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    Altering the feedback line on a VTA boarded ST-70/ST-120 amp

    Bob Latino
    Bob Latino
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    Post by Bob Latino on Mon Apr 11, 2011 5:13 pm

    The VTA ST-70 and ST-120 amp kits come with a 7500 ohm feedback resistor. (R7 > LEFT channel and R8 > RIGHT channel) This value has been found to be a good compromise for most applications - giving accurate, smooth tube amp sound and good speaker control for most speakers. I have at times altered the value of this feedback resistor and tried to give an objective and subjective evaluation of what I heard.

    1. The value can be changed but IMHO you should keep the value between about 3500 ohms and 10,000 ohms. Below 3500 ohms and above 10,000 ohms, the character of the VTA ST-70 and ST-120 will change.

    A. Below 7500 ohms will increase feedback, lower the gain of the amp but if you go below about 3500 ohms the amp will sound "harder" and less "tube like". The lowest I have ever gone was to 2200 ohms and I did not like the way the amp sounded - too hard, less mid bass ..

    B. Above 7500 ohms will decrease feedback, the gain of the amp will go up slightly, the bass seems to get a little sloppier above 10,000 ohms. The amp sounds a little more "tubey".

    2. You can also remove the feedback line by just removing the wire that goes to the two "NFB" eyelets on the front of the driver board. Beware, however, because there are some issues that could arise if you do this. The amp MAY now not be stable with certain speakers. If the amp "motorboats" (low level cyclic noise) with your speaker or exhibits any other abnormalities, you better reconnect the feedback line. I haven't had any troubles with the speakers I have here but the load that some speakers present to the amp could cause problems for the amp (without feedback). Another issue is that without any feedback the gain of the amp will rise. The VTA ST-70 and ST-120 sound warm and fuzzy but not particularly accurate without any feedback.

    Bob


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    kaner
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    Post by kaner on Fri Apr 27, 2012 8:53 pm

    Would it make any sense to put a pot on the feedback to give you the ability to adjust it depending on volume levels, type of music, speakers and the like?
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    zx

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    Post by zx on Sat Apr 28, 2012 9:23 am

    yes...
    you can then setup the sound for your speakers, an other speakers you may get goodluck

    Thanks for the site Bob
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    Post by sailor on Sun Apr 29, 2012 10:52 am

    If you use a pot you will want to put it in series with say a 2200 ohm resistor so the value never goes below that amount. You will also need a linear taper pot rather than an audio [volume] pot which tapers way too fast for this application.
    kaner
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    Post by kaner on Sun Apr 29, 2012 10:45 pm

    Makes sense. I wouldn't drill a hole in my chassis for it but at least I could find out the resistors that work best, then pick one.

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