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Thread: Fletcher-Munson compensation

  1. #1
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    Fletcher-Munson compensation

    I was curious if anyone ever built a guitar amp that takes into account that humans have trouble hearing low and high frequencies at low volumes but not at high volumes (as described by Fletcher-Munson curves).

    I envision the amp gradually boosting highs and lows as you turn the volume down and attenuating them as you turn the volume up. It would be cool if you could set your tone the way you want and have that tone remain consistent as you increase and decrease your volume since the circuit would retain the perceived volume balance of the different frequencies. I think it would make life simpler, you wouldn't have to tweak your amp as much.

    I looked on the internet for some stuff and I saw some patent applications for a circuit that gradually compensates as you increase/decrease the volume, but I don't think it wasn't geared towards guitar amps.

    How about a simple loudness control like found on some stereos? I was reading about those and it seems like they are a circuit with a fixed bass and treble boost that kicks in when the volume control is set at a certain point. I guess it uses a pot with four connections. It doesn't sound as nice as something that gradually compensates, but I suppose it's better than nothing.

    - Clint

  2. #2
    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    For all the extra circuitry trouble,... if you need that much extra compensation, just sit closer to the darn amp.
    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

  3. #3
    Better Tone thru Mathematics bob p's Avatar
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    thiele-small alignments have diminished bass response with with large signals vs. small signals. i like to think of them as an automatically correctiing for FM phenomena.

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    How about a volume control that mainly acts on mids?

    On the Electar Tube 10 amp rebuild project I'm working on I have a gain control, a master volume, and no tone controls at this point. On the gain control, I added a treble bypass capacitor so when I turn the amp down it retains the highs. That works pretty well. In contrast, the master volume doesn't have a treble bypass capacitor, so when I turn that down, I lose the highs pretty quickly.

    Since a treble bypass capacitor retains the highs when lowering the volume, I figured I should be able to make a bass bypass too so that it retains bass as I lower the volume too. So if I had a volume control that maintained treble and bass, then it appears that the volume control would really only be attenuating mids.

    I decided to experiment. As I said before, the amp has no tone controls at this point. At loud volumes, I like the balance of treble and bass, I wanted to see if I could retain that as I lowered the master volume (the volume without the treble bypass cap). As I lowered the master volume, I lost highs and lows. But then I compensated by using a graphic EQ pedal with accentuated lows and highs and diminished midrange. That worked pretty well. I got a decent tone at lower volumes. Using an Xotic BB Preamp pedal with gain turned down and bass and treble boosted sounded even better.

    But the EQ pedal and the Xotic BB Preamp pedal are "active", they can boost stuff. Using passive components, instead of boosting highs and lows, I would need to cut mids. I have a Torres midrange control kit for guitar that is designed to scoop mids. I think I'll try adding that to my amp and see how that works.

  5. #5
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    I reckon if one wanted to do something like this - my 1st ( feeble ) thoughts would be:

    Since typical passive RC tone circuits are attenuators, one might be able to run the circuit ground connection ( like the mid-slope resistor in Fenders ) through some device...like a transistor.
    From there, you could use a ganged volume pot and as you adjust the volume of the amp, you could scale the transistor's response to bring the tone circuit in and out of the the signal path.

  6. #6
    Lifetime Member Ray Ivers's Avatar
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    Clint,

    The Marshall 30th Anniversary amps are the only guitar amps I know of that have low-volume compensation, but there are probably others as well.

    Below are some circuits - the first set I got off the Web somewhere, the second one - which is not strictly 'loudness compensation', just a simple bass boost - is what I use in my amp; IME small-value pots have little or no treble attenuation at different settings. This circuit can be easily implemented in an Rk MV circuit, and suitable inductors were available from Mesa-Boogie last time I checked - the smaller the pot value, the smaller the L value needed for the same LF boost frequency.

    Ray

    Attached Thumbnails Attached Thumbnails loudness-circuits.jpg   inductor-loudness-circuit-my-amp-.jpg  
    Last edited by Ray Ivers; 09-16-2007 at 04:24 PM.

  7. #7
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    Check out the channel 2 gain control of Hughes & Kettner Vortex:

    http://www.freeinfosociety.com/elect...ervortex80.pdf

    It's not a solution for F&M compensation but does attenuate low frequencies when gain is increased and accentuate highs when gain is turned down.

  8. #8
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    Ray,

    I'm interested in your bass boost circuit and had some questions about it.

    You said
    This circuit can be easily implemented in an Rk MV circuit
    .
    I'm a newbie at all this, so I don't know all the lingo, but what does "Rk MV" mean? I assume "MV" means master volume. But what do you mean by Rk?

    Is your circuit supposed to be a master volume pot with a bass bypass? If so, isn't a 2.5K pot way too small to turn down the volume?

    I've been messing around with rebuilding an Electar Tube 10 amp. Based on that schematic (http://www.freeinfosociety.com/elect...iew.php?id=375), where would you put your bass boost?

    Thanks,
    - Clint

  9. #9
    Lifetime Member Ray Ivers's Avatar
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    Clint,

    Rk stands for 'cathode resistor'. Replacing a cathode resistor with a small-value pot (wiper = pot output) can make an excellent MV control in some circuits, although the Electar Tube 10 is not a good candidate for this setup IMO.

    Ray

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