Wednesday, 11 December 2019

Synthesizer Build part-11: ECHO and SOUND FX UNIT and LINE OUT.

100 different sound effects combined with a line-out and head-phones connection for the DIY synthesizer.

Here's a little item I found on eBay and thought it would work great in the synthesizer; and luckily I was right. It is a fantastic asset and makes the synth sound really professional and full. 
A word of warning though, this is not a beginners project. You need to know your electronics to follow my plans. This unit also needs two voltages to run on. The effects unit runs on +5V and the stripboard runs on +/-15V or +/-12V. So you must make sure you have those voltages available in your modular set-up.

The unit goes under different names but mostly as the Cara OK ("Karaoke" get it?) or DSP 5V Red Digital Stereo Mixer Reverberation Karaoke Reverberation Board Module (if you copy and paste that into the search bar on eBay, you'll find it). The prices vary from $15 to about $30 us.
I bought mine for $ 15,82 including shipping here.
It offers a 100 presets with reverb, echo and even chorus, phaser, flanger, phase shift and reversal effects and combinations of them together. There's a rotary encoder with which you choose the preset of your choise and then you just press to confirm and engage the effect. It's a favourite with many synth builders I noticed.
I made a special panel for it and combined a 'Line-Out' control and bypass switch option with the module. Here is the schematic or wiring diagram to make this unit part of the synthesizer:

I had to put extra attenuation on the bypassed (normal) signal because it was much louder than the output from the Echo Module, but that was easily fixed as you can see in the diagram above. The panel that I built into my synthesizer also includes a headphones out stereo jack which is not included in this article but it's just an output jack soldered straight to the negative poles of the Left and Right output Electrolytic Capacitors. Weirdly enough, if you listen through the headphones, the echo module is much louder than the normal line out. This is probably some impedance matching issue but it doesn't bother me. It's easy enough to turn a volume knob so I'm not bothered.
The Stripboard can work on +/-12V also. It doesn't matter if you use 15 or 12V. The CaraOK effects unit must have it's own +5V powersupply! You can use a 7805 voltage regulator and connect it to the 15 or 12V of the stripboard to get 5V for the effects unit. This is not further described in this article. I'm assuming you have the knowledge to make powersupplies. If not, check out my article on this subject here.

Here's the stripboard layout (new version):
(All potmeters viewed from the front)

Please note: The GROUND connection of the Effects Unit must be connected to the GND of the stripboard. Everything must share the same ground. I myself made a central grounding point with a solder eye connected to one of the M-3 bolts holding the Effects Unit to the panel. To put it in a simple way: the ground of the 5V powersupply for the Effects Unit must be connected to the ground of the +/-15 or 12V powersupply of the stripboard.

Print only:

(Last revised: 14-Aug-2020: Corrected mistake with negative voltage supply to the TL074.)

I used a 3 pole double throw toggle switch (ON-ON type) to be able to switch the synthesizer between normal output and output through the effects unit. This is the best solution to be able to switch the input straight through to the output buffer or into the effect unit and at the same time connect the outputs to the right signal source. This way there is no risk of the synth falling silent because one of the switches is set wrong (which can happen if you use more than one switch).

I've put in 4 buffer stages, using the TL074, for the input, normal output and FX Unit outputs Left and Right channels and I gave the latter two adjustable gain by means of two 50K potmeters in the feedback loops of the opamps; one for each channel. The gain is adjustable from 2 to 5.3 times. You can increase that by using 100K potmeters instead of 50K ones. That would give a maximum gain of 8.6 times.

When I first tested this unit I noticed I was receiving an FM broadcasting signal through the effects unit. (There's an FM Broadcast transmitter and antenna on a flat 100 meters from my location). So I took a ferrite ring and wound the audio input wire around the ferrite ring about ten times. I also put ferrite beads in the 5V power-supply line to the effects unit and to the print with the opamps on it. This solved the problem. One more little thing: beware of the little crystal X1 near one of the screw holes on the circuit board. It is rather flimsy and fragile. Take care not to damage it.

The Cara OK is a really versatile unit with lots of really cool sounding effects. Here's an overview of the possibilities it offers. I myself printed a small version of this list out, laminated it and stuck it at the bottom of the panel I made for it. Handy to have around I thought :) :

This picture shows all the connections to the circuitboard:

It's small so it won't take up too much space. Beware that it needs just +5V for power supply. Luckily in my synth build I made a power-supply that delivers dual 5, 12 and 15 Volts so I can feed it right from there. I can really recommend you picking this up and trying it in your build project. It will add a lot of options and is a very useful addition to the filters and its output is in stereo! The sound quality is just great so no problems there. The only thing is the difference in volume I mentioned earlier but that is easily fixed. You can use opamps buffers with it, like I did, but it's not absolutely necessary. I did without them at first but then installed output buffers with variable gain as I mentioned before.
The audio response of this module is so good that it even lets through the ultra low frequencies the Korg MS20 filter produces (see next article) and that can go as low as 10Hz. You can really see the speaker cones move bigtime!
Before I installed the 3-pole toggle switch I had a single pole and a double pole switch side by side to switch between FX-unit and normal line out. So after installing that 3-pole switch I had a hole in the panel left over. I mounted a 3,5mm stereo output jack in that hole as a connection for head-phones. The output jack is connected straight to the audio output on the stripboard. One thing I noticed with this arragement is that the normal line-out through the head-phones, sounds a lot quieter than when the effects unit is switched on. That's probably due to a difference in output impedance because we're effectively putting an 8 Ohm resistance between Line-Out and Ground in the form of the head-phone speakers. This doesn't occur when I listen to it on the normal audio amplifier, at least not if the head-phones are not plugged in. It would be a good idea to build a little head-phone amplifier for this purpose.
Here is a picture of the finished module in my synthesizer:

Here's a picture of what's behind the panel. Now you understand why this is not a beginners project ^_____^  You can see the yellow Ferrite ring with the black 'audio in' wire wound around it above the blue circuitboard and there's also one on the stripboard. I advise you to include these in the power supply line and audio in line. In red you can see the 3 pole toggle switch. This panel works really well like this. In the future I might add a Dry/Wet control which would be useful.

Here's the Line Out Panel I made on the back of my synth, with two gold plated RCA outputs and a 6,3mm (1/4") Stereo Output Jack, which is connected straight to the RCA left and right outputs.

Okay, that's the 11th module I put in the synthesizer. We're nearly there. I have just room enough left for one more module and that has got to be the Korg MS20 filter. But I'm waiting for some supplies from China before I can build it. (Circuit boards for one, coz I'm fresh out at the mo.)

Okay here's an excellent video by Juanito Moore that shows you how you can circuit-bend this device and make it voltage controllable. Click here

Right, that concludes this article. Thanks for stopping by and while you're here, why not leave me a comment or if you have any questions put those in the comments too and I'll get back to you asap.


  1. Hey Eddy! By chance do you have a picture that you can show of how the boards are connected. I'm assuming Lout and Rout are Line outs that are connected on row 22, from the main board. Where do the Fx outputs go or connect too? Thank You.

    1. I'm sorry I don't have a picture. A picture of my board wouldn't help you because it's a mess of wires. The FX outputs go into the main board at the left side. There they are buffered by the main board TL074 and then the line outs so into an audio amplifier and so out the speakers.

  2. Hallo Eddy,
    soll der 9 Pin Schalter für den Ausgang ein On/On oder On/Off/On Schalter sein ?
    Vilen Dank im voraus


  3. Hallo Eddy meine Teile/Schalter sind aus China eingetroffen,wollte nochmal fragen,wenn ich auf die Stripboard Zeichnung schaue,genau gesagt auf den 9 Pin Schalter,schaltet dieser von links nach rechts oder von oben nach unten,habe angst verkehrt zu verkabeln.

    1. Hallo Maik! The switch goes up and down. The middle pins connect to the upper pins if you turn the switch up and to the lower pins when you push the switch down. It will make sense if you just follow the patch that the electrons take on the circuit schematic. I hope this makes it clear for you.

    2. * Path not patch. My mistake :)

  4. Ich denke von oben nach unten,richtig?

  5. ...danke,hatte auch unüberlegt gefragt,habe den Pfad verfolgt,scheint alles richtig verkabelt zu sein von mir.Leider stimmt irgentwas nicht,ist das Stripboard Layout korrekt ?
    Wenn ich das FX nur mit 5 Volt betreibe ist das Display unauffällig,sobald ich aber die 12v +/- anschließe blickt es nach 20 Sekunden und es lässt sich nichts einstellen,ich werde wohl was verkehrt verbunden haben,konnte aber noch nichts finden,werde mal weiter nach dem Fehler suchen.

    1. That is strange Maik. You know that the stripboard needs +/-15V or +/-12V and only the effects unit needs +5V right? Do not connect the CaraOK effects unit to 12V. You might damage it.

    2. I noticed that the stripboard 'print only' image was a but unclear because the '1' of the '15V' was cut off so it looks like 5V instead of 15V. I have corrected that and made a new cut-out of the 'Print Only' image. I do hope that didn't confuse you. The wiring of this module is a bit complicated and certainly not easy for beginners. That's why I wrote the warning right at the beginning of the article.

    3. The stripboard layout has been used by many people successfully, and I used it of course for my own build, so I know that it's okay.

  6. Hallo Eddy,
    nein 5v Und 12V bringe ich nicht durcheinander,alles gut! Bei mir hatte das Display wild geflackert,weil ich dem Karaoke Modul 5,17 Volt gegegeben hatte statt 5 Volt das war wohl zuviel,jetzt versorge ich das Modul mit stabilen 5V und es ist alles ok!
    Ich habe auf Deinen Stripboard was Du gelötet hast (Foto) 6 Widerstände gesehen,kann das sein ? Auf der Zeichnung sind aber nur 5 Widerstände abgebildet !

  7. Felht bei mir ein Widerstand ?

    1. Maik you have great eyesight!! I don't know why the extra resistor is there, probably for the headphones output that I put in later and which is not on the drawing, but I'm not sure. Anyway, don't worry about that one.
      Funktioniert jetzt alles gut oder gibt es noch probleme irgendwo?


  8. Hallo Eddy,

    leider höre ich nur das normale Audio Signal ohne FX egal wie der Schalter
    geschaltet ist,ich habe alles mehrfach kontrolliert nach Kurzschlüssen,alle Verbindungen
    sind richtig,alle Widerstände und Kondensatoren auch voll ok,habe alles mit der schematc
    abgeglichen,alles ok ! Es ist wie verhext,bin ratlos,ich werde das FX Modul erstmal auf Eis legen,vielleicht
    werde ich es später einmal ganz neu löten

    1. Oh damn, I'm so sorry to hear that. I think you're right. Come back to it later with a fresh mind and re-wire the whole thing. (You tried the diferent preset numbers right? Because I believe setting 0 is a clean setting without effects.)

  9. ...ja habe verschiedene Effekte probiert,leider nichts! Werde wie Du es sagst später mit neuer Motivation und frischen Geist neu beginnen.