MATRIXSYNTH: ARP 2500 Styled MFOS Music From Outer Space Modular Synthesizer

Friday, January 14, 2022

ARP 2500 Styled MFOS Music From Outer Space Modular Synthesizer

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"Here's an interesting one-of-a-kind synth. We bought this from a customer who was into DIY electronics. He always admired the design of the ARP 2500 but couldn't find one, so he built his own! Well, kind of. He bought MFOS Music From Outer Space circuit boards and assembled the circuits. He made his own front panels to look similar to those on the arp2500 and wired them up. They're installed in a nice wooden cabinet he made as well.

This system is a real pleasure to play. If it doesn't sell, we're going to use it in our studio as a secret weapon, as it sounds great and quite different from our other synths.


Oscillator 104
Dual LFO 103
8 Stage Phaser 111
Oscillator 104
Dual T/G/T/S/H (We believe this circuit is the Ian Fritz module aka Threshold/Gate/Trigger/Sample/Hold Circuit)
Oscillator 104
Low Pass Filter 107
Dual Envelope Generator 133
Dual VCA 106
Clocked Sequencer 116

DIY synths often have lots of problems due to the fact that some of the people who build them have never had any prior experience assembling electronics, and even when they have, there are often mistakes that the builder didn't bother to correct. Fortunately, the person who assembled this unit had good soldering and decent assembly skills and, for the most part, this unit is working quite well and sounds amazing - we were very surprised by how good it sounds!

There are a few anomalies, most of which seem to be wiring errors that should be easy to correct or features that were never implemented. We are selling it as is, though we could probably have our tech fix any issues for an additional fee:

-The dual T/G/T module's LED is not working, but the module itself works fine. A couple of the other modules that have spaces for an LED never had LEDs installed.
-In reverse mode, the sequencer's step 1 does not play, and the sequencer's random mode triggers step 16 a lot more than it should if it was truly random, but we suspect this may be normal operation.
-There are 3 volume pots for each of the 3 audio inputs in the VCF module. Pots 1 and 2 do not seem to work unless something is plugged into input 3. The signal going into input 3 seems a bit muffled.
-VCF trigger and gate input jacks do nothing as they were never wired. This function may not be possible with this circuit.
-The LFO sawtooth and reverse sawtooth jacks (or panel labels) seem to be reversed on both LFOs.
-The phaser distorts when the input is high, but this is likely normal and is an interesting effect.
-We noted that VCO2's pitch was drifting in the past but we didn't notice this when this unit was tested recently. It's possible we fixed this a long time ago or that this may be intermittent.
-VCO2 sine wave sounds like a sawtooth - we suspect the wiring was wrong.
-Each of the oscillator modules has a waveform mixer on the front panel, but there doesn't appear to be a circuit for this function in vco2 or vco3. This function in vco1 results in lower level volumes than the direct waveform outs. There's a connector inside that's not attached and this may or may not have something to do with this.
-The resonance CV pot seems to pass a signal when it's at zero position but works as expected throughout the rest of its range.
-Envelope trigger 1 jack triggers envelope 2 and trigger 2 jack triggers envelope 1.
-VCA audio input pots seem like a simple tone control, which may be normal. Both pots crackle a bit.
-The VCF's audio input signal seem to frequency modulate the filter when the resonance is high.
-There could be more, but that's all we noticed when going through it.

It's hard putting a price on this. We're guessing the parts costs for everything - circuit boards, panels, components, power supply and cabinet may have been around $1000, but all the time spent building these things is valuable. We asked our tech what he felt it should be worth and he valued it a lot higher than its current price, but as someone who builds things like this professionally, he also knows how much time and effort these things take. If our asking price is too high, feel free to make a reasonable counter-offer, though we're also tempted to keep it and use it in the studio, so this unit is valuable to us."

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