hexinverter AteOhAte: Mutant HiHats User manual

HEXINVERTER.NET
presents:
AteOhAte: Mutant HiHats
a dual analogue hihat module based on the circuitry of the TR-808
ASSEMBLY MANUAL v1.0

Introduction
This manual exists to aid DIYers in creating their own AteOhAte: Mutant HiHats module from a bare
PCB acquired at hexinverter.net
Please note that this is an advanced project and a lot of mechanical and electronics ability is taken
for granted. If not purchasing a dotNET 4U format panel from me (coming soon), you will have to
craft your own panel and of course decide how to mount the PCB to the panel when you are done.
You will need to figure out how to mechanically construct your module. I have made a video
demonstrating how I make brackets from sheet metal to mount PCBs to front panels. Find it at the
hexinverter.net DIY FAQ by clicking (HERE). Should you have any further questions, please ask the
Muffwiggler synthDIY forum! Someone there will be happy to help you!
Technical Notes
The AteOhAte: Mutant HiHats circuit is based on the analogue circuitry of the legendary TR-808
drum machine. Functionally, it can create sounds virtually identical to the 808, however, a number of
features and enhancements have been added that can transform the sounds created into something
different! Most notably, voltage control over the decay parameter for the open hihat has been added.
This effect is achieved using a VTL5C3 analogue optocoupler (aka: “vactrol”):
A control voltage at the CV input is summed with the initial decay amount from the potentiometer
by opamp summer IC6A. This voltage is inverted and differentially amplified with an offset voltage
created by trim potentiometer T1 by opamp IC6B. This voltage level is converted to a current by PNP
transistor Q18 and associated components and is allowed to flow through the indicator LED (T.LED)
and vactrol (LDR2 on the schematic).
Simplified, this circuit turns a control voltage into a resistance at the vactrol’s output terminals. A
higher voltage at the input of the circuit produces a higher resistance at the vactrol’s output. This
higher resistance results in a longer decay time.
The response of the decay time to the input control voltage is not exactly linear or perfect. Some
careful tuning of trim pot T1 will be necessary to get the response you want, and, the input is quite
sensitive to CV. This is a good thing in my opinion and simply means you may have to dial down more
“hot” modulation voltages with an attenuator!
The indicator T.LED should be used as a tuning aid to dial in the proper offset voltage for the vactrol
driver circuit. If you use a red, orange or yellow LED like I suggested in the Bill of Materials, T.LED
should serve as a status indicator for the vactrol’s internal LED. To tune the circuit, follow these
easy steps:
• Remove any decay CV you might have plugged in
• Turn the decay control all the way up (clockwise)
• Adjust trim pot T1 while watching indicator T.LED
• T.LED should just begin to light when you turn the decay control back down
• Try some input CV sources and see how you like it. Adjust T1 to to get the response you want!

Notes About Parts/Substitutions
Vactrols aren’t sold by most large electronics distributors, but many small shops still sell them.
Look around and you will find some! You can get singles on eBay, even. If you cannot find anything
from a shop, post in the Muffwiggler DIY forum and I am sure someone will have one to sell you. I do
not have any to sell so please do not ask!
You should change R87 to about 590R if you’re using 15V.
You can substitute different transistors for the BC549/559 without any perceived changes, but make
sure the pinout matches by comparing the datasheets!
Every other part is pretty common and you should be able to find them at all retailers. I do not
recommend substituting other component values. Because this design uses resistors, capacitors
and transistors to create the hihat sounds from scratch, different component values will alter the
sound.

PCB Overlay

Wiring Diagram