Unibox question....

Discussion in 'Archived Threads 2001-2004' started by Joel Grice, Feb 15, 2002.

  1. Joel Grice

    Joel Grice Extra

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    In Unibox 3.0, there's a field that's labeled "Series Resistance". The note states that this number represents the added resistance of cabling and X-over. Playing with this number greatly affects the response curve of a given driver. My questions are:

    How is this number accurately determined?

    Can resistors be added to the system to increase this number to achieve a certain value for optimal frequency response?

    If so, how and what effect will this have on the amp?

    Thanks for answering a newbie's questions

    Joel
     
  2. Jon Hancock

    Jon Hancock Stunt Coordinator

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    Hi Joel,
    You can just use a WAG (wild *ssed guess), or you can actually measure your cables, (or use the company's spec) and if available, use the specs for the output impedance of the amp.
    Or, more likely, you can enter some appropriate numbers to get you in the ball park- for example, I use some 14 ft runs of Kimber 8TC, and the total resistance going out and coming back is about 0.035 ohms (measured- there are reference tables for copper properties, so you can calculate a cables resistance if you know the wire gauage). Now, this is a braided cable equivalent to 9 AWG. For 12 AWG, the same length, (say, Monster cable) you're looking at a bit over double. If you go up to AWG 14 or 16, roughly double the resistance for every two wire sizes. And if you have longer or shorter cables, scale accordingly.
    Now, while it's true that you can manipulate the Q of a system somewhat by changing the series resistance, in general this isn't a great idea. The resistors involved would be sized to a significant portion of the total load resistance, and would have to dissipate a lot of power in most cases- if they're big enough to have a significant effect on the Q.
    Now, remember, when you change the Q this way, you're altering the transient response. That is, high Q is longer transient settling time (i.e., "looser bass"). This is, without question, a coloration. But if you like it....
    Also, resistors have a temperature coefficient, which is just a fancy way of saying they change value with temperature- so, to avoid signal level modulation of the value (i.e., distortion), the resistors should be sized large enough that this modulation is small- which means a few time bigger than the nominal power handling requirement.
    It's probably much better to consider other facets of your design, such as the selection of driver, enclosure size, whether it's ported, what sort of tuning/alignment you use, rather than massaging the series resistor value to get closer to the desired result.
    That said, it's a free country, don't let the audio fascists tell you what to do or not to do. Sometimes the best teaching comes from experience. [​IMG]
    Do you have a copy of a good basic reference on speaker desgin, like Vance Dickason's Loudspeaker Cookbook? If not, highly recommended.
    Best regards,
    Jon
     
  3. Dan Wesnor

    Dan Wesnor Second Unit

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    The series resistance box is intended for use with crossovers. You'd enter the series resistance of the low-pass crossover. Some designers use series resistance to change the Q of the woofer.

    For a subwoofer, leave it at zero. If there's measurable resistance in your wiring, it will have a much lower effect on your speaker than any number of other things (box errors, T/S parameter errors, VC heating effects, etc.)
     
  4. Joel Grice

    Joel Grice Extra

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    Thanks all for the help and info. I was modeling a dipole speaker from Adaire. With the series resistance way up and a very large ported box, I was getting an F3 of around 9Hz but the "Max Power" was way down.

    I tried it again with the series resistance set to 0 and got it to get an F3 of just over 10Hz and pretty flat to that point.

    Thanks again,

    Joel
     

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