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    preamp/amp impedance interface

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    Captain Coconut

    Posts : 209
    Join date : 2009-10-13
    Age : 64
    Location : Great White North

    preamp/amp impedance interface

    Post by Captain Coconut on Sun Jul 21, 2013 3:38 pm

    It seems that a minimum 1:10 ratio of preamp output impedance to amp input impedance is desirable. The Dynaco ST-35 has a whopping 500,000 ohms input impedance and I'm using a phono stage (with a volume pot) that has 450 ohms @ 1kHz output impedance. That's a ratio of over 1:1000.

    Is this okay?

    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: preamp/amp impedance interface

    Post by tubes4hifi on Sun Jul 21, 2013 3:53 pm

    certainly OK, fantastic!

    sKiZo

    Posts : 1287
    Join date : 2013-04-01
    Location : Michigan USA

    Re: preamp/amp impedance interface

    Post by sKiZo on Sun Jul 21, 2013 7:54 pm

    Does it work for ya?

    [Check]

    It's good then!! tongue 

    I'm told I'm crazy to be using the .75ma setting on my McIntosh MC2205 when driving it with a Sansui quad. "Normal" people use the 2.5ma input. But ... although I do get a more civilized ratio of volume to control settings, I also lose a lot of crispness and detail.

    If you think there's an issue and would like to experiment, you can always get a good 50k pot and put that inline with a chopped RCA cable to adjust the line level input to the amp. If that makes a difference, then you can think about do an output mod on the pre ... probably just a simple resistor swap.

    I ran into a situation on my Oppo SACD ... wanted to drive a center channel speaker and had an old Carver cube ... VERY LOUD and that can be dangerous at a couple hundred watts. Did the split cable thing and just tapped a hole in an old lunch size applesauce cup for a "custom case". Works great, and been like that for years ... keep thinking I should improve on the case, but ... why fix what ain't broke? It DOES have a really nice knob. tongue 

    (and ya ... they ARE all right about the crazy part) geek

    arledgsc

    Posts : 330
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: preamp/amp impedance interface

    Post by arledgsc on Mon Jul 22, 2013 11:03 am

    For voltage transfer it is ideal to have low output impedance and high input impedance.   Otherwise you get into voltage divider issues so 1:10 (or higher) is a good rule of thumb.   For example if you have 1:2 impedance ratio the amp input would only see 2/3 of the preamp's output.   1:10 ratio sees 91% voltage transfer.   1:1000 sees 99.9% transfer.  Vin = Vout x ( Zin/(Zin + Zout) ).   You also get into frequency response issues as well with narrowing impedance ratios.   And a variable attenuator on the amp's input changes the input impedance - usually by lowering it.  


    Just for clarification, maximum power transfer is always best with matched 1:1 impedances as in output transformers and speakers.  We are only dealing though with voltage gains in the preamp/amp interface.

    Jim McShane

    Posts : 154
    Join date : 2011-10-19
    Location : South Suburban Chicago

    Re: preamp/amp impedance interface

    Post by Jim McShane on Mon Jul 22, 2013 11:50 am

    arledgsc wrote:For voltage transfer it is ideal to have low output impedance and high input impedance.   Otherwise you get into voltage divider issues so 1:10 (or higher) is a good rule of thumb.   For example if you have 1:2 impedance ratio the amp input would only see 2/3 of the preamp's output.   1:10 ratio sees 91% voltage transfer.   1:1000 sees 99.9% transfer.  Vin = Vout x ( Zin/(Zin + Zout) ).   You also get into frequency response issues as well with narrowing impedance ratios.   And a variable attenuator on the amp's input changes the input impedance - usually by lowering it.  

    Just for clarification, maximum power transfer is always best with matched 1:1 impedances as in output transformers and speakers.  We are only dealing though with voltage gains in the preamp/amp interface.
    Great post! Thanks.

    People following this thread should also keep in mind two things about the high input impedance of the amp in this equation:

    1. The higher input impedance means less current is needed from the preamp's output. This is good.

    2. The higher the input impedance of the amp the more sensitive it can be to noise pickup. This is bad.

    So it's always a compromise, but as the post above states so clearly - for voltage transfer you must be sure the impedance is adequately high. The 10:1 ratio works well in most cases.

    arledgsc

    Posts : 330
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: preamp/amp impedance interface

    Post by arledgsc on Mon Jul 22, 2013 12:53 pm

    1. The higher input impedance means less current is needed from the preamp's output. This is good.

    2. The higher the input impedance of the amp the more sensitive it can be to noise pickup. This is bad.
    Excellent clarifications.   Studios use 600 ohm (balanced) inputs sometimes for a reason and that is to lessen noise pickup on the inputs with long cable runs.  It is much easier to induce current in a circuit at high impedance.  So very high input impedance is not always a good thing.   So yes always a compromise.

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