Monday, 30 March 2020

Synthesizer Build part-25: DUAL BUFFERED MULTIPLE with LEDs.

A very practical module that replicates any voltage on the input and splits it 4 ways. Also known as a CV/Gate Expander but it's the same thing =) After seeing Sam Battle's version with the bi-coloured LED's I've added a second version of my own with the bi-coloured LEDs because I think it will be in demand. =) The version with LED's is half way down this article.

I have finished the case for the second stage of my DIY synthesizer so now I can continue building modules and writing about them here. So the first thing I wanted in the new case was a Buffered Multiple. I have built a number of filters that need a 1V/Octave signal but I only had one output to provide it so I designed this little circuit. You can connect anything you want to the inputs: Audio signals, LFO signals, 1V/Oct. CV, Gate signals, you name it. Any voltage presented at the input is replicated at each of the four outputs and if you need more you can connect one of the outputs of the first stage to input 2 and so get a total of 7 outputs for one input.

This module will work on both +/-15V and +/-12V.  Running it on 15V just means it can handle a bit higher voltages but as the average synth only uses control voltages with a maximum of +/-10V it really won't matter.

Here is the schematic drawing showing one stage. As you can see it's just 4 non-inverting buffers with all the inputs connected together. The module is just two of these on one piece of stripboard. I've put 100 Ohm resistors on the outputs for some extra protection. Because there's (practically) no current flowing these resistors won't influence the output voltage. I've tried 1K resistors but they influence the voltage and bring the notes down by a few Cents. I did a lot of testing on this and 100 Ohm is really the maximum. Of course you can use 1K and retune your VCO's but I want to keep it as clean as possible. If you don't feel comfortable with resistors on the outputs you can leave them out all together. In my own version I left them out too and replaced them with wirebridges. The IC's have built-in short-circuit protection anyway.

I've had some kind feedback in the comments of the Buffered Multiple not working right with Gate signals in combination with the Behringer TD-3. The Gate pulse would stay high. The solution is very simple, just put a 1MOhm resistor between the input and the Ground.  You can instal that as standard just in case.

Here's the verified stripboard layout. Just two IC's and some wire bridges and resistors. I recently added de-coupling capacitors for the chips because it was kindly remarked upon in the comments below that they were missing and it is good practise to include them although, if you use a normal linear powersupply, there's really no need for them. But many people use switch-mode power supplies and in that case it's good to have them included. Also if there is quite some distance between your powersupply and the module, so the wires can act as antennea and pick up noise. In that case it's good practise to use decoupling capacitors too.

(Last revised 14-April-2020: Added de-coupling capacitors for the ICs on the power rails. 18-May-2020 Added 1K resistors to the outputs. 24-June-2020 replaced some wirebridges with direct connections underneath the IC's, 28-June-2020 replaced 1K resistors for 100 Ohm ones.)

Print only. The 100 Ohm resistors can be replaced with wirebridges. In fact I would use wirebridges instead of resistors to begin with. I chose 100 Ohm because that's the highest value that still doesn't cause a drop in voltage but you can easily do without them and use wirebridges instead:

Bill of Materials:


Here's the layout for the version with the bi-coloured LEDs:
Wiring Diagram:

(Last revised: 24-June-2020: Replaced some wirebridges for direct connections underneath the IC's., 28-June-2020: Replaced 1K resistors for 100 Ohm ones. 13-Jan.-2021: Upped the gain of the LED stage to 6 times so LEDs are always on.)

Print only:

BOM for the LED version:

Make sure you choose the bi-coloured LED's with two legs and not the ones with 3 legs. They won't work in this set-up. They must be two-legged. You may need to alter the value of the 18K resistors according to the brightness of the LED's you're using but I find 18K to be a nice middle value. Not too bright not too dim either. I've altered my own Buffered Multiple to include the LED function and These 18K resistors work just fine with the Red/Blue LEDs I'm using. 
If you take a look at the demo video in the article about the "Really Good AS3340 VCO" you can see this Dual Buffered Multiple at work to the left of the VCO I'm demonstrating.
Below note E1 the Red light, which is positive voltage, doesn't light up but above E1 you can see it light up. The higher the note, the brighter the LED gets and it can handle voltages as high as 15V without problems. Above the lowest 2 octaves the LED shines about as bright as it will ever get. For the blue part of the light the threshold for the LED is higher, about 3,5 Volt. So it will need a tiny bit higher initial voltage to turn on, but that's not really a problem. But for accuracy it's better to use Red/Green LED's because of the lower threshold voltage of the green LED vs the blue one.
I've added 6 times gain to the LED opamp stage so that the LED will start shining at the lowest of voltages. The 510K resistor determins the Gain. The formula for calculating the gain is 1+(510K/100K)=6.1
Of course you could also use normal LEDs. That will just mean that, in the negative cycle, the LED won't light up but other than that it'll work just fine. Or you can use two LEDs; one for the positive cycle and one for the negative cycle. It's up to you.

Here's the schematic for the LED version. I've connected it to output 4 of each of the two sides but you can choose any output that's convenient. Because the LED is buffered it takes no voltage away from the output. This schematic drawing shows only one side because the second side is identical to this one.

This module is very useful to have in your setup because sooner or later you're going to need at least one of these, just like you need at least one mixer/passive attenuator.

Here are a few pictures of the finished product. I made a little L-Bracket (also visible on the layout) so I could mount the print at 90° to the panel.

This is the version with the LED's installed. The right LED is displaying the voltage of the Gate signal which is +5V. The left LED is displaying the 1V/Oct signal which is a bit lower.

The print up close. As you can see mine has more wire bridges because I forgot that some connections can be made directly under the IC. So I did it the hard way. The first picture below is the original module without LEDs. The second one is after I converted it and added the LEDs.

Because I added the LEDs later I had to use some jumpwires on the print to connect it all together.

Okay, that's an other one done. Number 25! A bit of a milestone for my synth build :)
Got any questions or comments just leave them in the comments below please. See you on the next one!


  1. what about the two capacitors next to each opamp? value? purpose? thx.

    1. What capacitors? There are no caps in this design.

    2. they are not in the stripboard schematic but I can see them on the fotos! connecting plus and minus 15 to ground?

    3. Yes, you mean the decoupling caps. You can put them in. They are usually 100nF between plus and ground and between ground and minus. But if you use a normal linear power supply like I do, there's really no need for them. But it can't hurt to put them in you're right. You know what, I'll adapt the layout and put in those caps so you know where they should go. Thanks for mentioning it!

  2. Hi Eddy, I recently discovered a problem with the buffered multiple in combination with the Gate Out on the TD-3. The output of the TD-3 gate would stay high even after released notes. Adding a 1M resistance between the input and ground seemed to solve the problem. Kind regards, Johan