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This is a "basic transformerless 5-band EQ using gyrators" schematic.

I think I understand the top part (pots remove the frequency from either input of the op-amp, either increasing or decreasing it from the output), but I don't understand what the bottom part is supposed to do, and why it requires those op-amps.

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    \$\begingroup\$ Welcome! Have you tried to simulate it? \$\endgroup\$
    – winny
    Aug 28, 2022 at 8:20
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    \$\begingroup\$ Have you looked into gyrators in general and how they are implemented? \$\endgroup\$
    – jramsay42
    Aug 28, 2022 at 9:24
  • \$\begingroup\$ There are not enough Gyrator filters to cover the entire range of hearing frequencies. Deep bass (below 40Hz) and very high (above 13kHz) audio frequencies are not adjusted. \$\endgroup\$
    – Audioguru
    Aug 12, 2023 at 16:40
  • \$\begingroup\$ I know this thread is almost 1 year old, but . . . a 10-band variation of this circuit is on page 13 of the LM833 datasheet. \$\endgroup\$
    – AnalogKid
    Aug 12, 2023 at 17:06

4 Answers 4

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This link to a document by Texas Instruments explains the design of equalizers using gyrators. Also, at this page you will find a link to reference 2 of the first document. Chapter 2, section 2.17 has more details.

One final note: It's a very unfortunate thing that this knowledge seems to have bee lost ...

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    \$\begingroup\$ Not even close to lost, just out of favor. A DSP can do things the opamp generations never dreamed of, and the marketplace never is going back. TI is doing a decent job of maintaining support for us NS-crazy dinosaurs, but note that the app note you linked is over 20 years old, the handbook is over 40, and the circuit is over 50. \$\endgroup\$
    – AnalogKid
    Aug 13, 2023 at 14:40
  • \$\begingroup\$ Yet, op-amps have still something DSPs will never dream of: analog sound. Back in the nineties, many used to tell me that vinyl was dead and CDs was the then future. Today, CDs are mostly used to scare birds at the balconies, while vinyl still plays. And don't forget the hi end of hi-fi is still valves, yes, the thermionic ones. \$\endgroup\$ Aug 13, 2023 at 14:54
  • \$\begingroup\$ @TasosKipriotis - Hi, (a) Please do not try to respond to a comment in an answer. Answers are only for answering the question. Nothing more. If you want to respond to a comment (which must be done politely & constructively) and following the comment policy, then you can do that when you have 50 points. (b) Do not make fun of someone's username in any way. Everyone must follow the Code of Conduct at all times. No "ad hominems" are allowed. || For these reasons, your post has been edited to remove the problematic part. \$\endgroup\$
    – SamGibson
    Aug 13, 2023 at 16:44
  • \$\begingroup\$ @TasosKipriotis - (c) Your answer is basically a "link only" answer & those are not suitable as answers on Stack Exchange - see here. Please can you edit your answer to include more information from the material you linked, so that if the links failed then your answer would actually still answer the question on its own. Links are only for supplementary material - please see here: "Always quote the most relevant part of an important link, in case the external resource is unreachable or goes permanently offline." Thanks. \$\endgroup\$
    – SamGibson
    Aug 13, 2023 at 16:44
  • \$\begingroup\$ @SamGibson: Hi. (a) The reason I answered to AudioGuru's comment in my answer is that his comment is totally misleading and the proof for it is in the linked documents. I did not know I had to do it separately or that I had to have some points(?) (b) If by "make fun of someone's username" you refer to the word "justify" I used, then, the Oxford Advanced Learner's Dictionary's definition for that word must be wrong. \$\endgroup\$ Aug 14, 2023 at 6:43
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Gyrator is a theoretical circuit element (like ideal voltage sources, resistors, capacitors etc... ) which has simple and well defined operation equations. Read them here https://en.wikipedia.org/wiki/Gyrator

The same article presents ways to have approximately the same operating equations with practical circuits. You seemingly know one such circuit, the opamp gyrator.

The same article shows as a practical application the "simulated inductor". In your example the simulated inductors are used in series LC resonators. A multiband equalizer made with real LC resonators would need plenty of inductors which would cost much more than simulated inductors.

The capacitors connected to the wipers of the potentiometers are the capacitive halves of the resonators.

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Here is the behavior of one "block", measured input impedance.
One can easily retrieve essential data.

enter image description here

And an approximative "equivalence" of this circuit ...

enter image description here

UPDATE: And here is the behavior of the filter (only varying a1).

enter image description here

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  • \$\begingroup\$ And what is the idea? \$\endgroup\$ Aug 12, 2023 at 14:15
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Circuit idea

Op-amps and the elements around them at the bottom of the circuit act as virtual inductances. They are connected in series with the capacitors in the upper part and form series LC filtering networks. By means of the potentiometers, the LC networks are connected in parallel to the input of the output op-amp.

Gyrator (simulated inductance)

Basic idea

The gyrator (simulated inductance) is a capacitor circuit that mimics the behavior of an inductance through time.

schematic

simulate this circuit – Schematic created using CircuitLab

It uses the fact that the voltage drop across the resistance of an RC differentiator circuit varies with time in the same way as the voltage drop across an inductor in an RL circuit. The voltage across RC actually represents the current through the capacitor (as the voltage across RL represents the current through the inductor).

The voltage across the inductor:

STEP 1a

The current through the inductor:

STEP 1b

The voltage across Rc (proportional to Ic); can be used as the voltage across the future simulated inductor (see the conceptual schematic below):

STEP 1c

The voltage across the capacitor; can be used to obtain the current through the simulated inductor (through the resustor Ri in the conceptual schematic below):

STEP 1d

Conceptual circuit

So, a simulated inductor can be build by connecting two elements in series - resistor Ri and following voltage source VRc. The input voltage Vin is applied across this network so the voltage Vc appears across Ri. In this way, the voltage across the simulated inductor is proportional to the current through the capacitor and the current through the simulated inductor is proportional to the voltage across the capacitor, i.e. the capacitor voltage and current are swapped and an "inductor" is obtained. In short, the simulated inductor is a capacitor with voltage and current swapped.

schematic

simulate this circuit

Implementation

The OP's circuit is a possible implementation of the conceptual circuit above. It consists of a voltage divider of two 10k resistors, an op-amp follower and a capacitor differentiator.

schematic

simulate this circuit

The op-amp copies the input voltage and applies it through the capacitor to the divider's output; thus the derivative of the input voltage is obtained. This is actually an "ideal" inductor that is connected through the top 10 k resistor to the circuit (inductor's) input. This resistance represents the internal ohmic resistance Ri of the "simulated real inductor". The graph below shows the current flowing through it.

STEP 2

LC series filter

schematic

simulate this circuit

STEP 3

Some history

In 2010, I shared a detailed intuitive explanation of this weird circuit with the so-called "wikipedians" and then wrote it in the old gyrator Wikipedia page. Unfortunately, as is usually the case there, wikipedians gradually have phased out the explanation of the idea over the years and replaced it with formal "explanations".

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