This dual Low Pass Gate from Low-Gain continues filling out my 4U rack. Logan Erickson designed this interpretation of the famous Buchla low pass gates. Rummaging in my files, I found a schematic for Don Buchla’s original LPG. Logan’s version is very close to this original. I like that it features an offset pot for the filter cutoff and also a CV attenuator. The module includes two other circuits, one is a mixer for the two LPG outputs whose output is on a 3.5mm jack. The other is a preamp for external signals with a 3.5mm jack for input and a banana jack output.
The Build
You can purchase just the circuit board, if you want, and make your own panel. The mixer circuit has four inputs that could all be brought to a panel, if desired. But I bought the panel PCB for holding the pots and 3.5mm jacks, which connects to the main PCB with headers, simplifying wiring, but leaves no room for extra mixer inputs. Mechanically, it’s similar to the Slightly Nasty modules I’ve previously blogged about. The back side of the main PCB is also the back of the module and has the power connector and trim pots, as well as the rest of the components. Headers go on the opposite side for connecting to the panel PCB.
In the picture above, notice the two wires I added. Why? It’s not a circuit modification, but a simplification for the board interconnection. The two DO (direct outputs) from the LPGs need to connect to two of the mixer inputs. If you put a single header pin and socket at each of those locations, then the circuit is completed through traces on the panel PCB. I wanted to avoid having two single header pins and so I just wired them directly here.
Notice also two added resistors. I did this because I had only 100K trim pots, where 20K were called for. Here’s the trick. By adding a 30K resistor in parallel, the resulting resistance is about 22K. And in this circuit the trim pots then adjust from 0 to 22K.
Finally, notice the 1206 size SMD 100nf capacitors. Nice way to get good bypassing on a dense PCB.
The top side of the panel PCB holds the pots and two 3.5mm jacks. Female headers go on the other side. Notice I’ve mounted the standoffs and secured the screws with lockwashers. After the panel is fitted, these screws aren’t accessible.
The DPDT center-off switches are NKK M2023SS1W01. Cudos to Low-Gain for a design that does not need a 3PDT switch, like many LPGs do!
Fitting the front panel to the panel PCB entails first securing the panel with the pot and jack nuts, and then soldering flying pigtails from the jacks and switches to lands on the PCB. Once this is done, it would take a lot of desoldering work to undo.
Yellow for inputs, Blue for CV inputs, Green for outputs. Medium blue Davies 1900H clone knobs for the CV attenuators, Black for the Offset pots, and a Dark Red knob for the preamp Gain. The 3.5mm nut are Befaco Bananuts, Gold for the preamp input and Black for the mix output.
Preamp modification
As designed, the preamp has a lot of gain, minimum of 5X and max of well over 100X. I modified it to have between 1X and 10X gain. The preamp is DC-coupled, which I think is odd for a high gain input amplifier. The input has two protection diodes, one to +12V and the other to 0V (which I think is a mistake, because it will clip a bipolar signal coming in). I omitted those diodes.
Sound Test
Although on the bench, the two LPGs seemed to perform very much alike, when sound testing I discovered that the XVIVE vactrol has much more gain than the original Vactrol. I had one of the older VTL5C3/2 Vactrols and ordered a new XVIVE one for the other side. I had to add more initial offset to LPG1, the older Vactrol. In the recording LPG1 is on the left ear. I might decide to replace it with a second XVIVE.
I found there’s a high frequency bleed-through with Gain and CV set to zero on both LPGs. What’s up with that, Logan? Can it be fixed?
First part is in VCA mode, second in Both mode, and third in Filter mode.
