Inductors vs gyrators in EQ filters

[Fancy Lime]

Hi there!

I've been messing around with different types of multi-band EQs lately. My current focus is on a gyrator based graphic EQ (see R.G. article here: http://www.geofex.com/Article_Folders/EQs/paramet.htm). I've been wondering what the pros and cons of using "real" inductors instead of gyrators in such an application might be. Practical considerations such as cost, size and availability limits use of inductors the upper mid and treble bands. For anything else we would need >100mH inductors, which either have too much series resistance to achieve high enough Q values, or are very large or prohibitively expensive. Might inductors still be the way to go for the upper bands, though? I am mainly thinking about noise here. Less active parts *usually* means less noise and noise tends to be most annoying at and above 1kHz. The question is: does it matter here? And what potential drawbacks would such a mixed topology EQ bring?

Thanks and cheers,
Andy

[antonis]

Just scratching the surface..

(+) Real inductors can handle much more current..
(if needed..)

(-) They have copious amounts of resistance and may also have significant capacitance..

(-) They are affected by magnetic fields..

(-) They can't be easily variable..
(e.g. via a single potentiometer)

P.S.
About "mixed" topology EQ..?? - I haven't the slightest idea..!! 

[Fancy Lime]

Just scratching the surface..

(+) Real inductors can handle much more current..
(if needed..)

(-) They have copious amounts of resistance and may also have significant capacitance..

(-) They are affected by magnetic fields..

(-) They can't be easily variable..
(e.g. via a single potentiometer)

P.S.
About "mixed" topology EQ..?? - I haven't the slightest idea..!! 

Well, the topology is not really mixed. I mean: using both inductors and gyrators for different bands of the same EQ.

Current is not really an issue in an EQ run at maybe 9-24V, is it? The capacitance may indeed be an issue. The resistance of the inductors needs to be taken into account in the design but there are inductors with up to 100mH and resistances in the single digit Ohm region on the market. Of course the ones with hundreds of ohms are only a tenth of the price and size but I think careful planning could deal with this particular problem. I generally try not to have strong magnetic fields in my effects but I get your point. Need to look into this some more. As for variability: that gets rather complicated with gyrators as well due to the interactions of parameters. So adjusting gyrators to move center frequencies is not really an option I consider viable anyway.

Thanks and Cheers,
Andy

[amptramp]

Any indictor with a large enough value for audio service would be a closed magnetic field, so I doubt external magnetic fields would be a problem unless you use air-core inductors (which would require a briefcase-size enclosure).

Gyrators may have some noise but iron-core inductors with magnetic domains flipping individually.  Neither approach appears to have a noise problem.

Inductor resistance may prevent the filter from having a narrow enough bandwidth.

I don't think variability is an issue since other parts of the stage can be redesigned around the values you have.

You can use ordinary parts readily available from most suppliers to make a gyrator.  Getting a group of inductors of the sizes you need would be a special order.

My choice would be to go with a gyrator.

[Rob Strand]

What you already said pretty much sums it up.

Without writing a large slab of technical crap there's some points worth noting about gyrators.

There's many forms of gyrators.   The common forms have the inductor grounded at one end,
which simplifies the circuit.  Other forms of gyrators don't force a grounded inductor.

One thing about gyrators which is not common knowledge is gyrators are not perfect inductors.
- the ground resistor effectively put s a parallel resistor across the inductor (there's actually details than that).
- In an EQ, this parallel resistance causes a high frequency shelf boost/cut on top of the peaking boost/cut.
- The parallel resistance causes losses and requires the series resistance to be considerable reduced to get
  the expected amount of boost.
- The effect will worsen as the ratio of the RF (the feedback resistor on the EQ opamp) to the parallel resistance RL (the resistor to ground on the gyrator) becomes larger.
- Transistor based gyrators further skew reality from the ideal L + C + R behaviour.  It can be quite significant for high-ish Q's and low gain transistors.  It is easy to see the effect even at hFE=400.

You really need to simulate the the gyrators in spice to appreciate what I'm saying here!   You won't use a gyrator calculator again, well only to get the initial values.

I am mainly thinking about noise here. Less active parts *usually* means less noise and noise tends to be most annoying at and above 1kHz.

The gyrator resistor to ground is one source of noise.  As you try to make the gyrator resistor to ground larger, in order to stop the above-mentioned effects, the noise gets worse.  However, what often turns out to be the case is the gyrator opamp itself is the source of noise.   A BJT input opamp with a larger gyrator resistor to ground only makes that worse.    So low noise JFET input opamps are a good choice.  If you aim for high amounts of boost from the EQ the noise gets amplified in that EQ topology.

As with any circuit with many opamps in the signal path the noise the noise accumulates.  A simple was it can be reduced is using pre-emphasis and de-emphasis. 

The question is: does it matter here? And what potential drawbacks would such a mixed topology EQ bring?

That EQ topology has many downsides but it's simple and therefore commonly used.  One trick often used for multibands use two circuits in cascade.  You then assign every second band to one circuit and the alternate band to the other circuit.

[antonis]

@Andy: With all the respect about your queries, I think you over-deal with DIYs "minor importance" issues, like noise..
(it should be called for an EQ incorporated into a Hi-Fi audio amp but for an effect placed somewhere between Fuzzes, ODs, Reverbs, Phasers, Delays, Flangers, A/B switches etc, the amount of noise reduction is purely transmuted into amplified mind trouble..)

The above, of course, doesn't, by any mean, oppose to any creative argument concerning  items use or circuits design..
:beer: :beer: :beer:
(:cider: for Stephen..) 

BTW, chapter 3 of https://sound-au.com/articles/gyrator-filters.htm clears some of "real inductor vs gyrator" vacillations..

[thetragichero]

thank you for this topic!
i picked up an solid state pa head with graphic eq awhile back with plans to convert it to a bass head (after i cheat about 15 other projects lol) so this exact question has been kicking around the back of my head (not so much concerned about size, and I've got a ton of various inductors from organs I've stripped)
I'll continue to follow the discussion

[Fancy Lime]

@Andy: With all the respect about your queries, I think you over-deal with DIYs "minor importance" issues, like noise..
(it should be called for an EQ incorporated into a Hi-Fi audio amp but for an effect placed somewhere between Fuzzes, ODs, Reverbs, Phasers, Delays, Flangers, A/B switches etc, the amount of noise reduction is purely transmuted into amplified mind trouble..)

The above, of course, doesn't, by any mean, oppose to any creative argument concerning  items use or circuits design..
:beer: :beer: :beer:
(:cider: for Stephen..) 

BTW, chapter 3 of https://sound-au.com/articles/gyrator-filters.htm clears some of "real inductor vs gyrator" vacillations..

A cider a day keeps the lobsters away, bucko 
Well you are not wrong in pointing out that I tend to overthink things. It just really bothers me when I know what I want to do and I kinda-sorta know how to, but not how to do it properly. The way someone who knows what they are doing would do it. Learning electrical engineering, or at least some of the particular subsection of it that we are dealing with in this forum, is half the fun of building pedals for me. Actually, I do not really build all that many pedals. I mostly design circuits, which I have no idea how to design when I start. So maybe learning how to design stuff is more than half the fun for me. That aside, the thing for which I am trying to design an EQ right now, is going to be a studio preamp. So noise is not as much of an issue as it would be in, say a hydrophone preamp for deep sea marine biology, but it is not quite as irrelevant as in a fuzz used for live gigs with a punk band.

Cheers,
Andy

[PRR]

Think less. Build/play more.

[Fancy Lime]

Think less. Build/play more.

Been hearing that advice for all my life. But the "think less" part seems to require chemical assistance that impedes the building and playing even more than the thinky bits.

[Rob Strand]

Well you are not wrong in pointing out that I tend to overthink things.

Like most things ignorance is bliss.

These sims shows the difference between an inductor, opamp gyrator, and a transistor based gyrator for various gains (hFE = 200, 400, 800).   There's two cases: one with a low value feedback resistor (3k3) and one with a high feedback resistor (10k).

What is shows is you can't fix the peaks and the HF boost at the same time by playing with RF.

What you can do is tweak the parts for the transistor case (not shown in these examples).   You can tweak the series resistor to fix the gain then tweak the gyrator cap to fix the frequency.   For high Q designs or when RF is too small you can reach a point where no matter what you do you can't tweak the circuit; attempts to do so usually result in the HF boost rising.

[antonis]

Really nice job, Rob..!! 

(maybe I should think again about those fool's tools..)

maybe learning how to design stuff is more than half the fun for me.

I'm on your side, Andy..!! 

[Rob Strand]

Really nice job, Rob..!! 
(maybe I should think again about those fool's tools..)

Some stuff is so easy to tweak with simulators.

One thing I probably should point out is I've tweaked the opamp version to match the ideal LCR version.  Notice how much lower the series resistance of the opamp version is compared to the LCR version to get the same gain.   So even after doing that tweak the BJT version still falls over.

If you just used the values out of a gyrator calculator the results would be worse again for all cases.

[tubegeek]

I've got a ton of various inductors from organs I've stripped

Then you're going to want to build a vibrato circuit a la Sears Silvertone/Hammond organ instead of an EQ. WAY cooler. If I can find my reference URL I'll edit it into this post.

EDIT: here's one discussion...

https://www.diystompboxes.com/smfforum/index.php?topic=95124.0

And:

https://sluckeyamps.com/warbler/warbler.htm

And:
http://www.nshos.com/X6618.htm

[ElectricDruid]

Those links seem to be to a transformer-based vibrato not the inductor-based "scanner vibrato" from the Hammond B3 and similar. That works by cross-fading from one position on the delay line to another to produce a variable phase shift and hence vibrato.

The phase shift stages in the other one are interesting though - I've never seen that done with a transformer before. I'll have to study that a bit and get my head round it.

[tubegeek]

Those links...

A few years back I recall going down a deep rabbit hole trying to learn about the Magnatone vibrato and how it might be replicated using parts available today - honestly I'm not sure whether those are the projects ones I was remembering - it's been a while. I never built it, I didn't have any of the parts that you have to salvage from old organs. I just remember it was pretty interesting....

If you really want to get in there, here's a pretty thorough discussion:

https://dalmura.com.au/static/Magnatone%20vibrato%20design.pdf

I found a link to another old forum chat forum with a lot of info and of course wouldn't you know it, PRR was laying down The Knowledge.