It is yet another clone of the well known Covox Speech Thing. I wanted to get some practice in EDA and what is one of the simplest part to make, which you could have fun with? If you are in retro community, you would understand, why it was a Covox clone.
Well, it is still a simple Covox clone. However, I wanted to make something brute, what you can solder easily, using some through holes parts, but still with some space for experiments. So I made it with a quad switch, three of which can be used to control bypass capacitors, to play with low-pass filtering and one for switching between mono and (fake-)stereo sound.
I made a video on my Youtube chanel about this Covox clone.
I have a good news for someone, who is especially keen to solder. Instead of using two types of resistors with 1/2 relation, I just used all the same types, but doubbled one row to maintain the relation. This way you don't need to search for resistors with a proper 1/2 relation, but just can take any resistors, you have at hand. The parallel port connector is not a through hole part, but rather a normal connector, like used in any printer cable. It is quite easy to find and can be directly soldered to the edge of the PCB.
| Part | Count | Remarks |
|---|---|---|
| R1..R26 | 26x | 7.5kOhm resistor |
| C1 | 1x | 0.01uF tantalum capacitor |
| C2 | 1x | 0.1uF tantalum capacitor |
| C3 | 1x | 1.0uF tantalum capacitor |
| C4 | 1x | 1.0uF electrolytic capacitor |
| SW1 | 1x | Quad-Switch (like LCSC C559139) |
| AUDIO1 | 1x | 3,5mm Audio jack port (like LCSC C145816) |
| LPT | 1x | DB25 parallel port M-plug |
Yes, I actually made two variants. The long board has switches, variable low pass filtering and uses three rows of same resistors. If you are not interested in all of that, you have a set of two types of resistors with a proper 1/2 relation and space is more important for you, then take the short variant. You will need less parts and get a more compact solution.
| Part | Count | Remarks |
|---|---|---|
| R1..R8,R17 | 9x | 15kOhm resistor |
| R9..R16 | 8x | 7.5kOhm resistor |
| C1 | 1x | 0.1uF electrolytic capacitor |
| C2 | 1x | 1uF tantalum or electrolytic capacitor |
| AUDIO1 | 1x | 3,5mm Audio jack port (like LCSC C18166) |
| LPT | 1x | DB25 parallel port M-plug |
I used EasyEDA to create this PCBs. If you want to derive from this work, you can find all the files in the project subfolders of the boards. To order the parts I used LCSC.
It is reasonable to play around with another resistors and filter capacitors. The higher the resistors value, the less noise you will get through the parallel port. However, this also would mean less power on the audio output and necessity of an amplifier. If you want to use passive headphones, try to go with lower resistance. I used 1kOhm (and 2kOhm) resistors and the audio quality was still quite good. Also the low pass filter capacitors are variable. It depends very much on the speakers, what's the best, but I found, that it's not worth it to go over 1uF. Even then, filtering starts to be very aggressive. The higher resitor values you take, the lower capacitor values you should need.