[sdiy] A question about phasers/notch filters

[Tom Wiltshire]

Hi All,

In the typical phaser design, the allpass filter stages are connected in series. Since each pair of stages produces a 360 degree phase shift and hence one notch, we could equally use notch filters - and if you do, you get the expected phasing effect. So far, so good.

My question is this - Why do the phaser stages have to be in series?

If filters are paralleled up, their responses add up. If they're in series, they multiply. Does this mean that the series notches are deeper than paralleled notches?
The other significant difference I can see is that in one case noise is fed through all the stages and accumulates, whereas in the other the noise (which would presumably be uncorrelated) is summed. This would be a motivation for a parallel connection, especially with noisy old seventies tech - so if they did it the other way, they must have had a good reason why (I hope!).

Thanks,
Tom