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    PAS3 Passive conversion?

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    jerkbag

    Posts : 6
    Join date : 2012-04-19

    PAS3 Passive conversion?

    Post by jerkbag on Mon Jun 26, 2017 3:06 pm

    Hi All,

    I have a pas 3 driving my ST-70, and I've been wanting to play around with a passive preamp instead as the PAS seems to be a weak link in my system (I much prefer the sound I get plugging a device to the ST-70 directly).

    Has anyone experimented with converting some (or all) of the pas 3 inputs to passive? Would the 100K Blue Velvet ALPS volume pot I have be suitable? I was thinking that I could wire an input on the selector switch to bypass the boards and go just from the pot to the outputs.

    Would that work? Or am I missing something? Any pitfalls I should consider?

    Thanks!
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    peterh

    Posts : 877
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: PAS3 Passive conversion?

    Post by peterh on Mon Jun 26, 2017 4:20 pm

    jerkbag wrote:Hi All,

    I have a pas 3 driving my ST-70, and I've been wanting to play around with a passive preamp instead as the PAS seems to be a weak link in my system (I much prefer the sound I get plugging a device to the ST-70 directly).

    Has anyone experimented with converting some (or all) of the pas 3 inputs to passive? Would the 100K Blue Velvet ALPS volume pot I have be suitable? I was thinking that I could wire an input on the selector switch to bypass the boards and go just from the pot to the outputs.

    Would that work? Or am I missing something? Any pitfalls I should consider?

    Thanks!
    Yes, there is a problem with your reasoning.
    To destroy a working PAS for no reason when you can sell the pas and purchase
    a passive "pre" AND have money to spare.
    avatar
    Peter W.

    Posts : 530
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: PAS3 Passive conversion?

    Post by Peter W. on Mon Jun 26, 2017 4:52 pm

    jerkbag wrote:Hi All,

    I have a pas 3 driving my ST-70, and I've been wanting to play around with a passive preamp instead as the PAS seems to be a weak link in my system (I much prefer the sound I get plugging a device to the ST-70 directly).

    Has anyone experimented with converting some (or all) of the pas 3 inputs to passive? Would the 100K Blue Velvet ALPS volume pot I have be suitable? I was thinking that I could wire an input on the selector switch to bypass the boards and go just from the pot to the outputs.

    Would that work? Or am I missing something? Any pitfalls I should consider?

    Thanks!

    If your devices have volume controls, those will come in two flavors:

    a) A volume control that limits output to 2V only. This is typical of those devices with a VC *and* a single output - such as many tuners including the Dynaco FM3, FM5 and AF6.

    b) A "line-level" output and a "variable" output. In this case, the variable output will often allow you to boost above 2V. I have a couple of Sony CD players with this feature.

    If you have the latter device, you will be unhappy with a line-level pre-amp, most likely.
    If you have the former device, you may purchase a simple switch-box that will take multiple inputs to a single output. Some include level controls, some do not. But they are quite cheap.



    jerkbag

    Posts : 6
    Join date : 2012-04-19

    Re: PAS3 Passive conversion?

    Post by jerkbag on Tue Jun 27, 2017 9:37 am

    peterh wrote:
    jerkbag wrote:Hi All,

    I have a pas 3 driving my ST-70, and I've been wanting to play around with a passive preamp instead as the PAS seems to be a weak link in my system (I much prefer the sound I get plugging a device to the ST-70 directly).

    Has anyone experimented with converting some (or all) of the pas 3 inputs to passive? Would the 100K Blue Velvet ALPS volume pot I have be suitable? I was thinking that I could wire an input on the selector switch to bypass the boards and go just from the pot to the outputs.

    Would that work? Or am I missing something? Any pitfalls I should consider?

    Thanks!
    Yes, there is a problem with your reasoning.
    To destroy a working PAS for no reason when you can sell the pas and purchase
    a passive "pre" AND have money to spare.

    Yes I suppose you are right -- I was thinking of just switching one or two inputs to passive, like the way the line ones skip the phono PC-6 board, making those inputs skip the PC-5 as well, but I think to keep the volume I would need to have the volume pot bypass the PC-5 which would affect all the inputs.

    jerkbag

    Posts : 6
    Join date : 2012-04-19

    Re: PAS3 Passive conversion?

    Post by jerkbag on Tue Jun 27, 2017 9:40 am

    Peter W. wrote:
    jerkbag wrote:Hi All,

    I have a pas 3 driving my ST-70, and I've been wanting to play around with a passive preamp instead as the PAS seems to be a weak link in my system (I much prefer the sound I get plugging a device to the ST-70 directly).

    Has anyone experimented with converting some (or all) of the pas 3 inputs to passive? Would the 100K Blue Velvet ALPS volume pot I have be suitable? I was thinking that I could wire an input on the selector switch to bypass the boards and go just from the pot to the outputs.

    Would that work? Or am I missing something? Any pitfalls I should consider?

    Thanks!

    If your devices have volume controls, those will come in two flavors:

    a) A volume control that limits output to 2V only. This is typical of those devices with a VC *and* a single output - such as  many tuners including the Dynaco FM3, FM5 and AF6.

    b) A "line-level" output and a "variable" output. In this case, the variable output will often allow you to boost above 2V. I have a couple of Sony CD players with this feature.

    If you have the latter device, you will be unhappy with a line-level pre-amp, most likely.
    If you have the former device, you may purchase a simple switch-box that will take multiple inputs to a single output. Some include level controls, some do not. But they are quite cheap.  



    Thanks. I am using a music server through a DAC so I can control the volume though the server. If I connect the DAC directly to the ST-70 I have plenty of volume control though the server. Sounds like a switch box is the way to go rather than messing with the PAS too much.
    avatar
    daveshel

    Posts : 152
    Join date : 2011-11-06
    Location : Tucson AZ USA

    Re: PAS3 Passive conversion?

    Post by daveshel on Tue Jun 27, 2017 12:13 pm

    Ah, the cognitive dissonance of PAS ownership. Perception of the value of a stock unit is skewed, a bubble that has been inflating for decades and has shown no signs of popping. The original unit was pretty good in its day, but it had some shortcomings, mostly in the power supply. I've had half a dozen of these over the years, some stock, some totally rebuilt.

    The double whammy is that the weak links in the design are also those that suffer more severely the effects of the passage of time - an old original power supply is way worse than a new one. Last year I started with a stock unit that worked. After listening for several days to evaluate the stock performance, I removed the incandescent pilot lamp and replaced it with an LED. This step dramatically improved the sound quality - mainly tightening very saggy bass. So just removing part of the load on the old original power supply improved the sound.

    Then there are lots of technologies that are available today that weren't available or cost effective 50 years ago, like LEDs and regulated power supplies. And there are numerous power supply and audio circuits that are new designs, standing on the shoulders of the originals. I replaced the entire power supply with an expensive regulated one, and I noted that the effect of this step was not as great as that of eliminating the lamp. To me, this highlighted the shortcomings of the original audio boards. By the time I finished that one, I had spent more than $600, in addition to my original outlay for the old stock unit. (But I love the sound of my Aikido linestage.) So the sad reality is that what is best for the PAS investor is at odds with what is best for the PAS listener.
    avatar
    Peter W.

    Posts : 530
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: PAS3 Passive conversion?

    Post by Peter W. on Wed Jun 28, 2017 8:10 am

    Then there are lots of technologies that are available today that weren't available or cost effective 50 years ago, like LEDs and regulated power supplies. And there are numerous power supply and audio circuits that are new designs, standing on the shoulders of the originals. I replaced the entire power supply with an expensive regulated one, and I noted that the effect of this step was not as great as that of eliminating the lamp. To me, this highlighted the shortcomings of the original audio boards. By the time I finished that one, I had spent more than $600, in addition to my original outlay for the old stock unit. (But I love the sound of kmy Aikido linestage.) So the sad reality is that what is best for the PAS investor is at odds with what is best for the PAS listener.[/quote]

    I did 'overdo' the power-supply in my PAS, and even eliminated the tube rectifier. I did not regulate, but I did make it resilient and robust enough not to sag - at all. Then, I cleaned up the boards by carefully matching capacitors (tested under full load) and a few other relatively minor tweaks. After which, cleaning up the controls and switches finished the job. As I have the true X-version, I left the tone controls in place.

    What one gets for all this is the option of another phono input (the tape-head input is easily modified), a CD input (AM/FM input), a decent blend control and other niceties omitted in modern stuff.

    LEDs are also quite noisy under some conditions.

    I doubt I have $50 in the above mods, and I did get about 3 hours of good therapy in the execution.

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