cowbell

This is a super-modified analog cowbell based on the TR808.  Bloat-ware or an educational experience?

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The Roland TR808 cowbell is the 808’s cheeziest sound, but its design is interesting to study because Roland engineers were masters of minimal analog synth design. They rolled this one up tight! … with just 2 simple chips and 4 transistors, the 808 cowbell contains two square wave oscillators, two VCAs, a triggered envelope generator, Hi-Q bandpass filter, and buffer amp. Nice one Roland… but too bad it barely sounds like a cowbell.  Its more of a buzzy-trebly-detuned electronic bleep.  Way back in 1993, one of my favorite sounds on the Boss DR-660 drum machine was its 808 cowbell sample, pitched down 2 octaves so it gets stretched into a melodic statement.  Looking at the TR808 cowbell schematic, its clear that there are many options to bend & tweak its sound.  What if I built a sequencer to activate all of these bends in interesting patterns?

One of my goals is to understand how Roland stuffed all that magic into the analog guts of the 808.  Lets look at the original cowbell schematic and try to understand how it works.  Why the cowbell?  Because its a good example of several analog drum building blocks integrated together.  AND, thanks to that SNL skit, we can all snicker while we read this article and imagine a white-coated Japanese engineer in Will Ferrell’s role.

cowbell and JBoT sequenced by birdbox

Look at the schematics and identify these sections:

• square wave oscillators (white)
• trigger circuit (blue)
• envelope generator (purple)
• VCA (green)
• filter (light blue)
• buffer amp (yellow)

The square wave oscillators are the usual CMOS schmitt-trigger type that you can build with 4584, 40106, 4093, etc.  The schematic specifies their period instead of frequency.   But you know that frequency = 1/period, so what they’re really asking for is FREQ.1 = 540 Hz and FREQ.2 = 800 Hz.  These frequencies are variable with internal trim controls TM1 and TM2.  As specified, the ratio of FREQ2/FREQ1 is 1.48, a  detuned perfect 5th.   This seems like its intentionally out of tune to enrich the tone.

Transistor Q9 is supplying +5V to the 14584 chip via a reference voltage generated by dividing the 808’s main 15V power supply thru R61 and R60.  Since the 14584’s oscillation frequencies are dependent on its supply voltage, an easy mod to tune the cowbell, hats, and cymbal would be to tweak the values of R61/R60, or replace them with a potentiometer configured as a voltage divider.   Although, this would affect the volume of these instruments as well, higher pitches becoming louder…. but I haven’t tried this mod yet.  I just throw away Q9 and run the circuit on +12V…

The trigger generator is a generic circuit that is repeated throughout the 808 for each drum sound, with little variation. Its job is to provide a pulse of current to charge up the envelope generator (EG) capacitors. The amount of current is controlled by the voltage on the ACCENT bus; higher voltage here gives more current into the EG, giving a higher peak volume level.

The EG has two capacitors, C9 and C34. The voltage on these caps decays as it bleeds away thru the VCA, giving an exponential volume envelope. In fact it appears that the cowbell envelope has two time constants, because R82 is in series with C34. The smaller value of C9 will cause a faster decay, for an initial ‘punch’, followed by the delayed decay of C34 as it bleeds thru R82, giving a longer ‘tail’ on the sound.

The “swing-type VCA” is very minimal.  In fact, there are two VCAs - one for each oscillator input.  Before the EG has been triggered, Q14 and Q15 are not conducting and therefore the VCA is silent.  When the cowbell is triggered, current from the EG flows through R28-D3 and R29-D4, lifting the collector voltages of Q14 and Q15 up so they may conduct.  The cowbell oscillator frequencies are coupled thru the bases of the transistors with caps C32 & C33, and thus assume the volume envelope presented by the EG.  The tradeoff for such a simple design is that the incoming audio is half-wave rectified as it passes thru D3 and D4.  This isnt a VCA you’d want to use for typical purposes because it would severely distort normal audio.  But since we’re dealing with square waves only, the rectification effect is useful to add color.

The BPF is a Bridged-T RC network around opamp 2. Variations of the bridged-T circuit are used throughout the TR-808 as both filters and drum oscillators (i.e. snare, toms). In this case it is used as a filter. The cowbell frequencies, after being gated thru the VCA, are mixed together at the junction of the four capacitors C28-C31. The center frequency of the BPF is easily tunable by varying R24.  If you look at Figure 11 below, you’ll see similarity between the cowbell’s BPF and the generic 808 drum oscillator in the figure.  Our BPF has a high enough Q-factor (resonance) that it adds a thump of its own when the cowbell is triggered.  This is a damped sine wave oscillation and seems to simulate the air cavity resonance of a real cowbell… an important effect that adds to the overall sound.

Finally, there is a seemingly unnecessary buffer amplifier made of opamp 6. It is configured for unity gain and seems to be used only to lower the output impedance of the cowbell circuit to 1-kohm before it hits the CB OUT jack and the 808’s mixer section.  I left it out of my design.

Ok … so its still a little foggy how the VCA and BPF work?  The TR808 Service Manual actually explains it in detail.  This is from back in the day when they built things to be fixed.  Actually its almost like a textbook; these two building blocks are the heart of many 1970’s and 80’s analog drum machines. They even give you the math to design your own bridged-T networks!  Here’s the diagram:

So Where Are The Bends??

The following figures show how to modify the original circuit to change its oscillator frequencies, filter, envelope, and accent.  Changing the capacitors circled in red is one way to adjust the oscillator frequencies.  Changing the resistor circled in light blue does interesting things to the filter.  Cutting the wire where the purple “X” is shortens the decay time.  And by varying the voltage on the ACCENT bus, the cowbell’s volume can be controlled.

Awesome.  But I don’t have an 808 to bend, so I’m just going to build this from scratch.  And since that means plenty of room for mods, why not get crazy and make it bend itself?  If I had an 808, I’d probably just install some pots and toggle switches to control the bends manually.   But this is really an exercise in design, so I get to test out some other ideas I’ve had recently.

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