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Thread: Impedance and passive volume control

  1. #1
    Join Date: Apr 2016

    Location: Bishops Stortford

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    I'm Chris.

    Default Impedance and passive volume control

    I need a bit a help understanding this.

    I gather, so far, that in an ideal world a bit of kit should have a high input impedance and a low output impedance, so when connecting two bits of kit together the cable joins a low to a high impedance. The cable may influence that impedance, but that's a separate topic.

    My question is, does a passive volume control affect impedance or is it 'invisible' in that consideration.
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  2. #2
    Join Date: May 2008

    Location: Surrey

    Posts: 6,833
    I'm Rob.

    Default

    My understanding is that a passive pre provides no gain to the following amp, but that an active pre provides signal gain. I am not aware that an active pre varies the impedance. However someone might be along to say that I am talking complete bollocks (a nod to Mark Williams - The Fast Show for the last comment)

    EDIT: Blimey, just read this which if it is correct I was talking doo doo

    https://www.audiosciencereview.com/f...ive-one.14047/
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  3. #3
    Join Date: Feb 2013

    Location: W Lothian

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    I'm Grant.

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    a passive's pot will affect the impedence as its a resistor but if its not too high it shouldnt affect it too much... A pot value of 1 to 2K should be fine. the impedance ratio is normally between 5 and 10 times i think.
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  4. #4
    Join Date: Apr 2016

    Location: Bishops Stortford

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    I'm Chris.

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    Quote Originally Posted by struth View Post
    a passive's pot will affect the impedence as its a resistor but if its not too high it shouldnt affect it too much... A pot value of 1 to 2K should be fine. the impedance ratio is normally between 5 and 10 times i think.
    So if a source having an impedance output of 1k is married to a power amp with 50K input its a good impedance match (50 times)

    Now place a PVC in line with say 1 or 2 K pot. Is there now an impedance mismatch?
    Source
    SW1X Universal Music Server UMS I Signature with Power Supply Unit PSU I Signature
    SW1X USB II
    SW1X DAC III Special
    Amps
    Pre amp -- Hi fi Collective twin mono ladder stepped attenuator, with Charcroft Z-foil and silver wired.
    Power amps -- Welborne 45 SET monoblocks 1.8W / Decware Taboo 6W / Elekit 300B TU-8600SVK plus further improved components 9W / ICE Power 1000W
    Speakers
    Highly modified Endorphin P17 open baffle speakers containing both vintage and modern alnico drivers and paper cones. All silver wired - 8" Cube Audio FC8 full range drivers and vintage 15" Altec VOTT 416 bass drivers. All sat on Townsend Audio Podium seismic isolation platforms.
    BK Electronics XLS400FF Sub.
    Cabling
    Silver mains cables, interconnects and speaker cables by SW1X
    Headphones
    HRT HeadStreamer and SennHeiser HD650 headphones

  5. #5
    Join Date: Jan 2009

    Location: Essex

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    I'm openingabottleofwine.

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    I would choose a potentiometer having an impedance that is the geometrical mean of the output impedance of the source and the imput impedance of the load. For the example you cite, that would be √(1000 ∙ 50000) ≈ 7000, or 7kΩ. However since this is not a standard value, either 5kΩ or 10kΩ ought to be suitable.
    Barry

  6. #6
    Join Date: Apr 2016

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    Quote Originally Posted by Barry View Post
    I would choose a potentiometer having an impedance that is the geometrical mean of the output impedance of the source and the imput impedance of the load. For the example you cite, that would be √(1000 ∙ 50000) ≈ 7000, or 7kΩ. However since this is not a standard value, either 5kΩ or 10kΩ ought to be suitable.
    Getting a grip on the topic now , but isn't it true that a volume control of say 10K changes its impedance at different volume settings?
    Source
    SW1X Universal Music Server UMS I Signature with Power Supply Unit PSU I Signature
    SW1X USB II
    SW1X DAC III Special
    Amps
    Pre amp -- Hi fi Collective twin mono ladder stepped attenuator, with Charcroft Z-foil and silver wired.
    Power amps -- Welborne 45 SET monoblocks 1.8W / Decware Taboo 6W / Elekit 300B TU-8600SVK plus further improved components 9W / ICE Power 1000W
    Speakers
    Highly modified Endorphin P17 open baffle speakers containing both vintage and modern alnico drivers and paper cones. All silver wired - 8" Cube Audio FC8 full range drivers and vintage 15" Altec VOTT 416 bass drivers. All sat on Townsend Audio Podium seismic isolation platforms.
    BK Electronics XLS400FF Sub.
    Cabling
    Silver mains cables, interconnects and speaker cables by SW1X
    Headphones
    HRT HeadStreamer and SennHeiser HD650 headphones

  7. #7
    Join Date: Aug 2009

    Location: Staffordshire, England

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    I'm Martin.

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    Quote Originally Posted by bumpy View Post
    Getting a grip on the topic now , but isn't it true that a volume control of say 10K changes its impedance at different volume settings?
    Yes but it won't necessarily be audible.

    If there's a problem you'll hear it as no bass, rolled off top end, or both.

    IME a passive with a buffer or an active pre with low gain are the best solutions for a digital source, sidesteps most potential issues.
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  8. #8
    Join Date: Jan 2009

    Location: Essex

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    I'm openingabottleofwine.

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    Quote Originally Posted by bumpy View Post
    Getting a grip on the topic now , but isn't it true that a volume control of say 10K changes its impedance at different volume settings?
    Yes it does, but the 10k, for example, is the impedance at the maximum volume setting. At lower a volume setting, that part of the potentiometer not tapped appears in series with the source impedance, with the combined series impedance appearing in parallel with the portion that is tapped. So the effective source impedance is reduced further.
    Barry

  9. #9
    Join Date: Apr 2016

    Location: Bishops Stortford

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    I'm Chris.

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    That's great
    Thank you all
    Source
    SW1X Universal Music Server UMS I Signature with Power Supply Unit PSU I Signature
    SW1X USB II
    SW1X DAC III Special
    Amps
    Pre amp -- Hi fi Collective twin mono ladder stepped attenuator, with Charcroft Z-foil and silver wired.
    Power amps -- Welborne 45 SET monoblocks 1.8W / Decware Taboo 6W / Elekit 300B TU-8600SVK plus further improved components 9W / ICE Power 1000W
    Speakers
    Highly modified Endorphin P17 open baffle speakers containing both vintage and modern alnico drivers and paper cones. All silver wired - 8" Cube Audio FC8 full range drivers and vintage 15" Altec VOTT 416 bass drivers. All sat on Townsend Audio Podium seismic isolation platforms.
    BK Electronics XLS400FF Sub.
    Cabling
    Silver mains cables, interconnects and speaker cables by SW1X
    Headphones
    HRT HeadStreamer and SennHeiser HD650 headphones

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