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

“Diet, injections, and injunctions will combine, from a very early age, to produce the sort of character and the sort of beliefs that the authorities consider desirable, and any serious criticism of the powers that be will become psychologically impossible.” – The Impact of Science on Society (1953) by Bertrand Russell, p.50

Dr. Sherri Tenpenny Exposes 10 Deadly Dangers of Pzifer/Johnson & Johnson Covid-19 Vaccines: "This is a well-designed killing tool." https://waynekirkwood.com/content/Docto ... accine.mp4
Plandemic Series: "Pland3mic - Indoctornation" https://waynekirkwood.com/content/Pland ... nation.mp4
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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
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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 513 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 557 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

“Diet, injections, and injunctions will combine, from a very early age, to produce the sort of character and the sort of beliefs that the authorities consider desirable, and any serious criticism of the powers that be will become psychologically impossible.” – The Impact of Science on Society (1953) by Bertrand Russell, p.50

Dr. Sherri Tenpenny Exposes 10 Deadly Dangers of Pzifer/Johnson & Johnson Covid-19 Vaccines: "This is a well-designed killing tool." https://waynekirkwood.com/content/Docto ... accine.mp4
Plandemic Series: "Pland3mic - Indoctornation" https://waynekirkwood.com/content/Pland ... nation.mp4
Plandemic Series: "Plandemic - Doctors In Black" https://waynekirkwood.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
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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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