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    dual linear potentiometer for adjusting feedback

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    tajanes

    Posts : 12
    Join date : 2015-02-07
    Location : Chicago

    dual linear potentiometer for adjusting feedback

    Post by tajanes on Sat Apr 11, 2015 8:49 pm

    I've seen topics on this forum discussing adjusting feedback resistors, including suggestion of reducing resistors ( 7 & 8 values) and running in series with a pot. Has anyone done this with a ST-120 and/or any specific recommendation - manufacturer and value? I'd expect one would want to use a very high quality pot for the application.

    Thanks in advance

    MontanaWay

    Posts : 767
    Join date : 2014-02-06
    Age : 58
    Location : Cameron, Montana

    Re: dual linear potentiometer for adjusting feedback

    Post by MontanaWay on Sat Apr 11, 2015 10:15 pm

    any particular reason why you are considering such a mod on the ST120?
    The ST120 is pretty much as good as it gets.

    sKiZo

    Posts : 1285
    Join date : 2013-04-01
    Location : Michigan USA

    Re: dual linear potentiometer for adjusting feedback

    Post by sKiZo on Sun Apr 12, 2015 12:47 am

    If I'm reading this right ...

    There was a dual pot mod you could use on older VTA boards to balance the AC, but the newer VTA boards have Constant Current Source (CCS)  which is adaptive and performs this function automatically under normal operation. It also has the mystery R39 for tailoring the board to a specific tube set if required ...

    Seems to me you'd be better served just upgrading the VTA board to the CCS version?

    How old is your ST120? This feature was added a couple years ago. Look for the two transistors between the tube sockets ...


    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: dual linear potentiometer for adjusting feedback

    Post by Bob Latino on Sun Apr 12, 2015 8:08 am

    tajanes wrote:I've seen topics on this forum discussing adjusting feedback resistors, including suggestion of reducing resistors ( 7 & 8 values) and running in series with a pot.  Has anyone done this with a ST-120 and/or any specific recommendation - manufacturer and value?  I'd expect one would want to use a very high quality pot for the application.

    Thanks in advance

    We have been down this road before on another post somewhere on the forum .. I have swapped resistors in and out of R7 and R8, the two feedback resistors for the VTA ST-70/ST-120 board and have consulted with Roy about this. ALL the VTA amps use a 7.5K feedback resistor which is a compromise value and gives the amp the ability to drive any speaker system. It also gives low distortion and that "tube sound" that people are looking for. IMHO after a number of feedback resistor swaps in and out, the value should not go below about 2.2K or above 10K. If you go below 2.2K the amp starts to lose its "tube character" and sounds a little "harder" and less tube like. If you go above about 10K, the bass starts to sound a just little "loose" and "sloppy". Some have suggested a variable pot in the feedback line. Well, if it is your amp and you want to do it, give it a shot. Some have also suggested a switch for running the amp with and without feedback. The problem is if you run the amp without any feedback the gain rises, the distortion rises, the bandwidth shrinks and the ability to drive complex loads like planar magnetic and electrostatic speakers is reduced.

    Bob

    Bob Latino
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    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: dual linear potentiometer for adjusting feedback

    Post by Bob Latino on Sun Apr 12, 2015 8:17 am

    sKiZo wrote:

    Seems to me you'd be better served just upgrading the VTA board to the CCS version?

    How old is your ST120? This feature was added a couple years ago. Look for the two transistors between the tube sockets ...


    What sKiZo is referring to is not a transistor but a small LM334 integrated circuit which keeps the current balanced. The LM334 draws a constant value of current regardless of source voltage or load resistance ..

    Bob

    tajanes

    Posts : 12
    Join date : 2015-02-07
    Location : Chicago

    why am I considering / looking for comments on an adjustable feedback

    Post by tajanes on Sun Apr 12, 2015 9:43 am

    Thanks for everyone's comments, and why is a fair question.

    While my current speakers do not incorporate a HF compression horn, I'm leaning that way for my next build (and using my ST120 for the mid and high frequency sections, so not too concerned about affects on bottom end), and I've read that slight adjustments to feedback can make subtle enhancements to sound of a horn (not looking to remove all NFB). Similar are comments of people positively comparing their horns when moving from solid state amps to tubes.

    Before my ST-120 I was using a small adjustable tube buffer pre- my solid state amp and found- subjectively of course, an improvement in sound.

    Then I saw this Hafler HA75 Tube head amp with adjustable negative feedback;
    http://audioxpress.com/article/Hafler-HA75-Tube-Head-Headphone-Amplifier.html
    and got me to post this inquiry.

    If I could only leave well enough alone- but that's 1/2 the fun of this.
    Andy

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