azlan Posted November 30, 2012 Share Posted November 30, 2012 Hello all, Basically, I was trying to figure out what effect on impedance using a crossover has, for example If I were to have a PA consisting of two stacks, each with a single 4ohm sub and 8ohm top, with a hi-pass filter built into one of the outputs of each sub, would the amplifier see a 4 ohm load, or would it see a load of 2.66ohms, or would it vary by frequency (so for a given input amplitude, the subs would get nominally double the power as the tops) and if it is the latter, does the impedance load drop significantly around the crossover frequency where both sets of drivers might be receiving power. The reason I am asking is that a friend is looking at adding some subs to his PA, but his budget currently wont stretch to buying a new amp, so in the interim, he is looking at using his existing amp to power all 4 of the speakers that will form the system Link to comment Share on other sites More sharing options...
Stuart91 Posted November 30, 2012 Share Posted November 30, 2012 Impedance always varies by frequency, even across a single driver. That's why manufacturers often refer to a nominal impedance on their spec sheets. Some publish graphs of how the resistance varies over the frequency range, and there can be quite dramatic dips and peaks. When the tops are fed via the crossover built into the subs, the amp will only "see" the impedance of the subs, it certainly won't be the same as connecting both cabinets in parallel. If you are in any doubt, stick a multimeter across the speaker cable. The resistance reading you get will be pretty close to the nominal impedance of the entire load. Link to comment Share on other sites More sharing options...
Brian Posted November 30, 2012 Share Posted November 30, 2012 ... a single 4ohm sub and 8ohm top, with a hi-pass filter built into one of the outputs of each sub...It's more usual to use drivers of equal impedances. In your scenario, the amp will always see the 4 Ohm load of the sub. If the sub is in a sealed box it's impedance plot will be something like this... ...whereas a ported box will look like this... Now, add a crossover into the mix, with it's own impedance variations, and it all gets more complicated. What you haven't told us is what is the top box configuration? I'm assuming that it might even have its own crossover inside to do a two-way split to LF and HF drivers. If this is the case then your 2.66 Ohm figure is likely to be right at some frequencies. More information (including amp model) is needed... Link to comment Share on other sites More sharing options...
jamesperrett Posted December 3, 2012 Share Posted December 3, 2012 In your scenario, the amp will always see the 4 Ohm load of the sub. If the sub is in a sealed box it's impedance plot will be something like this... It will only see 4 ohms when it is outputting frequencies that are below the sub's crossover point (I'm assuming the sub has a crossover here). The crossovers mean that the drive units should never be in parallel. The main difficulty with using mixed impedances is that the high frequency output from the sub will probably be intended to work with 4 ohm speakers so, with 8 ohm tops, you'll end up with a little too much overlap between the 2 systems at the crossover point which means things may sound a bit bass heavy in some places (assuming the speaker efficiencies are otherwise matched). James. Link to comment Share on other sites More sharing options...
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