Parametric eq with only two opamp sections

[Cuelist]

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.

Audient_ASP_mid_eq.pdf

(197.75 KiB) Downloaded 1298 times

[mediatechnology]

Thank you for joining and posting cuelist!

You might want to try and re-upload the image.
Looking forward to seeing it...

[mediatechnology]

I just had a chance to look at this.
I think I've seen something similar to it and will look for it.

[JR.]

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.

Yes the SVF topology can provide complete independence between EQ parameters. (I sold a SVF based parametric EQ kit in 1979.)

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.

"John" Petrucelli, RIP a good friend who is missed. (He was also the "P" from APB consoles, founded by a few other ex-Crest/Peavey employees).
viewtopic.php?f=11&t=855&p=10070&hilit= ... lli#p10070

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.

We used fixed Q wein bridge mid-sweep EQ at Peavey back in 90s, our topology used one op amp per EQ band. We also tooled up a custom pot where the two variable Rs for frequency control were done with a single pot section (common wiper shorting across two screened resistive tracks on single wafer.) This allowed a single concentric pot to vary both boost/cut and center frequency

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.

Audient_ASP_mid_eq.pdf

From glancing at the schematic I'm not so sure the Q is completely independent but easy enough to prove with sim (by someone else).

JR

[Cuelist]

Here's a SIM I did on this some years back on the Crest Century V12 which has variable Q in this topology:

Crest_Century_V12_mideq_SIM.pdf

(387.86 KiB) Downloaded 1176 times

Looks to me like Q and gain are not interacting too much.

Here's the schematic (also a JP design):

Crest_Century_V12_mideq.pdf

(333.64 KiB) Downloaded 1239 times

[mediatechnology]

The interaction doesn't look to be enough to be of any real consequence.

Thanks for posting this!

[JR.]

Here's a SIM I did on this some years back on the Crest Century V12 which has variable Q in this topology:

Crest_Century_V12_mideq_SIM.pdf

Looks to me like Q and gain are not interacting too much.

Here's the schematic (also a JP design): Crest_Century_V12_mideq.pdf

Thanks for the sim... That is definitely good enough to call the Q independent. I've seen and used Wein bridge EQ, but not with that extra op amp buffering the high impedance node.

Back in the 80s I put full parametric EQ in one console input strip (LOFT) but I used more conventional SVF topology... the high and low bands had switchable shelving that made the Q adjustment more than a little weird.

JR

[Cuelist]

FYI - The Q-range on that Crest EQ is 0.7 to 3.0

[Bill Wilson]

I found schematics of Crests Century console EQ on Group DIY. R 14 & R 15 are actually 10k rthere than the 4.7k shown.