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bass output versus amp input impedance questions

Discussion in 'Amps and Cabs [BG]' started by Magneto, Jul 23, 2004.

  1. Wondering about a problem I'm having.

    My brother lent me his new Line6 Spider 112 (50 watt) guitar amp to play around on with my ESP F-204 bass (active electronics).
    The amp has one input. After connecting to it and powering up, the amp makes a screech sound when I turn my bass's EQ controls. (It's not a feedback problem).
    This happens with the amp at low volume levels, all effects off, even with the bass's volume at low levels.

    I'm thinking I have a mismatch between the bass and amp. The amp's manual says it has an input impedance of 1 MegOhm. I haven't the foggiest what my bass's output is.
    I guess this amp doesn't like the hotter active electronics. I've tested the bass on various amps, and it doesn't make any noise. The amp is in good working order, however, I won't be connecting to it unless I can figure out this issue.

    Any thoughts??

  2. xyllion

    xyllion Commercial User

    Jan 14, 2003
    San Jose, CA, USA
    Owner, Looperlative Audio Products
    It is probably not an issue of impedance, but rather of signal level. It is quite possible that the signal level out of your bass is too high for the amp's input. Try turning down the volume knob on your bass (possibly almost all the way off).
  3. 1 Meg input impedance is fine. And the amp should easily handle hotter electronics, after all many people use effects pedals that put out a signal just as hot or hotter than your bass. Something else is causing this. Bad cord? Dying battery on bass?

    edited: Should handle hotter electronics...
  4. Thanks for the replies and the ideas.
    I've just replaced my battery, checked (like always) with a meter. Good energizer.. Don't think this is the issue.
    Tried experimenting with various volume levels, various EQ settings. Screech is at its worst when I try to increase/decrease my bass boost control, but does it on the mid and/or treb controls too. It almost sounds like it's overpowering the input or something.
    Turning the amp down low or the bass's volume down low does not help. Still screeches when I adjust my EQ controls.
    All effects on the amp are off, eq controls in mid postions, drive and master volumes in mid positions, etc..

    Could it be my cord? This does not seem likely because the same cord works fine on my little stereo practice amp. No crackling, no problems.

    Might do some more testing in a bit. I just don't want to fry anything by accident...

    Thanks.. Mag...
  5. Did some playing around with the bass+amp this evening, and I'm still getting the noise when I try to turn my bass boost up higher than about 1/3. If I turn down my mid boost to OFF, I can almost get the bass up to approx. flat, but no higher. The noise I'm getting sounds like a TV's noise at high volumes. It does not matter if I barely have the bass's volume up at all, if I try to increase my bass, the amp has fits. As long as I keep the bass EQs down, the amp sounds fine. Weird, right?
    Maybe it has something to do with the signal levels or something. I don't know.
    The sad part is that the little amp actually sounds killer when it's not making the noise. With a bass speaker it would be super fun to practice on.
    My concern now is whether I'm doing any harm to this amp by playing on it at all. I'm surely not cranking the volume, and the sound is very clean..

  6. Flourescent lighting? Bad ground?

    Have you moved the amp to a different room?
  7. RevGroove

    RevGroove Commercial User

    Jul 21, 2002
    Burlington ON Canada
    Manager, Account Services: Long & McQuade Ltd. (Burlington); MTD Kingston Basses International Emerging Artist; Bartolini Electronics Emerging Artist
    I can't believe no one else has asked this question yet, but why on earth are you playing a bass guitar through a 50 watt g**t*r amp?
  8. notanaggie

    notanaggie Guest

    Sep 30, 2003
    If it is noise-like, do you have the amp gain way up?

    Some of those digital modeling amps have input noise gates. Part of the reason is they can be pretty noisy! When playing, the noise may be drowned out, or else some compression may be set on, and the wanted signal "squashes" the noise.

    If you crank up the bass's controls, you may have enough residual noise output from its active electronics to trigger the noise gate to open, and suddenly there is a loud noise.

    If this sounds possible, back the amp gain way back, and put it on a model with low gain to begin with, then see what happens.
  9. Haha.. I guess everyone was waiting for YOU to ask the question RevGroove :)

    Reason: No bass amp at this time. Saving my money for a real amp. Only home stereo with home stereo speakers (actually doesn't sound bad) for practice. This guitar amp is a welcomed change.

    Flourescent lighting?: No
    Bad ground: Don't think so..
    This isn't present noise that increases as I increase volume on the bass or amp. It comes out of nowhere, suddenly, quite loud. There is no hum or hiss or anything of that sort.

    "notanaggie": The amp gain is not very high, nor are any of the other amp controls. It does it even at very low volumes, and happens when I try to raise/lower the eq controls on the bass. It does it on ALL the amp models, with or without effects on. I'm using the mostly-clean models.
    It could be some sort of noise gate like you've suggested, and the amp has no manual compression controls to adjust, so unless it's automatic?

    Thanks for all the ideas..
  10. i got a similar thing happen to me when i tried using a zoom guitar pedal!

    i guess i wasn't bothered about making it work so i just stopped using it... i was using a bass amp passive bass and this pedal non of the modellers made much difference i just got crappy soundswhen i hit the strings!

    personally i wouldn't use the guitar amp neways... you might damage a friends amp even if you loose the crap sound