NINE CHANNEL STEREO AUDIO EQUALIZER

Thank you for joining us today at Fresh's Workshop. If this is your first visit, welcome! If you're returning, we appreciate your continued support.

This is the spot where we build and repair all kinds of cool stuff, along with making a ton of saw dust. (Man Glitter.)

During our last session in the Workshop, we worked on a cart for my Hi-Fi turntable.

To see that episode, just clicky clicky here:

 

Before we begin today's project, let's take a moment and talk about shop safety:

Be sure to read, understand and follow all of the safety rules that come with your power tools. Knowing how to use your power tools properly will greatly reduce the risk of personal injury. And remember this, there is no greater safety rule than to wear safety glasses.

Hope you all had a very nice Christmas and New Years. It's been a bit chilly here in Central Florida, so the shop time has been limited.

We were going to work on a Lazy Susan for the kitchen table, but the cooler weather (and other things) kept me out of the workshop.

So, we went another direction. We are exploring vintage electronics reminiscent of the 1970s. Over the past few years, I have been designing and constructing a 6-channel audio Mixer for the Radio Room. You can follow the progress of that project on my other blog, "Building an Audio Console." 

Occasionally, we will provide updates on our progress here at workshop.

Today, we commence work on an accessory for our 6-channel Audio Mixer. This will be a multipart effort. First, we will design and build a prototype unit. Later we will order professionally looking circuit boards to aid in the construction of our Equalizer, and finally, we will get back to the shop and build a nice-looking cabinet. In between, we will get out to the shop for more woodworking, and the garden for some outside time.

Designed for a stereo system, our Equalizer has nine 1-octave adjustments in each channel. Using integrated circuit (op amp) active filters, the equalizer has an internal power supply. Boost and cut limits are ±12 dB; voltage handling limit is 2 V RMS; and the total harmonic distortion is a low 0.05 percent. The frequency response of the equalizer is from 20 to 20,000 Hz (±3 dB), hum and noise is 65 dB below 1 volt RMS, input impedance is 100,000 ohms and output impedance is less than 10, ohms.

 Connection to an operating audio system can be made between the preamp-out/power-amp input jacks or between the tape-out/tape monitor input jacks. 

The nine gain adjustments are centered at 50, 100, 200, 400, 800, 1600, 3200, 6400, and 12,800 Hz. Although the lowest and highest frequency filters are bandpass types, their use in a feedback loop gives them a low-pass/high-pass response. The enclosure of the entire array of active bandpass filters in a feedback loop also provides low noise and distortion.

The arrangement of the potentiometer knobs for both channels on the front of the equalizer provides an accurate graphic representation of the tonal compensation. The equalizer can also integrate into an electronic musical instrument system by connecting between the preamp and the power amplifier.

As many of you are aware, we are constructing this project utilizing older technology, specifically from the era when Op Amps and digital IC dominated the electronics world.

 Our project is based on an article from the May 4, 1974, issue of Popular Electronics.

Popular Electronics was an American magazine published by John August Media, LLC, and hosted at TechnicaCuriosa.com. The magazine was initiated by Ziff Davis Publishing Company in October 1954 for electronics hobbyists and experimenters. It soon became the "World's Largest-Selling Electronics Magazine." 

Back in the 1950s and 1960s, every nerd like me always had a copy of the latest issue. We would spend hours poring over all the schematics and projects. And in some cases, we even made some of those projects.

Let us return to 1974, when I constructed this particular project. 

It was a valuable addition to my home stereo system and provided excellent service for many years.

However, one day it stopped functioning. I had upgraded to a more compact and professional unit, and this project was sent to the scrap heap.

In retrospect, I regret having done that. 

So, let's revisit this project and make it a contributing part of our quest to construct our 6 Channel Audio Mixer.

If you would like to revisit those thrilling days of Popular Electronics, you can download ALL the issues here.  Make sure you have a couple of Adult Beverages and plenty of time. This web site is MASSIVE.

Let's begin with a look at our schematic.

First the Original.

1974 POPLAR ELECTRONICS

Since our mixer already has a power supply, we will not be constructing the P.E. version. If I were building a new power supply, I would utilize voltage regulators.

Additionally, I will not be placing the two sections side by side. I am planning for this unit to fit into a 19-inch rack space. Since the circuit board for just one channel is 13 inches long, two of them side by side will not fit my allotted space. (Spoiler Alert: I will change my mind several times) We will address those issues when we build the cabinet. For now, we shall proceed with the electronic part of our project.

FIG 1  

How It Works. (Courtesy of Popular Electronics) 

The schematics for one channel and the power supply are shown in Fig. 1. The input to the channel is coupled through capacitor C1 to voltage divider R1-R2. One of the two op amps in IC1 buffers the input from the voltage divider and provides a low-impedance source for the nine active filters. Each of the latter is composed of an operational amplifier (IC1 and both of IC2) with the related resistors and capacitors. The outputs of the bandpass networks are then summed in one half of IC6, whose output is fed back through R-R11. Slide potentiometers R12 through R20 vary the overall gain of the feedback loop at the operating frequency of each filter.

Since the filter circuit has unity gain 0-dB equalization settings, it is necessary to follow the summer with an amplifier made up of the second half of IC6. The amplifier also provides the signal inversion necessary to keep the input and output signals in phase.

An EQUALIZER IN-OUT (SI) is provided so that the unit can be bypassed if desired.

Simpler explanation. Remember the old record players that had a volume AND a TONE control? Think of this as a record player with NINE tone controls. Get it?

And a simple explanation for those who went to UCF:

Audio goes in and audio comes out. Don't tough anything.

I have shown the basic schematic with two of the adjustment sections shown. Now we can add the other 7 sections.

The first 5 sections.

And the rest.

Now we can design a circuit board. P.E. used two different boards (L & R) Since JLC PCB has five-board minimum order, I am redesigning the board so that the same board design can be used for both channels. All we had to do was move the bypass switch off of the board. We do not need a power on/off switch. The final board measurement is 13 x 5 inches.

One of the changes I made to the design was to add a .1uf capacitor on each power input on each chip. (+ & - V) Most ICs need to be decoupled from their power supply, usually with a 0.1 uf capacitor between each power pin and ground. Decoupling is usually used to remove noise and smooth power fluctuations. That is the reason you place these capacitors as close to the chip as possible. You can see that in the schematic below.

The other major change in the design is use of the NE5532 Op Amp instead of the 5558. The 5558 used in the original was MIL-SPEC. I believe it was equivalent to 4558. However, the 4558 is hard to find. I tried a 14558 (with the same pinouts and voltage), but it failed on power up. (all the internal smoke came out) Therefore, I revised the design to use the NE5532 Op Amp.

And finally, I thought the 4558 (NE5532) op amps would press up against the front panel. In order to avoid an overheating situation, I moved all the chips and the larger components to the reverse side of the board.

I later found some RC4558's on AMAZON and decided to give them a try as well. More on that experiment later.

Here is the board (back side)

And the front.

Now let's put together a new parts list.

CAPACITOR ELECTROLYTIC         C-01        1 UF. 50 V

CAPACITOR ELECTROLYTIC         C-21        1 UF. 50 V

CAPACITOR FILM                              C-02       .22 UF

CAPACITOR FILM                              C-11         .22 UF

CAPACITOR FILM                              C-03        .12 UF

CAPACITOR FILM                              C-12         .12 UF

CAPACITOR FILM                              C-04         .056 UF

CAPACITOR FILM                              C-14         .056 UF

CAPACITOR FILM                              C-05        27000 PF

CAPACITOR FILM                              C-14         27000 PF

CAPACITOR FILM                              C-06        15000 PF

CAPACITOR FILM                              C-15         15000 PF

CAPACITOR FILM                              C-07        8200 PF

CAPACITOR FILM                              C-16         8200 PF

CAPACITOR FILM                              C-08        3900 PF

CAPACITOR FILM                              C-17         3900 PF

CAPACITOR FILM                              C-09        1800 PF

CAPACITOR FILM                              C-18         1800 PF

CAPACITOR FILM                              C-10         1000 PF

CAPACITOR FILM                              C-19         1000 PF

CAPACITOR FILM                              C-20        4700 PF

CAPACITOR ELECTROLYTIC         C-22         10 UF 60 V

CAPACITOR ELECTROLYTIC         C-35         450UF 50V

CAPACITOR ELECTROLYTIC          C-36        450 UF 50 V

CAPACITOR                                       C-23        .1 UF DISC

CAPACITOR                                       C-24        .1 UF DISC  

CAPACITOR                                       C-25        .1 UF DISC

CAPACITOR                                       C-26        .1 UF DISC

CAPACITOR                                       C-27        .1 UF DISC

CAPACITOR                                       C-28        .1 UF DISC

CAPACITOR                                       C-29        .1 UF DISC

CAPACITOR                                       C-30        .1 UF DISC

CAPACITOR                                       C-31        .1 UF DISC

CAPACITOR                                       C-32        .1 UF DISC

CAPACITOR                                       C-33        .1 UF DISC

CAPACITOR                                       C-34        .1 UF DISC

I.C. OP AMP                                         IC-1           4558

I.C. OP AMP                                         IC-2          4558

I.C. OP AMP                                         IC-3          4558

I.C. OP AMP                                         IC-4          4558

I.C. OP AMP                                         IC-5          4558

I.C. OP AMP                                         IC-6          4558

CONNECTOR                                    J-01        MOLEX 2 PIN

CONNECTOR                                    J-02        MOLEX 2 PIN

CONNECTOR                                    J-03        MOLEX 3 PIN   

CONNECTOR                                    J-04        MOLEX 2 PIN

CONNECTOR                                    J-05        MOLEX 2 PIN

CONNECTOR                                    J-06        MOLEX 3 PIN

RESISTOR ¼ WATT 1 %                 R-01          100 K

RESISTOR ¼ WATT 1 %                 R-78          100K

RESISTOR ¼ WATT 1 %                 R-02           10K

RESISTOR ¼ WATT 1 %                 R-76           10K

RESISTOR ¼ WATT 1 %                 R-77           10K

RESISTOR ¼ WATT 1 %                 R-03          470

RESISTOR ¼ WATT 1 %                 R-04          470

RESISTOR ¼ WATT 1 %                 R-05          470

RESISTOR ¼ WATT 1 %                 R-06          470

RESISTOR ¼ WATT 1 %                 R-07          470

RESISTOR ¼ WATT 1 %                 R-08         470

RESISTOR ¼ WATT 1 %                 R-09         470

RESISTOR ¼ WATT 1 %                 R-10          470

RESISTOR ¼ WATT 1 %                 R-11          470

RESISTOR ¼ WATT 1 %                 R-21         470

RESISTOR ¼ WATT 1 %                 R-22         470

RESISTOR ¼ WATT 1 %                 R-23         470

RESISTOR ¼ WATT 1 %                 R-24         470

RESISTOR ¼ WATT 1 %                 R-25         470

RESISTOR ¼ WATT 1 %                 R-26         470

RESISTOR ¼ WATT 1 %                 R-27         470

RESISTOR ¼ WATT 1 %                 R-28         470

RESISTOR ¼ WATT 1 %                 R-29         470

POTENTIOMETER SLIDE POT    R-12          10 K

POTENTIOMETER SLIDE POT    R-13          10 K

POTENTIOMETER SLIDE POT    R-14          10 K

POTENTIOMETER SLIDE POT    R-15          10 K

POTENTIOMETER SLIDE POT    R-16          10 K

POTENTIOMETER SLIDE POT    R-17          10 K

POTENTIOMETER SLIDE POT    R-18          10 K

POTENTIOMETER SLIDE POT    R-19          10 K

POTENTIOMETER SLIDE POT    R-20         10 K 

    I found the slide pots on AMAZON

RESISTOR ¼ WATT 1 %                R-30          3900

RESISTOR ¼ WATT 1 %                R-31           3900

RESISTOR ¼ WATT 1 %                R-32           3900

RESISTOR ¼ WATT 1 %                R-33           3900

RESISTOR ¼ WATT 1 %                R-34           3900

RESISTOR ¼ WATT 1 %                R-35           3900

RESISTOR ¼ WATT 1 %                R-36           3900

RESISTOR ¼ WATT 1 %                R-37           3900

RESISTOR ¼ WATT 1 %                R-38           3900

RESISTOR ¼ WATT 1 %                R-39           47 K

RESISTOR ¼ WATT 1 %                R-40           47 K

RESISTOR ¼ WATT 1 %                R-41            47 K

RESISTOR ¼ WATT 1 %                R-42           47 K

RESISTOR ¼ WATT 1 %                R-43           47 K

RESISTOR ¼ WATT 1 %                R-44           47 K

RESISTOR ¼ WATT 1 %                R-45           47 K

RESISTOR ¼ WATT 1 %                R-46           47 K

RESISTOR ¼ WATT 1 %                R-47           47 K

RESISTOR ¼ WATT 1 %                R-48           6800

RESISTOR ¼ WATT 1 %                R-49           6800

RESISTOR ¼ WATT 1 %                R-50           6800

RESISTOR ¼ WATT 1 %                R-51            6800

RESISTOR ¼ WATT 1 %                R-52           6800

RESISTOR ¼ WATT 1 %                R-53           6800

RESISTOR ¼ WATT 1 %                R-54           6800

RESISTOR ¼ WATT 1 %                R-55           6800

RESISTOR ¼ WATT 1 %                R-56           6800

RESISTOR ¼ WATT 1 %                R-57             680

RESISTOR ¼ WATT 1 %                R-58             680

RESISTOR ¼ WATT 1 %                R-59             680

RESISTOR ¼ WATT 1 %                R-60             680

RESISTOR ¼ WATT 1 %                R-61             680

RESISTOR ¼ WATT 1 %                R-62             680

RESISTOR ¼ WATT 1 %                R-63             680

RESISTOR ¼ WATT 1 %                R-64             680

RESISTOR ¼ WATT 1 %                R-65             680

RESISTOR ¼ WATT 1 %                R-66             22 K

RESISTOR ¼ WATT 1 %                R-67             22 K

RESISTOR ¼ WATT 1 %                R-68             22 K

RESISTOR ¼ WATT 1 %                R-69             22 K

RESISTOR ¼ WATT 1 %                R-70             22 K

RESISTOR ¼ WATT 1 %                R-71             22 K

RESISTOR ¼ WATT 1 %                R-72             22 K

RESISTOR ¼ WATT 1 %                R-73             22 K

RESISTOR ¼ WATT 1 %                R-74             22 K

RESISTOR ¼ WATT 1 %                R-75            100 OHM

RESISTOR ¼ WATT 1 %                R-76            10 K

RESISTOR ¼ WATT 1 %                R-77            10K

RESISTOR ¼ WATT 1 %                R-78            100 K 

RESISTOR 1/2-WATT 1 %              R-79            330 OHM

RESISTOR 1/2-WATT 1 %              R-80            330 OHM

SWITCH D.P.D.T.                             S1

            Found these on AMAZON         

CONNECTOR MOLEX                    J-03 3 PIN

CONNECTOR MOLEX                    J-04 3 PIN

CONNECTOR MOLEX                    J-05 3 PIN

We now have a design, a schematic, and a parts list. Let us commence with the prototype.

We will be using our standard Proto Board for this part of the project.

Here is the prototype. I do not enjoy creating prototypes due to the numerous connections that need to be made. However, I have found that in most cases building a prototype may discover errors that you can fix PRIOR to ordering the boards.

After experimenting with operational amplifiers, repairing many poor solder joints, my prototype came to life. It doesn't sound too bad either.

When building a prototype like this, I usually start by laying the ICs and the power input sections. I then run all those wires and test out the power. I always put power LEDs on my projects as a diagnostic feature.

And the more I consider a side-by-side layout the mixer, the more I prefer it. (Told you I would change my mind.)

We have a bit more "tweaking" on the boards before we can order them. Too save on shipping I usually order boards for other parts of the project at the same time. (I.E. Controller boards for my digital clock project - WATCH THE SPACE FOR THAT ONE) So it may be a bit before we can continue this project. But we do have a great start on it.

So, the design is done, and the prototype works. It is time for a break.

As soon as I get the boards, we will resume this project. Until then, we'll go out to the shop and make some man glitter. (If it warms up)

See you then!!!

Be sure to visit some of my other blogs:

AROUND THE KITCHEN  - SEE WHAT I COOK

BRIARWOOD - YOU CAN READ MY NOVELS

AUDIO CONSOLE - WHERE I AM BUILDING AN AUDIO MIXER

DISCLAIMER The information contained here is for ENTERTAINMENT purposes only. Working with woodworking tools can be dangerous. The user of these tools should have a basic knowledge of woodworking and be familiar with the proper use and safety precautions associated with these tools. Always wear appropriate personal protective equipment, such as safety glasses, ear protection, and a dust mask. Always use the tool in accordance with the manufacturer's instructions and guidelines. Always maintain a safe and organized work area.

I am counting the days!!! Happy Birthday to me!

God Speed, Mother Nature