JP6 filter allpass

 Since breadboarding the first cell of the JP6 filter was successfull, it's time to revisit the allpass circuit. I sort of got this working in an earlier post: http://atosynth.blogspot.com/2019/01/jupiter-6-all-pass-filter.html, but had to remove resonance feedback it seems.

I tried the same circuit in LTSpice again, mixing Bandpass output with the input tapped after the input mixer. It gives a nice phase change but the response is -10dB for the "normal" case, up to 4dB at the cutoff frequency, not very usefull. 

I then realised that mixing with the input may refer to the original input, not what is seen after the mixer. However, the input at this point is inverted, so we don't get the phase reversal effect we're looking for

NB: Not original BP but unity gain one

This is easily fixed by inverting with unity gain. Now we're talking. We get a flat response with a -6dB notch and the phase reversal we're after

I started thinking that this could be the best option, but the notch made me uncertain. After a bit of googling, I once again found the multimode filter pages at electricdruid. There, the following is stated: 

"Phase shifters produce a notch in the frequency response by mixing this phase-shifted signal with the input, so technically they’re a type of notch filter." 

Ok, so a notch is to be expected. I then found the link to "Craig Anderton's Multiple Identity Filter" article. I thought for a moment this confirmed my -6dB notch as it says -6dB notch at the end, but I mixed up notch and allpass. It is worth a closer look though.

But then I realised, what if I make sure BP output is the same as HP/LP etc, though my non-inverting amplifier, just as I did with the BP output from cell 1?

That works great!

The closeup doesn't look that great until you look at the axis. The input is just very slightly attenuated (-170mdB). Looking at all combinations together shows that it is essentially flat:

Red line is unity BP + inverted input

Now, this is great news. Of course this is just a simulation, and as I have seen earlier, the actual result on the breadboard is different, but I think I have the best starting point at least - mix the inverted input with the unity gain BP and we get a good all pass version!