (SS) amps do have a 10k input impedance, but of course their preamps are designed to drive that load.
     
    Sean Sandoval likes this.
  7. That kind of output impedance is what a 6DJ8 will have when pulling the signal straight off of the plate of the tube. That's what the first Budgie does.
     
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  8. Sean Sandoval

    Sean Sandoval Senior Member Thread Starter

    Location:
    Sweden
    Thanks for your excellent answers guys. I have omitted one thing in my question which I should have mentioned. The amp is a Lyngdorf TDAI 2200 which is an integrated digital amp (a powerDAC). The ADC circuit with the analog input is a AKM based, and know very little on how they handle input impedances. However I read on the Lyngdorf specsheet that they want to keep the input impedance low to mitigate noise. But that indirectly suggests an output impedance that is low so stupid me who got a high impedance tube amp.... ;-)

    My main music taste is Rock, so maybe the topsy-turvy frequency response doesn't bother me to such a degree, but I've compared source material in digital form to vinyl playback (OM40) and I do notice some dB dropoff in upper frequencies (cymbals, hi-hat). I haven't heard any problems with low bass yet, and midrange vocals and piano instuments sound awesome.

    What to do! I really enjoy the tube colorations in the mid-range, so maybe I should seek out another phono stage with lower output impedance. I found one example made by the same guy at Parks Audio (Budgie Hybrid) which has a JFET follower to the tube to push down the output impedance to 600. Then I have at least 10+ ratio which sounds more ballpark in theory. I really like my digital amp, so I would be hardpressed to replace it for vinyl playback.. But at the same time I enjoy the swapping aspect of tubes to match different music. Telefunken 7308's is a marvel when playing Neil Young Live at Massey Hall for instance.

    What would you guys recommend? Should I switch to a SS phonostage or would the Hybrid Tube stage be my ticket? I have collected various NOS tubes already so it would be a bummer to sell them off and go full SS... But at the same time I don't want added noise or HF rolloff. Choices, choices!
     
    Last edited: Dec 1, 2014
  9. Warren Jarrett

    Warren Jarrett Audio Note (UK) dealer in SoCal/LA-OC In Memoriam

    Location:
    Fullerton, CA
    Since you'd like to keep everything you have now, I would add a "buffer" which is specifally designed to provide the impedances you need. Such as:

    http://www.upscaleaudio.com/musical-fidelity-x-10-v3

    I would buy it on Ebay, listen and compare, then if nothing improves by its addition, just sell it again and continue to enjoy what you already have.
     
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  10. chervokas

    chervokas Senior Member

    I have to second Warren's suggestion. A buffer is a circuit designed to do exactly what needs doing here --buffering one circuit from another so that the second one doesn't deleteriously load down the first. Sounds like that's what Park Audio is offering: a JFET buffer (which frankly should have been designed into the preamp in the first place, I mean, 6K ohm output impedance isn't really a "real world" product in that you don't see many preamps with 60K ohm or greater input impedances). How much do they get for that? Personally I'd try that. You could also try some other buffer like Warren suggests above, though $350 for that Musical Fidelity buffer seems excessive to me and a buffer designed to inject "tube warmth" into a chain sounds like a buffer designed to do more than just buffer.

    Of course you can try other phono stages -- there are plenty of other phono stages, including tube phono stages, with output impedances below 6K ohms. But if you like the gear and the designer is offering an aftermarket buffer, I'd try it (I'd also use the shortest, lowest capacitance cables possible between that phono stage and whatever comes next in the chain, be it the buffer or a preamp or integrated).
     
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  11. Sean Sandoval

    Sean Sandoval Senior Member Thread Starter

    Location:
    Sweden
    Many thanks Warren! 470k input impedance should be more than enough for my system. Has anyone tried it?
     
  12. Sean Sandoval

    Sean Sandoval Senior Member Thread Starter

    Location:
    Sweden
    Good point chervokas. I try to keep capacitance down by using short distance plus low capacitance rca cables. What occurs to me would be to replace the phono stage altogether and avoid the impedance mismatch. I have to be honest and say that I didn't even know about this when I bought the Budgie. Many thanks for the input guys!
     
  13. jupiterboy

    jupiterboy Forum Residue

    Location:
    Buffalo, NY
    Another option.

    http://www.eeaudio.com/eeaudio_009.htm
     
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