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Discussion in 'Electronic Design' started by Lost'n Found, Aug 8, 2006.

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  1. Lost'n Found

    Lost'n Found Guest

    Hello. I want to ask about few things that have been bothering me recently a

    1) I've seem some solid state guitar schematics. My question is, why don't
    they ever use op-amps in solid state amplifiers?

    2) I've seen some tube schematics tubes are used, and on the gird of the
    input stage tube, there is a resistor (about 100k-500k) that goes from the
    grid straight to the ground. My question is, if they want high input
    impedance, why would they put a resistor and not just connect the input to
    the grid resulting in very high input impedance?

    3) Assume there is a capacitor before the input stage, and we want to have
    lots of negative feedback. Does it matter were we feed the signal back to
    increase the bandwidth of the amplifier?

    Thank you for your response!
  2. Phil Allison

    Phil Allison Guest

    "Lost'n Found"
    ** Look harder - most SS guitar amps are chock full of op-amps.

    ** Because 500 kohms is high enough for electric guitar pickups.

    Having NO resistor would result is loud BANGS when a lead was plugged or

    ** Yes.

    ......... Phil
  3. Bob Eld

    Bob Eld Guest

    Op-Amps are sometimes used, however they are not easy to incorporate into a
    power design because they have too much open loop gain and insufficient
    voltage swing. It's easier to design a circuit using discrete components
    that provides the required voltage drive and the right amount of open loop
    gain than it is to use what is pre-canned in an op-amp. This comes about
    because all gain stages have a natural "pole" or break point in their
    response characteristics each with up to 90 degrees of phase shift. For
    stability reasons, it is problematic to cascade more than two of these
    within one amplifier feedback loop structure. Most op-amps already have
    these two stages but more are required to get the necessary voltage range
    for a power amp. This makes the design messy with at least three poles in
    the response. So, the simple answer is to not use op-amps, design exactly
    what you need. In most amplifiers only two voltage gain stages are within
    the feedback loop.

    It is necessary to have a DC path from the grid to ground to insure that the
    contact potential of the tube is "leaked" off to set the required bias for
    the tube. If this is not done, the grid will aquire an increasingly negative
    charge because of the electrons wizzing by that will eventually cut the tube
    off and no signal will pass. It is possible to have a DC path that includes
    the source, but, since the source is unknown and undefined, it is not done
    this way. Usually the source is AC coupled with a capacitor and a known
    resistor provides the DC path.
    Yes it matters how the feedback path is created. See number one above.
    Feedback must never accumulate more than 180 degrees of phase shift at any
    frequency while the loop gain is above unity or your amplifier will be an
  4. Lost'n Found

    Lost'n Found Guest

    Very clear and helpful, thanks
  5. Ken Smith

    Ken Smith Guest

    That one is easy to solve with a fastish op-amp. You can locally close
    the loop on the op-amp to make a stage with the gain you need.
    This is the one that is more trouble. You have to go to a discrete part
    to get the swing so you loose a lot of the advantage of the op-amp.
    Make that 'at least one natural.." The smaller discretes often work up to
    higher frequencies where things like the inductances of leads and the time
    it takes for carriers to get through the device start to effect the
  6. Bob Eld

    Bob Eld Guest

    That's right, not only do you still have to come up with the required
    voltage gain but you also have to come up with lower voltage (15 Volt) power
    rails. You also have to figure how to drive both positive and negative from
    center. Everything about op-amps complicates the design. Of course they can
    be used but they bring little to the table in this application so why use
  7. Phil Allison

    Phil Allison Guest

    "Bob Eld"

    ** It is VERY COMMON to see op-amps used as the first stage in power

    There are several popular ways to use standard audio op-amps as the gain
    stage in a power amp.

    1. The op-amp acts as a pre-amp prior to the class A gain stage.

    2. The op-amp's +/- supply pins produce anti-phase drive signals for the
    output stage.

    3. The op-amp *drives* the output stage directly - here the normal
    output point is linked to ground and the speaker signal appears at the PSU
    centre point.

    # 1 is used in Phase Linear Series 2 amps, the Crown DC300A and many other

    # 2 is used in Cerwin Vega power amps, early Jands amps and others.

    # 3 is used in many power amps by QSC and guitar amps from Seymour Duncan
    and Peavey.

    Your lack of familiarity with commercial audio design practice is very

    ........ Phil
  8. Not really that common. However, granted it is done by designers that
    don't really know what they are doing though.
    Not relevant to the actual power amp design point being made. However, a
    high performance opamp preamp to a power amp is quite a good idea.
    Yeah... nasty...
    I believe this last amp came out around the late 60's. Hardly
    representative of modern amp design.
    And these form good standards?
    Bob made very valid points. Using an opamp contained in the feedback
    loop of a main power amp is suicide. You lack of familiarity of how to
    design high performance audio amplifiers is very clear.

    Kevin Aylward B.Sc.
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    "There are none more ignorant and useless,than they that seek answers
    on their knees, with their eyes closed"
  9. The Duncans and Peaveys that I've looked at are pretty good.
    Oh, dear, stand by for nuclear holocaust!
  10. t.hoehler

    t.hoehler Guest

    I've had to work on several gtr amps in the last few years, and the Crate
    amps all used 1458's thruout the preamp, tone and effects stages. They used
    high power zeners to derive the plus and minus 15v rails from the power amp
    rails. Not elegant, but the damn things worked pretty well.
  11. Phil Allison

    Phil Allison Guest

    "Kevin Aylward = Manic Pommy Psychopath

    ** It is very commonly seen - you fucking pommy ass.

    ** Of course it is relevant - you insane pommy pile of puss.

    ** Go drop fucking dead - you asinine shithead.

    ** Wrong and WRONG again.

    ** They represent current commercial practice - you fucking MORON.

    QSC are one of the largest amp makers in the world.

    ** BOLLOCKS.


    ** That Kevin Aylward is seriously mentally ill and a know nothing pile
    of manic pommy shit has been ABUNDANTLY clear to the whole world for

    .......... Phil
  12. Yes indeed John:)

    Kevin Aylward B.Sc.
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    "There are none more ignorant and useless,than they that seek answers
    on their knees, with their eyes closed"
  13. Rich Grise

    Rich Grise Guest

    Yabbut, that's "effects" - by the time the signal gets to them, it has
    already been preamplified, so is up out of the noise floor.

    And really, what kind of frequency response do you need for a guitar?

  14. You need a much wider full-power frequency response than you might
    imagine, especially at the low-frequency end.

    But I would vote for a strict limit on gain. (;-)
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