Frequency-specific monitor amp problem?

Discussion in 'Monitoring' started by Bobby Yarrow, Jan 23, 2004.

  1. Bobby Yarrow

    Bobby Yarrow Guest

    I'd noticed a slight unevenness in my monitors, just a general sense that the center image had drifted a little to the left. My little radioshack spl meter confirmed that the left monitor was producing a little more volume, maybe 3db, particularly with test tones around 200hz. My first thought was that it was problem with the right monitor, but, remarkably, when I switched the speaker leads, the problem switched sides.

    By testing with other monitors and by replacing all the cables, I think I've pretty much isolated the problem in the amp. The thing that has me scratching my head is the frequency-specific nature of it. The right side of the amp is definitely coming up short between 200hz and about 300hz or so. At 100hz the monitors test out the same, and above 1K they test the same. Between 300 and 1K, I sometimes got a spread of 1 or 2db, but of course I'm not using the greated decibel tester.

    Does this make any sense? It's occurred to me that the frequency-specific test results might have more to do with the meter than the amp. Could one side of the amp just be falling short at some frequencies but not others?

    Thanks a million.
  2. jonyoung

    jonyoung Well-Known Member

    Dec 31, 2003
    Bob, It's just a guess, but there may be a capacitor or inductor on that channel that's not behaving properly. They are frequency dependant, reactive devices. Their values are specified to produce flat frequency response within the circuit, but if the capacitance or inductance goes out of whack without total component failure, it could produce a curve at any given frequency. EQ circuits are just combinations of resistors, capacitors and/or inductors. The frequencies contolled by a band on an EQ is determined by the values of the reactive parts.
  3. MisterBlue

    MisterBlue Member

    Mar 15, 2003
    It is possible that there are some bad components in your amp although it is surprising that the problem only appears in a somewhat narrow frequency range. Are you sure that the difference is not coming from the signal source itself? It is a much more likely origin than the power amp. Did you try new/other cables ?

    Just some ideas (most of which you might have already investigated).

  4. Bobby Yarrow

    Bobby Yarrow Guest

    Thank you both. I've done all I can think to do to isolate it in the amp, except swap out the amp of course. It doesn't seem be any higher up the chain -- problem happens with a cd player patched directly into the amp, and the problem doesn't switch sides if I swap the left and right ins going to the amp. Checked it with different speaker cables and monitors, and there it is. Sounds like I'm going to need a new amp!

    Thanks again.
  5. dayvel

    dayvel Member

    Mar 25, 2002
    Post deleted.
  6. mkruger

    mkruger Guest

    It is possibe that your "idle current alignment" or more likely your "offset allignment" needs to be adjusted. Some amps this can be adjusted with a set of variable resistors. Some amps are only built with regular resistors makeing the adjustment impossible. Heat causes this change more commonly on VR's because they are mechanical, and the constant expansion and contraction of the contacts wheres them out. They only last so long, like capacitors they need to be changed which is why you only find this problem in more expensive high-end amps. Anyway, you need to find the bad resistor and change it out. If this is the case do both channels with resistors that have no more than a 2+-% tolerance. Use ceramic to replace the foil resistors. I suggest you try to find a service manual for your amp or bring it to a pro to get fixed. It's a fairly simply fix but you need to have the power on to read the current so if your hand slips you'll blow your amp and possibly get the shock of your life! If you want to know more about this just ask...

    I think riversedge is also on the right track with his capacitor theory. Definatly change all of them throughout the entire amp. It's just good to do for sonic integrity.
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