CAUTION

But any color organ that plugs into the wall deals with hazardous voltages. If you aren't prepared to know and follow high-voltage precautions, you shouldn't be poking around in a line-operated appliance!

Preliminary Examination

• Is it physically damaged? Cracked case or busted parts? Waterlogged?
• Is it plugged in and turned on?
• Does the pilot light come on?
• Is the fuse OK?
• Are the lamps plugged in to it?
• Have you tested the lamps to make sure they work?
• Did you overload the output?

For testing color organs, I have found it handy to get a set of small, identical incandescent lamps. Small night-lights from your local discount store would be fine. Plug them all in, power up the color organ, and see if some of the channels work and others do not.

You Need A Schematic

General Theory Of Operation

Fancier color organs have multiple channels, each of which is sensitive to a different sound frequency (pitch) range. The high note of an alto flute might light a red light, while the low-pitched thump of a bass drum might light a blue light.

General block diagram of a three-channel color organ.

Why Many Color Organs Are Poor

How can color organs get away with not working properly? I believe that the reason for this is that color organs are simply not a demanding application, so defects are not readily noticed.

Consider a very high-end color organ with 12 channels, with a light for each channel lined up in a row. The color organ is running at a piano recital, part of which includes a smooth run from the lowest note of the keyboard to the top. As the notes run higher, the lights follow, and the crowd loves it. This is a demanding application. I'll bet this kind of thing is rare.

Consider a D.J. using a common three-channel color organ. Each channel drives a couple of PAR cans, shining on the dance floor. The music is loud and fast. So are the dancers. Do you think that anybody will really notice precisely how the lights correspond to the sounds? Do you think anybody will really care?

So, most of the time sloppy operation is good enough. It's cheaper and easier to design and build something sloppy. And if nobody ever notices a defect, they will never complain, and the defect will never be resolved.

Low Overall Sensitivity

Color organs vary in how they get the sound input that drives them.

• Some use a microphone, which is quick and convenient to set up, but can pick up background noise.
• Some color organs are wired directly to the sound source, such as your stereo. This eliminates background noise, but brings up a new problem: you don't want a malfunctioning color organ feeding 110 VAC into your stereo and blowing it up.
  • Some circuits use transformers to isolate the stereo from the color organ.
  • Some color organs have a low-voltage section that interfaces directly with the stereo, a high-voltage section that controls the lights, and an interface in between.
  • The modern solution is to use an optocoupler (also known as an optoisolator).

But before we go into the details, please make sure that:

• You have hooked up the color organ to the right type of audio source. If the unit is designed to run from speaker-level signals, it won't work when plugged into the "line" output.
• Have you tried turning up the volume?

Microphone-Input Color Organs

• electret microphone (also called a "condenser" microphone)
• crystal microphone
• small loudspeaker, used as a dynamic microphone

Transformer-Isolated Color Organs

Two-Section Color Organs

Optocoupler-Based Color Organs

Here are some typical optocoupler-based input stages:

Low Sensitivity On Some Channels

• Your sounds don't extend into the region that the channel is designed for.
• The channel's filter is listening to the wrong part of the frequency spectrum.

Your Sound Is Off

Envision a nice three-channel color organ:

frequency range	light color	responds well to
high	red	alto flute
medium	yellow	trumpet
low	blue	bass drum

You're in the middle of a concert, and everything is going fine.

Then, there is a flute solo and all you see is red light for 10 minutes.

You have several choices on how to deal with this:

• Enjoy the red light. Sooner or later, you'll get 5 minutes of blue light when the bass drum takes over.
• Turn up the sensitivity of the other channels, so that the little bits of medium and low frequency sound that are there produce a visible response.
• Shift the frequency response of the other two channels (or perhaps all three), so that all the high-frequency band occupied by the flute is itself broken into smaller sub-bands, one per channel.

If you want to preserve the general-purpose nature of the color organ, you will probably want to leave it alone, or perhaps turn up the sensitivity on the lower channels.

General Theory Of Frequency Selection

These are the schematic symbols for "capacitors". They acts like resistors (capacitive reactance) with a value depending on the frequency of the AC signal. The higher the frequency, the lower the resistance.

Capacitive reactance is given by the formula:

X_C = 1 / (2πfC)

• X_C = capacitive reactance, measured in Ohms
• f = frequency of AC in Hertz
• C = capacitance in Farad

These are the schematic symbols for "inductors", They act like resistors with a value depending on the frequency of the AC signal. The lower the frequency, the lower the resitance.

Inductive reactance is given by the formula:

X_L = 2πfL

• X_L = inductive reactance, measured in Ohms
• f = frequency of AC, in Hertz
• L = is the inductance, in henry

This is a simple L/C "lowpass" filter. Low frequencies encounter little resistance from the inductor and pass easily through. Any high frequencies that get through the inductor are shorted out by the capacitor.

This is a simple L/C "highpass" filter. High frequencies encounter little resistance from the capacitor and pass easily through. Any low frequencies that get through the capacitor and shorted to ground by the inductor.

If you prefer to think of it in a slightly different way, this is a voltage divider.

This voltage divider uses resistors.

The output voltage (taken across R2) is R2 / (R1 + R2).

The L/C circuits are also voltage-dividers. Instead of using resistors, the effective resistance is provided by frequency-sensitive components.

Well, inductors cost a few cents more than resistors. I have yet to see a kit that uses them. Instead, kits tend to use R/C filters, where a voltage divider is composed of a resistor and a single frequency-sensitive component (a capacitor). The filter is not as precise and sharp, and that's probably just fine for the intended purpose.

For the sake of discussion, we will consider a color organ that responds to frequencies from 500 Hz to 2 kHz.

Changing The Low Frequency Channel

Changing The Mid Frequency Channel

This is the mid-frequency channel of a design that will go nameless.

This design would probable be acceptable if it used an inductor instead of a resistor. The inductor would short out low frequencies, the capacitor would short out high frequencies, and we would be left with some stuff in the middle.

But the substitution of a resistor instead of an inductor ruins things. All we have here is a poor low-pass filter, with additional across-the board attenuation. Yuck!

This is the mid-frequency channel of another design that will go nameless.

I believe that the designer was trying to make a band-pass filter by routing the signal through first a low-pass filter (boxed in blue), and then a high-pass filter (boxed in red). Unfortunately, the ordering of the components in the schematic make little difference to where the electricity is going. Looking closer at the filter, and rearranging the components a little...

We really have a resistance and capacitive reactance in parallel. Ultimately, it provides a combined resistance with a value depending on the frequency of the signal.

The same R/C combo, with identical values, appears on the top and bottom. This forms a voltage divider that wastes half the signal coming in to it.

Since the top and the bottom have the same value (which varies depending on the frequency), the circuit will always divide the signal in half. The divider will just draw more current at high frequencies.

Changing The High Frequency Channel

Output Driver Issues

Things to check:

• A well-designed color organ has a fuse per channel. Make sure that all fuses are OK. Replace only with a fuse with the same rating.
• Check the TRIAC, SCR, transistor, or whatever driver to make sure it works. If you don't have facilities to test the part, swap the part with one from a working channel.
• Note that the surest way to blow a channel in a color organ is to try to drive more watts than the unit is rated for. It might look really nice for a minute or two, then it just stops working.

Related Pages