Sound Lab MODIFICATIONS (You already built it... time to tweak it)

Back to "Music From Outer Space" Analog Synth Pages

Back to the Sound Lab Mini Synth home page

My Modified Sound Lab

I added the VCO scaling, Osc 1 modulated by Osc 2, and the fine tuners modifications. The Sound Lab is shown with my reed switch keyboard.

I removed the original controls and then carefully drilled some new holes. I then pasted the portion of the panel that changed onto the original. I had to use small body pots for all six of the pots involved with the oscillator mods. Normal ones might have fit but it would have been very tight.

PDF of the modified faceplate.

Protecting the CV inputs from over voltage application.

If you are afraid of applying too much voltage to the external CV inputs then protect them with zener diodes as shown. If the CV voltage exceeds the zener voltage the zener diode will conduct and limit the input voltage to 9.1 volts. The CV Summer OpAmp inputs are IC5-B pin 6 for oscillator 1 and IC6-B pin 6 for oscillator 2.

Adding More CV Inputs to The Sound Lab Oscillators

Patched Out Sound Lab

If you're wondering how to make a patched out version of the Sound Lab Mini-Synth instead of the normallized version here are some guidelines. I will be adding to this page as I have opportunity and ideas. Obviously this is just my first shot at this. I think if you have some electronics savvy you will be able to do whatever you want after reviewing these guidelines. You can use banana jacks or you can use phone jacks. Just ground the ground lead of the jacks to the ground of the unit and you will be fine.

LED Indicators

If you want to add LED indicators here is how. Bear in mind that the LEDs will eat current and kill your batteries faster though. Since the LFO indicator is driven by the ramp/tri wave it will fade in and out at the LFO frequency.

Printing

When printing these pages You need to set your page orientation to landscape and set the margins as low as they will go. If necessary use cut and paste to put the larger images into your word processor or image editor and then print them from there. I have not been able to figure out why Internet Explorer adds several blank pages (does Bill have stock in some paper companies?).

Sound Lab Mini-Synth AR (Attack - Release) Envelope Generator

Sound Lab Mini-Synth LFO (Low Frequency Oscillator),
Noise Source, and +/- 9 Volt Battery Power Supply

Sound Lab Mini-Synth VCA (Voltage Controlled Amplifier) and
VCF (Voltage Controlled Filter)

Sound Lab Mini-Synth VCOs (Voltage Controlled Oscillators)

Sound Lab Mini-Synth Intra-VCO Modulation

To get some cool modulation effects you can modulate VCO-1 with the output of VCO-2.

This first circuit assumes the patched out version. You need to add a new 100K pot, two switches (SPDT and SPST) and the 100K resistor and 2uF capacitor. This configuration allows you to choose the mod source independent of the wave setting of Osc 2. "Osc 2 Out" is IC6-D pin 14 and "Osc 2 Out Pulse" is the junction of R80 pin 2 and R81 pin 1. (You could also use IC2-A pin 1 for a non-attenuated version of the "Osc 2 Out Pulse" signal).

This circuit assumes the non-patched out version. You need to add a new 100K pot and one switch (SPST) to the front panel and the 100K resistor and 2uF capacitor. I recommend you solder the resistor and cap to the switch and pot terminals and then run wires to the PC board as needed. This configuration causes the mod source to be controlled by the wave setting of Osc 2.

Sound Lab Mini-Synth VCO Fine Tuners
Add these extra pots in order to allow fine tuning of the oscillators.

Sound Lab Mini-Synth VCO V/Oct Scale Adjustment Modification

As soon as people starting talking about using the Sound Lab Mini-Synth as a voice module I started thinking... Oh Oh I did not put the volts per octave scaling adjustment into the unit. Actually this was not an oversight. The intent behind the Sound Lab was to make sounds and it does that excellently but in order to be used for music you need to be able to scale the oscillators. So... here you go.

In order to do this modification you need to clip the base leads of Q1 and Q2 as close to the circuit board as possible so that you have as much of the base lead of each transistor to solder a wire to. Fold the leads up. Don't bend and rebend the transistor leads very many times or they will break and you will need two new 2N3904s. I soldered the two 100 ohm trimmers and the two 475 ohm resistors onto the kludge area of the board and ran wires to other circuit points. Remember that the wiper of each trimmer goes to the respective base of its corresponding transistor and the connection to each trimmer resistive element goes to the respective output of the corresponding op-amp.

Be careful soldering the wires directly to the bases of the transistors. Solder the wire going to the output of the op-amp to the respective feedback resistors (R60 and R61) on the side that connects to the output of the op-amp (pin 7 of IC5-B and IC6-B).

Connect the external control voltage to a 1V/Octave keyboard and play C at several octaves. Adjust each oscillator separately by turning down the volume of the oscillator not under test. Adjust the appropriate trim pot until the pitch corresponds correctly to the key you are playing. When you play the octaves or a scale you will notice that the scale will go flat or sharp at first but then as you adjust the oscillator's scale trimmer you will be able to bring the scale into tune.

If you want temperature compensation too you will need to use 2K tempcos for R60 and R61. Mount the one for R60 in physical contact with Q1 and Q3 using some epoxy or heat sink compound. Mount the one for R61 in contact with Q2 and Q4 in the same manner. The intent is to keep the thermistor and the transistors at the same temperature. Since the oscillators have a wide range you can reduce the value of the C8 and C9 integrating caps (for example to .002uF) and the oscillators will track over a wider range. You could try the Precision Resistor company in Florida.

To obtain the most accuracy of scaling of the oscillators for control via Ext CV1 and Ext CV2 do as many of the following mods as possible:

  • Implement the 1V/Oct scaling modification.
  • I found that some randomly picked 2N3904 transistors (from a batch of 100 I ordered) gave me 4 to 5 octaves worth of "in tune" range (the oscillators track worse and worse as the frequency gets higher) but many people argue that matched trannies are best so... You can use matched pairs of NPNs for Q1-Q3 and Q2-Q4. While a little messy you could solder wires to the legs of the matched differential pair in a couple of SSM2210 or LM394. (CA3046 used to be mentioned but it has "died on the vine" production wise).
  • Use a 2K Temperature Compensating Resistor (tempco) +3500ppm per Deg. C (Precision Resistor 2K PT146) for both R60 and R61. R60 should be thermally coupled to the Q1-Q3 pair and R61 should be thermally coupled to the Q2-Q4 pair.
  • You can also go even smaller on the integrating caps (as low as 300pF) and use silvered mica, poylstyrene, or polycarbonate. If you go smaller on the integrating cap you may need to increase the value of R50 and R52 to as high as 100K in order to allow the frequency adjustment pot to maintain a useful range.
  • Use 1% metal film resistors for all resistors in the oscillator circuits.

As you can see from this drawing you can mount the 100 ohm trimmers in the kludge area like this. Mount the 475 ohm resistors vertically as shown. One of the leads of the 475 ohm is soldered to trimmer lead 3 and the other lead is soldered to ground (the PC land toward the bottom of the figure). Notice that the green color is used to show which circuit points get connected together. You solder wires to the leads of the trimmer and run them to the circuit points shown in the drawing and in the modification schematic.