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Headroom Questions/ Observations/Discussion

Discussion in 'Amps, Mics & Pickups [DB]' started by Chris Fitzgerald, Feb 14, 2002.

  1. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator Gold Supporting Member

    Oct 19, 2000
    Louisville, KY
    This has sort of been covered before, but not in this exact way here at TB, so I hope this doesn't seem terribly redundant. Having said that...

    With every amplification setup I've ever owned, seen or heard, there is a point below or at which you get good, clean, and relatively uncolored sound - but once you go above that point, the sound starts to get "hot" and "electric" and "muddy" sounding. I've always thought of this point as the "headroom ceiling" of the particular system in question, and have noticed that each amp system has its own point where the sound starts to go to hell.

    With almost every amp I've ever owned, that point has been at about 12 o'clock on the Gain and Master volume controls. What I'm wondering is, which part of the signal chain is really going to hell at that point? In my case, I've got three parts: Preamp, Amp, and Speakers. It could well be that all three are getting maxed out at that point, but I suspect that either the amp or speakers get to that "overload" point first, and I'd love to know how to figure out which one it is so I could upgrade that part to something that includes more clean headroom. I play some fairly high volume gigs on DB, and since I love the music that's okay (even though it's certainly not ideal)... but when I get to the POINT OF MUD RETURN, I always simply refuse to turn up because I hate playing with $#!++y tone, and I'd like to be able to get a bit more CLEAN volume when needed.

    Thoughts or ideas on the issue? Which part of the signal chain do you think fails first? What are some of the high-end options for upgrading that part?
  2. Joe Taylor

    Joe Taylor

    Dec 20, 2001
    Tracy CA
    As an Electronics Engineer here is what I know about amps and distortion.

    you can look at the amp as a chain of smaller amps, and signal conditioners.

    Distortion can be introduced at any point along the chanin when the individual stage gets over driven. What this means is that the amplifier input is such that when the stage addes it gain to the signal the amplitude reaches in maxmimun level and starts to clip the signal off. The sound we all like is some kind of wave form sine, triangle, square, etc. When the signal in amplified to a point where the top of the signal is chopped off distrotions occure. As the signal increases it gets to the point where, for all practicle purposes, the signal turns into a square wave which is kind of bad as a square wave is made up of lots of harmonics. (all odd and even harmonics)

    So, if you over drive the preamp the signal is distroted from the start and as the signal passes through each stage of amplification the distrotion gets amplified.

    There are lots of other things that can cause signal distortion such as the type of couppling between stages, crappy power supplies, bad amplifier components.

    If you ever get a chance to fool with a wave form generator run it through a speaker and change from one type of wave to an other and check out the diffrent sounds. Signal processing of these wave forms is how electronic musical instruments work. Any type of wave form can be made from a sine wave.

    Now, when one of our beloved bass strings vibrate it is a sine wave. The string vibrabrades at the primary frequency pluss all of its harmonics the body of the bass signal process all these frequencies and gives us that tone that is so warm and rich and a few odd sounds when the harmonics add or subtract up to make woof notes.

    enough dribble

  3. Joe Taylor

    Joe Taylor

    Dec 20, 2001
    Tracy CA
    I for got the head room is the diffrence between there the signal normaly is and the signal level wher distrotion occures.

    Say, you have a normal input signal that runs at .75 vpp (volts peak to peak) and your amp distorts at 1 vpp you would have .25 head room. What this means is in your normal playing the signal you generate is .75 vpp and you hit a loud passage you will most likely distort your signal. What you do to fix this is go buy a larger amp (more watts rms.) OR, turn down the preamp so that when you hit the loud part you don't overdrive you amp.

    To test an amplifier you need a signal generator and an "o" scope. Then you can figure out how large a signal you amp can take befor distrotion happens. And you can figure out what the normal output for your pickup or microphone is.

    Best of all to just listen to set your amp to what sounds good.

    Of course all of us who grew up in the 60's just love the over driven amp with lots of feed back so go buy a real big amp and over dirve it ;-)

    All this stuff is about right for analog amplifiers when it comes to digital amplifiers you are on your own.

    Joe - who gave away his last amp years ago.
  4. Chris Fitzgerald

    Chris Fitzgerald Student of Life Staff Member Administrator Gold Supporting Member

    Oct 19, 2000
    Louisville, KY
    I hear you, and I agree to an extent. But Tuesday night I was experiencing the same phenomenon on a gig with a big stage and no drummer (still, the ROOM was loud). I had a 25' cable and walked out to 20' in front of the stage while the guitar player twiddled with the amp knobs at my request, and the same thing happened even when the top of the bass wasn't directly in the line of fire. This leads me to believe that there's more to the equation than just the top of the bass vibrating. I could be wrong, though.
  5. Bob Gollihur

    Bob Gollihur GollihurMusic.com

    Mar 22, 2000
    Cape of New Jersey
    Big Cheese Emeritus: Gollihur Music (retired)
    Maybe I'm a slow learner, but as I've graduated to larger and higher powered amplifiers these last few years I've discovered that very often what I'd thought was a "speaker reaching its limits" sound was actually an "amplifier pooping out" sound.

    I hesitate to do so, but I suppose a "Tim the Toolman More Power" sound is due here. It certainly does make a difference IME.

    Mr. Taylor enlightens the thread with his comments; the audible artifacts of overload can occur in any stage of the signal processing.
  6. Sam Sherry

    Sam Sherry Inadvertent Microtonalist Supporting Member

    Sep 26, 2001
    Portland, ME
    Euphonic Audio "Player"
    Chris, I see I'm a bit late here -- been sick.

    My $0.02 is that sounds clean up when you back off the preamp and open up the master. I agree with you that on the amps I have used, some audible compression occurs at 12:00 ON THE PREAMP. On many amps, things start to heat up a bit earlier, even.

    However, if your preamp's at 11:00 and your master's only at 12:00 you should have BOATLOADS of room left on the master before any power-amp clipping kicks in. If that's not true for you, take your amp to a good tech and get it checked out.

    Punchline for me: Set the master at about 1:00 and open up the preamp until you like the sound. Then, as the gig heats up, don't touch the preamp -- open up the master volume instead.
  7. Chuck M

    Chuck M Supporting Member

    May 2, 2000
    San Antonio, Texas
    My experience has brought me to the same conclusion as Bob. I've tried a load of different amps and have settled (for now) on an SWR Grand Prix>Stewart PA-1000>Eden 210MBX.

    I put an RCA 7025 in the SWR and it has beautiful clarity and detail and none of the harshness I usually hear in SWR equipment. The PA-1000 was the turning point in the amplification system for me. Prior to that step up in power, my amps never seemed to have the ability to really reproduce the dynamic range of a bass (electric or acoustic).

    I heartily second Bob's recommendation that you try more power. Choose that power amp carefully as some power amps simply do not sound very good or live up to their power ratings. Crown, QSC, Stewart and Crest all have good reputations. I'm sure there are other good brands as well but those are the ones I've heard.

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