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Thread: Switchable Global NFB options

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    Switchable Global NFB options

    What would be the best way to wire up a switch to negate the global negative feedback in an amp - I have tried a simple switch that either connects the NFB to the PI tail, or leaves it disconnected. It works fine other than a pop on the switch.

    Would a better method be to add such a large resistance that it still sound disconnected, perhaps a 2.2MΩ resistor, or is simply leaving it disconnected a preferred method?

    I am just wondering if a disconnection - with the circuit not having any type of path - is okay or not.

    Thanks

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by Gtr0 View Post
    What would be the best way to wire up a switch to negate the global negative feedback in an amp - I have tried a simple switch that either connects the NFB to the PI tail, or leaves it disconnected. It works fine other than a pop on the switch.

    Would a better method be to add such a large resistance that it still sound disconnected, perhaps a 2.2MΩ resistor, or is simply leaving it disconnected a preferred method?

    I am just wondering if a disconnection - with the circuit not having any type of path - is okay or not.

    Thanks
    I've done it two ways. The first, is adding a 10M (or some high value resistor like 5M6, or something) in series with the feedback resistor for the -basically equivalent- "no feedback" position. With the switch shorting it out for the global feedback option. I liked this option in that there was no audible switching noise when the switch was engaged.
    The second way was basically to switch in a large shunt capacitor to ground the relevant AC in the feedback path, preventing.. well, the feedback.
    I used a a DPDT switch to simultaneously adjust the input level to the PI to keep the output levels pretty equal when switching in and out of global feedback as well.

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    Supporting Member The Dude's Avatar
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    Sure it's ok. There are amps without NFB. I'm not sure I'd ever want NO NFB in an amp that was designed with NFB in mind. Switching in a different resistor might be a better idea. You'll have to experiment. As to the noise, are you actually going to switch NFB in and out in the middle of playing? If not, I wouldn't worry about the switching noise.

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    Quote Originally Posted by SoulFetish View Post
    I've done it two ways. The first, is adding a 10M (or some high value resistor like 5M6, or something) in series with the feedback resistor for the -basically equivalent- "no feedback" position. With the switch shorting it out for the global feedback option. I liked this option in that there was no audible switching noise when the switch was engaged.
    The second way was basically to switch in a large shunt capacitor to ground the relevant AC in the feedback path, preventing.. well, the feedback.
    I used a a DPDT switch to simultaneously adjust the input level to the PI to keep the output levels pretty equal when switching in and out of global feedback as well.
    great! thanks for the info. I am failing to see the DPDT/volume thing though... what was/is attached to the switch?

    Quote Originally Posted by The Dude View Post
    As to the noise, are you actually going to switch NFB in and out in the middle of playing? If not, I wouldn't worry about the switching noise.
    Right... not switching in the middle of playing... there is a huge volume jump without NFB which I can't imagine to be useful.

    Thanks!

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    Senior Member SoulFetish's Avatar
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    Oh i just switched in a voltage divider simultaneously when switching out feedback. Ill try and find a schematic of it

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    Lifetime Member Enzo's Avatar
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    Like Dude says, you are not likely to switch it during play, so who cares if it pops?

    I think also, like a lot of various mods to amps, you will play with it at first, but most of us will decide we like one way better, and never touch the switch after that.

    I agree with Soul, if you want to kill the pop, put a high value resistor across the switch. I think a 1 meg is plenty, as most of you don't normally have a bunch of larger resistors. I know in my shop I had a good selection of 1.2M, 2.2M 3.3M, 4.7M and 10M and probably others, and over 30 some years I might have used five of them.

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    Quote Originally Posted by SoulFetish View Post
    Oh i just switched in a voltage divider simultaneously when switching out feedback. Ill try and find a schematic of it
    Right, of course. The volume jump between a 47k NFB resistor and none seems to be like - double. Of course that's my impression... i'm sure that there is a mathematical equivalent.

    Quote Originally Posted by Enzo View Post
    Like Dude says, you are not likely to switch it during play, so who cares if it pops?
    Agreed

    Quote Originally Posted by Enzo View Post
    I think also, like a lot of various mods to amps, you will play with it at first, but most of us will decide we like one way better, and never touch the switch after that.
    So right. However my tastes do change day to day. Sometimes I feel like the NFB is too dampening, other times I feel like without NFB is way too... hairy. Perhaps it is time to install a 3M pot and find a good setting that I like, setting up a balance between the two with whatever caps to perhaps getting rid of a touch of that high pitched fizz... taking the readings and wire that in proper.[/QUOTE]

    Quote Originally Posted by Enzo View Post
    I agree with Soul, if you want to kill the pop, put a high value resistor across the switch. I think a 1 meg is plenty, as most of you don't normally have a bunch of larger resistors. I know in my shop I had a good selection of 1.2M, 2.2M 3.3M, 4.7M and 10M and probably others, and over 30 some years I might have used five of them.
    yea, I have probably some 2.2M, which is why I used that particular value. I never had a reason to have higher value - but I have such a stock of resistors, I suppose it is a good time to grab a few each of those higher values.... since we're talking all of about $2 here.

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    Lifetime Member Enzo's Avatar
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    I should also point out, ther is not A feedback resistor. The feedback is usually through a voltage divider. That is made of the feedback resistor and another resistor to ground at the end away from the speaker. That extra resistor may be shared by some other function. So it is a ratio between two resistors, knocking down from the speaker voltage.

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    Quote Originally Posted by Enzo View Post
    I should also point out, ther is not A feedback resistor. The feedback is usually through a voltage divider. That is made of the feedback resistor and another resistor to ground at the end away from the speaker. That extra resistor may be shared by some other function. So it is a ratio between two resistors, knocking down from the speaker voltage.
    Right. I understand - Thanks!

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    Here's a technique that works well if you have a presence control. The presence pot works the same as usual, but when you turn the presence to zero the switch shorts to ground and takes out NFB and presence. Just use a switched 25kl pot. Avoids having to add a separate switch and the wiring is easier.

    Cheers

    Click image for larger version. 

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    Supporting Member loudthud's Avatar
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    The reason you get a pop when you throw the switch is that there is DC on the feedback path that gets shorted to ground by the output transformer.

    Edit: Peavey used the same idea on their amps equipped with a Resonance control.
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    Last edited by loudthud; 10-06-2019 at 12:08 PM.
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