The No.1 Website for Pro Audio
Simple Inductor EQ schematics for DIY
Old 3rd May 2020
  #1
Simple Inductor EQ schematics for DIY

Hi Friends.
I have recently started learning the basics of electronics and have built a few simple circuits from schematics on the web and I´m seriously thinking of quitting my job This is way too much fun!

I have a big love for Inductor based Eq´s, but as soon as I start to investigate if it is something maybe I could try to build it always ends up being way to complicated, for my very very limited knowledge.

Question 1:
is there a simple way to implement inductors into, a simple EQ design consisting of only caps and resistors, lets say Baxandall for example?
Question 2:
Is there anyone that know of some less complex Inductor EQ schematics a
newbie like me could build?

I have attached an image of an old tone control design from Gibson that makes my head spin.

Question 3:
Can you call the attached schematic design for an "Inductor eq"?
In other words, is the inductor in this schematic being used/playing the
same role as in other more complicated/classic designs?

Thank you all for your time.
//Ted
Attached Thumbnails
Simple Inductor EQ schematics for DIY-gibson-varitone-1961-2862193610_64cb78d779_z.jpg  
Old 3rd May 2020
  #2
Gear Head
 
rob f's Avatar
 

Quote:
Originally Posted by Ted Krotkiewski View Post
Hi Friends.
Question 1:
is there a simple way to implement inductors into, a simple EQ design consisting of only caps and resistors, lets say Baxandall for example?

//Ted
Baxandall eq's are by nature active, so you couldn't make one with just caps resistors & inductors. The eq element is in the feedback loop of an active element.
Old 3rd May 2020
  #3
Quote:
Originally Posted by rob f View Post
Baxandall eq's are by nature active, so you couldn't make one with just caps resistors & inductors.
Right.
The bax designs I built had TL072 opamps. So I should add that to the list.

Quote:
Originally Posted by rob f View Post
The eq element is in the feedback loop of an active element
ooooh this makes my head spin so much.
Care to elaborate this for a dummie like me?

Thanks a lot!
//Ted
Old 4th May 2020
  #4
Gear Head
 
rob f's Avatar
 

Read this. It's Peter Baxandalls original document
https://learnabout-electronics.org/D...edbackTone.pdf
Old 4th May 2020
  #5
An intuitive explanation of filters in a negative feedback loopj
- As negative feedback increases, output drops
- A passive filter selects what frequencies to attenuate
- That filter, put in a feedback loop, decides which frequencies have less negative feedback.

Less negative feedback = more gain

For example, a lowpass filter in a feedback loop makes the amount of low freq gain less, which is equivalent to making the high frequency gain larger in comparison.

There is more to it; phase plays an important role in design. But that's a basic concept overview.
Old 14th May 2020
  #6
Thanks a lot for your explanation Philbo. I am still not sure about how this works, but I am sure I´ll figure it out and end.

Below, I found a very basic schematic for a Mid Control circuit that uses an Inductor.
Since Inductors are very expensive and I can´t play around with values like I would with Caps and resistors, my question is. What area of Henries are common for mid freq´s in this case?

Are we talking 10mH - 200mH or higher around 1-5H?

Anyone know of cheap inductors that can be used for audio circuits like this that I can use for experimentation?
The only one I know of is the rather expensive Carnhill ones that can be bought through AML audio.

If I search for inductors on the web that can be used in audio circuits like this, what is it that defines it being suitable for audio? what are the markings and materials etc to look for?

Many questions in one go.
Sorry and thank you so much for your help.
I am sure others will find this interesting too.
My best.
//Ted
Attached Thumbnails
Simple Inductor EQ schematics for DIY-screen-shot-2020-05-14-3.51.00-pm.jpg  
Old 14th May 2020
  #7
Lives for gear
 

inductors for eq

Quote:
Originally Posted by Ted Krotkiewski View Post
If I search for inductors on the web that can be used in audio circuits like this, what is it that defines it being suitable for audio? what are the markings and materials etc to look for?
Opamp Labs in Hollywood has or had audio grade inductors.
Mouser used to stock various values of Xicon inductors that were suitable for the UA model 508 feedback equaliser.
Quad/Eight's model 312 used what appears to be inexpensive ferrite pot core inductors (in no way a diss as that eq sounds amazing).
Attached Thumbnails
Simple Inductor EQ schematics for DIY-inductor.jpg  
Old 14th May 2020
  #8
Quote:
Originally Posted by Ted Krotkiewski View Post
Thanks a lot for your explanation Philbo. I am still not sure about how this works, but I am sure I´ll figure it out and end.

Below, I found a very basic schematic for a Mid Control circuit that uses an Inductor.
Since Inductors are very expensive and I can´t play around with values like I would with Caps and resistors, my question is. What area of Henries are common for mid freq´s in this case?

Are we talking 10mH - 200mH or higher around 1-5H?

Anyone know of cheap inductors that can be used for audio circuits like this that I can use for experimentation?
The only one I know of is the rather expensive Carnhill ones that can be bought through AML audio.

If I search for inductors on the web that can be used in audio circuits like this, what is it that defines it being suitable for audio? what are the markings and materials etc to look for?

Many questions in one go.
Sorry and thank you so much for your help.
I am sure others will find this interesting too.
My best.
//Ted

It depends on the impedance of the circuit surrounding the inductor. The impedance (technically, reactance), in Ohms, of an inductor, is 2*pi* F* L. F is the frequency (Hz), and L is the inductor value (H). Your guesses are in the ballpark. For a coil to have 100 Ohms at 10 Hz it would have a value of 6.28 H.

There are a lot of specs for inductors besides value. Interwinding capacitance, core matrrial & permeability and current rating come to mind. There's a lot of complexity in analog filter design, but it's fun to play with.

I suggest starting by doing a little reading. This is a decent reference for designing passive LCR filter circuits:
https://web.stanford.edu/class/ee133...erCookbook.pdf

There are many similar 'cookbooks' that move on to filter design using opamps as well. One classic reference that is still relevant is Don Lancasters Active Filters Cookbook:
https://www.tinaja.com/ebooks/afcb.pdf

There are also an online calculator on analog.com that allow you to enter filter specs and will give you a completed schematic and parts list.
The books will give you understanding; the calculator will give you results. Both are good...
Old 15th May 2020
  #9
Awesome!

Quote:
Originally Posted by Philbo King View Post
...The books will give you understanding; the calculator will give you results. Both are good...
Excellent info here Philbo!
Thanks so much for explaining and giving me te books.

I am completely lost when it comes to math and logic, (kind of skipped it at school) but, I just started to understand Ohms law. So I hope in time I will grasp how the calculating aspect of this works as well.

I am so happy that you provided the "Cookbooks" That makes it possible for me to actually try the things mentioned in there and hopefully through trial and error, understand what is written.
I am a hands on type of person.

Have a fantastic day.
//Ted
Old 16th May 2020
  #10
I'm glad you found it helpful
Old 27th May 2020
  #11
Center Tapped VS Multi Tapped Inductors.

Hi.
One more question.

Am I right in guessing that the first schematic "Varitone Monaural from 1961" uses a Multi Tapped Inductor?
and the last schematic I posted some posts above uses a "Center tapped" inductor?

These inductors, that I intend to buy from Carnhill, are all "Multi Tapped" with different henry values per coil.

Can I use a Multi tapped inductor for a schematic wanting a Center Tapped and just choose to use one value by connecting it at "Coil start" and then, tap it out at the Henry value I want, lets say at "C" @80mH?
Would that be the same as Center tapped?

Apologies if this is too obvious.

Thank you again for all your help everyone.
Your knowledge super valuable!

Have a great day!
//Ted
Attached Thumbnails
Simple Inductor EQ schematics for DIY-carnhill-inductor-values.jpg  

Last edited by Ted Krotkiewski; 28th May 2020 at 08:50 AM..
Old 30th May 2020
  #12
Lives for gear
 
ruffrecords's Avatar
Quote:
Originally Posted by Ted Krotkiewski View Post
Thanks a lot for your explanation Philbo. I am still not sure about how this works, but I am sure I´ll figure it out and end.

Below, I found a very basic schematic for a Mid Control circuit that uses an Inductor.
Since Inductors are very expensive and I can´t play around with values like I would with Caps and resistors, my question is. What area of Henries are common for mid freq´s in this case?

Are we talking 10mH - 200mH or higher around 1-5H?

//Ted
It is the series combination of an inductor and capacitor that determines the frequency that particular mid EQ works at. There is a huge range of possible values for both for any given frequency. However, the sharpness of the the EQ curve is determined by the value of the inductor. So for the typical range of values of sharpness (or Q as it is known) you are likely to find useful there is a small range of inductor values for this circuit. They are typically in the one to two Henries range.

Cheers

Ian
Old 31st May 2020
  #13
Quote:
Originally Posted by ruffrecords View Post
It is the series combination of an inductor and capacitor that determines the frequency that particular mid EQ works at. There is a huge range of possible values for both for any given frequency. However, the sharpness of the the EQ curve is determined by the value of the inductor. So for the typical range of values of sharpness (or Q as it is known) you are likely to find useful there is a small range of inductor values for this circuit. They are typically in the one to two Henries range.

Cheers

Ian
Awesome info here. Thanks Ian.
I remember that the tech Eddie Ciletti wrote in an article that he felt the "Q" curve being to sharp in my Raindirk console eq´s, and that by changing the Inductors would broaden the shape.

So, am I right to assume that the higher the Henry values is, the broader the shape will become? Or is it much more complex than that, depending on other factors as well?

I have some Inductors on order now, So will experiment and find out as I go I suppose.
In the meanwhile, I´ll hang out here, trying to grasp how it works.
Thanks again.
//Ted
Old 31st May 2020
  #14
Lives for gear
 
ruffrecords's Avatar
[QUOTE=Ted Krotkiewski;14773050]
Quote:
Originally Posted by ruffrecords View Post

Awesome info here. Thanks Ian.
I remember that the tech Eddie Ciletti wrote in an article that he felt the "Q" curve being to sharp in my Raindirk console eq´s, and that by changing the Inductors would broaden the shape.

So, am I right to assume that the higher the Henry values is, the broader the shape will become? Or is it much more complex than that, depending on other factors as well?


//Ted
Nearly all inductor based EQs belong to the general class of RLC circuits which contain resistance, capacitance and inductance in a series. In the EQ you posted the resistance part is determined by a combination of the resistor in series with the inductor, the dc resistance of the inductor itself and and the resistors around the the op amp. The actaul Q is given by:

Q = wL/R where

w = 2 x PI x frequency
L is the inductance
and R is the resistance

So Q is directly proportional to L - double L and you double the Q

Q is also inversely proportional to R, so double R and you half the Q.

Varying R is a common way of varying Q. The Pultec EQP1 varies Q using a pot in series with the inductor.

If you check out the DIY section of my web site you will find lots of info about simple and more complex passive EQs using inductors

http://www.customtubeconsoles.com/diy

Cheers

Ian
Old 31st May 2020
  #15
[QUOTE=ruffrecords;14773072]
Quote:
Originally Posted by Ted Krotkiewski View Post
Nearly all inductor based EQs belong to the general class of RLC circuits which....

...If you check out the DIY section of my web site you will find lots of info about simple and more complex passive EQs using inductors

http://www.customtubeconsoles.com/diy

Cheers

Ian
Again, Invaluable info here.
Thanks Ian. This is awesome.
Old 31st May 2020
  #16
[QUOTE=ruffrecords;14773072]
Quote:
Originally Posted by Ted Krotkiewski View Post
Yaayy!
THIS! is exactly the type of EQ design I have been looking for! https://drive.google.com/drive/folde...1FTTWNWSWduSTg

With this schematic and the excellent explanation provided, I can also learn and experiment with different values, as is suggested in the article.
📝 Reply
Topic:
Post Reply

Welcome to the Gearslutz Pro Audio Community!

Registration benefits include:
  • The ability to reply to and create new discussions
  • Access to members-only giveaways & competitions
  • Interact with VIP industry experts in our guest Q&As
  • Access to members-only sub forum discussions
  • Access to members-only Chat Room
  • Get INSTANT ACCESS to the world's best private pro audio Classifieds for only USD $20/year
  • Promote your eBay auctions and Reverb.com listings for free
  • Remove this message!
You need an account to post a reply. Create a username and password below and an account will be created and your post entered.


 
 
Slide to join now Processing…
🖨️ Show Printable Version
✉️ Email this Page
🔍 Search thread
♾️ Similar Threads
🎙️ View mentioned gear