Amps with no negative feedback and guitar vol knob

Discussion in 'Amps and Cabs' started by 5F6-A, Aug 17, 2019.

  1. lefty kwan

    lefty kwan Member

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    My Pro jr has been modified to have a nfb off switch, on position turns on nfb pot. The nfb pot goes from regular nfb to about .5 the amount of nfb. It’s a great mod and most of the time I run the amp with the nfb off, which gives it a more tweed like sound and response.
     
    Daytona57 likes this.
  2. HotBluePlates

    HotBluePlates Member

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    I know I don't know. Kuehnel's 5F6-A Bassman book doesn't guess at a reason but points out the input 12AY7 of the Bassman was exchanged for a 12AX7 (more available in Europe?), which then implies Marshall had room to offset that with more feedback around the output section.

    Gotta double-check their work:


    Problem is, the JTM45 (another, harder to read, schematic) has the same 27kΩ series resistor as the 5F6-A Bassman, and sources voltage from the 16Ω tap. OT tap voltage (with constant power through-put) varies as the square-root of impedance, so the move from the 2Ω tap to the 16Ω tap results in √8 (or ~2.83) times the source voltage for the feedback loop. This is ~9dB more feedback, and the effects are explored in the Kuehnel book linked earlier.

    Robinette seemed to be talking about other, later, 50w Marshall amps.

    @hogy's post may be misleading unless you're very familiar with amp designs, and see he's talking about overall topology. That is, a not-terribly-gainy, mostly wide-frequency preamp circuit into a long-tail phase inverter, into a power section that is supplied by a somewhat under-filtered power supply.

    EDIT: And if you're talking about the Top Boost channel of an AC30, there are more similarities. That is, a couple of gain stages with a Volume control between, into a cathode-follower driving a tone stack. The stack and a great many other things differ between the amps, including Vox's use of both inputs of the long-tail (which Hogy has said before contributes to the AC30's sound).​

    @Nolatone Ampworks' post spends a lot of time explaining how the AC30 and JTM45 are quite different amps, but share fairly low capacitance/filtering in the power supply relative to the current being drawn, resulting in some nice sonic effects they have in common.

    It can be hard to understand this stuff & how it works together, until you already understand this stuff & how it works together.
     
    Last edited: Aug 20, 2019
    5F6-A, rumbletone and bobotwt like this.
  3. RayBarbeeMusic

    RayBarbeeMusic Member

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    In fact, that's exactly what it controls over a certain frequency range.
     
  4. jnovac1

    jnovac1 Supporting Member

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    people like goodsells. no nfb. that’s one thing they like about ‘em. i am people.
     
  5. rollyfoster

    rollyfoster Silver Supporting Member

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    Not in the context of this discussion, though. Also depends on how it’s implemented.
     
  6. HotBluePlates

    HotBluePlates Member

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    You're both right. But the original question is the problem:

    If you haven't already read Aiken's info that @rollyfoster linked:

    Most often, negative feedback is a fixed amount of speaker signal applied at an earlier point in the power section, usually at the phase inverter. This is implemented with 2 resistors that tap off a fixed amount of the speaker's signal. The resistors form a voltage divider that delivers a fixed % of the speaker signal to the earlier stage (absolute amount of feedback voltage is proportional to the output at the speaker).

    A "Presence" control reduces the high frequency content in the feedback signal, in accordance with the control's position. Now that less highs are in the feedback, there is less gain reduction for those highs, and the player has the impression of a "treble boost" when Presence is turned up. It's really just "less cut" than the gain reduction received by the rest of the signal.

    One could extend the concept further: Make part of the feedback loop variable so that there is more/less feedback for all frequencies, or perhaps put a couple of preset options on a switch. Some amps have "Resonance" controls that do the same as Presence, but reduce the low-frequency content of the feedback signal, to give the impression of boosted lows. A potential drawback is the speakers could sound loose & uncontrolled with excessively-high Resonance settings, depending on amp/cab/speakers. After all, I pointed out feedback tightens the amp's control around the speaker's bass resonant frequency, which is right where the Resonance control is reducing feedback.
     
    Last edited: Aug 21, 2019
  7. RayBarbeeMusic

    RayBarbeeMusic Member

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    Well, I actually work on this stuff for money so. Your presence, or resonance control, works in your negative feedback loop. They control how much of certain frequencies are in the negative feedback signal. When you turn up presence, or resonance, you are reducing the amount of high end (low end) present in the negative feedback signal, thereby increasing gain at those frequencies.

    So no, we aren't 'both right'. I'm right. Presence controls negative feedback at frequencies set by the size of the RC network involved.

    A global negative feedback control affects all frequencies.
     
  8. rollyfoster

    rollyfoster Silver Supporting Member

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    Lighten up.

    And read the question I responded to.
     
  9. HotBluePlates

    HotBluePlates Member

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    @rollyfoster seemed to mean, "No, the presence control doesn't control negative feedback at all frequencies." Because @Bryan T's question appeared to imply that meaning/action for the Presence control.

    Perhaps Rolly's statement was imperfect, but he was saying the same thing you were, that the Presence control is in the loop, and winds up controlling EQ. Perhaps your statement of it was more-better, but you guys were saying the same thing.

    Isn't the goal helping Bryan understand the feedback loop & the Presence control better?
     
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  10. rollyfoster

    rollyfoster Silver Supporting Member

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    Yeah I read it as he was asking whether presence controls the NFB like having an actual NFB control, which isn’t the same thing.

    Setting a value or making an amp’s NFB adjustable isn’t the job of a presence knob. Presence affects NFB but not in the sense that it’s controlling the amount of it in an amp.

    My aforementioned KR12 has both. The effectiveness of the presence control is governed by the amount of negative feedback set by the NFB control. Presence is less effective when there’s less NFB and more effective when there’s more NFB.
     
    Last edited: Aug 18, 2019
  11. Bryan T

    Bryan T guitar owner Silver Supporting Member

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    Thanks for the clarification. I knew that the Presence control was done via negative feedback. What wasn’t clear to me was if bypassing the Presence control removed all of the negative feedback from the circuit.
     
  12. HotBluePlates

    HotBluePlates Member

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    No, actually the Presence control does control the amount of negative feedback, but only at high frequencies. It reduces the amount of counter-acting voltage in the feedback loop at those high frequencies, while leaving the feedback voltage at lows alone. And that is why there is a treble/presence boost as a result.

    And that is an example: the Presence control is less effective, and provides less boost, because there is less feedback voltage in the loop and so less ability for the Presence control to "boost" by reducing feedback at high frequency.

    In this case, both controls are impacting feedback but the NFB control can increase/reduce all of the feedback.
     
  13. LonesomeCraig

    LonesomeCraig Member

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    It seems like a sweeping variable NFB on a pot that you can set where you think it sounds best with different instruments would be the best solution.
    Cutthroat's "Little Wing" amp has this feature. His "Down Brownie" amp has a 3 way NFB toggle.
     
  14. VICOwner

    VICOwner Supporting Member

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    The Jtm45 actually has much more negative feedback than the 5F6-A. Marshall ran his feedback from the 16 ohm tap producing way more voltage than Fender at the 2 ohm output. Both use a 27k/5k ratio. The JTM45 has a 12ax7 v1 which gives more gain than Fender’s 12ay7 v1. The JTM45 has a different feel because of these two differences, but there is more voltage feedback in the loop.
     
  15. VICOwner

    VICOwner Supporting Member

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    I personally like an amp with little or no NFB. I like the slower transition from clean to overdrive if that makes sense. They are more aggressive.
    I have several different amps that range from 0 NFB to large amounts of NFB. They each have different character.
     

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