Speakers in Parallel
Parallel wiring has the opposite effect of series wiring – load impedance drops when speakers are wired in this fashion. And the more speakers you wire in, the lower the impedance. The most common reason for wanting to lower impedance is to raise acoustical output. Speaker output increases because the amplifier's power output rises as the load impedance decreases.
The number of speakers that can be connected in parallel is limited by the minimum load impedance that the amplifier is capable of driving and the power-handling capacity of the speakers. In most cases, load impedance should be held to a minimum of 2 ohms – provided the amplifier can handle impedances that low.
Figure 2A shows how to wire a pair of speakers in parallel. A wire from the positive terminal of one channel of the amp is wired to the positive terminals on speakers A and B. (The simplest way to do this is to run a wire from the amp terminal to Speaker A and then run a second wire from that terminal to Speaker B.) Then the negative terminal of the same amp channel is wired in like fashion to the negative terminals on both speakers. The second channel is wired the same way.
Calculating the load impedance for the parallel-wired channel in Figure 2A is a bit more complicated than doing so for speakers wired in series. Using Equation 4, multiply the impedances of each speaker and then divide the result by the sum of the speakers' impedances. You can visualize the result as a single imaginary speaker (Figure 2B), whose impedance is represented by Zt. Zt stands for the equivalent-load impedance, while Za and Zb represent the impedances of speakers A and B, respectively.
Equation 4: Speakers in Parallel
Zt = (Za x Zb) / (Za + Zb)
Turning again to our subwoofer install, say you want even more oomph from your system. So you trade in the original amp for one that has the same 4-ohm power rating (100 watts x 2) but is also 2-ohm stable. Since the power output of most amps increases as impedance decreases, you could boost the amp's power output and the system's bass response simply by switching to a parallel wiring scheme. Doing so would drop the net, or equivalent-load, impedance for each channel to 2 ohms. Mathematically, you substitute 4 for Za and Zb in Equation 4 and work it through:
Zt = (Za x Zb) / (Za + Zb)
Zt = (4 x 4) / (4 + 4)
Zt = 16 / 8
Zt = 2 ohms
To calculate the new amplifier's power output into 2 ohms, refer to Equation 2. Plugging in the appropriate numbers, the calculation goes as follows:
Po = 100 x (4 / 2)
Po = 100 x 2
Po = 200 watts
As you can see, by upgrading to a 2-ohm-stable amplifier and wiring the same four 15-inch woofers in parallel – two per channel – power output jumps fourfold – from 50 watts x 2 to 200 watts x 2.
Now, to find out how much power each subwoofer will receive when wired in parallel, we must use Equation 5, which is actually a scrambled version of Equation 3 (remember, we'll be working the equation for just one speaker (Pa)):
Equation 5: Power Applied to Each Speaker
Pa = Po x (Zt / Zn)
Substituting 200 for Po, 2 for Zt, and 4 for Zn, the equation works through as follows:
Pa = 200 x (2 / 4)
Pa = 200 x 0.5
Pa = 100 watts
Since both subwoofers are rated at 4 ohms, the second one (Pb) would also receive 100 watts.