Inductor/Gyrator EQ questions.

[Bill Mountain]

Hello,

I am new to this forum and I am not much of an effects guy but I love to wire up my own bass preamps and amps for fun.

You seem like a knowledgeable bunch so I thought I would ask you guys and gals for some help.

I love the Duncan Tone Stack Calculator.  When I'm bored at work I play with this more than solitaire!  I have spent the last week studying the Steve Bench inductor tonestack.  The more I play with it the more excited I am to try it.  One major problem has been finding the right inductors.  I have found some options but the resistance is pretty high (up to 4k ohm for 20L!).  I have also found some chokes that may work but they will be pretty big and could have noise issues.

I have tried changing the circuit (with the help of some LC calculators) to use available audio grade inductors and the frequencies might stay the same but the q gets super wide and I lose the peak (according to the TSC).  The top of the midrange gets rounded over and I'm not sure how that will sound.

Here is the schematic I am working with:



It is designed to go between miniature tube gain stages but I believe it should still work well after an opamp buffer.  I may try a low power preamp tube if need be (I'm looking for a reason to try this anyway!)

I had though about using simulated inductors but this tonestack needs floating inductors.  I spent some time researching floating simulated inductors and I could not find anything useful.

Steve Bench himself suggests the use of some really small power transformers but I don't understand how that works.  Also, the products he mentions seem to either be out of stock or have a huge minimum quantity.  I don't quite grasp how this works so I have not been able to find suitable alternatives.

I called Cinemag and emailed Sowter about custom wound inductors.  Cinemag said it's not worth it and Sowter has not responded yet.

Here are the questions that I am left with (forgive me!):

How would I change the circuit to use simulated inductors/gyrators?

If I use different sized (available) inductors and adjust cap values how can I restore the peak?

If I want to use a different value inductor for the bass control how do I scale the bass circuit without a capacitor in series?

Could I use the power supply chokes (smallish chassis mount ones from Hammond) as inductors in this circuit?

Will the super high resistance (and low spec) audio inductors that I found be a good representation of this circuits potential?

Would I get the same effect if I used a FMV or Bax tonestack and added an active midrange?

What is the inductor sound?  Is it worth the trouble?  Will gyrators get you there?

Is there anything else that I'm not thinking about?

Thanks for reading.

I look forward to reading any and all responses!

[Bill Mountain]

Just to make sure I'm not over complicating things (which I often do!).

This synthetic inductor circuit here:



Can not be used in the eq circuit above.  Correct?

But this raises another question.  If the output is connected to virtual ground then where does the output of the gyrator come from?

[sault]

You mentioned that you already had a chance to see my calculator, excellent.

What I built the calculator for is the bandpass/notch filters on the Metal Zone... it took a fair bit more research on my part to understand exactly what a gyrator is. Turns out its pretty simple - it simulates an inductor. So a capacitor + inductor = resonance filter, and that's exactly what the Metal Zone gyrators are doing.

Starting to think about writing a version 2...

(link, btw, is http://awasteofsalt.com/gyrator for those who have no idea what we're talking about)

My ability to do that is completely dependent on the amount of time that my child allows me (I'm a very tired single father), and my understanding of the theory involved. I'm in the process of teaching myself electronic theory, so as I learn more I can do more.


My eventual goal, btw, is to actually write an equivalent program for the TSC... but that's where my lack of knowledge is really taking a beating, and why I'm going back and learning everything from the ground up. When I'm able to get a better grasp of what exactly is going on, at least in terms of being able to model it mathematically, then I can write a javascript/web version of the TSC, or at least of the circuits that I do understand (ie, FMV tone stack at least!).

I'm kind of big on "just go to a stupid website and show me what happens when I twiddle some knobs already" philosophy. I want to keep it as simple as reasonable, because sometimes I just don't want to think... and apparently I'm willing to spend dozens of hours sweating and cursing and making my brain hurt on this kind of thing now so I won't have to at some theoretical future date.   



Saul T

[Bill Mountain]

Thanks for the reply.  I think your calculator is fantastic.  I already figured out some values I could use but I still don't know how to implement gyrators into the original circuit.  I may have to roll my own (which never works how I want it to!).

[sault]

Well, going by the equation you have listed, pick values for R1, R2, and C that end up with the correct L value. L is in Henries, right?

In the schematics that I have shown (click the 'schematic' tab on the gyrator calculator page, I lifted them straight from Jack Orman's AMZ site), ignore C1, ie   L = R1 * R2 * C2

So since you've already got a 51k resistor, let's make R1 and R2 51k resistors as well. Let's look at that 20 H inductor...

R1 = R2 = 51k
L = R1 * R2 * C
20 = 51k * 51k * C
C = 0.000000008 =  * 10^-9 = 8nF

Therefore, take the schematic, plop two 51k resistors in for R1 and R2, then when C is around 8nF, L = 20 Henries. I see 8.06 at 1%, or you can go with 8.2 in 10% or 5% as the closest component values.

R1 doesn't have to equal R2, and doesn't have to be 51k... but hey, might as well keep it simple, right? At least, that's how I'd figure it. Does that answer your question?


Saul t

[Bill Mountain]

That helps a lot but my main problem is that gyrators are grounded on one side.  The schematic needs floating inductors.  I don't think I can use them with this schematic.

I will just need to make low and high shelving controls and then a bandpass filter for the mids.

In the schematic in your calculator it looks like the signal exits through R1, is this correct?  Why is there a voltage rating?  I imagine it has something to do with a 1/2 voltage reference in a single voltage power supply.  I use dual supply +/- 15V in my designs so I'm not sure if it will work or not.

[Bill Mountain]

I like your numbers but how do I put C (C = 0.000000008 =  * 10^-9 = 8nF) in your calculator.  It doesn't go that low.

Thanks!

[sault]

If your calculator has exponents (and it hopefully should, should be something like 'yx' with x a little higher than y), then it's pretty straightforward...

51,000 * 51,000 * 8 *  10 yx -9 =

On mine I leave the exponent for last, because hitting the equal button completes the equation automatically. I do have parentheses buttons, which could be used instead, so the button sequence would be

51000 * 51000 * 8 * ( 10 yx -9 ) =


If your calculator doesn't have an exponent button, go out and buy one that does. I highly recommend a 'Pi' button and parentheses '(' and ')' buttons. My favorite is the TI-30XA, I have something like 3 or 4 of them scattered around my life, so if I lose one I can still find one with a bit of searching. Doing electronics, you will constantly need the exponent and pi buttons, which is why I recommend them.

Alternatively, the Windows calculator has a scientific mode that has an exponent button and parentheses, or you could even do it manually. The trick to that is remember your exponents... the way I remember them is

m u n p = -3 -6 -9 -12

So 1 uF is 1 * 10^-6, because u = -6.

So you could manually type 1 * 0.000001 and get the same answer.

If your calculator won't display enough digits... get a new calculator, because you're going to need to hit those lower digits and maybe even go into scientific notation for some of your responses, especially if you drill down to the pF level.

Does this answer your question?


Saul t

[Bill Mountain]

This is a wonderfully thorough answer.  Very informative!

I absolutely love your willingness to help but I was referring to your gyrator calculator.  I couldn't figure out how to put such a small number for C2.

Thanks!

[sault]

Okay, the number in C2 is entered in uF.

So   0.001 uF = 1 nF.

I probably should have gone further down than I did... people do use components below 10 pF....  ah, well, version 2....
Maybe next time around I'll format it differently, make it a little more intelligent?


Saul t

[Bill Mountain]

Damn your right.  I had my micro to nano conversion all wrong.  A buddy of mine is an engineer and he says it happens to pros all the time!

[wavley]

Necro posting here, but I actually ran across this doing research for the day job, where I'm working on replacing inductors with active components, both for precision and real estate purposes.  (Actually, the research started because of a theoretical filter using synthesized negative inductors, which I actually had limited success with a breadboard before stray capacitances and inductances got in my way)

If you still need to float a synthesized inductor, you can do it by using two gryators of half the value with the "grounded" elements tied together.  There seems to be a decent amount of information about it on the web now, looks like the folks that have the most success are using OTA's for the gyrator.  I have access to the IEEE database of papers, if you can get to those through a library or something it would help, but google is actually turning up decent amount of information.

Good luck.  I am by no means an authority on the subject of Gyrators, but I'm in the middle of figuring out a lot of stuff about them, so I'll help where I can.