Negative Feedback (?)

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Electric I, Oct 10, 2006.

  1. Electric I

    Electric I Member

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    Is this a desired effect ? What does it do,exactly ?
     
  2. kt77

    kt77 Member

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    for one thing...it makes your amp run more linear

    the 'desired' effect is subjective

    PP amps can get brash w/ less NFB

    some people put a pot in place of the NFB resistor and dial it in until they like what they hear....effing good idea
     
  3. teleamp

    teleamp Senior Member

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    NFB, negative feedback simply put is feeding some of the output back into the input. It can be used to; improve stability, help regulate impedance, control internally generated noise, and increase bandwidth, all at the cost of overall gain. Some people will mod their amps by removing or placing a switch or pot inline to have the option of no NFB or variable NFB. Removing NFB can icrease gain, but, the NFB loop might be there for a reason, see the second sentance.

    I once saw an amp makers advertisement claiming there was no Negative feedback in his amp, he also said his amp was 15 watts because thats what the OT was rated at.

    I do not use a NFB loop in a couple of my amps because I am trying squeeze every last bit of gain out of those particular amps.

    Hope this helps.

    OTM, are you out there?
     
  4. uberpict

    uberpict Member

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    Isn't this what the presence knob is connected to on most amps, changing the amount of feedback?
     
  5. donnyjaguar

    donnyjaguar Member

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    I just have to pipe up with this lone correction. NFB does not improve stability, but it can sure create instability.

    I can explain why. The NFB signal that is applied to the input of a power amplifier is out-of-phase with the input. This of course reduces gain. As you go up higher in frequency, much higher, what can happen is the signal through the transformer will start to change phase. This is due to the reactive nature of the transformer itself and something a well designed piece of iron will limit to a large extent. When this change-of-phase happens the signal now applied to via the NFB circuit will be in-phase with the input. When this happens you get another useful circuit in electronics called an oscillator. :) I say this tongue in cheek as this is not something you want happening in a power amplifier. I think you can see that if there is no NFB circuit in a power amplifier that this oscillation can't occur, hence my statement that NFB does not improve stability.

    Generaly speaking, your guitar does not produce these ultrasonic signals but it is possible that a transient can cause the amplifier to burst into oscillation. This could be as simple as a pop created when you plug your instrument in.

    There are a few approaches to stopping this from happening. The first is to filter out any signal from the PA that would cause this to happen. This could be bandwidth limiting going into the amplifier itself. It could be to place a resistor in series with the signal(s) going to the output tubes thereby creating a low-pass filter by virtue of the inherent internal capacitances of the tube(s). Another way, which is often applied to hi-fi amplifiers (where you want lots of bandwidth) is to create a capactive network in the NFB circuit that will negate the problem of phase reversal at ultrahigh frequencies. This can be a bypass cap on the NFB resistor or a second NFB route extracted from the plate of one of the output tubes with component values carefully chosen to apply a larger degree of NFB, in the correct phase, back to the input stage of the power amp.

    Now you know!

    DJ
     
  6. CarlH

    CarlH Member

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    My understanding is that the JTM45 has more NFB than any other amp ever made (at least the ones made by major manufacturers). Some seem to think it was a mistake made by Marshall.
     
  7. reaiken

    reaiken Member

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  8. uberpict

    uberpict Member

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    :AOK Know I remember where I read about NFB. Great articles, BTW, you really have a way of bringing it down to a layman's level without losing the intent.

    Totally OT question, but I need to ask if you have any ideas on why my amp has blown three tubes in the same socket within two months of each other? :messedup :messedup:messedup Blew one last night after switching strings and was really digging the tone. :(

    I've tried to retension the sockets to see if that would help but no dice. They were questionable before I replaced the last tube but they're tight as they can be now. Did this with SED's and GTE34LS's so I don't question the tubes at all.

    The one thing I can see that's wrong inside the chassis is that the resistor near that socket (very big and appears to be carbon comp) looks "burnt". I don't remember it looking that way when I put the first set of GT's in but I'm really not sure what to look for. I noticed it when replacing the last tube (didn't bother with a new set to see if the new tube did the same) and not sure if the resistor is a cause or consequence of the problem.

    Sorry to hijack the thread, just PM me if you could, I would appreciate it greatly. I'm taking it to the shop and will have the resistor replaced (along with any other bad components) but value your input tremendously. :)
     
  9. uberpict

    uberpict Member

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    Probably the bigger of the two, it's a Rivera Knucklehead. That's what I was thinking the resistor is fried for sure. Could have been one of any prior tubes which burnt the resistor; used GC trade-in (roll the dice). There was a strange residue in the chassis I couldn't identify. Got tooo eager to hear the new tubes and wiped it off. (Kicks dirt) Carbon. Doh! And it was sounding sweeeeet before it blew.
    :dude:
     
  10. drfrankencopter

    drfrankencopter Member

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    One thing to note is that NFB makes your amp run more linear until the power stage (and phase inverter) starts to distort, after that, it can actually add distortion, or at least change the character of it.

    When you install NFB in an amp that formerly didn't have it you might notice a more sudden transition between clean and crunch (assuming a relatively clean preamp here).

    Cheers,

    Kris
     
  11. Electric I

    Electric I Member

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    Thanks for all the responses. All is clear now.
     
  12. Electric I

    Electric I Member

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