Odds and Ends

Back to "Music From Outer Space" Analog Synth Pages

This is where I will put little bits of information that don't fit anywhere else and which may be useful.
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Five Octave Switch and One Octave Adjust Pot

This is the general solution for a five octave adjust switch and one octave adjustment pot. The voltage reference creates a stable 5 volta reference which is used by the circuit.

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DC Coupled Level Shifter

This is the general solution for level shifting a signal. The first op amp inverts and cuts or boosts the signal to the desired amplitude. The second op amp re-inverts to restore the original polarity and also allows application of DC offset.

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Coarse Range Control Customize

If your coarse range control is not giving you the range you want then read this. When using +/-15 volts on a project that was originally designed for +/-12V you may need to customize the coarse range control.

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Simple Audio Mixer

You can add more inputs to this circuit. The gain is set to x1 in this schematic. You can change the gain by adjusting the resistor values. By lowering the value of an input resistor you increase the gain for that input. By raising the value of an input resistor you decrease the gain for that input. The gain formula is feedback resistor divided by input resistor. The 47pF is to stop high frequency oscillation of the OP-Amp (you might not need it).

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Using a pot for a voltage divider
(i.e volume control, voltage adjuster, etc)

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5V to 12V Gate Booster

With no connection or with the input to ground (or less) the 2N3904 is turned off (no current flows from collector to emitter). Thus 12 volts via 100K is applied to the base of the 2N3906. This causes the 2N3906 to turn off (no current from emitter to collector). Thus the output sits at ground. When a 5 volt gate is applied to the input (base of 2N3904 via 100K) it turns on and its collector sinks current from the base of the 2N3906 (turning it on). 12 volts is delivered (via 2N3906 and 470 ohms) and applied across the input impedance of the 12 volt gate input (which is probably at least 10K).
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12V to 5V Gate Reducer

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Simplest Oscillator On Earth

U1 is a hex inverting schmidt trigger. There are 6 (hex) inverters in the IC. An inverter means a high input will give you a low output and a low input will give you a high output. With +9V volts on the input you get ground on the output. Conversely, with ground on the input you get +9V on the output. The "schmidt trigger" parts means that the trip point for when the output goes high is input voltage less than 3 volts (1/3 +V). The trip point for when the output goes low is input greater than 6 volts (2/3 +V). This oscillator works as follows: When you apply power U1-A's output goes high because the .01uF capacitor is discharged. U1-A's output charges the .01uF cap through both the 1Meg pot and the 1K resistor. When the .01uF capacitor charges to greater than 6 volts U1-A's output goes low (to 0 volts). This causes the .01uF cap to discharge through both the 1Meg pot and the 1K resistor until... the voltage on the .01uF cap gets to less than 3 volts at which time... U1-A's output goes high again and the cycle repeats and repeats and repeats... you get what I mean. Adjusting the 1 Meg pot causes the .01uF cap to charge and discharge faster or slower thus changing the frequency of the oscillations.

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Know Which Switch Is Which

Switch Images borrowed for educational use from Futurlec, Jameco and All Electronics
Buy lots of parts from them!

I only show toggle switches below because they are both easy to mount and widely available. Slide switches (example at left) of the same ilk (SPST, SPDT, DTDT, etc.) are cheaper and will provide the same function but they are a pain to mount (unless you have a rectangular drill). Shop around switch prices can vary A LOT. If you can find surplus even better! This is NOT a comprehensive switch index there lots of variations and types (foot switches, micro-switches, limit switches, etc.) but this will get you going and cover 99% of your synth-diy needs.

Switch Type Schematic Symbol Example
Single Pole Single Throw (SPST) These just switch one thing at a time. Its either on or off. Search parts supplier sites (or Google) for "SPST" or "switch".
Single Pole Double Throw (SPDT) These switch the center pole to one or the other side poles. They also make this as a special center off version that has three positions. The center position causes the center pole not to touch either pole (thus center off). They are listed in catalogs as either ON-ON or ON-OFF-ON (center off version). Schematics will specifically call out the center off version when it is used. Search parts supplier sites (or Google) for "SPDT" or "switch". Notice that SPDT can be used as SPST if you find a good deal on SPDT switches.
Double Pole Double Throw (DPDT) These act like two SPDT switches with one bat. They also make this as a special center off version that has three positions. The center position causes the center poles not to touch either of their corresponding poles (thus center off). They are listed in catalogs as either ON-ON or ON-OFF-ON (center off version). Schematics will specifically call out the center off version when it is used. Search parts supplier sites (or Google) for "DPDT" or "switch". Notice that DPDT can be used as DPST, SPDT or SPST if you find a good deal on DPDT switches.
Single Pole Single Throw Momentary Push Button (SPST PB). These pushbuttons have two poles. For N.O. or normally open, when you push the button the two poles are connected until you release the button (thus momentary on). For N.C. or normally closed, when you push the button the two poles are disconnected until you release the button (thus momentary off). Top symbol is N.O. and the bottom symbol is N.C. Search parts supplier sites (or Google) for "SPST" or "pushbutton" or "switch".
Single Pole Single Throw Push-On Push-Off (SPST PB). These pushbuttons have two poles. Push it once to short the poles (close the switch) push it again to release the connection. These are good for the power button for your synth. The schematic will specify that the switch is push-on push-off (or latching) if this is the type needed. Search parts supplier sites (or Google) for "SPST" or "push-on push-off" or "switch".
Rotary Switches (nPnT) (that's n poles n throws). These are switches that you rotate to connect the common pole (or poles) to the switched poles. They come in many flavors of nPnT. Here 1P12T (1 pole 12 throw) is shown. Sometimes when you need, say for example, a 1 pole 4 throw you have to buy a 3P4T (or 3 Pole 4 Throw) switch and only use part of it. Search parts supplier sites (or Google) for "Rotary Switch" or "switch".

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No scope? No problem. Use your amp as a signal tracer.

Sometimes you have to just use your ears to see if a circuit is putting out a signal. And you know what? They work pretty darn good. With practice you can tell a sine from a triangle from a square with nothing more than your ears. And you can also hear if it is distorting. And of course you'll hear when a signal is NOT present. Use an alligator clip to connect the ground to you circuit's ground. For the other alligator lead clip a short piece of stripped wire at the other end and use it as a probe. NEVER SCREW AROUND WITH ELECTRICITY IN YOUR BARE FEET OR IN A WET AREA. Insulate yourself from earth when working on projects. Getting shocked is no fun and can be lethal!

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Fundamental OP-Amp Information

The greatest book about OP-Amps (in my opinion) is Thomas M. Frederiksen's "Intuitive IC Op Amps" from the National Semionductor Technology Series. If you can beg, borrow or (and more regrettably) steal one it is worth it. I got mine back in the days when I was working for an implantable defibrillator company in Pennsylvania (Intec Sytems). We had a commercial electronic supply store that would let us browse through a veritable library of data books and have ALL WE WANTED FOR FREE everytime we visited. I had the most extensive library imaginable at the time. Every manufacturer, every technology, I horded them like some kind of information miser. At some point, it may have been after a sharp blow to the head, or the realization that the wall of databooks was beginning to take up a bit too much space, I GOT RID of many of them (... as in threw away). When I got back to my electronic roots and began synth building again I bitterly regretted having done that but thankfully I kept many of the better ones including the Frederiksen and others. National Semiconductor (at least) will still send you some if you request them and will also send you a CD with an extensive library on it. And of course if you have a fast connection to the internet you can get all you want on line anytime. Enough rambling... here is some fundamental Op Amp information.

Here you can see a typical dual op amp (TL082). There are single Op-Amp packages, dual Op-Amp packages, and quad Op-amp packages. The figure shows the Op-amp powered from two nine volt batteries. You can, of course, power the op amp with up to + and - 18 volts from a power supply but this is just an example. There are Op-amps that work with single supplies too. You can combine various gain blocks to do what you need to do. The figure shows how to connect an op amp for inverting or non-inverting operation. Also shown are two methods of connecting the input (AC coupled or DC coupled). You use the one that meets your requirement. Just to reiterate this is really fundamental information for using an op-amp as a gain block. There are literally thousands of things you can do with an op-amp so read the data sheets provided by National and TI because they show TONS of example circuits that you can breadboard and learn from. Also many op-amps will oscillate at a very high frequency when you have a gain greater than one (and sometimes that oscillates). To get rid of the oscillation place a small capacitor (10 to 100pF) across the feedback resistor (the one connected between the inverting input and the output).

Inverted means upside down so when you use an inverting op amp gain block the output is upside down from the input and multiplied by whatever gain you are applying. The inverting input is the one marked with a minus sign (-).

Inverting a signal that is oscillating about ground. (Gain = -1)

Inverting a signal that is oscillating about a positive DC offset. (Gain = -1).

Inverting a signal that is oscillating about a positive DC offset. (Gain = -2)

Some OP-Amp Package Diagrams

As you can see from these package diagrams the inputs and outputs of Op-Amps are clearly marked on the manufacturer's data sheets. You can get the data sheet from most manufacturer's like National and Texas Instruments.