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Capacitor question for my rig

Discussion in 'Amps and Cabs [BG]' started by andruca, Dec 16, 2004.


  1. andruca

    andruca

    Mar 31, 2004
    Madrid (Spain)
    Hi everybody! I need some advice regarding capacitor values for my rig. I use a Hartke 3500 head + 4.5XL cab (4x10" + 5") + 215XL cab (2x15"). I have plenty of power with it (and never used the master past 3 -goes up to 10-). But I use this very heavy sound with plenty of sub-low frequencies and I note that sometimes there's a little farting on the 4x10" cab. In theory its frequency response starts at 30Hz (exactly the same as my 2x15) but let's be serius about it, they have plenty of excursion for reproducing such sub-lows while my 15"s are hardly moving. So I'm thinking of cutting some of thise sub-low pressure on the 4x10" by simply adding a capacitor in the cable (so I don't have to modify the cab itself and also, easier to test), but here's where my electronics know-how ends. I mean, I know how to wire the entire thing, no problem, but I totally ignore what values should I use in order to cut everything under (let's say...) 250/500Hz. I know I'll have to try quite a few values 'till I find the one that brings out the balance I like, but without knowing where to start it could be endless. I read some high pass filters' table that states values for 4ohms (my rig is running in 5.2ohms) the following way:

    Freq(Hz). Capacitor (uF)
    100 400
    125 320
    150 265
    200 200
    260 150
    400 100
    600 70
    800 50
    1000 40
    1500 25
    2000 20
    3000 12,5
    4000 10
    5000 8
    6000 6,5
    8000 5
    10000 4

    Is this even near to right? If so I'm trying every value between 400 and 40 uF. Any setting that is more accurate? What frequency do "sub-lows" officially start at? Thanks so much!

    ANDRUCA

    EDIT: No money for bi-amping :) and, in fact, just needing to release the sub-low pressure on my 10"er cab. One more thing: what voltage should the capacitors handle given that my amp puts 350W@4ohm and here in Europe, AC power is 220V? Thanks again!
     
  2. slinkp

    slinkp

    Aug 29, 2003
    brooklyn, NY, USA
    I have to say I find the idea a bit sketchy... just because I've never heard of anybody doing this. But anyway:

    voltage = current times resistance (or impedance).
    And watts = volts times current.
    A little algebra from that tells us that
    voltage = square root of (watts * impedance).
    Which comes out to 37.416573867739416 volts.

    But I would double or triple that if possible to have a safety margin.
    Actually I wouldn't do it at all, but hey, it's your gear :)
     
  3. andruca

    andruca

    Mar 31, 2004
    Madrid (Spain)
    Thanks so much. I wouldn't do this if I wasn't sure I won't damage anything. After all, I'm just implementing a passive high-pass filter with a very low frequency cut :) Thanks again!

    ANDRUCA
     
  4. slinkp

    slinkp

    Aug 29, 2003
    brooklyn, NY, USA
    hmm, now that I think about it, there is plenty of precedent for what you are doing... since it's basically just half of a first-order Butterworth crossover :)

    http://www.trueaudio.com/st_xov_1.htm

    The only difference is you are leaving out the inductor for the low-freq driver so it's being used full-range. I *think* this won't matter, but I Am Not An Electronics Enginner - at least not at this time in my life :)
     
  5. andruca

    andruca

    Mar 31, 2004
    Madrid (Spain)
    I did the test and didn't like how it sounded at all. I guess I'm having a power balance issue between both cabs. I mean:

    - 4.5XL handles 400W@8ohm
    - 215XL handles 400W@4ohm

    But I modified the 215XL and put both 15"s in series (were in parallel) so the cab became a 16ohm cab which, in parallel with the 4.5XL would end up working as a whole in 5,3333... ohms (nearest to the "optimal" 4 ohms for the maximum power output of the amp). But the unwanted result of this is that 2/3 of the power are going to the 4.5XL cab while only 1/3 goes to the woofers. So today I'm trying to wire the 215XL back in parallel as it was from the factory (4ohm), keeping the cab still in parallel with the 4.5XL (8ohm). I know this will result in my system running in 2.67 ohm so I must be careful with volume (not an issue as I always keep it low and never used it past 3/10, Hartkes are LOUD!). Another option can be to attach a 1.5ohm resistance in series with the speakers (in such case only 230W will be usable for the speakers -the rest would be lost in the resistance-, but would keep my amp and/or cabs to blow). Is there anything wrong in my deductions? Any other suggestions? Thanks so much!

    ANDRUCA
     
  6. andruca

    andruca

    Mar 31, 2004
    Madrid (Spain)
    slinkp, thanks so much for the link! I learned a lot from it! Very instructive for us, non-electronic guys!

    ANDRUCA
     
  7. andruca

    andruca

    Mar 31, 2004
    Madrid (Spain)
    If the balance 2/3 (for woofer) and 1/3 (for mids and highs) works ok, the last thing to do would be to rewire the 4.5XL cab (4 8ohm speakers wired series parallel -resulting in and 8oms cab-) to leave it working in 2ohms (all of them im parallel). This way I can wire it in series with the other cab (2ohm) and it would all be 6ohm now and as the power balance is proportional to resistance when wiring in series, so it'll still be 2/3 for woofer and 1/3 for mid-high. This way I can have some 280/300 watt max power and I wouldn't be risking my head. Am I right?

    ANDRUCA
     
  8. andruca

    andruca

    Mar 31, 2004
    Madrid (Spain)
    Correction: when I said...

    "This way I can wire it in series with the other cab (2ohm)..."

    ... I meant to say...

    "This way I can wire it in series with the other cab (4ohm) ..."

    ANDRUCA
     
  9. slinkp

    slinkp

    Aug 29, 2003
    brooklyn, NY, USA
    I think you worked that out right.
    You are an adventurous guy :)
    I hope ya don't fry anything...
     
  10. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Personally I'm not a fan of wiring mismatched sized speakers in series. In theory you end up with a nominal impedance that works, but in parctice you could have issues. As you know, the true impedance changes greatly depending on the frequency produced and the speakers characteristics. The impedance can go sky high at or near the speakers resonant frequency, and for a ported cab, another peak at the cabs tuneing frequency.

    Lets say the 2x15 had an impedance peak at 40Hz for example. The 10's may have an impedance peak at maybe 70Hz. At 40 Hz, the 15's draw all the power and leave nothng for the 10's. The same applies in reverse - the 10's can suck the 15's dry when they're at or near their impedance peaks. So when one cab is peaking, the other may go silent, creating all sorts of holes in your frequency response, particularly in the low end, which is where you're problems are at the moment.

    Speakers wired in parallel don't hav this problem. They recieve the same amount of voltage across the terminals regardless of the speaker's peaks.

    Having said that, it's worth a try. Just do it at low volumes to start with and keep a close ear out for any complaints from the cabs.
     
  11. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    What you want to do has value but you need to roll off the tens around 100-150 Hz; lower than that doesn't help them enough. Don't use resisitors, you'll blow them in a heartbeat, and don't wire the cabs in series, that fouls everything up. You need a real live crossover to do any real good. Wire the 215s back to parallel, leave the 2x10 stock. Have them fed through a 10mH inductor (Parts Express # 266-946) and feed the 4x10 through 140 mfd of capacitance, consisting of a large and small poly cap parallel wired (Parts Express # 027-447 and 027-442). Mount them in a separate box with jacked ins/outs to feed the cabs so you can run without the crossover when desired.
     
  12. andruca

    andruca

    Mar 31, 2004
    Madrid (Spain)
    OK, I tried it. I wired the 2x15 back in parallel as it was from factory (4ohms) and did nothing to the 4x10 (so it's 8ohms as it's allways been). I connected both cabs (in parallel) to my amp (2,67ohms) and checked it out. I went no further to 9 o'clock in the master to keep it safe. And the balance is now WAY more pleasant to my ears. Sounds smoother and I don't need to beef up the low end at all as I was doing previously. Ok, so now the balance is right (2/3 of power to the 2x15 and 1/3 to the 4x10). Modifying the 4x10 to make it 2ohms and wiring it in series with the 2x15 would give me a 6ohms impedance which falls in between the "legal" limits for my head, but as you state (didn't know such a fact) there are these impedance peak issues (I knew impedance is resistance at a given frequency but my electronics don't go as far as deducting such things, you know). So back again to a parallel configuration. The only other possibility is to wire both cabs all in series (32ohms for the 4x10 and 16 for the 2x15). This would give me a total load of 10,67ohms on my amp, but a very poor power rating (less than 200W). So, still thinking of keeping the 2.67ohms configuration I liked so much. What if I put some "safety" in between? A fuse I guess. Let's do some more math. I don't have to exceed the maximum 350W my amp can deliver so I should use a fuse that burns at sqrt(350/2.67)=11,45AMP. I could take some extra safety and use a 10AMP fuse. Anything wrong in doing such a thing?

    ANDRUCA

    P/D: I know all of you love my experiments and fear to do some on your own. Consider me you unconscious (and rather ignorant :)) alter-ego! Now seriously, forgive me for being such a continuous harassment.
     
  13. andruca

    andruca

    Mar 31, 2004
    Madrid (Spain)
    billfitzmaurice, I did some low-cutting to the 4x10" cab (tried from 100Hz to 1.5KHz) and was never satisfied as the cab still was WAY LOUDER than the 2x15" so I did some reasoning and found this unavoidable "unbalanced power" situation. I found it's not a matter of dividing frequencieds between cabs (in fact, both are intended to work with full range signal) but of unequal power applied into each of them. I'm still unbalanced now but certainly like this situation much more. Thanks so much!

    ANDRUCA
     
  14. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    That's actually not the case. The average 15 is designed to work a full octave lower than the average 10, the average ten an octave higher than the average 15. Sending the same signal to both speakers is wasting both amp power and speaker capacity, as the tens will not reproduce the low bass and the 15s can't reproduce the highs. There are at least a half dozen other reasons why running both full range is technically wrong and sonically inferior to letting each driver work it's own partcular range independantly of the other. Your unequal power situation was caused by having the impedance of the 2x15 too high, and rewiring obviously has corrected that, but your system is still a long way from delivering optimal performance.
     
  15. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Bill I respectfully disagree. If we were talking about speakers for playback of recorded music, then yes, I see where you're coming from. But for bass rigs, I think you'll find a majority bassists run their 15 + 4x10 combo's with a full range signal going to both speakers, many of whom will tell you they've tried Bi-amping and didn't like it, myself included.

    We agree that the whole idea is to remove the harmfull bottom end from the smaller speakers - In practice, there's a certain amount of frequency filtration going on anyway when you match a big speaker with smaller ones. When I add a 15 to my rig, I immediately have to wind off some low frequencies with the EQ. The 10's therefore get fed less bottom end than they would be otherwise recieving. Mission acomplished.
     
  16. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    True, but they don't do so because they should, they do so because their equipment doesn't allow them any option to do otherwise.
     
  17. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Not really. You cut my quote short:-

    "I think you'll find a majority bassists run their 15 + 4x10 combo's with a full range signal going to both speakers, many of whom will tell you they've tried Bi-amping and didn't like it, myself included."

    Don't get me wrong, some of my home made cabs have high pass filters on them. There are definitely situations where they're a good idea. I just don't believe it's essential in all cases.
     
  18. andruca

    andruca

    Mar 31, 2004
    Madrid (Spain)
    I agree but my Hartke 4.5XL's specs state that its frequency response ranges from 30Hz to 12KHz. The 30Hz lower response limit is the same in the 215XL cab (its upper frequency response limit is just 1KHz -clearly a sub-low module-). Anyway I achieved what I needed, 1) a nicer balance to my ears and 2) not beating my 4x10" that much. The only unsolved problem is the amp's safety? Any opinions on my fuse idea? Thanks so much again!

    ANDRUCA
     
  19. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Up till now I've been assuming the 3500 is rated safe for a 2 ohms load. A quick google search shows it isn't (350w at 4 ohms). I've not seen any other advice in this situation other than "don't do it, you'll fry your amp". But we're now dealing with something I know little about. Any friendly techs out there who can help us out on this one?
     
  20. IvanMike

    IvanMike Player Characters fear me... Staff Member Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    if your amp isn't rated below 4 ohms, there really is no way to be safe running a 2.76 load on it, even if you keep it really quiet.

    Anyhow, currently the 4x10 is getting 1/3 the power, and the 2x15 is gettign 2/3 of the power. If both cabinets had equal sensitivity, the 2x15 shoudl be just a bit louder (+3dB) than the 4x10. If the 4x10 is 3 dB more sensitive than the 2x15, they should be equal volume. If the 4x10 is louder, it's probably a much more efficient cabinet than the 2x15. The other thing to consider is that the 4x10 may have more mids than the 2x15, making it SEEM louder.