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    Dynaco St-150 (35kOHM input impedance) ok match for PAS 3Z?

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    oldsmobuick

    Posts : 12
    Join date : 2015-03-19

    Dynaco St-150 (35kOHM input impedance) ok match for PAS 3Z?

    Post by oldsmobuick on Wed Mar 25, 2015 7:09 pm

    I'll be building the Z-mod with regulated PS this week and wonder if I can use the ST-150 with its 35k input impedance. I know it was not a good match with the stock PAS 3, even with the 62k and 510k resistors removed.

    In general, is there a guideline (50k, 100k) for what might be a good match for the 3Z loading/capacitance wise? Also, I know the Z is based on the SP6 VTA unit, are the specs for the SP6 still available and will they be the same with the Z-mod?

    peterh

    Posts : 642
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Dynaco St-150 (35kOHM input impedance) ok match for PAS 3Z?

    Post by peterh on Thu Mar 26, 2015 3:24 am

    No.
    You need a buffer between them for high-quality sound.

    oldsmobuick

    Posts : 12
    Join date : 2015-03-19

    Re: Dynaco St-150 (35kOHM input impedance) ok match for PAS 3Z?

    Post by oldsmobuick on Thu Mar 26, 2015 7:20 am

    Thanks, is there a documented modification to raise the 35k up to something more universally friendly? I like the amp and would be up for tinkering if it meant I could use it natively with the 3Z.

    peterh

    Posts : 642
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Dynaco St-150 (35kOHM input impedance) ok match for PAS 3Z?

    Post by peterh on Thu Mar 26, 2015 7:56 am

    oldsmobuick wrote:Thanks, is there a documented modification to raise the 35k up to something more universally friendly? I like the amp and would be up for tinkering if it meant I could use it natively with the 3Z.

    One example is this :
    http://www.ebay.com/itm/NEW-MK4-Indeed-Class-A-Valve-Buffer-Amp-Amplifier-electro-harmonix-6922EH-tube-/290827123772?pt=LH_DefaultDomain_0&hash=item43b6a5bc3c


    This will make your PAS3 able to drive any amp without loss of fidelity

    oldsmobuick

    Posts : 12
    Join date : 2015-03-19

    Re: Dynaco St-150 (35kOHM input impedance) ok match for PAS 3Z?

    Post by oldsmobuick on Thu Mar 26, 2015 8:00 am

    Interesting, thanks for that link! Specifically, I'm wondering if the Z-Modded PAS will also need such buffering?

    peterh

    Posts : 642
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Dynaco St-150 (35kOHM input impedance) ok match for PAS 3Z?

    Post by peterh on Thu Mar 26, 2015 8:09 am

    oldsmobuick wrote:Interesting, thanks for that link! Specifically, I'm wondering if the Z-Modded PAS will also need such buffering?
    That you will have to ask your vendor ( or examine the schematics which i do not have)

    Myself found no suitable buffer, instead i built one : http://n.manet.nu/fm1/fm1.html

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: Dynaco St-150 (35kOHM input impedance) ok match for PAS 3Z?

    Post by PeterCapo on Thu Mar 26, 2015 10:39 am

    However, going back to the original Dynaco PAS, note the following excerpt from the Dynaco Stereo 150 manual, under the "CONNECTIONS" section, "INPUT" subsection:

    "The amplifier's nominal input load impedance is 35,000 ohms.  The input sensitivity is 1.0 volt for full output.  The Stereo 150 may thus be easily driven by control preamplifiers such as the Dynaco PAT-5, PAT-4 or PAS-3X.  The Dynaco PAS-3X tube type preamplifier requires an easy internal modification.  To change, simply locate and remove the pair of 62,000 ohm (blue-red-orange) resistors on the PC-5 printed circuit board in the preamp.  Earlier PAS-2 and PAS-3 preamplifiers cannot be modified for use with the Stereo 150.  Some other tube type preamplifier brands may require modification for optimum performance into a 35 K ohms load."

    Based upon this excerpt, it seems like Dynaco thought that their PAS with the X-mod, and with the 62K resistor removed, should be able to drive their Stereo 150.  If your PAS has the X-mod OR has had the tone controls correctly removed so that it emulates the X-mod (including the 1.0uF capacitor), then, along with the removal of the 62K resistor from PC-5, it seems like the PAS should work with the Stereo 150.

    http://www.updatemydynaco.com/documents/Stereo150EarlyManual.pdf

    aguaazul

    Posts : 45
    Join date : 2012-07-08
    Location : Livermore, CA

    Re: Dynaco St-150 (35kOHM input impedance) ok match for PAS 3Z?

    Post by aguaazul on Thu Mar 26, 2015 3:04 pm

    I have a PAS3. I connect it to a Bozak N-106 electronic crossover between the PAS & ST-70. As I live in the bay area, I asked Joe Curcio his thoughts on a CF. He had built a few and had great success with them. His CF board adds a 2nd transformer for an all new beefier power supply. The tube is a 12AT7 (6201). Since then I've added VTA boards (with a bit of Roy's expert help). I have pretty good pictures if you would be interested in looking at them.

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: Dynaco St-150 (35kOHM input impedance) ok match for PAS 3Z?

    Post by tubes4hifi on Fri Mar 27, 2015 7:40 pm

    if you have or obtain a Zmod line stage for your PAS preamp you can easily drive the Dynaco ST150 amp.
    The Zmod has an output impedance of around 1K and can drive any amp with an input impedance above 10K.
    www.tubes4hifi.com/PAS3-CAT.htm

    oldsmobuick

    Posts : 12
    Join date : 2015-03-19

    Re: Dynaco St-150 (35kOHM input impedance) ok match for PAS 3Z?

    Post by oldsmobuick on Fri Mar 27, 2015 7:46 pm

    Thanks Roy! Good news! And thanks for the other replies also, this is the info I need as I'm builing the Z-mod.

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