Introduction: Build an Analog Vocoder
Step 1: Design and Schematics
The first thing we need is a circuit design, which I am posting as a series of images. I designed it from scratch, although I did investigate as many vocoder schematics I could get my hands on. I focused on using easily obtainable components that are inexpensive, and I tried to keep the circuit as simple as possible without sacrificing functionality.
A vocoder basically has two inputs and one output. The first input is the program (usually a connected to a microphone) and the second input receives a carrier signal (usually a keyboard). The program signal is then fed to an analysis section, which extracts the spectral information from the sound and applies it to the carrier signal. This vocoder will analise the signal on 14 bands, but the design can easily be modified to include more channels, or actually fewer (if you are in a hurry).
First, we must have input amplifiers for each of the two inputs. Then, we must build each channel, and finally mix all the channels. These are the three main blocks of the circuit. All the 14 analysis channels are identical, except for the values of the capacitors (I purposely designed it this way to make it easier to build).
Each channel in the analysis section consists of two identical band pass filters. The first receives the program signal which then goes into an envelope follower. The output of the envelope follower then controlls an VCA (voltage controlled amplifier) which amplifies the carrier signal coming in through the other band pass filter.
The next step contains a parts list, however, I will tell you right away that ALL the op amps in the circuit are TL074. So all the components you will need are the TL074's, resistors, condensers (capacitors), and NPN transistors (2N2222). I am trying to keep it simple. Also, you will obviously need wires, connectors, pots, jacks, and some material to put the whole thing in (I used plywood, MDF, and acrylic for the casing.
This circuit also requires a bipolar power supply (+/- 10-15 volts). I pulled out the power supply from a damaged old computer, which is bipolar, I used the +12v, ground, and -12v from this power supply (they are standardly marked yellow, black, and blue respectively in a computer power supply). Using an existing power supply will save you a lot of time and money.
In this step you should download and study the schematics just to get acquainted with the project.
Step 2: Buy the Components
Besides the condensers listed in the previous step, I used the following components. You may want to buy a little more than the quantities I list here just in case you damage some while working.
17 x 4.7u
2 x 10u
14 x 150n
14 x 0.1u
16 x 50K
1 ganged 50K
Small calibration pots:
14 x 50K
14 x 1K
27 mono phone jacks (because each channel is patchable)
1 stereo phone jack
3 meters of 10 wire ribbon cable
30 male 10 wire ribbon cable connector
30 female 10 wire ribbon cable connector
A bunch of wire to make all the connections (I destroyed an Ethernet cable to get this copper cable, its great because it is color coded and all).
28 x 1.2K
28 x 2K
2 x 1K
123 x 10K
28 x 20K
56 x 33K
28 x 120K
28 x 100K
42 x 200K
28 x 220
42 NPN transistors (2n2222 or similar)
Diodes: 42 signal diodes
Step 3: Test Build
Do not try and solder the circuit before testing the individual parts on a breadboard!
Step 4: Solder It Up
Since I am self-taught I used standard circuit boards instead of printing the boards myself. These photos show the process of soldering up the filter bank.
Someone asked for images of the underside... so I have posted 3 more images to this step, the images are of one completed bank board (which actually has 2 filter banks on it). The first image is the top, the corresponding is the underside (flipped vertically, not horizontally), the third image is a copy of the underside with some visual cues.
Step 5: Make the Casing
Depending on your style and the way you decided to wire the circuit up, I recommend you make your own design. I routed channels into two plywood boards to hold the filters vertically and hold the side panels. The front and side panels were cut from 4mm acrylic, and the back is cut from an 4mm MDF board. Then bolted the top and bottom lids together.
Step 6: Enjoy
Now just get down and mess around with your new vocoder. I had never played around with one of these and I am just about scratching the surface... This vocoder (with patch cables) will allow you to invert the spectrum or transpose it, (even use external CV from analog gear like sequencers).
The design borrows ideas from several vocoder schematics on the net. Many of these vocoders used difficult to find parts and many more components. I used a filter calculator provided on the net by Okawa Electric Design (http://sim.okawa-denshi.jp/en/OPtazyuBakeisan.htm) to design the filter banks. Also, I borrowed the concept of only varying condensers on the filter from the Okita vocoder, this makes it very easy to make all the banks because they are identical except for 8 capacitors in each; making the voltage control patchable was borrowed from the Elekvoc. The voltage controlled amplifier is based on a circuit made available from Rene Schmitz.
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