Matching diferent impedance cabs in Bi-amp mode

Discussion in 'Amps and Cabs [BG]' started by baxter_x, Dec 7, 2013.

  1. baxter_x


    Nov 27, 2013
    Hi all,

    I recently sold one of my SWR Goliath and miss it way too much. So I'm looking forward to get another one. I currently have a good opportunity on a G3 4ohms. I already own 8ohms version.
    Still, I would use the bi-amp (stereo) feature of my Ampeg SVT4 Pro, so my question is not about matching impedance as this way both impedances will remain the same.

    It's more about the "matching" tone.
    Is it a problem to run both 4 and 8 ohms cabs at the same time?
    I know volume might be a diferent but I can use the balance knob to compensate that "issue".

    What do you guys think about this?

  2. Your amp is rated at 2ohms.
    You should be fine as long as you don't bridge.

    My suggestion would be to stack the 4ohm cab over the 8ohm cab.
    At least the 4ohm cab will be at ear level so you can hear if this cab starts to fart out since it will be drawing out more power, you can bring the overall volume down.
  3. baxter_x


    Nov 27, 2013
    Thanks for your answer but I think you miss the point ;)
    I DONT CARE about impedance as I'm going to use stereo mode!
    That way BOTH cabs will remain at their nominal impedances. BOTH will remain at 4 and 8 omhs as they will be powered by two complete independant power amplifier. That's the bi-amp thing.
    I just want to know if it's not going to sound to weird to have both cabs at different impedances. That's it ;)
  4. Dave Curran

    Dave Curran Lilduke

    Jul 27, 2013
    It will work fine. You may have to play with the balance, as you will be sending more power to the 4 ohm than the 8ohm. Or like orangejulius3 said, put the 4ohm closer to your ears so you can hear the 4 ohm.
  5. If you Bi-Amp, send the high Frequencies to the 8ohm and Low on the 4ohm.

    I think you'll be fine in any configuration, just becareful with the 4ohm of farting and enjoy your rig!
  6. baxter_x


    Nov 27, 2013
    As long as there no noticable change in sound, I'll use the balance knob.
    Thanx you two.
  7. baxter_x


    Nov 27, 2013
    That's what I call a great advice as that answers the second question I was about to ask!!

    But why would the 4 ohms would tend to fart more than the 8?
    If I use on a cleaver way the balance knob that shouldn't happen right?
  8. This will only happen if you push your amp pretty hard.

    The 4ohm cab will be withdrawing about 150w per speaker.
    The 8ohm cab will be withdrawing about 80w per speaker.
    If you push your amp hard, the speaker receiving 150w will start to sound like its reaching its peak power, while the speaker receiving 80w will still have room or more play.

    Like I said though, this will only happen if you go overboard on the amp.
    You should be fine.
  9. baxter_x


    Nov 27, 2013
    That's where the balance knob steps in ;)
    Anyway, thanks for the tip.
  10. SunnBass

    SunnBass All these blankets saved my life.

    Aug 31, 2010
    Columbia, Mo
    He actually fully understood and gave you a thorough answer. You should care about impedance as the 4 ohm cab will be receiving more power than the 8. Sound weird? No. Unless the 4 ohm cab starts farting. Which is the point he was making...
  11. dincz


    Sep 25, 2010
    Czech Republic
    As the OP keeps mentioning, the balance control will allow the power to the 2 cabs to be balanced. In that setup, there's no reason for the 4 ohm cab to be getting more power.
  12. flatfender

    flatfender Ad eundum quo nemo ante iit Supporting Member

    Years ago I had to do a 10 days of outdoor gigs (Sturgis Motorcycle Rally) and I had a 8Ω 410 GK cab and borrowed an identical 410 GK cab except it was a 4Ω. Same wattages just different ohms.

    I used a RBI>Crown XLS2000 in Y input mode

    Sent a 200hz or so (IIRC) test tone from a tone generator into the RBI. Could of used a synth keyboard too.

    Set a mic plugged (back about 6' in front of the stack) into a little 4 channel yamaha mixer with an VU meter.

    Turned the 8Ω cab side of the power amp all the way up and adjusted the mic/Yamaha mixer output until I saw about 0db on the meter.

    Left everything the same, turned off the 8Ω side on the Crown and turned up the 4Ω side carefully until I got the same 0db reading on the Yamaha meter. It was about 2/3 as high as the 8Ω side. Marked the 4Ω side on the amp.

    Worked well on balancing the cabs for the week of shows and it sounded good. Rented an 810 fridge for the following years.
  13. baxter_x


    Nov 27, 2013
    Great feedback you're providing me here! That totally answers my request ;)
    But what's this 0db thing actually. I don't understand the trick? Excuse me for what I'm about to say but, isn't 0db equal to no sound??
  14. Passinwind

    Passinwind I Know Nothing Supporting Member Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Just saying 0dB has no concrete meaning -- it needs a reference of some kind. If the reference is 1 volt, 0dBV = 1 volt. If the reference is .775volts (dBu), 0dB = .775 volts. If the reference is sound pressure level (dB SPL), 0dB = the average threshold of human hearing. In all these cases the spec could be negative rather than positive: -10dBu, for example.

    Does that help a little?
  15. flatfender

    flatfender Ad eundum quo nemo ante iit Supporting Member

    I should've used "0" rather than "0db"

    it was a digital VU (level) meter and the "0" reference was when all the green leds were lit up and it was just starting into the red leds.

    I used "0" as my reference, but you can use any other spot on the meter as long as they're the same for both cabinets.

    A rough dial in for setting power going to the different cabs, but was more accurate than my ears.