Filter Bleed

Citation : I ran this by Dave, and here is what he had to say...

There have been some customers asking about the internal circuitry of the Prophet ’08, specifically on the issue of oscillator leakage through the filter when the filter cutoff is fully off. First, there are many sources of filter control, so of course there will still be some signal coming through if filter keyboard amount, filter envelopes, lfos, pressure, wheels, or any other modulation is currently on. If ALL filter controls are fully off, there will still be a little oscillator sound coming through, but it is at a very low level, around 67 dB down from full signal.

This is due to the analog ICs that we use in our instruments (the Evolvers all do the same thing; interesting that no one mentioned that in the last 5+ years of shipping thousands of units). Since the oscillators and filters on on the same IC, there can be this small amount of feedthrough. Of course, the fact that we use this Curtis IC allows us to make an instrument with 8 full analog voices for significantly less that what it costs for a single voice from other companies!

Many vintage synths have the same properties. Some, like the Prophet 5, do not have the same issue, but that’s partly because the basic signal-to-noise level is much lower than the Prophet ’08. In other words, the noise level on the P5 is actually 6 db higher than the feedthrough level on the P8! If there is leakage at the same level, you can’t hear it since it’s buried in noise.

Most players never notice the feedthrough, since it doesn’t show up in normal playing of normal programs. After all, the VCA is there to work with the filter as necessary. When the VCA is off, the noise is 90 db down on the P8, which is very quiet for a real analog synth.
In summary, yes there is a little oscillator feedthrough in the filter. No, it can’t be improved since it’s internal to the analog ICs we use. If you twist knobs a lot and find it interferes occasionally, try tweaking the VCA controls at the same time.

Cheers,

Andrew

Citation : Since the oscillators and filters are on the same IC, there can be this small amount of feedthrough.