Cloning a Roland CR-78

in the meantime, the good old CR-78 and it's preset-sister CR-68 archieve prices which are beyond my personal point of pain. I got a broken drumvoiceunit from a 70ies organ quite cheap on the bay and first tests showed that many of its voices sound VERY similar to the ones on the CR78 (clave, bongos and conga). I decided to complete the missing voices and add a midi-trigger-interface, some individual outputs and an accent-unit.

Misunderstandings concerning Roland TR-77 and CR-78

You often hear the TR-77 has the same drumvoices as the CR-78 and even sounds the same. Thats not quite true. The cr78 is the successor of the tr77, has the same drumvoicetypes (see the 14 below) and many of its voices sound quite similar (MetalBeat, Guiro and Tambourine e.g.). But BD, SD and HiHats are totally different. The TR-77 is much softer and subtle, the cr78 has much more punch in its BD. Whereas i dont hear a dramatic difference on Clave, Bongo, Conga, the BD and SD don't have any parallels, they are totally different on these mashines.

When looking at the schematics we see that the tr-77 voices for BD, SD, Clave, Bongo, Conga and Rimshot are built around coils (inductors) whereas these voices are generated around transistors on the cr-78, propaply to save productioncost. Coils are only used to bandpass-filter some sounds on the cr-78 (TB, Cymbals, MB...) whereas in the tr77 they are directly used to generate sounds together with some capacitors. Exceptions are Claves and Rimshot on the cr78, they are still built with coils and just sound ...cool!

Press yellow button again to STOP playing audiofile

Got some *bay-luck: that's the soundgenerater board of a drumunit from a 'viscount 160' organ for 3 EUR. I bought them because i spotted coils on the board and wanted to experiment a bit with them. After ripping apart all the old organ cabling and putting some voltage supply and triggern onto it i was quite positively suprized. Warm and ploppy.

Although it has a BD (reminding me to the TR77 BD) it doesnt have a SD. The Snare was simply simulated by triggereing CYM1 and HB together.

I cut the board in 2 parts and removed the half with its Tango, Waltz and Bossabrain. There already was a preamp on the voiceboard which summed all voices. I wanted to have Cymbals separate to give them some external FX like chorus or sth. So i added another very simple preamp which does the job quite well. Schematics to the right.

I had a coil remaining from a voice on the waltz/bossa-side. Didnt find out what it did, i just took the coil and rebuilt the very simple and missing cr78-Rimshot-Voice, one of my favourite sounds.

Here is a soundfile with the Viscount-board sounds alone

viscount-drumboard cut in half

voiceboard with added rimshotvoice, preamp and triggercable

detail of rimshot and preamp

simple cymbal-preamp, the 47p-capacitor is optional, i didnt mention any difference when removing it

Tambourine and MetalBeat

These 2 are probably the most typical CR-78 sounds. Everybody knows them from 80ies charthits. I built them right away from the cr-78 schematics. All transistors are bc547a.

The Tambourine has two trigger inputs discharging its envelope with 2 different decays. The decayenvelopes open a transistor which amplifies white noise. The trick to get the typical sound is now, that the amplifying transistors collector (which outputs audio, overlayd by positive voltage) doesn't go onto plus over a resistor like it normally would. It gets it's plus through a COIL paralleld by a CAPACITOR. I am not an expert, i guess its a bandpassfilter which shapes the noise into its typical CR-78-Tambourine shape. Now i didnt have a coil. So i tried to build a simple filter with an opamp to shape the noise-frequency before it gets into the TB-scheme. It worked quite well. See schematic to the right.

The MetalBeat uses the quite often used ordinary simple "decay-envelope into transistor VCA"-unit. But it won't eat ordinary white nouse but a 3-squareoscillator-mix-timbre produced by a 40106 schmidttrigger-ic. Thats why we also need 5V on this board. With some external parts (trimmers, capacitors) the schmidttrigger outputs three squarewaves which can be tuned by the 3 trimmers. Some years later, roland also used this principle for feeding its hihat units on the later machines cr5000, 606 or 808.

Now the MetalBeat also has this bandpassfilter in the output unit. Although the nonfiltered MetalBeat sounds quite similar to the original MB. l nevertheless experimented a bit and found out, that the coil of an old relais paralled with a capacitor also did an interesting job, hsaping the sound a bit more in the right direction.

Here is a soundfile of the cloned CR78-sounds alone

TB and MetalBeat. The white square thing is an old relais misused for bandpassfiltering MB

the simple, unipolar noisefilter for TB to (sort of) overcome the lack of coils. positive input of opamp gets a half of voltagesupply to lift the audiosignal up.

Midi-Trigger-Interface, BD, SD and Guiro

Midi-Trigger-Interface

I had a 8515 left which was delivered for experiomenting with the atmel-progranmingboard. It has lots of legs. Now it reads Midi and spits out some triggers and drives the 3 leds. Veloicity over 100 activates a negative-high accent trigger. The 2 notes for GUIRO (slow and fast) are sensed for ON and OFF.

BD, SD

They are built rideaway from the scheme. Their out is quite weak, i put the same preamp on them as for the viscountcymbals.

GUIRO

Also built from schematic and like on cowbell problems to get the multivibrator swing. I replaced R550/R549 with a pot for calibration. It turned out that i had to put the filter behind the guiro unit because the leven of noise the unit wants seems to be important to get it work.

There are two wires going into the Guiro. One STOPS the guiro when set high (+5 volt). The other halfens the frequency of the multivibrator when set high.

mainboard with miditriggerinterface, BD, SD and Guiro

for keeping desktop clean i recycled an old salatbowl into a developingunit

Cowbell, HiHats, Cymbal, Zap

Cowbell

The cowbell consists of 2 sine-oscillators which go into the often used one-transistor-vca. The transistortypes seem to be more important here than on the other instruments. I had lots of problems to get them to swing. Had to change capacitors and make the voltagesupply for the sineoscillators adjustable with a pot to 12v, normally they just go fixed to 5V. In the end it worked, although on other frequencies than the original one, but i dont mind.

HiHats

The viscountcymbals are filteres through coils. Because i had some trigger-outs left i decided to rebuild the cr78 hihat and cymbal and filter them through another relais. Came out as quite 'dark' Hihats, good contrast to the viscount cymbals.

Zap

To populate the last triggeroutput i added a kraftwerk-like simple and short 'zap'-voice.

voiceboard with 4 voices

Detail of relais-bandpassfilter

Housing and frontplate

I had some box left from a recently killed synth (had some malfunctions). The front is simple 3 mm wood with a laserprint glued. I only can print A4 so there is a seam in the front which i dont mind.

Everything can be reached from the front. I just HATE to have to climb behind anything to connect something in the dark.

There are 3 pots for

• cr78 BD and SD
• viscount Cymbals
• cr78 percussion

The percussion has 3 additional individual outs:

• Tambourine
• Metal Beat (which normally should read 'Metallic Beat' bur i like the short form better in memory of the John Foxx Song)
• cr78 Cymbals, from which i only made HH and CYM. I dont have coils so i added a relais instead, sounds cool.

Here is a soundfile with all 18 sounds played in a row

a look inside