Voltage-Controlled Inductance via OTA's (Gyrator)

[Analog_Fan]

I saw this video from Professor Aaron Lanterman some 3 weeks ago:



This is what i found on the subject:
https://www.mdpi.com/2076-3417/11/1/291https://inst.eecs.berkeley.edu/~ee100/fa04/lab/lab10/EE100_Gyrator_Guide.pdfhttps://ijrar.org/papers/IJRAR19D1397.pdf
it's also shortly mentioned in: 450007017-Steve-Dove-Console-Design-pdf.pdf, but no complete circuit.

I can't find a complete circuit that uses OTA's for a Equalizer, i assume this allows you to play with the "cuttoff" frequency and have somewhat a better or variable EQ than you can find standard on a mixing desk, DJ mixer. But my electronics math skills are not up for the task.
Anyone that can help?

 

[Analog_Fan]

simulator: https://tinyurl.com/yt58lfb8

I implanted the OTA's in the usual form like done in Voltage voltage.
100K/1k on the input pins of them and voltage followers as Professor Aaron says and build it like mentioned here: Gyrator Filters

But, i don't see any effect.
Would be cool to have a Tunable Filterbank.

 

[KA-Electonics.com]

From the Aphex 1537 data Sheet:

 

[abbey road d enfer]

I can't find a complete circuit that uses OTA's for a Equalizer

The problem with OTA's is that you soon have dynamic range limitations, you have to make a compromise between headroom, distortion and noise.
I experimented with a State Variable Filter (SVF) based on Blackmer VCA's (THAT), which are at least an order of magnitude quieter than OTA's.
The goal was to make a modular dynamic EQ allowing to cascade as many stages as (reasonably) desired.
Every parameter (frequency, BandWidth, Cut/Boost) was DC controlled.
I made a working prototype but came soon to the conclusion that the noise performance was not good enough for the intended purpose (vinyl mastering).

 

[Tubetec]

I took a closer listen to the noise floor out of a graphic based on gyrators ,
even if your cutting the noise goes up . Weakest point in the entire signal chain and its often between your desk output and FOH .

 

[JohnRoberts]

The problem with OTA's is that you soon have dynamic range limitations, you have to make a compromise between headroom, distortion and noise.
I experimented with a State Variable Filter (SVF) based on Blackmer VCA's (THAT), which are at least an order of magnitude quieter than OTA's.
The goal was to make a modular dynamic EQ allowing to cascade as many stages as (reasonably) desired.
Every parameter (frequency, BandWidth, Cut/Boost) was DC controlled.
I made a working prototype but came soon to the conclusion that the noise performance was not good enough for the intended purpose (vinyl mastering).

did you ever consider using digital pots?

JR

 

[abbey road d enfer]

did you ever consider using digital pots?

JR

Yes I did. The problem is that it was a dynamic EQ, so it would have required some form of A/D between the side-chain, which would have required all the paraphernalia related to digital feedthrough, zero-crossing, and so on. So I considered it was more complicated, with less promising results than a fully digital solution.
But most vinyl ME's insist on fully analogue audio in their chain so I dumped the project.
It's a conclusion several designers reached when trying to develop digitally controlled analogue mixers.

 

[JohnRoberts]

Yes I did. The problem is that it was a dynamic EQ, so it would have required some form of A/D between the side-chain, which would have required all the paraphernalia related to digital feedthrough, zero-crossing, and so on. So I considered it was more complicated, with less promising results than a fully digital solution.
But most vinyl ME's insist on fully analogue audio in their chain so I dumped the project.
It's a conclusion several designers reached when trying to develop digitally controlled analogue mixers.

I did an automatic mixer design using DPOTs that required dynamic gain changes. To my ear the prototype sounded adequate without zero cross synchronization (while I built that capability in). The product never made it to market so who knows if the analog only true believers would have accepted it?

We need to find an analog DSP platform

JR

 

[terry setter]

Engineer, producer, designer George Massenburg is credited with having created the first parametric EQ and coined the term "Parametric EQ". He did so using gyrators. His company, GML, makes the highly regarded 8200, and they now have a "mastering version" called the 9500 (Model 9500 – George Massenburg Labs). I believe the 8200 still uses gyrators, but I am unfamiliar with the 9500. Apparently, they can be made to work very well.

 

[Tubetec]

Its been a long time since real inductors were used in commercial studio gear ,
Im generally happy with channel eq using gyrators but graphic eq needs an op amp for each band , if its 31 band thats a lot of extra noise adding up at the output .

 

[abbey road d enfer]

Engineer, producer, designer George Massenburg is credited with having created the first parametric EQ and coined the term "Parametric EQ".

That's correct.

He did so using gyrators.

Did he?
Do you have evidence ot it?
The circuit GM got a patent for used bridged-T circuits.

His company, GML, makes the highly regarded 8200, and they now have a "mastering version" called the 9500 (Model 9500 – George Massenburg Labs). I believe the 8200 still uses gyrators, but I am unfamiliar with the 9500. Apparently, they can be made to work very well.

The term "gyrator" has been largely misused. The common simulated inductor based on the input impedance of a Sallen & Key HPF is too often misnomed "gyrator", which it is not.

 

[abbey road d enfer]

Im generally happy with channel eq using gyrators but graphic eq needs an op amp for each band , if its 31 band thats a lot of extra noise adding up at the output .

I'm not aware of any pro audio equipment using gyrators.
I've seen them in audio test measurement products and in telecom equipment.
The most common implementation is the Antoniou gyrator, which uses two opamps. Its advantages are low sensitivity to component drift (important for high-Q filters) and low sensitivity to restricted GBW of the opamps.

 

[firerunner]

I'm not aware of any pro audio equipment using gyrators.
I've seen them in audio test measurement products and in telecom equipment.
The most common implementation is the Antoniou gyrator, which uses two opamps. Its advantages are low sensitivity to component drift (important for high-Q filters) and low sensitivity to restricted GBW of the opamps.

From their marketing I believe Phoenix does Neve style gear with gyrators. I saw some in a Vestax mixer too

 

[abbey road d enfer]

From their marketing I believe Phoenix does Neve style gear with gyrators. I saw some in a Vestax mixer too

From the description (singular amplifier), I believe they use erroneously (and somewhat deceptively) the word gyrator.

 

[gyraf]

He did so using gyrators

No, he used twin-t notches in feedback circuit
(edit, as Abbey says: Bridged-T)

I'm not aware of any pro audio equipment using gyrators

I do extensively - but I confine myself to good old-fashioned "real" LC gyrator, not those new-fangled analogue-computer-simulations with opamps

/Jakob E.

 

[firerunner]

From the description (singular amplifier), I believe they use erroneously (and somewhat deceptively) the word gyrator.

Thanks for the insight!

 

[terry setter]

I recently mentioned George Massenburg using gyrators in circuitry and Abby Road asked if I have evidence that he did. Well, yes and no. My statement was based on a conversation I had with Massenburg during which he said that he used gyrators when developing the parametric equalizer. So, is that evidence of him having used them? I'm thinking that it is. He also told me that the central problem in successfully doing so was creating an exceptional operational amplifier.

It's entirely possible that Massenburg used gyrators in developing his circuit ideas, then went to a variation of that circuit in the patent application and for various production models. The point I was making in my previous post was that Massenburg, the man who gave us parametric EQ, successfully implemented gyrators in some of his designs.

 

[abbey road d enfer]

Or could it be that he simply misused the name gyrator?
After all, the common one-opamp circuit used in many equalizers has no commonly accepted name. However, it is quite often referred to as "gyrator", incorrectly.
GM may well have tried several topologies, e.g. SVF before opting for the bridged-T arrangement, which is clearly not a "variation of that circuit".

 

[terry setter]

Or could it be that he simply misused the name gyrator?

If you read the cover article on Massenburg in "Tape Op" No. 54, July/Aug 2006, it says: "... George had a DIY attitude from the beginning. He dropped out of an electrical engineering program when he realized that his professors wouldn't seriously consider innovative design ideas. At one point, a professor told him that the circuit he was working on (a gyrator) could not be realized. That circuit was already in his prototype of the first parametric equalizer - and it worked just fine. After that encounter, George could see the writing on the wall, so he put together a couple of prototype boxes and headed to France...."


This was taken from the audio recording I made in preparation for writing the article, and we had additional conversations in which George referred to using gyrators in his prototype for the parametric EQ. If he misused the word "gyrator" when describing a circuit that provided variable inductance without the use of wound inductors, he was, at least, very consistent about doing so. I always felt sure that he knew exactly what he meant.

 

[abbey road d enfer]

I always felt sure that he knew exactly what he meant.

OK, you had it from the horse's mouth. Nevertheless, all commonly known iterations of his parametric EQ use bridged-T.
It would be interesting to know why he dropped the idea of gyrators. I would have, too, considering a biquad using gyrators takes four opamps when a bridged-T takes only one. At the time, it was important.
In the video on the GML website, GM never mentions the word gyrator, although he specifically mentions the "T-filer".