CHAPTER 18 - - - NINE-CHANNEL STEREO EQUALIZER

Today, we commence work on an accessory for our 6-channel Audio Mixer. This will be a TWO-part episode. 1)Design and testing a prototype and having the circuit boards made and parts installation. 2) Testing and building a cabinet.

We are incorporating a NINE-Channel Stereo Audio Equalizer.

Designed for a stereo system, it has nine 1-octave adjustments in each channel. 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 our mixer project utilizing older technology, specifically from the era when Op Amps and digital IC's 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.

I have not yet decided on mounting scheme for the equalizer. Initially, I considered mounting it in a 19-inch rack. However, since the circuit board for a single channel is 12 inches long, placing two of them side by side will not fit in that space., I am contemplating-by-side arrangement above the VU meters. We will address these 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.

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) I have my boards made in China by JLC PRO. I can get 5 boards for about 25 bucks plus shipping. And I receive them in 5 days. 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 12 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.

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.

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

When I began working the prototype, I discovered that the potentiometers could not extend through the front panel due to the height of the other components. Consequently, I redesigned the board with the slider potentiometers on one side and the components on the other. I ensured that the spacing of potentiometers allowed for the usage of knobs This is the redesigned board

I know they look the same. It will make sense when you see the completed boards.

Now let's put together a new parts list. (FOR ONE CHANNEL)

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

I.C. OP AMP                                         IC-1           4558 - NE5532

I.C. OP AMP                                         IC-2          4558 - NE5532

I.C. OP AMP                                         IC-3          4558 - NE5532

I.C. OP AMP                                         IC-4          4558 - NE5532

I.C. OP AMP                                         IC-5          4558 - NE5532

I.C. OP AMP                                         IC-6          4558 - NE5532

CONNECTOR AUDIO                       J-01 RCA OR MOLEX

CONNECTOR AUDIO                       J-02 RCA OR MOLEX

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-74             22 K

RESISTOR ¼ WATT 1 %                R-74             22 K

SWITCH D.P.D.T.                             S1

            Found these on AMAZON   (YOU ONLY NEED ONE)      

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 commence with the prototype.

We will be utilizing our standard Proto Board for this segment of the project. My proto boards are insufficient in length, necessitating creative parts placement. Remember, this is NOT the final version but is intended to verify the design of both the circuit and the board.

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.

Now we can order some circuit boards from China. 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, 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)

I have finalized the board design for the Equalizer.

One item I neglected to mention previously was the bypass switch.

Below is the diagram of the switch.

And the final Board version.

Now, we wait for the boards. Once I place the order, I will include a Mixer Board that I am currently designing, so we can order them all at once.

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We are progressing.....As the boards are almost ready to ship

Now we wait some more. We did not have to wait very long, as the board showed up on our door step a few days later.

Here is the board direct from China.

We will begin by adding all the power components. Then, we can apply power and verify its functionality.

The two beige connectors, one goes to the power supply, the other will go to the other board. (right channel)

I insert most of the parts, securing them with a piece of tape. Then, I turn the board over and solder one lead. After removing the tape and adjusting the part, if necessary, I solder the other lead. This process takes some time.

See how the power connectors lean? I can straighten them up with just a touch of the soldering iron, while pushing down on the part.

Here, I have applied power to the board. The LED's indicates proper functionality.

Next, I move on to the terminals and jacks.

Then comes the resistors and the remaining capacitors.

We are making some progress. Just take your time.

I have positioned one of the potentiometers to ensure it fits perfectly. I will solder it in later.

Now that all the resistors are installed, we should proceed with adding the small capacitors. 

OK, let's move onto the potentiometers.

How about a look at the finished board?

Now we can add the LEDs, and power it up. NOTE: We have NOT added the chips yet.

Everything OK? LEDs are showing power? Check for the voltages at each chip location.

All is well?

Now we can add the chips. The 4558 chips were a tad 'noisy', so I went with the NE5532 chips.

Now power it up and see if the imbedded smoke stays INSIDE the chips. I had a 4558 get blown to bits. I thought we had a board disaster, but it turned out to be a defective chip.

We hook up the audio goes ina and comes outa.

Let's find an audio source and give it a try.

And it works!!!!

It's a very short video, as Blogspot has a minimum size for videos. 

We now have an audio equalizer. Well, one channel at least.

We need a break.

This is impressive. But we are not done yet, we need do the RIGHT channel.

I did the right channel a bit different. I started by placing ALL the resistors.

Then the bypass capacitors.

And the Audio Filter capacitors for each audio band.

Here I have taped down the 8 pin IC sockets, so I could turn the board over.

I only solder two pins. (each corner) then turn the board over. I can adjust any of the sockets if needed.

Now I can finish up the sockets.

Now I can move on to the connectors and the larger capacitors.

The last thing to go onto the boards are the potentiometers.

Before I plug in the IC chips, I want to power it up and make sure the voltages are where they need to be. The was no smoke, so I proceeded to hook it to the audio and see how it functions.

Here it is under test. Sounds great! I did get some new 4558 I.C.s, and they sound fine. So, use either the 4558 or the 5532.

Are we done....... Not quite... There is more thing to be done.

We need to design and construct a cabinet for this project.

And we can remove parts from the prototype that we can use later.

I will run this board (as I did the other) for 24 hours

I am constructing a 'meter bridge' that will sit on a shelf above the mixer. I believe that is an appropriate location for the equalizer. But more on that at a later time.

I think we will leave things as they are for now and make the cabinet as Part 2.

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