L-1 Stereo VC Mixer replaces the Verbos Scan & Pan for my output mixer.
Features
- 4 stereo channels with voltage controlled level and panning
- Voltage controlled sum out level
- Stereo level bar graph, 10 LEDs per channel
- Headphones out
- Audio signal flows only through VCA chips and summing amps, all controls through CV
Why switch?
Verbos Scan & Pan is a nice, compact VC mixer, having four channels with VC level and VC panning for each channel. It also features a scanning function that I didn’t use much. I liked it a lot. But it has a few drawbacks. The knobs for panning are tiny. The channel level sliders are inoperative when a level CV is patched in. And there’s no global output level control. I never took advantage of the input preamp overdrive feature. So, not to disrespect the Scan & Pan. It just didn’t have the right feature set for me.
I heard Paul Schreiber, of Synthesis Technology, raving about the quality of the L-1 VCA designs, based on THAT 2180 chips, and that influenced me to switch. The L-1 is larger than the Scan & Pan, partly because of its large knobs. It also had to be larger to accommodate all of the circuitry, which lives on two PC boards that plug together. The functionality is straight-forward: Each of four input channels has a level pot and and pan pot, with corresponding CV inputs. The interaction of the pots and CV inputs turns out to be very useful. Unity gain is set on each channel with no CV in, the level pot full and the pan pot fully left or right. If you add a CV, the panel controls impart an offset (an initial) setting.
Construction
A partial kit can be purchased from the designer, Aleksei Laman, at L-1. Please see the L-1 Stereo VC Mixer page. I purchased a full kit from SynthCube. The SynthCube kit at $395 is a good value, considering that the module is $600 assembled. Assembly is greatly simplified by all SMT parts being already soldered to the boards. Your job is to solder the through-hole components and assemble the two boards together with the face plate. It seems simple enough, but there are some things to watch out for.
Assembly Tips
Follow all the assembly tips on the L-1 page. The jacks legs need to be cut off, as well as tabs on the pots. There need to be thin washers (not supplied) under the standoffs going between the boards. This helps to raise the separation just a tad, to keep the trim pots from hitting the board above. I have more assembly tips.
- Mount the boards together, using the standoffs, for soldering the headers that connect the two boards. This can be done early on. Having them together while soldering insures alignment.
- The back board can be soldered with organic, water-washable solder. I used Kester 331, which is getting harder to find.
- Before soldering them, check the 1uf 50V electrolytic capacitors to see if they are short enough to clear the front panel. I had to replace the ones that SynthCube supplied.
- To solder the jacks and pots, mount them all loosely at first, affix the front panel over them and put washers and nuts over all the jacks, and a washer and nut on the two corner pots. Holding the panel flush to the parts, solder the corner jacks and pots. Recheck alignment. Now solder all the jack and pot leads. This way you are assured of alignment.
- Leave the LEDs for last, after you’ve soldered all the jacks and pots. Put all the LEDs in place (the long lead goes to the + symbol), then reassemble the panel. You can adjust the height of all the LEDs at this point. I set them to protrude just a bit above the panel. Watch out for LEDs that expose one of the leads at the side, which can short to the panel.
- Look carefully at all of the photos, to make sure you’re soldering things on the correct side of the front PC board. (All parts insert from the same side on the back board.)
PC Board Photos (click to enlarge)
Top of the front board. I put the LED driver chips in sockets.
Back side of the front board. Notice the washers underneath the standoffs. You can also see flux residue from the no-clean solder I used.
Front side of the rear PC board. Notice I did not use IC sockets.
Back side of the rear PC board. Squeaky clean, due to washable flux. This side faces the rear. Notice the little holes for accessing the trim pots. This board will be covered by a protective metal plate.
The back of the assembled module. The screws were not supplied, but are standard m3 panel screws.
Here is a side view of the sandwich.
The Custom Power Header
Because the power has to connect to the front PC board, the rear board and its cover plate have rectangular holes for the header to poke through. And, since this height is needed, Aleksei supplies a custom header, pictured to the left below.
To the right is a normal 10-pin Eurorack keyed header. If you did use one of these, it would be difficult to connect the power cable. Unfortunately for me, the custom header did not get included in the kit. I had to wait for three weeks for one to arrive from Aleksei, mailed from Belarus! He was quick to send it, free of charge. I let Synthcube know about the omission, and they were responsive, but didn’t have the custom header.
Hi Richard,
Thanks for posting your experience about the l1 stereo mixer build. Do you recall about how many hours it took to build it? I’m thinking about commissioning someone to build it for me, but I would like to know what I might be looking at for time charged. Thank you!
Edgar, thanks for visiting my site. Well, since I’m not in the business of building modules for other people, I don’t have much idea how a builder sets prices. I don’t track my time when building. Since the L-1 is available assembled, why not buy an assembled one? Sure, it costs $200 more, but you know it will be assembled and calibrated properly. Finding a builder who would do it for less than $200 might be difficult. I think the assembled price is a fair one.
Hi Richard, thanks for this! Just a question, you say “notice I didn’t use IC sockets” but you don’t mention why?
Short answer: higher reliability.
Long answer:
Not using sockets comes from the days of building MOTM kits from Synthesis Technology. Paul Schreiber did not supply sockets and recommended against them. Sockets can contribute resistance into a circuit. I once had an old computer with IC sockets. That made it easy to change chips, but I had to periodically press on all the chips to re-seat them firmly. Heating and cooling loosened them.
Integrated circuits are pretty reliable. I’ve only had one bad IC in all of the MOTM kits I built. And there was a circuit design flaw in the Blacet Time Machine that caused a chip to burn out. I had to replace that by the tedious process of desoldering it from the board. But, since I only omitted sockets on PC boards of high quality, such as Synth Tech, Blacet and now L-1, it is possible to desolder them without damaging the board. I always put sockets on boards with single-sided etching, because it’s too easy to damage that type of board during rework.
A little extra information:
The master output level control works internally to limit the output of all 8 VCAs. This means that, if you have hot envelopes generating more than +5V, all you have to do is turn down the master gain to keep the outputs at or below unity gain.
To get unity gain, the channel level knob has to be full OR be zero with +5V on the CV input, the master level has to be full, AND the pan pot must be fully panned left or right. If the pan pot is centered, the max gain with +5V CV is around 0.75.
The pan CV input takes a -5V to +5V (bipolar) CV to fully pan left or right (with pan pot centered). Since we mostly use bipolar LFOs for panning, this design is perfect.
As for overdrive, the L-1 will go over unity gain with enough total CV. It appears to start clipping around 2X gain, a 10V p-p signal input being amplified to 20V p-p.
Is there much calibration to be done on this build? I don’t see reference to any anywhere? I don’t have a oscilloscope (only the one of the computer) but i do have a mutlimeter would you say i will be ok to build this?
Thanks
Robert, calibration is simple. There are five trim pots, one for each of the four channels, and one for master. Use a 1 KHz sine wave from a VCO. Measure its amplitude with your multimeter set to AC voltage. Set the four level knobs and the master level knob to maximum and the four panning knobs to full left. Patch the sine wave to channel one input and measure the voltage at the left output. Adjust the master and channel one trim pots to get unity gain (matching the level on the input). You only have to adjust the master once. I set it around the mid point. After you have unity gain on channel one, move the input to channel two and adjust the channel two trim pot for unity gain. Repeat for channels three and four. That’s it.
Note that with the pan pot centered, gain is less than unity. This is for equal power panning.
Thanks! Great description. Also I have one other question about the build, how do you get the LEDS to the right length so they are all neat and at the right level because i see the legs are left fairly long is it a case of attaching the front panel and seathing them, soldering, them, then taking the panel back off so you can posistion the jacks?
How did you approach this part of the build?
See above for the LED tips. LEDs are soldered in last. You first mount all jacks and pots and affix the front panel temporarily to make sure all are aligned and seated properly. Then you can solder the jacks and pots. Remove the front panel to insert the LEDs loosely (watch the polarity), letting the leads poke out the back. Replace the panel and secure with a few nuts. Now push the LEDs up through the panel. At this time you decide how far you want them to protrude.
Thank you!
Hello, what’s the depth of the assembled module?
Depth is 35 millimeters, measured from the panel to the back of the seated power connector.