LM1036 VC Mixer & Signal Processor

Introduction

About the LM1036

Schematics

Vectorboard

Faceplate

Introduction

Thanks goes to Batz Goodfortune for motivating me to get into the LM1036 - his reply to my Request for Comment on the chip can be read here.

National Semiconductor's LM1036 was designed for use as a volume, pan, and tone control chip for consumer electronics. It's a nice little chip that seems to have gone largely unnoticed by the synth building community. The fact that all inputs and outputs use voltages rather than currents makes it very simple to use, and the fact that it attenuates normal level input signals rather than amplifying tiny ones helps keep noise to a minimum. The chip can be use to pan a single input to two outputs or to x-fade two inputs signals to a single output.

Below are a few variations on the LM1036 circuits that I've built showing different configurations for different purposes. These circuits expand on those in National's data sheet by providing additional summing and bus circuitry optimized for a +/-15v power supply, 0 to 5v CV synth environment.

The chip has two audio inputs, two audio outputs, voltage control inputs for volume, pan, treble, bass (V/P/T/B), a control input for "loudness" (a combination of treble and bass), and two audio outputs. It also has a variety of "support" connections, for a total of 20 pins. You can read all about it in the datasheet. The most important things to note are;

1. The chip is intended for use with a 0 to +15v power supply, it cannot handle +/-15v
2. The control voltages expect values from 0 to +5.4v. With 0v resulting in the minimum V/P/T/B, and 5.4v being the max. An cv of 2.7v (or 5.4v / 2) results in centered pan and flat treble and bass response.
3. The "loudness" is ON when the loudness input is grounded, and OFF when it is at +5.4v
4. While the chip is stereo, it is not two independent audio processors, there is only one CV input for each V/P/T/B that affects both signals.
5. The two audio signals are not mixed within the chip. The best way to think of the pan CV is to think of the volume CV as being run through two VCAs, one controlled by the pan CV, and the other controlled by the inverse of the pan CV, then these go on to control the VCAs for the two audio signals.

Schematics...

I've split up the circuit into two parts - the audio parts and the CV summing circuits. I've made two different types of CV summing circuits. The amount of control you want over the circuit and how you want to layout your faceplate will determine the type of CV summing circuit you want to use. There are a lot of different ways to use the chip, but I've followed one basic theme, that being the "Audio Processor" concept. The "audio processor" is a single chip module that is switchable between pan and x-fade modes, and allows very complete control over all of the CV inputs. Another option might be to use the chip in a multi-channel mixer module, where you might go for less options on the control voltages (to save panel space), and where you'd want to sum up the output busses - I've left this sort of module as an exercise for the reader.

The Core Circuit

In every case, you want to build the "core" section around the chip the same way. I haven't used the "loudness" input at all, and I've hardwired it to reference voltage to disable it, you are welcome to make a switch for it, or even another CV input. I've also used .33uF caps for the bass, rather the .39uF reccommended in the datasheet, only because they're easier to find.

Here's the core circuit that is more or less straight from the datasheet;

You'll notice that several pins are unconnected; the CV inputs, the audio inputs, and the audio outputs. I will go over each of these below....

The CV inputs

For complete control over the CV's it's nice to have an "inverting scale" on the CV input, This is an opamp and pot setup so that when the pot is centered, the CV amount is 0, when is fully clockwise, the CV is at +1 (unattenuated or even amplified), and when it's fully counter-clockwise the CV is at CV * -1 (inverted). You also want another pot to be used for setting the initial level of the CV, for controlling the parameter when no CV is present. That's the tricky part, because the voltage coming out of that pot has to be scaled to 0 to +5.4v, and you want to have a high impedence relative to your pot value so that the effect of parallel resistors on the input attenuator will be minimized (you can check out my Java applet for calculating input parallel resisance here). Actually, it's more important to have the 50% point of the pot's travel create an output voltage of 2.7v - as that will be center for the panning and flat response for the treble and bass. I've carefully selected resistors values that were both commonly available and that meet this funky voltage scaling critia. I've factored in the "parallel resistor effect" on the pot - the result is that when the "initial level pot" is at 100%, the output is a little high (5.76v, as opposed to 5.4v), but at 50% it's damn close to 2.7v.

The LM1036 also doesn't take too well to negative voltages (as I sadly discovered through experiment), so I've added a diode to protect it. It seems to handle overdrive of the CV's (beyond 5.4v) just fine. This can distort the audio a little, but the distortion is not a hard clipping, and is actually a nice effect - much nicer than an overdriven op-amp.

Here is the schem for the CV inputs - you will need four of these for each LM1036, and this can be done with two TL074's.

Audio Inputs and Outputs

To allow for switching between x-fade and pan modes, a DPDT switch is used to switch the inputs and outputs. When panning, Input 2 is disabled, and when x-fading Output 1 is disabled. Note that Input / Output #1 is the left input / output on the faceplate and because of the way the pan-CV works and the way the knob is turned, this should be run through Channel #2 on the LM1036!

Each channel has an input section that consists of two resistors and a cap to attenuate the inputs (10v p-to-p inputs are expected) into the low distortion range for the 1036. The cap sets the DC bias for the input up to the 7.5v level found on the input pins. The diagrams below shows the wiring for the switch and the output sections. The switch is shown in the x-fade position.

Output sections - one for each channel. The "Aux. Input Channel" is optional, and would be used if you were making a mixer module that you wanted to be able to chain with the output of other mixer modules (like the Serge system mixers). It is not included in the vectorboard layout or the faceplate.

Vectorboard Layout

Be sure to check the Vectorboard Building Tips before you start. The actual Visio file for the faceplate layout is available here.

Faceplate

Below is the faceplate I used for my module. To give you an idea of the scale, this should be about 5.1" high, and is made for a 3u rack panel (5.25"). I also printed it straight from CorelDraw, so mine doesn't have the pixation and clarity problems that this image will if you print it as a gif. If you want the actual CorelDraw v7 file for this faceplate (and a couple of others), click here.