More High End Under Load

Discussion in 'Amps and Cabs [BG]' started by Remus_Redbone, Feb 26, 2014.

  1. Remus_Redbone


    Dec 27, 2010
    Western AR
    I purchased a GK MB500 head a few months ago to replace the amp on top of my old Ampeg B15T Portaflex cabinet.

    Something I have noticed; when driving the single Eminence Legend CB 158-8 ohm speaker I now have in the Portaflex cabinet, the amp sounds fine, but has less high end than when I have it driving the Eminence and any other 8 ohm speaker. I know the amp is producing more power into the 4 ohm load, but I haven't owned an amp that changes its "EQ" when driving a lower resistance load.

    As an experiment, I got a longer speaker cable and faced the extension speaker away from me and about 10' away. I sat directly in front of the Portaflex, and played the amp with the extension speaker connected, then with it disconnected. The result; significantly more high end out of the Eminence in the cabinet when the amp is driving the 4 ohm load.

    The extension speaker is an old JBL E140 - 8 ohm in a homemade cabinet with no tweeter. Nothing special, but good reinforcement when I need the extra kick for a big room or outdoors. I even faced it into the floor one time to make sure it wasn't the source of the additional highs. Same results; the Eminence in the main cabinet sounds much brighter when the amp is loaded at 4 ohms.

    Are my ears playing tricks on me, or is it not so unusual for an amp to have a different frequency response when under a different load?
  2. Hi.

    When combined without a crossover, the two different cabs are slightly out of phase with eachother on certain frequencies.
    The lows are affected (attenuated) more than the highs.

    Rather common phenomenon when mix-matching cabs.

  3. Remus_Redbone


    Dec 27, 2010
    Western AR
    I think I pretty well eliminated any out of phase issues by laying the extension cabinet face down several feet behind the Portaflex cabinet, and playing / testing with my face (ears) directly in front of the Eminence speaker in the Portaflex cabinet.

    In my original post;
  4. Hi.

    Actually, You IME did not.
    Depending on the space the test is done and the dissimilarities between the two speakers, there's no telling how high the unpredictable frequency reproduction extends.

    The lows, being of course omnidirectional in nature, are affected the most. The speaker orientation then obviously has very little or no effect on that.

    Only by the aid of a steep cross over(s) You can partly eliminate that unpredictability.

    If You do the test with two alike cabs/speakers, and still get the same results, I'd look at the amp as the cause of that rather weird phenomenon.

    OTOH, by feeding the existing mis-match rig from a signal generator and using something as simple as the dB meter on Your phone, you can gather some empirical data on the affected frequencies.

  5. teemuk


    Mar 1, 2011
    Class-D amp? Their output LC filters are usually tuned for a certain load and response to higher or lower loads is different.
  6. The polarity on early JBL speakers, by the way, is opposite of conventional speaker polarity. For conventional speakers, the cone moves outward when a positive signal is applied to the + terminal on the speaker. For these JBL, the cone moves inward. (This is in JBL Technical Notes, Volume 1 Number 12B.)

    Try reversing the polarity on the JBL.
  7. Remus_Redbone


    Dec 27, 2010
    Western AR
    Ya, well aware of the JBL polarity issue; been using the same type speaker for over 20 years. I did the 9v batt test to insure have it in phase with the Eminence in the main cab and all other cabs I own.

    That would have probably resulted in a major thinning of the overall sound. I'm not experiencing that at all.

    When I connect ANY other 8 ohm speaker I have, I'm hearing a significant increase in the brightness / treble out of the Eminennce speaker in the Portaflex cab on which the amp sits.
    This is like hitting a bright switch when I plug in the second speaker; any other speaker I own.

    A previousl post by teemuk seems to suggest this is a characteristic of a class D amp, which the MB500 certainly is at 7" x 11" x 2" and 4 lbs.
  8. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Might be your extra power handling and sensitivity is meaning less distortion, which means less high harmonics.
  9. Grab a matching pair of GK 112 Neo cabs and run the same experiment.
  10. Remus_Redbone


    Dec 27, 2010
    Western AR
    I'm doubting my wife would buy the rationalization for "needing" two more speaker cabinets considering I've got 8 sitting around here already.
    My line; "I've got to have these two matching speakers to figure out why my new amp does this unusual treble boost when I plug in both at the same time".

    Might work....
  11. Passinwind

    Passinwind I Know Nothing Supporting Member Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    With a purely resistive load Class D amps typically make less high treble with lower impedance loads. Here's an article that illustrates that. Rod Elliot also mentions that speakers are not ideal loads, and how that can affect things. Maybe Teemuk can expound on what he thinks might be going on. I have a theory or two that might explain it, but would rather not go off half-cocked. ;)
  12. AstroSonic

    AstroSonic Supporting Member

    Dec 10, 2009
    rural New Mexico
    The answer to this may be that the parallel loads are not equal. The voice coil inductance causes a marked impedance rise (impedance rises with frequency) starting at a few hundred Hz. For drivers with markedly different voice coil inductance, the impedance two different 8 ohm drivers would diverge rapidly as frequency increased. The load seen be the amplifier at higher frequencies would be markedly unequal. The driver with the lower voice coil inductance would receive more power from the high mids upward. The voice coil inductance of the CB158 is specified as 0.33 mH, while that of the JBL E140 is 1.1 mH. The impedance of the JBL would be higher than the CB158 at high frequencies. The CB158 would receive more power at higher frequencies as a result.

    This phenomenon could largely explains why non-identical speaker cabs often blend unpredictably. The oft sighted phase difference may be a contributor, but experience with many phase coherent and non-coherent speaker systems suggests that the ear largely ignores phase differences (at least up to a point).

    The suggestion to test using two identical speakers would likely help settle this.
  13. Solution is simple. You NEED another identical JBL cab ;)
  14. I think AstroSonic might have nailed it. Sounds like a reasonable explanation.