Roland - Digital Chorus DC-50
“poor man’s Space Echo”
“This is a cheery little unit - a poor man's Space Echo if you will. Curiously named Digital Chorus it's actually a BBD based delay and chorus unit. There's not a lot of information about this unit as it does appear to be quite rare. A quick summary therefore: It's based on two MN3001 BBD units controlled with a common clock circuit which is similar to that used on the Boss CE-1. The MN3001 is actually two separate 512 stage BBD lines, so you have four sets here giving a maximum of 2048 stages. The clock circuit doesn't run particularly fast though - around 7KHz to 22KHz. That limits the maximum frequency the unit will affect so don't expect particularly bright chorus or delay sounds. The low pass filters, and there are a great many of them in the signal path, do not really let much above 3KHz through. The straight through signal is clean enough though. You can't have both chorus and delay, it's one or the other. And oddly the chorus takes its output from the final BBD which means it too will have a noticeable delay in comparison to the straight through sound. This is actually quite a nice effect on longer notes but sounds somewhat odd with short plucky sounds. I have modified mine to allow the chorus effect to come from either the first (inverted to maintain phase), second or fourth BBDs. This gives quite a nice tonal variation and I used the somewhat redundant HML output level switch to select between the three. Still, even on the first tap, with the relatively slow clock speed and large amounts of LPF, it's not going to sound like a CE-1. It is also possible to modify the unit to take an external CV and allow for manual control over the delay time and chorus depth. To save the rather lovely front panel, I re-functioned the mic input socket to be the CV input, and the mic level pot to control modulation depth of the chorus, delay time and CV input sensitivity depending on the mode the unit is switched to. The maximum delay time now is only around 140mS and the minimum around 45mS. It's not a trivial modification though - I had to use a quad op-amp and a handful of passives as well as some minor changes to the main PCB. If I get some time I'll put the mod details up on line. Oddity: There's a opto based compressor at the front of the BBD chain. While compression is not unusual in a BBD device so as to improve the SNR, one would normally find an expander at the end of the chain to undo the compression at the input. There is no such thing in the DC-50. So in chorus mode you get a strange, and actually quite nice, effect of having a deeper chorus sound for quiet input levels and less effect when the input signal is high.”
cred: matrixsynth.com/1976-roland-dc-50-digital-chorus-echo, modwiggler.com/forum/
