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    Altering the feedback line on the ST-70/ST-120 VTA driver board

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    Bob Latino
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    Altering the feedback line on the ST-70/ST-120 VTA driver board

    Post by Bob Latino on Sat Jan 09, 2010 12:11 pm

    Hi,

    I got this Email from someone and I thought that the answer would be an informative post here for those of you that use a Dynaco tube amp with a VTA driver board.


    "Bob, On your amp kits can the amount of negative feedback the amp uses be changed or can the
    feedback be removed completely?"


    Hi,

    "Yes" to the first question and "usually yes" to the second question.

    The amp uses a 7500 ohm feedback resistor which gives about 13 dB of negative feeback. This is just enough to keep the amp stable under any speaker load situation. You can, however, change the amount of feedback on any VTA driver board amp by altering the value of R7 (left channel) and R8 (right channel). To INCREASE the amount of feedback in the circuit you have to drop the values of R7 and R8. Lowering the value of those two resistors will also drop the gain of the amp. On an ST-70 you will still get your 35 watts per channel but it will require more output from your preamp to achieve the 35 watts. To DECREASE the amount of feedback you would have to raise the value of R7 and R8. Understand also that you will get an increase in the amp's gain if you choose to reduce the amount of feedback. You may want to try altering the values of R7 and R8 somewhere between 3000 and 15,000 ohms. Staying within these parameters for your feedback resistor won't really alter the "character" of the amp. I have tried going below 3000 ohms for R7 and R8 myself and the amp seems to sound just a little "harder" in character.

    You can also TRY removing the feeback line which goes to the two "NFB" eyelets on the front of the driver board. I have done this before and my ST-70 amp was stable with all the speakers that I had here. This may not be the case with the speakers that you have. Without any feedback SOME push/pull amplifiers can become unstable and go into osscillation or behave strangely IF the speaker presents a load that the amp can't deal with. You will also get a noticeable increase in gain without feedback. A gain increase can tend to magnify any upstream hum or noise your system may have. If your preamp has hum or noise it will be even more noticeable with a no feedback situation. OTOH now and then I have a customer that uses very efficient Klipsch, JBL or Omega speakers with sensitivities ranging from 95 to 105 dB. Using a VTA boarded ST-70 (or a tubed ST-120) with these speakers usually results in pretty loud music in just the first 1/4 turn of the volume control on your preamp. With highly sensitive speakers like that you may want to try dropping the value of R7 and R8 on the VTA driver board. My usual recommendation to them is to try about a 3500 ohm resistors in R7 and R8. Gain goes down noticeably and now your "loud" position on the volume control is maybe at 12 o'clock instead of 9 o'clock. Check the photo below for the location of R7 and R8.

    Bob



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