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Beat Box / Rhythm Machine
How to modify to get individual outputs
Question: How much would it cost to get info on creating individual outs on my AceTone Rhythm Ace?
Answer: Thank you for your donation. To create all materials presented here took me almost 3 hours: search my files and pull materials, scan and edit, write this text. Much more time than expected and what your donation might have paid for, but I hope this helps to get your project going and completed. I am also available for specific questions.
1) As you likely already know: The unit is comprised of 2 boards: The rhythm "computer" and the instrument or drum sound generation board.
2) basically each drum instrument is excited by a trigger pulse that generates a short oscillator tone (for drums mainly comprised of a L C network).
3) The outputs of all instruments are mixed together with a mixing resistor onto a mixing sub bus. The mixing resistor connects at one end to the instrument and with the other end to the mixing bus. The machine uses pretty high impedance circuitry for the soud generation circuitry, which means that a buffer amp will need to be added per output, which means additional circuitry for 12 instruments needs to be fabricated. Thus the goal: hook into each instrument at the point before the instrument feeds into the sub mix bus and add a high impedance buffer amp to this point for each instrument .
My suggestion would be to find the mixing resistor for each of the instruments and then connect a high impedance resistor, such as 100K (or higher as not "load" the instrument circuit, which would alter the sound) at the end of the mixing resistor where it is connected to the instrument, to feed into a high impedance buffer amplifier. One buffer amp per instrument for which you want an output.
Buffer amplifiers can be easily constructed by using operation amplifiers. There are several op-amp ICs that house 4 op-ams in one 14 pin case, such as the LM324 (or 2 in one case, such as RC4558 LM4558, etc). Op amps however use a balanced power supply of usually +/- 12V. The Rhythm ace operates on about -18V (ground positive - use signal coupling capacitors to de-couple the ground negative circuit for the output buffer amps).
The most straight forward way to construct op-amp based output buffer amplifiers is information gathered form Forrest M. Mims III OpAmp cookbook, (Enfineer's Mini Notebook Op Amp IC circuits Radio Shack part # 276-5011) which explains the basics and has circuit examples.
If uncertain where to find the output of each instrument on the instrument board: a Crystal ear phone (can make suggestion on where to buy) is a high impedance device that can be connected to most any circuit point on the board. The other pin would be connected to ground and monitoring to through the ear phone allow to listen to various points. This is a fine piece of test gear for various work on audio gear. (continued below...)
|I created the following support documents:
1) Schematic of the Korg M7 rhythm machine, which is very similar to the Ace Rhythm in construction. (Don't have the schem for the Ace) Have look: the point labled in the schematic as "Pre-In" is mix output of the various instruments. Observe that each output has a resistor the bus between 82 and 330K.
2) A modified schematic of an audio distribution amplifier that I once designed. The schematic I modified today for you to get to an approximate buffer amplifier for your application. The power supply section needs more detail. Shown is basically ho to get +/- 12 from a single winding transformer. (You could tap into the AC output section of the transformer in the Rhythm Ace) Power supply filter caps of about 470 uF will have to be added. the RF filter caps are 47n.
The buffer amp has an input de-coupling cap. Likely it's polarity has to be turned around, as the Rhythm Ace has ground positive. The point before the mixing resistor of each instrument would get connected to the input coupling cap of the buffer amp. The output of the buffer amp to feed to the corresponding 1/4" separate instrument output.
3) An exerpt of the above mentioned Forest Mims book.
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