Parametric eq with only two opamp sections
Posted: Sun Jun 21, 2020 9:49 am
Hello,
Just registered on this forum and thought I should make a small contribution - perhaps it will be of interest to some of you.
Parametric eq's (with independently variable centre frequency, bandwidth/Q and cut/boost) are by large created using a state-variable filter consisting of to integrators and a summing section. This requires a total of four op-amp sections.
But a parametric eq section can also be realised differently. I am aware of the early "Twin-T" designs by Sontec etc but it can also be realised with a Wien-bridge filter. I first came across this in design for Crest Audio done by the late Joe Petrucelli. His version had switched Q but could be altered to variable. Later I came across a similar implementation in the Audient ASP range by David Dearden. David is still around and he gave me his comments as follows:
Looking at the ASP8024 schematic for the first time since around 1997, it is basically a WEIN bridge topology which was in common use in all Soundcraft consoles and also in by various other manufacturers in the 80's and 90's. However, they were all fixed Q with the Q and amount of boost/cut heavily dependent. The normal situation was to set the boost/cut to around +/-15db by adjusting the ratio of the capacitors (which I think was around a 2:1 ratio) and accept that the Q would be around 1.3. Both mine and JP's version have equal value capacitors.
I wonder whether it was JP or Chas Brooke at BSS came up with the elegant way of achieving independence between boost/cut and Q by simultaneously changing the gain of the boost/cut invertor opamp and the Wien opamp with a single resistor? Sadly, we will not be able to ask JP, but if I see Chas I will ask him.
Just registered on this forum and thought I should make a small contribution - perhaps it will be of interest to some of you.
Parametric eq's (with independently variable centre frequency, bandwidth/Q and cut/boost) are by large created using a state-variable filter consisting of to integrators and a summing section. This requires a total of four op-amp sections.
But a parametric eq section can also be realised differently. I am aware of the early "Twin-T" designs by Sontec etc but it can also be realised with a Wien-bridge filter. I first came across this in design for Crest Audio done by the late Joe Petrucelli. His version had switched Q but could be altered to variable. Later I came across a similar implementation in the Audient ASP range by David Dearden. David is still around and he gave me his comments as follows:
Looking at the ASP8024 schematic for the first time since around 1997, it is basically a WEIN bridge topology which was in common use in all Soundcraft consoles and also in by various other manufacturers in the 80's and 90's. However, they were all fixed Q with the Q and amount of boost/cut heavily dependent. The normal situation was to set the boost/cut to around +/-15db by adjusting the ratio of the capacitors (which I think was around a 2:1 ratio) and accept that the Q would be around 1.3. Both mine and JP's version have equal value capacitors.
I wonder whether it was JP or Chas Brooke at BSS came up with the elegant way of achieving independence between boost/cut and Q by simultaneously changing the gain of the boost/cut invertor opamp and the Wien opamp with a single resistor? Sadly, we will not be able to ask JP, but if I see Chas I will ask him.