AW: Re: butter and pole locations + filter information

[Haible_Juergen#Tel2743]

>But the Moog filter looks like this in small signal,
>
>   +-----+-R-Ibuff-+-R-Ibuff-+-R-Ibuff-+---+---
>   |     |         |         |         |   |
>  Iin    C         C         C         C   R  Vout
>   |     |         |         |         |   |
>   +-----+---------+---------+---------+---+---
>
> where Ibuff is the current buffering (low input impedance = resistance R 
and
> high output impedance )

Very good descriprion. I used to draw it with *parallel*
RC stages, separated by voltage controlled current sources,
but if you introduce a thing like a "current buffer", that's quite
elegant as well!


> Seems to me, that the Moog-ladder calculations are not based on the right
> small-signal equivalent circuit. The Moog filter transfer function should
> be identical to OTA-based 4-pole filters like SSM2040, SSM2044 and
> CEM-filters. The different sound is due to different non-linearity inside
> the filter.
>
> Moog-ladder:
> Pre-distortion in the input (differential pair) cancels the distortion in 
the
> active load transistors. Distortion due to transistor mismatch and base
> current 'leaking' from the signal path (= distortion cancellation not
> accurate enough).
>
> Diode ladder:
> Like the Moog, but no base current distortion. Also different transfer
> function. Signal is attennuated more in the diode ladder, so worse S/N.
>
> SSM2040:
> Basic OTAs connected as voltage followers. This leads to low distortion
> at low frequency (due to negative feedback), but distortion increases with 

> frequency. At high Q-values large distortion concentrated near the > 
resonance
>frequency and thus the fat sound.
>
>SSM2044:
>The folded current mirror ladder is very much like the Moog-ladder.
>Of cource distortion is significantly lower due to low on-chip mismatches
>compared to the discrete Moog-filter.
>
>CEM-filter:
>Like SSM2040, but uses linearized OTAs and therefore less distortion and
>not a fat sound. But high S/N anyway.
>
>Well, that's all, folks,
>---------------------------------------------------------------------------  
 ----
>--
>  Kimmo Koli                                 Email: kimmo at clara.hut.fi

Very good overview! Though I am not sure that diodes by themselves
imply a worse SNR.
A few remarks to OTA-based filters: 2040 also has less PN junctions
in the signal path than 3080 or CEM filters (less current mirrors!); this 
might contribute to the difference in sound.
The 2044 should indeed be similar in behaviour to *later* Moog ladders, 
where transistor arrays were used. But again, here you have more
PN junctions in the audio path than in the direct ladder: At low 
frequencies,
the Moog cascade acts like stacked cascodes (that have excellent
linearity by themselves!), while in the 2044 you still have all these
current mirrors.
Don't know if it really matters, but it might.

JH.