Andromeda Space Rockers


 

Now that I have already 7 sound generators tested and breadboarded, the analog drums project starts to look promising! There are 3 generators from Boss DR-55 (bass, snare, hi-hat) and 4 generators from Korg KR-55 (bass, snare, rimshot, claves). But something is still missing: at least another hi-hat would be needed, and maybe something more. In the previous post I was reasoning, why I did not want to implement Korg’s hi-hat. So on with the quest for searching analog drum schematics from the internet.

On that quest I stumbled upon a device series called Andromeda Space Rockers. While my sources of inspiration in this project have so far been classic commercial drum machines from the 70’s, these devices are something completely different. I don’t even know what they exactly are. I have understood that they have been sold as DIY kits, but nowadays they seem to be just schematics and instructions freely available from the internet.

This sentence describes these devices quite well: “The Andromeda Space Rockers are a range of simple analogue synthesizers and drum machines designed by Eric Archer that synchronize over a minimal infrared (IR) wireless network for communicating tempo / clock information.” Description taken from https://electroat.com/projects/space-rockers/. Also this video by Eric Archer gives a good idea about them:


 

The main idea behind Andromeda Space Rockers (I’m lazy and call them ASR from now on) is the IR syncing system, which makes it easy to accomplish all kinds of ad-hoc synth combinations and odd rhythmics. While this might be a funny concept to toy with sometimes, I’m now trying to find sound generators. And these devices do contain also those, so they might be useful.

All sound generators are quite simple, and they are implemented using common and generic parts, like TL06x-series op amps and BC5xx-series transistors. Much easier than commercial devices designed in the 70’s, like Boss and Korg.

 

MK-2 hi-hat

There are several different sound generators in the ASR series. Now I’m mostly after a hi-hat, so the MK-2 is closest to that, because it can produce a decaying noise burst. It is again breadboarding time!

The original schematic is quite straightforward, if we forget the syncing parts: noise generator, filter with one op amp T-bridge to shape the noise spectrum, and an envelope to generate the decaying burst. There are two LDR resistors to control the filter frequency and burst decay time. I don’t want LDRs now, so I replaced them with pots.

The original envelope generator part is in this picture and is a bit problematic:


When trigger pulse comes, Q8 conducts for a short period. Because R28 (pot in this case) is quite high-ohmic, voltage in point A rises immediately near the supply voltage. This voltage is also the output of the envelope generator and determines directly the amplitude of the output noise. This causes a sharp spike in the start of the envelope cycle and is heard as quite loud click. I moved R28 between point A and R42 and increased R38 to 1.8k, so C1 limits the rise of the voltage. This reduces the click significantly.

Because this is more a generic noise maker than a hi-hat circuit, the filters center frequency is not high enough. I changed the 3.3n capacitors in the T-bridge to 680p, which rises the center frequency. It is a bit low even after that, but works.

Here is the final schematic after modifications:


 And a sound sample. The decay time is adjusted during the sample.

The MK-2 hi-hat can be seen on the breadboard, with Korg bass drum and snare, in the header picture of the previous post.


MK-1 toms

There are also other interesting and simple sound generators in the ASR series. The MK-1 is a basic tom or bongo type of sound, implemented with two damped T-bridge oscillators, both using one op amp. The outputs are mixed passively.

The schematic lists several capacitor values, chosen according to what base frequency is wanted. I built two of those, the “LOW” and “LO-MID” versions. I’m calling these “Lo tom” and “Hi tom” although they are not resembling much of a tom, maybe more like bongos.

The only modifications done to the original schematic are replacing the LDRs with pots (and of course the syncing parts are left out) so I won’t include the modified schematic here.

In this sample, both oscillators are sounding. I’m adjusting the hi-tom during the sample.


In the next post, I'll try to figure out what to actually do with all these nice sound generators.

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