Monotron Delay, a few mods more

Jes joo, moving on with the Monotron stuff. First up, certainly worth mentioning is this mod by Stexen, that another modder Dan wrote me about: Replace the keyboard potentiometer with a 500mm version!

Drop in a Spectra Symbol SoftPot such as this one, and the added precision should aid landing specific notes way better. I certainly am tempted to try this, but adding one would make my planned extender module grow in size too much. Thus, adding external CV/Gate control makes more sense at this point for me.. But then again, mounting this SoftPot to some additional box and wiring it to the pitch CV mixer through a series resistor might be one option.

SoftPot. Must.. refrain.. from.. typing.. silly.. cannabis jokes!

On a completely different approach, adding this and turning the synth into some sort of a ‘mini-guitar’ might make a fun mod.

Other than that, I’m tired of typing ‘Monotron Delay’ all the time so I’ll start using the more convenient abbreviation ‘MD’ instead. With future scribblings in mind, whenever its this synth and the abbreviation, no I’m not talking about Sega Mega Drive ;)

LFO routing mod revisited

Since there have been requests about the details of this mod, I assume its best off described with a schematic.  If you’re unable to add the mod even with the help of these additional descriptions, may I suggest you stick with a stock MD..

LFO routing mod schematic

Like I suspected on my previous post, a 4k7 resistor proved a better alternative over a 10k one to interface the VCF CV summing mixer. If you add this mod to your MD, do test both alternatives to determine which one you prefer.

The way I ended up installing this mod was to first glue a small piece of pcb over the keyboard connector and then have the switch on top of that. Through-hole resistor and PCB feet leftovers are used to connect to the MD board. These help affixing the switch in place better than regular (flimsy) cable.

Side view of LFO routing switch. Stacked with a small piece of PCB on top of the keyboard cable connector.

One additional mounting suppor was to bend the lugs on both end of the switch. When the case top is installed, the downward pointing lug rests against the Power off/LFO shape select switch (SW1) and the upward pointing one against the slot in case top. Add this with the solder connections and together they form a mounting solution sturdy enough for the switch to withstand normal operation.

Slide switch is anchored in place with a few solder connections..

..but the bent mounting lugs give additional support once top cover is added.

Opening for the LFO routing switch.

To add a possibility of resetting the LFO by external signal, I connected a 100k resistor to wiper of VR3A (LFO rate knob). Pulling the other end of this resistor to GND will reset the LFO.  Didn’t bother testing whether connecting this to VCC would offset the reset to positive peak..

Triggerama

If you’ve been following my MD posts, you might recall from the first of them that I had the two unused inverter gates circled (section D3) in the schematic. What idea I came up for these was to test adding a Arp Odyssey style gate-to-trigger converter. Combining this with the LFO reset mod would allow using the MD keyboard to retrigger LFO. Additionally routing this through a switch would’ve allowed incorporating both free-run and retrig modes.

Arp Odyssey MKII trigger generator circuit

On the Arp Odyssey circuit, two NAND-gates Z1C and Z1D are configured as inverters and (if I got the following bit right), with a handful of support components, the combined propagation delay of these inverters is used to turn a incoming keyboard gate signal to a positive-going trigger pulse.

Gate-to-trigger circuit testing in progress..

Too bad I didn’t get the circuit working fully: The trigger pulse was there but the output also had a DC voltage component I was unable to remove. Whilst testing variations in the circuit, I managed to break yet another solder pad on the PCB. This lead me to decide that it’s maybe better to back away gracefully,  repair whats broken and not waste time possibly inflicting more damage. Seems the pads are easier-than-average to break with repeated soldering..

In general, if you wish to play around with these unused inverter gates, bending up at least pins 1 and 13 from IC3 is required. Depends on the intended use, of course. If you’re not comfortable with the idea, alternative approach would be to buy a replacement chip, cut away the existing one from MD PCB, and bend the pins up on the replacement prior to soldering that to the board.

Op-amp upgrade

One thing that bothered me from the get-go with MD was the high freq noise on the output. To illustrate this here’s a spectral analysis of  single (lowest) note:

FFT freq plot of stock Delay, lowest note held.  Voxengo’s free Span VST for analysis.

As you can see, there are two very distinct spikes; first at 8kHz and second at 16kHz. I have no clue where these harmonics might originate, but as the above reads, they sure are well present.. Actually even louder than the actual body of the sound!

Where upgrading the audio path and op-amps are concerned, only IC4 needs to be swapped. The circuitry has been (conveniently) designed so that the other op-amp (IC2) does control / calibration related stuff only.  For the replacement I chose MC33174.

I must admit I didn’t bother doing too much investigation into suitable replacements. Safe to say, there certainly should be other (better?) alternatives out there. It’s good to keep in mind that Monotrons use op-amps designed to be operated from a single-sided power supply and this rules out the most ‘classic’ op-amps, if you wish to retain ‘drop in replacement’

So initially comparing this with LM324 by spec, what raised my interest was that it has a better Common-Mode Rejection Ratio (100dB vs 80dB). Going for higher decibels, how scientific ;).. Anyhoo, pop in this alternative, and:

FFT plot for 33174-modded Delay.

Spikes gone! Also both the low and high freq roll-off curves are affected, especially for the latter things look pretty interesting. Can’t say I notice much difference in noise floor. Maybe the synth just is noisy due to low operating voltage etc.

Internet suggested (hi Michael!) that the disappearance of spikes might have to do with MC33174 having a higher bandwidth and slew rate, but the datasheet I have for LM324 doesn’t list the latter. Bandwidths are 1.8MHz and 1MHz, respectively.

In case you want to compare the audio files used for the above yourself, get the 44k/24bit wavs here: Original MD & MC33174 modded MD. Both are recorded by triggering the lowest note on the keyboard, but weirdly enough the tuning is slightly different.. Probably didn’t hit the keyboard spot on. RME Fireface 800 was used for the A/D conversion.

Bla-blabbage

Next up, I guess its time to see about that expander thingie. Must admit that the VCF FFT plots above make me want to modify the high-pass cutoff point (75Hz or so) to include lower freqs.. I’m guessing the cutoff freq has been set this way simply to lessen distortion on the internal speaker. As it is unable reproduce any real bass frequencies, cutting them off removes the good-for-nothing mechanical movement.

If you have any MD mods to share, do leave a comment!  Links much appreciated. If anyone takes up the task of building the mini-guitar-thingie (with the SoftPot), I certainly want to be among the first to see it :)