Filling out my 4U Serge rack project is a Serge design based Dual Negative Slopes plus Hills and Valleys from Low-Gain Electronics. Of course the original Serge module names were Dual Negative Slew and Peak and Trough.

The Negative Slew has always been one of my favorite Serge modules. I came across one around 1977 or 78 and took the trouble to draw out its schematic for my personal use. I built two Negative Slews for my 1980 DIY synthesizer. Somewhere around 1980 the Negative and Positive Slews were replaced by the Dual Universal Slope Generator. I’ve also built Ken Stone’s CGS82. The Low-Gain 4U version was much easier to build.

Negative Slew Functionality

From a 1970s Serge catalog: The DUAL NEGATIVE SLEW is one of the unique multi­-functional patch­-programmable modules in the Serge system. The module features two independent sections with wide range voltage controllable slew rates. The slew is active in the negative direction only and can be patched to perform a number of synthesizer functions. With the Pulse output patched to the Input, the module will oscillate for use as a voltage controlled sawtooth oscillator or pulse generator. An audio signal applied to the Input will be envelope detected, and the complex envelope will be available at the output. If a pulse is applied to the Input, the unit will function as an envelope generator with a fast rise time and a voltage controlled fall time.

• Negative slew as an envelope follower
• Attack/Release envelope generator with pulse to input
• LFO/oscillator with pulse out patched to input
• Trigger generator with very short pulses

Schematics

I have a low resolution copy of the schematic I drew around 1977. Today I recopied it for comparison to the CGS and Low-Gain circuits. This early circuit has some interesting differences. It used a 3046 transistor array. Later versions used the CA3086. It does not use a +6 volt supply, but just +/-12V. Some resistor and capacitor values were different. I’m not sure I correctly read all the parts values.

For comparison, here is the Ken Stone CGS82 schematic.

And here is the Low-Gain circuit (on two drawings). The schematic indicates the MC3401P chip, whereas in actuality an LM3900 is used. It’s almost identical to Ken Stone’s, except an LM78L06 regulator is used for the +6V.

The Panel PCB schematic shows how the cycle switch is wired. It very simply switches the input between the Input jack, J1, and the End pulse jack, J3. Simple but useful, so you don’t have to patch it.

Build

This build is similar to the Low-Gain Dual LPG, but it’s different in that the panel is 3 inches wide, but the main PCB is only 2 inches wide. Once again, the main and panel PCBs are connected with multiple headers and sockets and secured with 11mm standoffs.

There is a mistake in the etch that is described on the PEAK schematic.

Fixing the problem takes cutting the trace where R57 goes (incorrectly) to ground and jumpering it over to -12V.

The Panel PCB shown below has the four pots whose nuts hold the panel on. The two LEDs are also soldered here during final assembly.

The front panel gets loaded with banana jacks, the switches, and LED lenses before final assembly.

Below is the back of the front panel. Here I have some remarks about the switches. Why was it so hard to find a miniature SPDT switch on Mouser??? The one I bought, 642-5636AB16, surprised me. It has a short bat handle. Images are for reference only. See Product Specifications. Ha ha, that’s OK. The switch fits in a 0.25 inch diameter hole, which is normal. But the panel hole was quite a bit smaller and had to be drilled out. Note: the panel is made of PCB material, so be careful if drilling.

Let’s put the front panel and the front panel PCB together. After we wire the jacks and switches to the panel PCB, it is not easily taken apart.

The completed module is shown below. As usual, Yellow jacks for inputs, Green for signal outputs, Red for Pulse outputs and Blue for CV inputs. I was going to use Yellow LEDs, but ended up with Super Bright Orange. If you use a bright, efficient LED you’ll need to adjust RLED. Mine are 4K7.

Scope views

Here’s one negative slew oscillating. Yellow trace is the output and the Blue trace shows the very short end pulse. Vertical scale is 2V per division. The maximum frequency is set to 200 Hz, using the trimpot.

The next two images show the Peak (Hills) and Trough (Valleys), obtained when patching both outputs into the Hills or Valleys inputs. The Yellow trace shows the Hills output (maximum voltage) in both photos, and the Blue shows the Valleys. Notice that by default with no inputs Valleys sits around +6V and Hills sits close to zero.

The last scope image shows the outputs of both slews. The top one, yellow, is being CV modulated by the lower one, blue, which is running about 3X the frequency, producing bumps in the modulated slew.

Finally, here a pic of the three Low-Gain modules installed in the rack. I changed the CV attenuator knobs on the Dual LPG to Dark Blue, so they all match nicely. (There’s a mounting screw missing on the lower right side of the Negative Slopes. I need to adjust the rack a little to let the screw fit.)