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Board top with circuit traces Board underside with solder pads Component identification and placement. Note the square grid spacing of 0.1" per mark. The assembled board Placement of the inductor network The rotary switch and Stop position Proper orientation of voltage regulators
Board top with circuit traces
Board underside with solder pads
Component identification and placement. Note the square grid spacing of 0.1" per mark.
The assembled board
Placement of the inductor network
The rotary switch and Stop position
Proper orientation of voltage regulators

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Build An Active EQ
A DIY project to build your very own equalizer...
By Scott Dorsey

I’ve been building passive equalizer networks for years, and have always been a fan of passive equalization. But recently I have started playing with conventional active equalizer networks built with inductors. These circuits give you the best of both worlds: you get both cutting and boosting functions (obviously passive equalizers can’t boost since they can’t add power to a signal), you get excellent noise performance, and you get excellent stability. You also get some of the side-effects of inductor circuits that people like a lot in passive equalizers


One of the worthwhile properties of inductors in the real world is that their value changes based upon the DC level in them. This can be used for motor control, for example, in the case of magnetic amplifiers. These are basically inductors, which carry a large AC current but which can be driven into saturation by a small DC voltage so that the AC current is reduced or shut off.

In the case of a typical passive equalizer like a Pultec, the inductors in the signal path are affected by low-end components in the signal, so that heavy low end causes the filter networks to change frequency and also to become a bit less efficient. This causes a very interesting bass compression effect that a lot of people like on low-frequency instruments.

The project

This equalizer project is a three-band equalizer with a high-end shelf built with a resistor-capacitor network, a mid-range peak built with a resistor-capacitor-inductor network, and a low-end shelf built with a resistor-inductor network.

All three of these networks are adjustable in frequency, although the filter resonance (the Q) is fixed. However, the most interesting thing about this design is that the low-end shelf is constructed both with conventional inductors that are designed to minimize nonlinear effects, and also with “swinging” inductors that have a much more substantial amount of nonlinearity to them—and the user can select which to use at any given time. I have found this sort of equalizer to be extremely useful on drum submixes, as well as on bass guitar, because it gives you a great variety of different sounds.

Be aware that this is not intended as a surgical equalizer, like an Orban parametric. It’s intended as an effects equalizer, like a Pultec or Neve, and it can give a great deal of tonal control, especially on the low end.

This is also not a first-time project. I’ve tried to make it as easy as possible, and because of the way it’s laid out the electronics part consists just of stuffing a PC board. That makes it not too difficult to do, but it’s still got some delicate soldering work and there are still a lot of parts involved. It’s not that scary a job, but it’s a good couple afternoons’ work, and you’ll need to design your own enclosure for it.

How it works

In spite of what it says on the PC board, this isn’t really a passive equalizer (although the board can be used to build a passive equalizer). Instead, it’s a conventional network equalizer, just one which uses old-style inductors.

The power supply is straightforward: the power is stepped down from the transformer to two 22 VRMS outputs that are individually rectified. Since a sine wave with 22 VRMS has a 30.8 V peak voltage, the output of the rectifier is a set of pulses going up to 30.8 V from one rectifier and down to –30.8 V on the other. These go into C20 and C21 to smooth them down into only slightly rippled 30.8 V outputs. Regulator chips IC2 and IC3 contain active electronics that regulate them down to an accurate +24 V and –24 V output, then the C22 and C23 capacitors remove any additional noise that is left after regulation, to give a very clean ±24V supply for the op amp.

The basic circuit revolves around the op amp IC. To understand how it works, forget about the filter networks on the left hand side of the schematic and just think of the amplifier as a unity gain stage. The signal comes in the input, and it’s attenuated by the resistor network made by R19 and R16, which halves the voltage going into IC1. The signal is amplified and comes out the output, but then it’s returned into the input by R18 and R15, which is also a voltage divider. The negative feedback input always sees half of the output voltage, so the op amp is operating at a gain of 2 at all times if the filter networks are removed.

So, the input is attenuated, and then amplified, and comes out at the same level. If we were to run a resistor between the negative input of the op amp and ground, it would reduce the amount of feedback and the gain of the amplifier would increase. If we were to run a resistor between the positive input of the op amp and ground, it would reduce the level of input and the gain of the system would decrease. So, if we put a potentiometer between the two inputs and tie the center tap to ground, we can adjust the gain of the system very easily by turning the potentiometer.

This is actually a pretty ingenious feedback plan, and I believe it was originally figured out by George Philbrick in the 1940s.

So what makes this circuit an equalizer?

The trick to turning this feedback system into an equalizer is that we can stick something that is frequency-selective in between the tap of that potentiometer and ground. We can put in a network that has a higher impedance (let’s think of impedance as a sort of resistance to AC) at higher frequencies than at lower ones, and get a low-frequency shelving filter. We can put in a network that has a higher impedance at lower frequencies than higher ones and get a high-frequency shelving filter. And then we can take both of those networks and put them in series to make a midrange peaking filter.

You can see that the low-frequency filter is basically just a bunch of inductors in series, with a control to allow you to select the value that you want (and a separate set of inductors of different design, with greater nonlinearity, that can also be switched in). And you can see that the high frequency filter is basically just a bunch of capacitors with values selectable. The peaking filter is a bunch of networks made with capacitors and inductors in series; they form a high shelf and low shelf which crossover at the same point and form a little peak.

The slopes of all these filters depend entirely on the quality of the elements used (and the letter Q for filter slope originally came from it being used to indicate the Quality of an inductor), and the slope is going to change a bit, also depending on the boost and cut used. I selected the capacitors and inductors here for fairly low slopes but for more or less symmetric ones (so the high-pass and low-pass networks are fairly close to one another in rolloff).

The circuit itself is actually pretty simple once you think of it as an op amp and a series of discrete networks.

Let’s build it

First you need to get the parts (you can download a PDF of the parts list below!). Three orders are required, one from Magnetic Circuit Elements for the inductors which are the heart of the design, one from Digi-Key for all the miscellaneous parts, and one from Kludge Audio (namely me) for the PC board. I’m only accepting checks and money orders for the boards because I’m basically running a recording studio and doing this on the side. (And no, I’m not making any real profit off of boards either, but I will drop the price in the unlikely event that hundreds of people want to buy these things and I can get a price break on a large quantity.)

This project is fairly simple to construct because everything is on the PC board, and everything on the PC board is labeled. The easiest way to built it is to begin with the resistors and solder them in, then do the power supply rectifiers, the capacitors, the input and output jacks (be sure to bend the tabs down before soldering the jacks in, to make them a bit more sturdy), the regulators, the IC socket, and then all the inductors.

Each one of the inductors is marked on the boards as to value. Only after doing everything else should you put the switches and pots in; they block up a lot of the board and make it harder to get into the resistors and capacitors of the filter banks. The same goes for the power transformer.

When you put parts in, remember that the square pads indicate pin 1 for everything. In the case of electrolytic capacitors and the bridge rectifiers, it indicates the positive pin. In the case of the IC, it indicates pin 1 (on the side of the IC with the notch cut in the case). In the case of the switches and pots it indicates pin 1 too (not that you could easily put these in backwards anyway). In the case of C9 it doesn’t indicate anything vital—C9 can go in either way.

Handle with care

Be very, very careful to insert the electrolytic capacitors in the correct direction. IC2 and IC3 are indicated on the layout diagram for the correct direction, but the PC board isn’t very clear about it. When in doubt, take a look at the photographs.

When the switches are soldered into place, you will need to insert the pins that come with the switches into the correct stops so that the switches on ly move to the correct number of positions. Turn the switches all the way to the right, insert the pins, and screw them down. When the system is actually in place, you should have one stop more than the actual networks, so that the lefthand position totally disables the network, then the next steps switch different values of reactances into it. Check the third photograph on page 69 to see an example of the switch.

Be sure that the power transformer and the regulators go in correctly. Check them against the bottom photograph to make sure they look right.

There are power leads to the board; use the center tap and either one of the two side terminals if you are in a country like the US with 120 V power. If you have 220 V power, use the distant side terminals.

Apply power to the board, then use a meter to make sure that the correct pins on the IC socket are getting power. Measure between ground and each of the power pins and make sure ±24 V are there.

Insert the OPA604 into the socket, plug in an unbalanced input and an unbalanced output, and listen to it. I think you’ll like the way it sounds, especially on things like drum submixes.

You are on your own with supplying a cabinet for it, I am sorry to say. This board was actually designed to fit into a standard BUD box which was suddenly discontinued as this article was being prepared for press.

Custom shop

As mentioned above, I run the op amp rails at ±24 V, which limits the number of op amps that you can use. Still, I find the OPA604 to be an inexpensive and excellent sounding op amp which works well. If you want to try other op amps, though, you will probably want to reduce the rail voltages.

You can replace the 7824 and 7924 regulator chips with 7818 and 7918 chips respectively, to bring the rails down to 18 V. This will allow you to use just about all of the common single op amp chips. I can’t vouch that any of them will be stable in this circuit other than the OPA604 and the MC34081, but it won’t hurt to try them. I do know that the OPA604 has been backordered a lot recently, too.

If you want a stereo unit, you can get two PC boards, and cut one of them at the indicated line between the power supply and the electronics section. This can be done with a sheet metal brake or with a fine saw like the X-acto saws. After doing this, you can tie the ground, +30, and –30 V lines together on the two boards, so that the board with the power supply section provides power for the one without it. (You should, in fact be able to tie four boards together without any problems.)

When you do this, the only thing that is shared is the unregulated section of the board, and each individual channel is separately regulated. This reduces possible interactions between channels, and while it’s probably overkill, it doesn’t add much to the cost of the device.

Optional bands: The treble section can be set up to have an additional band at 1.5 KHz by adding a 0.1 uF polypropylene capacitor to the space marked “optional” on the PC board, on the bank of capacitors behind the HF switch.

An additional band can be added to the bass section as well. Stuffing the “Option” space in the inductor bank on the lower eq network with an additional choke can give you an additional low-end band. For a 12 Hz band, try a 2400 mH choke (OC18CL41 from MCE), and place a jumper in the “OPT” resistor position that it connects to.

In both cases, if you add the optional band, be sure to adjust the stop on the rotary switches so that you still have an “off” position for each section.


You’ll still need a cabinet with a nice front panel and lettering for this, unless you want to built it into a console channel strip. The board is designed so the power supply section can be cut away completely if you want to power multiple channels off of a single power supply; as I said above, one power supply should be able to run four boards.

The output on this can be very hot when boosting signals, and you can boost a whole lot—enough to actually clip a lot of unbalanced inputs. So be careful with aggressive use because this thing has more headroom than a lot of the gear you might be plugging it into. It’s got a variety of very useful sounds and it’s become my favorite box for drum equalization. Have fun!

You can reach Scott Dorsey at


Parts List

Quantity / Desc / Schematic Nos. / Description / Supplier / Price


4 OC18CL37 H1, H2, H3, H4 380 mH Audio Chokes MCE $6.61

2 OC18CL39 H5, H6 950 mH Audio Chokes MCE $6.61

2 OC18CL33 H13, H14 60 mH Audio Chokes MCE $6.61

2 OC18CL31 H11, H12 24 mH Audio Chokes MCE $6.61

4 OC25BL37 H7, H8, H9, H10 440 mH Chokes (optional) MCE $7.06

(total: $100.95)


3 CT2123-ND SW1, SW2, SW3 12 position C&K rotary switch DK $8.28

1 OPA604AP-ND IC1 Burr-Brown OPA604 op-amp DK $1.31

1 ED3308-ND Socket for IC1 DK $0.42

1 P3104-ND C1 0.1 uF (12.5 mm) DK $0.86

1 P3224-ND C2 0.22 uF (15 mm) DK $1.28

1 P3394-ND C3 0.39 uF (17.5 mm) DK $1.89

1 PF2824-ND C4 0.82 uF (20 mm) DK $2.89

1 P3473-ND C5 0.047 uF (10 mm) DK $0.64

1 P3333-ND C6 0.033 uF (7.5 mm) DK $0.59

1 P3223-ND C7 0.022 uF (7.5 mm) DK $0.59

1 P3103-ND C8 0.01 uF (7.5 mm) DK $0.53

1 EF1106-ND C9 10 uF mylar film cap (22.5 mm) DK $4.58

3 91A1A-B24-B13-ND R20,R21,R22 5Kohm potentiometer DK $4.74

(total: $54.64)


1 1.10KXBK-ND R1 1.1K resistor DK

2 1.00KXBK-ND R2,R3 1.0K resistor DK

1 750XBK-ND R4 750 ohm resistor DK

1 953XBK-ND R5 953 ohm resistor DK

1 562XBK-ND R6 562 ohm resistor DK

1 866XBK-ND R7 822 ohm resistor DK

1 357XBK-ND R8 357 ohm resistor DK

1 681XBK-ND R9 681 ohm resistor DK

1 475XBK-ND R10 475 ohm resistor DK

1 261XBK-ND R11 261 ohm resistor DK

1 255XBK-ND R12 255 ohm resistor DK

1 249XBK-ND R13 249 ohm resistor DK

1 237XBK-ND R14 237 ohm resistor DK

4 10.0KXBK-ND R15,R16,R18,R19 10K resistor DK

1 1.50KXBK-ND R17 1.5K resistor DK

(Note: you will have to order these resistors in packages of five—

A package of five is only $0.53, though.)

(total: $7.95)


Power Supply:

1 TE70005-ND TR1 22V dual secondary 1.6VA xformer DK $9.34

2 P10310-ND C20, C21 35V 2200uF caps DK $2.65

2 P10294-ND C22, C23 35V 100uF caps DK $2.65

2 W005GGI BR1, BR2 50V GI bridge rectifiers DK $0.42

1 NJM7824FA-ND IC2 +24V TO-220 regulator DK $0.63

1 NJM7924FA-ND IC3 –24V TO-220 regulator DK $0.96

2 SC1121-ND IN, OUT 1/4" Phone Jacks DK $1.68

(total: $25.73)


PC Board:

1 Equalizer PC board KA $60.00

Total cost: $249.27 for all parts.


Digi-Key • 701 Brooks Ave. S. • Thief River Falls, MN. 56701 • 1-800-344-4539

Magnetic Circuit Elements • 1540 Moffett Street, • Salinas, CA. 93905

Attn: Stacey Callahan • (831) 757-8752

Kludge Audio • PO Box 1229 • Williamsburg, VA. 23187-1229

(Only accepting checks and money orders for the PC board, at $60 postpaid.

Expect four to six weeks for delivery, please.)


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