Parametric eq with only two opamp sections

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Cuelist
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Parametric eq with only two opamp sections

Post by Cuelist » 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.

Audient_ASP_mid_eq.pdf
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Last edited by Cuelist on Sun Jun 21, 2020 10:19 am, edited 1 time in total.

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mediatechnology
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Re: Parametric eq with only two opamp sections

Post by mediatechnology » Sun Jun 21, 2020 9:54 am

Thank you for joining and posting cuelist!

You might want to try and re-upload the image.
Looking forward to seeing it...
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mediatechnology
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Re: Parametric eq with only two opamp sections

Post by mediatechnology » Fri Jun 26, 2020 7:28 am

I just had a chance to look at this.
I think I've seen something similar to it and will look for it.
https://ka-electronics.com

"States Ordered To Fraudulently Inflate COVID-19 Cases 15 Times Actual Rate"
https://banned.video/watch?id=5efab695672706002f367a0a
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JR.
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Re: Parametric eq with only two opamp sections

Post by JR. » Fri Jun 26, 2020 7:58 am

Cuelist wrote:
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.
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
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Cuelist
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Re: Parametric eq with only two opamp sections

Post by Cuelist » Sat Jul 04, 2020 10:39 am

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 23 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 33 times

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mediatechnology
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Re: Parametric eq with only two opamp sections

Post by mediatechnology » Sat Jul 04, 2020 11:08 am

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

Thanks for posting this!
https://ka-electronics.com

"States Ordered To Fraudulently Inflate COVID-19 Cases 15 Times Actual Rate"
https://banned.video/watch?id=5efab695672706002f367a0a
"Plandemic/Doctors In Black" https://proaudiodesignforum.com/content/Plandemic.mp4

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JR.
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Re: Parametric eq with only two opamp sections

Post by JR. » Sat Jul 04, 2020 12:34 pm

Cuelist wrote:
Sat Jul 04, 2020 10:39 am
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
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Cuelist
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Re: Parametric eq with only two opamp sections

Post by Cuelist » Sat Jul 04, 2020 1:08 pm

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

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