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NFB 4ohm/8ohm switch

Discussion in 'Shock Brother's DIY Amps' started by Jewellworks, Jul 12, 2020.

  1. Jewellworks

    Jewellworks Tele-Meister

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    I'm looking at adding a switch on my NFB loop to switch it on and off. Then it occurred to me, I also have an 8ohm tap on my OT. So why not switch between 4ohm, off and 8 ohm. ?
    I've got a 22k resistor on it now. What will happen if I switch to 8ohms?
     
  2. andrewRneumann

    andrewRneumann Tele-Holic

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    There’s more voltage on the 8-ohm tap than on the 4-ohm. With the same feedback resistor, you will end up with more feedback. You could make some calculations and figure out what resistor on the 8-ohm tap would give you the equivalent of the current 4-ohm set up. Then you could have your switch be “none”, “a bit”, and “a bit more”.
     
  3. andrewRneumann

    andrewRneumann Tele-Holic

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    Actually it’s an easy calculation. Use a 33k (31k1 to be exact) resistor on 8-ohm tap and you will have the nearly the equivalent of a 22k on the 4-ohm tap.
     
  4. Jewellworks

    Jewellworks Tele-Meister

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    Sonically, what's the difference? By "a bit more", it'll have a smoother, cleaner response? Is it even worth the trouble? If leave it at 22k, it'll be "a lot more", meaning very clean and smooth.. right??
     
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  5. robrob

    robrob Poster Extraordinaire Ad Free Member

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    You'll get about 40% more NFB with the 8 ohm tap so you probably will hear a difference.
     
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  6. andrewRneumann

    andrewRneumann Tele-Holic

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    Well it’s subjective. Too much feedback and you lose a lot of bite. Think Hi-Fi. I’m not too sure what 1.414 times more feedback will sound like for you. I think your original idea sounds like a good experiment though. Try disconnecting the speaker jack from the 22K and connect the 8-ohm tap to the resistor and see if you like it. It won’t hurt to try.
     
  7. James Knox

    James Knox Tele-Holic Silver Supporter

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    What is the calculation?
     
  8. andrewRneumann

    andrewRneumann Tele-Holic

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    Doubling the source nominal impedance, say going from 4-Ohm to 8-Ohm tap, you increase feedback by the square root of 2 (1.414).

    The winding ratios control the voltage on the secondary taps. An 8-Ohm winding has 1.414 times as many windings as a 4-Ohm tap. The same proportion can be divided if moving down the tap ladder too.
     
    Last edited: Jul 13, 2020
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  9. D'tar

    D'tar Friend of Leo's

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    20200608_192217.jpg
    From Rob's site
     
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  10. andrewRneumann

    andrewRneumann Tele-Holic

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    From what I’ve read, less distortion until you get to clipping and then it clips harder. What kind of amp is it? What kind of overdrive do you like?
     
  11. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    A part of the output signal is fed back and subtracted from the (input) signal. Any non-linearity/distortion introduced inbetween is counteracted. Up to the point where even NFB can't correct it.

    This results in the behaviour as described in the post above.
     
  12. James Knox

    James Knox Tele-Holic Silver Supporter

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    I have a simple on/off mini toggle on my 5F1 for NFB or no NFB. After reading through this thread, I am realizing that the new 5F1 I’m getting ready to build will have an 8ohm OT speaker tap (which I plan on using).

    What resistor do I replace the 22k NFB with?

    Also, I’m trying to go “next level” and replace the simple on/off NFB switch with a on/off/on NFB switch so I can have “no” NFB and 2 different levels of NFB. Still a little confused with figuring out the right values for the 2 resistors.

    After seeing @robrob pic:

    CE5EC24C-2403-4514-9675-21DD8087AC91.jpeg

    I put together this to try out in my current 5F1:

    AD0CF3E4-6DD7-40D4-A86C-7F225E257CC9.jpeg

    So the center lug of the on/off/on goes to the board where the 22k (removed) resides and the twisted/terminated resistors are soldered directly to the speaker jack.

    @andrewRneumann do the 2 Resistor I use here retain their original value, or since they are twisted together do they “together” create a different value?
     
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  13. andrewRneumann

    andrewRneumann Tele-Holic

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    Assuming all else equal, if all you are doing is changing from the 4-Ohm tap to the 8-Ohm tap, you just need to increase the negative feedback resistance by about 40%. The closest E12 is 33k. I don't think you could hear the difference between 31k and 33k.

    It appears to me that the resistor that is switched "off" is part of an open circuit and therefore doesn't change the resistance of the other "on" resistor. Is that a 68k or 6k8? It's hard to tell.

    Fill in the blank: on your 3 option feedback switch, you are looking for "none", "original" and ____ ?
     
  14. andrewRneumann

    andrewRneumann Tele-Holic

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    @James Knox You are keeping the same OT impedance load on the 6V6 right? (5k or 8k?)
     
  15. James Knox

    James Knox Tele-Holic Silver Supporter

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    I need to figure this out for both actually. My old build 5F1 is 4ohm and. Y new build is going to be 8ohm.
     
  16. James Knox

    James Knox Tele-Holic Silver Supporter

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    “Different”

    or maybe more specific

    “halfway between none and original”

    Thanks for you help man,
     
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  17. James Knox

    James Knox Tele-Holic Silver Supporter

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    68k cuz I had one. I’ve read some guys really liking 56k for NFB, but I didn’t have one.
     
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  18. andrewRneumann

    andrewRneumann Tele-Holic

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    That 56k will give you about half the amount of feedback that 22k will. (If I did my maths right. I calculated 53k for half.) So again, as long as you didn’t change anything else (like the OT primary impedance) you would multiply this by 1.414 if doubling the nominal secondary impedance. 68k isn’t exact but it’s pretty darn close. You could also try 82k. Just change that 22k to 33k and I think you will be golden.
     
  19. Jewellworks

    Jewellworks Tele-Meister

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    for what its worth, i just decided to keep it simple and add a toggle for the NFB On/Off. if anything i want the cleaner signal and a grungier signal with the NFB off. i dont see any reason to go "cleaner".
     
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  20. jtcnj

    jtcnj Tele-Holic

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    I have mine on a 25k pot in series with 2 - 4.7k resistors - for @10k minimum value.

    I usually use the pot to increase the NFB and tame things a bit with humbuckers.
    Allows a bit more adjustment along with vol pot and strength of input signal.

    Thats on the stock 4Ω tap; have yet to experiment much since I added the 4/8Ω switch.
     
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