Inspired by the Poor Man's Pultec (https://groupdiy.com/index.php?topic=43859.0) this is my take on the classic 1073 Equalizer. The main goal of this project is to give people an affordable option for building this EQ. This is achieved by replacing all inductors with gyrators aka 'simulated inductors'. I also want to help people gain a better understanding of the original circuit.

You can find the corresponding GroupDIY topic over here: https://groupdiy.com/index.php?topic=70096.0 as well as a topic on gearslutz: https://www.gearslutz.com/board/geekslutz-forum/1235388-diy-poor-mans-1073-eq.html

• I assume that you already have a suitable power supply for this circuit. As I'm mostly playing around with Eurorack stuff, I've been using this design for a linear regulated +/-12v PSU: http://electro-music.com/forum/post-389344.html
• I’ve indicated which parts of the circuit correspond to the original 1073
• I won’t link to any original 1073 schematics, as they are easy enough to find if you search for “1073 1084 user manual issue5”
• The BA211 board caused me quite a headache, until I finally figured out, that the original circuit only uses the 200mH tap of the T1280 inductor.
• The two amplifiers on the BA284 board, that are used for the EQ circuits, function like basic inverting Op-Amps (see https://repforums.prosoundweb.com/index.php/topic,34458.html#msg509550)
• TL07x opamps will get the job done, but I got better results (lower noise at line level), by using NE5532 Opamps for the amplifiers, as well as for the gyrator circuits.
• The presence band shows lower maximum gain at higher frequencies. This effect is a characteristic of the original circuit (see http://www.uaudio.com/webzine/2006/may/text/content2.html)
• Feel free to play around with the circuit using the LTSpice .asc schematic:) LTSpice is freely available from Analog Devices http://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html

As there are no potentiometers in LTSpice, I used serial resistors instead. So if you change the value on one side (turning the imaginary resistor) you need to change the other resistor accordingly.

• The following article from Rod Elliott has been very helpful in understanding and implementing gyrators http://sound.whsites.net/articles/gyrator-filters.htm
• The inductance and resistance values for the different inductors have been taken from here http://www.sowter.co.uk/eqinductors.php
• I’ve used the Waves Scheps 73 plugin as my main reference for the frequency response https://www.waves.com/plugins/scheps-73
• DDMF’s Plugindoctor has been a great tool for measuring and referencing my circuit with the plugin https://ddmf.eu/plugindoctor/
• Special thanks goes out to gearslutz user Cabirio, who helped a lot in optimizing the circuit

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