Stupid gyrator question

[stickjam]

Sorry if this turns out too basic, but I'm having a quarantine brain cloud.  

I am working on a design that calls for various beefier inductors (8...800 mH).  Rather than try to find such rare birds or wind my own coils, I'd like to use op-amp simulated inductors.  I've never delved into the use and design of gyrator circuits.  I can can easily comprehend how they operate, however they are always shown as simulating an inductor to ground.    Here's my stupid question: How does one simulate an inductor with one end tied to a power rail, or a series inductor where neither end is connected to ground?

I'm probably overthinking it and the solution is in front of my face (virtual grounds and/or the whole AC analysis concept that power rails are considered shorted to ground), but maybe I'm not and it is hard.  Any input or links to learning resources and/or schematics of devices that implement such things that would help get my derailed thought process back on the tracks would be appreciated.   


Thanks
-Bob

 

[abbey road d enfer]

Sorry if this turns out too basic, but I'm having a quarantine brain cloud.  

I am working on a design that calls for various beefier inductors (8...800 mH).  Rather than try to find such rare birds or wind my own coils, I'd like to use op-amp simulated inductors.  I've never delved into the use and design of gyrator circuits.  I can can easily comprehend how they operate, however they are always shown as simulating an inductor to ground. 

There's a good reason for that. Simulating a floating inductor is somewhat more difficult.

Here's my stupid question: How does one simulate an inductor with one end tied to a power rail, or a series inductor where neither end is connected to ground?

It's not a stupid question. I know there is an answer but I'm too sleepy to retrieve it.
Googling "emulating a floating inductor" gives many answers.

 

[abbey road d enfer]

Here's the answer: the Riordan gyrator, close cousin of the Hewland pump.
This circuit provides an almost perfect inductor, very low series resistance, very high parallels.
I have added these parasitics, R12 (series) & R3 (parallels).
Leq=C.R²
This circuit is used whenever high Q inductors are needed. I've never designed around this circuit, but I've seen it used in the Neutrik (now NTI) test equipment.
I think the reason people like them is because it allow them to use the "ancient" formulae for LC based filters. Also performance is less affected by GBW limitations of the opamps.

 

[Bo Deadly]

Here's the answer: the Riordan gyrator, close cousin of the Hewland pump.

That is an interesting and relatively simple solution. So the second op amp is driven common-mode except for the cap which temporarily shunts negative input current. Over time it settles on it's common mode state.

 

[abbey road d enfer]

Actually, this circuit does not really behave as a fully floating inductor. I need to investigate.

 

[john12ax7]

I haven't yet found a solution for a floating inductor,  would definitely be useful.

 

[PRR]

https://www.google.com/search?client=firefox-b-1-d&q=floating+gyrator

???

 

[john12ax7]

https://www.google.com/search?client=firefox-b-1-d&q=floating+gyrator

???

Have you tried any of those in real life audio circuits? It's been years,  but last time I looked into it they all had various shortcomings.

 

[abbey road d enfer]

https://www.google.com/search?client=firefox-b-1-d&q=floating+gyrator

???

I have simulated both the Riordan and the Antoniou. The problem is they both are not bi-directional; the input impedance is correctly emulated, but the output impedance is not, which is probably to be expected from an active circuit.
As a consequence, they cannot be applied directly as substitutes for real inductors in complex LC filters.

Regarding the specific OP's question, some thought must be given as to what is exactly expected from this simulated inductor, specifically in which direction it is meant to work.
For example, I would not think they are usable as a substitute for a choke in power rails.

Also, serious attention must be brought to the risks of clipping, since the output voltage of the opamps increases a lot at LF.

 

[JohnRoberts]

If you are trying to replicate an inductor EQ consider using an inductor... any emulation will not replicate the obscure distortions.

if you have to emulate an inductor with a capacitor and gain stage, perhaps make a cap based EQ.

JR

PS: Sorry to be a debby downer

 

[stickjam]

Thanks for the great input.  The word "floating" was eluding me.  The application is a audio frequency LC tank as used in old school analog rhythm machines.    I'll breadboard up a drum voice using a Riordan.  Audio sample is hopefully forthcoming.  ;D

 

Here's the answer: the Riordan gyrator, close cousin of the Hewland pump.
This circuit provides an almost perfect inductor, very low series resistance, very high parallels.
I have added these parasitics, R12 (series) & R3 (parallels).
Leq=C.R²
This circuit is used whenever high Q inductors are needed. I've never designed around this circuit, but I've seen it used in the Neutrik (now NTI) test equipment.
I think the reason people like them is because it allow them to use the "ancient" formulae for LC based filters. Also performance is less affected by GBW limitations of the opamps.

 

[abbey road d enfer]

Thanks for the great input.  The word "floating" was eluding me.  The application is a audio frequency LC tank as used in old school analog rhythm machines.    I'll breadboard up a drum voice using a Riordan.  Audio sample is hopefully forthcoming.  ;D

Can you post a schemo?

 

[stickjam]

Can you post a schemo?

Sure.  Here it is.  Because it's capacitively-coupled to the audio mixer stage, the idea occurred to me to level-shift the trigger and run the voice circuits on the negative supply so that a conventional gyrator would work, but there are other instrument voices for which that would end up being problematic.    I'm producing a Eurorack module containing the voicing circuits from a Roland CR-78 - one of those drum machines with an iconic sound.

The prime reason for wanting an emulation of the inductor was for miniaturization, but the footprint of a Riordan using 0603 SMT (not going any tinier than that) isn't that much smaller than the OEM inductors were.  (using OEM spares or parting out an original Roland unit is unconscionable).

Because "the sound" is what I'm after, perhaps real iron is what I really need.  However that reveals my second reason for seeking an emulation...laziness.  :  There are countless core options with lots of unfamiliar specs because it's been decades since I had to design coils--and then only theoretically in college.    I did more digging this morning, things came back to me a bit, and searched for cores similar to the Roland OEMs in size and design.  I went for a gapped since it's in a DC path.  Maybe you guys can check my math and thinking.    If I wrapped a bobbin with 267 turns and clamped it into a pair of these, would I end up with the 45mH inductor I'm looking for?  https://www.mouser.com/ProductDetail/EPCOS-TDK/B65813J0630J087?qs=sGAEpiMZZMt1hubY80%2Fs8LgRgZrZT%2FhekKrrobfa7D8%3D

 

[stickjam]

(ironically, it occurred to me that it's possible no major artist ever used the cowbell on the CR78, but they sure used the heck out of it on the TR808.  It's probably the only sound not occurring in this playlist.  https://open.spotify.com/playlist/7Lo6zah2ARz5L28rCNYwh1?si=0YSgiR9sTjazcw67vVapJA  )

 

[abbey road d enfer]

Sure.  Here it is. 

Thans. I had not seen one of these in years. Now that I know, I would say  it's very unlikely that a simulated inductor behaves like a real one. Since the transistor is run with a pulsed collector current, its inductance varies significantly. I'm not even sure the original ones were gapped...
I think the standard gyrator could be used there; it would just need to be powered with a 24V rail and referenced to the 12V rail.

 

[PRR]

47mH inductors rated for several mA of DC are about a dollar each at Mouser etc. Unless you are going into vast production, it is surely cheaper to buy than to wind or re-circuit.

22R476MC Murata Power Solutions Fixed Inductors 47mH 0.033A 154ohm Bobbin Wound SMT
652-RL622-473K-RC  Bourns  Fixed Inductors 47mH 10% 79KHz 30mA
11P-473J-50 Fastron Fixed Inductors RF CHOKE 47mH 5%

 

[PRR]

Hmmm. That "trigger" depends on raising the current in Q205, I assume quite percussively. The usual gyrators won't like that.

And of course this is not "floating". One end is nailed solid to AC ground.

A gyrator will not "kick" like a real coil.

 

[stickjam]

47mH inductors rated for several mA of DC are about a dollar each at Mouser etc. Unless you are going into vast production, it is surely cheaper to buy than to wind or re-circuit.

22R476MC Murata Power Solutions Fixed Inductors 47mH 0.033A 154ohm Bobbin Wound SMT
652-RL622-473K-RC  Bourns  Fixed Inductors 47mH 10% 79KHz 30mA
11P-473J-50 Fastron Fixed Inductors RF CHOKE 47mH 5%

Why I never found those in search earlier on Mouser, I have no idea.  :-[  Yes percussive by definition.  The triggers will be square pulses of various amplitude to alter volume and tonal dynamics.     

The only thing that seems not to be available similarly are in the two coils I need around the 700mH range.  However, those are in purely passive circuits so I'll order some 68mH Muratas to try, bumping the decimal point on the caps to see what it sounds like before resorting to winding my own.

Thanks PRR.  I knew you'd be the one bring me around to reason.    Agreed that nothing kicks like real L. 

-Bob

 

[stickjam]

Attached is the original schematic of one of such passive voices, the clave. "3R" is a Roland part number which is 0.7H.

Planning on taking C506 to 0.047uF with the 68mH Murata.   

 

[Bo Deadly]

The inductor for a wah pedal is ~500mH. A "Fasel inductor" on Ebay is $15 USD.