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    PAS 3 Output Impedance

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    myaddiction

    Posts : 15
    Join date : 2009-03-27

    PAS 3 Output Impedance

    Post by myaddiction on Mon Oct 05, 2009 6:49 pm

    I have a couple of questions about the high output impedance on the PAS3. I notice on all of the PAS3 that have been modded with the replacement of the selector and only the phono going through the RIAA, that the 510K resistors from the OUTPUT to ground have been removed. Does this raise the output impedance on the PAS?

    I ran into an issue using the PAS with the Dynaco ST120. Previously I had Advents or Dynaco A50 hooked to the ST120. Well I now have some Klipsch Forte II, and an unusual situation exists. When powered up, after warm-up of preamp, the woofers on the Klipsch start moving in and out about 1/2", This does not happen with the Advents or A50. I can only assume it is the high efficiency of the Klipsch.

    Should I re-install the 510K resistors? Will this lower the output impedance? I measured it with my DMM and got a reading of 678,000 ohms.

    Regards,
    Bill

    GP49

    Posts : 732
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: PAS 3 Output Impedance

    Post by GP49 on Wed Oct 07, 2009 11:19 pm

    The output impedance of the PAS, or any other preamplifier, is determined by the design of the last stage. Reinstalling the 510K resistors will put them in parallel with the existing LOAD impedance that you already have the PAS driving and will thus make whatever load even lower in impedance and even harder for the PAS to drive. It will have ZERO effect on the output impedance of the last tube stage. You would have to modify that stage itself, or add a cathode follower or buffer, to lower the output impedance.

    As to whether you should restore the 510K resistors, that's dependent on the load impedance you are trying to drive. If the resulting load impedance is too low, you will get a bass roll-off. In the shop I used to trim the output impedance on the PAS with the customer's own power amp and a variable resistor for flattest bass; then replaced the variable resistor with a selected 1% resistor to match it.

    Basic electronic theory...that's all it is. Not voodoo, which we discussed and had a laugh about, last week.

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