Doepfer

Hi Ingo

> Does this count? "[A] second audio feedback path to boost bass
> response at higher Q levels. This makes the filter "growl and rumble"
> even more!" Paul S. on the MOTM-440 VCF.
> 
> The MOTM-440 is inspired by the SSM2040 chip and I *guess* the A105
> (which is based on the SSM2044) is similiar in design. Anyhow, I was
> never able to give the MOTM-440 a listen but the A105 is impressing.

Ok, now we come closer: An increased resonance means, the output voltage 
of audio signals at the cutoff frequency is increased in comparison to 
the signals at lower frequncies than the cutoff frequency (speaking only 
for lowpass filters now). Now there the question is how the filter 
behaves regarding its complete outputsignal.

Several types are possible:
1.) The maximum output voltage of the signal stays the same, which means 
signals below the cutoff frequency (and below the resonance peak) will 
have a lower signal level - in other words: There will be less bass with 
higher resonance.
2.) The output voltage of signals below the cutoff frequency will stay 
the same, which means the resonance "peak" will have a higher voltage. 
With other words: higher resonance won't touch the bass. BUT: This will 
usually cause distortion in the following audio path, because the 
resonance peak will. This is generally unwanted (and no filter is really 
designed like that). It could be achieved by simply using an 
amplification after the resonating filter.
3.) the overall power[*] of the output signal stays the same. This means 
also that the low frequencies will have a reduced output level (as in 
1), but the amount of reduction depends of the resonance frequency: a 
high frequency in resonance will have less electrical power and will 
reduce the overall power less than a low frequency in resonance (I hope 
this description is not too confusing).

Dieter once explained to me, how one can determine in the schematic 
which type of resonance is used, but I don't remember it. To my 
knowledge the A-122 and A-123 are of type 1, the A-105 is type 3, the 
A-120 and other non-single-chip-filters are somewhere inbetween type 1 
and 3. I don't know about the A-121 (but for 12dB filters this issue is 
less relevant as for 24dB filters).

So I think, the A-105 is the filter that Carlos asked for.

For an A-122/123 one could use a CV to control the resonance; the same 
CV should control also the signal level in a following VCA (with less 
control amount). This might compensate the level loss at higher 
resonance settings.

Florian

[*]we remember physics lessons when we were 14 years old ;-): electrical 
power is product of voltage and current