Click here to purchase the entire book in PDF format. Parallel vs. Series Filters Let's take two reciprocal peak/dip filters, each set with a Q of 2 and a gain of 6 dB. The only difference between them is that one has a centre frequency of 700 Hz and the other has a centre frequency of 1.3 kHz. If we use both of these filters on the same signal simultaneously, we can achieve two very different resulting frequency responses, depending on how they're connected. If the two filters are connected in series (it doesn't matter what order we connect them in), then the frequency band that overlaps in the boosted portion of the two filters' responses will be boosted twice. In other words, the signal goes through the first filter and is amplified, after which it goes through the second filter and the amplified signal is boosted further. This arrangement is also known as a circuit made of combining filters. Figure 6.14: Block diagrams of two ways of connecting filters. The top diagram shows the two filters connected in parallel, the lower shows the two filters connected in series. Figure 6.15: The frequency response of two individual filters that are used in the two `circuits' shown in Figure 6.14. One filter has a centre frequency of 700 Hz, the second at 1.3 kHz. Each has a Q of 2 and a gain of 12 dB. Figure 6.16: The resulting total frequency response of two filters (with individual frequency responses shown in Figure 6.15). The red curve shows the result of the two filters connected in series. The black curve shows the result of the two filters connected in parallel. If we connect the two filters in parallel, however, a different situation occurs. Now each filter boosts the original signal independently, and the two resulting signals are added, producing a small increase in level, but not as significant as in the case of the series connection. This arrangement is also known as a circuit made of non-combining filters. The primary advantage to having filters in connected in series rather than in parallel lies in possibility of increased gain or attenuation. For example, if you have two filters in series, each with a boost of 12 dB and with matched centre frequencies, the total resulting gain applied to the signal is 24 dB (because a gain of 12 dB from the second filter is applied to a signal that already has a gain of 12 dB from the first filter). If the same two filters were connected in parallel, the total maximum gain would be only 18 dB. (This is because a the addition of two identical signals results in a doubling of level which corresponds to an additional gain of only 6 dB. Note as well that the overall gain of a parallel connection is 6 dB.) The main disadvantage to having filters connected in series rather than in parallel is the fact that you can occasionally result in frequency bands being boosted more than you're intuitively aware. For example, looking at Figure 6.16, we can see that, based on the centre frequencies of the two filters, we would expect to have two narrow peaks in the total frequency response at 700 Hz and 1.3 kHz. The actual result, as can be seen, is a (sort of...) single broad peak between the two expected centre frequencies. Also, it should be noted that a group of non-combining filters will likely a ripple in their output frequency response.