Total DC Resistance of bass cab.

Discussion in 'Amps and Cabs [BG]' started by Jim K, Jan 2, 2018.

  1. Jim K

    Jim K

    Jan 2, 2018

    I've been gigging with a Hartke HA3500 Head and 410XL cab for several years now and have been very happy with the sound.

    The system is not new, but has been used yet lovingly cared for (I bought it 2nd hand).

    Recently I have noticed a lack of bass depth in the projected sound and am working through the rig to see if anything is amiss during our gig downtime (volume is fine).

    I recently had a loud "crackling" during played notes at a gig which was stopped by turning off the amp, removing the volume pedal (my only pedal) and carrying on with a single lead from bass to amp head. I assumed muck in the volume pedal potentiometer or a dodgy lead. I've cleaned the former and replaced the latter - no more crackling, but I think I am hearing less bass depth in the projected sound since the episode.

    I've started by checking the cab first as the amp looks to be functioning correctly (visually anyway).

    All four speaker cones move in the same direction with the 9v battery test. One speaker seems to move slightly more than the others.

    I cannot see any visual tears on the aluminium cones.

    When gently pushed in and out the cones are all free to move. The speaker that has more cone movement on the battery test seems slightly easier to push in then the others than the others.

    I removed the above single speaker and tested it's DC resistance (whilst still connected to the cab internal wiring) and it was 4.8 ohms. This seems lower than I would expect, but I'm not sure if the fact it was still connected to it's internal wiring makes a difference?

    The DC resistance of the whole cab through the cab input jack socket is 6.8 ohms - which seems about right as DC resistance on an 8 ohm speaker seems to be about 2 ohms below impedance as a general rule.

    The cab impedance is rated at 8 Ohms and I understand it contains 4 x 8 ohm speakers connected in series/parallel to achieve this.

    Are my DC resistance measurements around what you would expect for this cab please?

    Is it normal for one speaker in a cab of identical speakers to have more cone movement / easier cone movement?

    Many thanks - I don't think there's anything wrong with the cab now but thought I'd ask people with more experience than myself!
  2. You must disconnect the speaker wiring from the rest of the cab to properly measure it's DC resistance.
    This can be done by disconnecting one lead from the speaker and then measuring across the speaker terminals.

    Most multi-meters are not very accurate, especially when measuring very low DC resistances.
    I would suggest, rather than trying to get a DC resistance of just the one speaker to determine a problem, that you should measure the DC resistance of each of the speakers individually, as described above.
    This will let you compare each speaker while also taking into account any inaccuracies in the measurement.
    The actual DC resistance you measure is not as important as how each speaker measures relative to each other.

    See what the three "good" speakers each measure by them selves and compare those measurements to the one "suspect" speaker.
    Let us know what you discover.
  3. Jim K

    Jim K

    Jan 2, 2018
    Thanks for your quick reply,

    I suspected as much.....

    In the meantime. Do the mechanical tests I have described (battery test and pushing on cones) and negative visual defects noted indicate that I probably have a normally working cab and it's something else?

    If it ain't broke....etc . I want to look for the obvious things first.

    Thanks again.
  4. It sounds like you did everything the right way to test the cones/drivers as best as you could. But that is not really testing things under operating conditions, so I would rule it as inconclusive. Those tests are more static in nature and intended to rule out some obvious problems.
    They won't necessarily tell you everything.

    If the DC resistance turns up that one of the drivers is significantly different than the others, then that may tell you something more. It is still not a good dynamic test though.

    Lets see what the DC resistance numbers are for each driver before we try to draw any conclusions.
  5. BadExample


    Jan 21, 2016
    6.8 Ohms is good on measuring the entire cab. You don't have a fully shorted or open voice coil, it seems. Like OGB said, it probably best to get on each driver with the wires pulled next and see if they are all the same.

    I don't know what to think about the driver with loosened up suspension. I'm kind of a hack at MI. I think it's odd, and I think that driver would make more bass sound not less, but fart out easier. You may not be hitting the fart out on it, so that may not be heard.

    I can't recommend it, but if it were my cab, with me being as much ape as man, I might try each driver one at a time maybe to a stereo rather than a powerful bass amp. My bloodline is way too messed up to advise you try this.
  6. basscooker

    basscooker Commercial User

    Apr 11, 2010
    Northern KY
    Cab fan, hobbyist
    Has the sound happened again?

    Take the measurements as OGB says, since you've already got the cab open.

    If the drivers all measure to roughly the same, get the rig back together and see if the sound returns.

    Try the effects loop test if so.
  7. BadExample


    Jan 21, 2016
    Was the crackling horrifically loud? Louder than the bass itself as you normally play it?
  8. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Generally all drivers in a cabinet suffer from mechanical deterioration at the same rate.

    The way to test for this is to measure the resonant frequency of each driver (search on the grain of rice test), deterioration causes da to decrease. It's helpful to know what it is when new.
    Aaarn and BadExample like this.
  9. Jim K

    Jim K

    Jan 2, 2018
    Thank you for your replies.

    I have now had a chance to unsolder one wire from each speaker and measure the DC resistances individually.
    They are ALL 6.3 ohms each, including the one with the (possibly) loosened up suspension.

    Interestingly, once reassembled the cab now has a TOTAL DC resistance of 6.3 ohms (was 6.8 ohms before) - which to my uneducated mind is probably too small of a difference to have been influenced in any way by my soldering, but nonetheless now indicates perfectly balanced speakers, and perfectly working internal parallel/series connections?

    So - no fault found it would seem. I am gigging this weekend and we shall see. Taking my spare combo amp just in case....

    It's only happened once. When it occurred, it came on intermittently towards the end of an indoor, but cold and damp gig. It seemed to stop initially after vigorously working the volume pedal a few times so I assumed that was the problem.
    However, when it re-occurred and was wrecking the performance, I eventually I turned the amp off, removed the volume pedal and plugged the bass straight into the amp for the remainder of the set and it didn't happen again.

    I've sprayed switch cleaner into the volume pedal and all the jack sockets I can find (!) and also replaced a Fender jack cable that had developed a loose end, as there was an occasional crackle at the bass guitar end when I wiggled the cable when it was in the bass's jack socket.

    I believe in only changing one variable at a time when fault finding and it's possible that the recent loss of bass quality sound from my rig was due to worn out strings (although the bass sounds fine on other amps) and the mid performance loud crackle issue is unrelated to my more recently perceived loss of bass sound quality.

    So to date I have:

    1) Switch cleaned every switch and socket.
    2) Replaced "iffy" cables.
    3) Inspected and cleaned out amp internally (no fault found apart from dust) and changed the valve.
    4) Checked cab as above.
    5) Renewed strings.

    The only thing I've not done yet is to inspect the bass guitar's jack socket internally.

    Fingers crossed for this w/e!

    Yes, the crackling was indeed horrifically loud, and louder than the bass itself as I was playing it. To my mind it sounded like either mechanical speaker failure, or a dodgy jack cable/speaker connection. Is this a definite sign of something else specific?

    If it DOES reoccur I shall be back to ask what the "effects loop test" is.

    Thanks again.
  10. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    The D.C. resistance is not an indicator of the condition of the suspension components of the speakers.

    Don't spray "switch cleaner" into any pot or control, otherwise you will be asking a whole different set of questions about ruined pots.
    BadExample likes this.
  11. TomB

    TomB Supporting Member

    Aug 24, 2007
    It sounds so much like a bad jack connection that I have to wonder if you weren't just much more careful in handling once the crackling began. Kind of a long shot for me speculating at this distance, but I've experienced many times where crackling at a jack becomes the sound of a minor explosion out in the house - especially when using an active bass. If so, I hope your finagling fixed it.
  12. From the whole tale, I would guess it was caused by the loose cable.