Hello to Friends -

Audio Amplifiers.

HERE is a very basic circuit which can be modified in dozens of ways, to make useful effects, by adding very simple components. A simple op-amp may be useful to make INTERESTING GAINS - interesting sounds and noises. None of these signals is big enough to shock or bite you, nor burn you.

Parts needed: A couple Dual Operational Amplifiers (op-amps) such as LF412, LF353, or TL082. Preferably in mini-dip packages. AND, an LM358N. And several sockets for them. Mini-Dip sockets are fine. You'll probably need two 9-volt batteries, and sockets for those batteries, and switches to turn off the batteries when not in use. You'll need a few resistors (1 or 5 or 10%) such as 20k, 50k, or 100k ohms, and a couple 1-turn potentiometers (usually called "pots"), such as 50k (but 100k or 200k could be used.) You'll also want a few RCA phono plugs, to connect to your existing amplifier jacks, inputs and outputs.

You need to connect the output to a conventional audio amplifier  and speaker, such as a "stereo  amplifier" or "hi-fi amplifier", plugging into the "AUX" or Auxiliary input, or to any input, such as the "tape" input, but preferably not the "Phono  input" - that is often too sensitive. When you are ready to start, turn the volume down, most of the way.

You can use the same batteries and switches for all various circuits, but you might use new sockets for each circuit, and move the amplifiers around.

Apply a volt or so of input signal to Vin. THIS could be from a sine wave generator, triangle, or square wave, or radio, or tape deck or CD player.

The input resistor, Rin, is chosen as about 100k. The feedback resistor is Rf.

If Rf = Rin, the gain will be about 1, so you might not be able to distinguish between Vin and Vout, using S1. (S1 need not be a real switch. You can use any clip-lead to connect to one signal or another, or an alligator clip to just clip onto various signal leads.)

What if you start adding components? Cf? Small, Big?

What if you add a load resistor? It might not make much difference because operational amplifiers are good at tolerating resistive loads as low as 2 kilohms. Adding a load heavier than that (lower ohms)might cause distortion and waste battery power.

What if you add a resistor across Rf? This will cut the gain a lot, if Rf gets low enough. You might have to turn up the gain.

What if you add a resistor across Rin? This will increase the gain a lot, if the total conductance of Rin gets low enough. You might have to turn DOWN the gain, after the output.

What if you add 1 or 2 DIODEs at Z1? If the signal is small enough, not much change occurs, but with a bigger signal (~ 2 volts) the diodes at Z1 can have a big effect - this is called DISTORTION.

Another kind of DISTORTION is called clipping. You can fool around by adding diodes at Z3 and/or Z2. If you put in BOTH diodes, the clipping will be significant. No matter how big you make Vin, the output doesn't get much bigger. Set aside the diodes for a while.

If you add a capacitor at CF (Z3), and bigger and bigger ones, the high-frequency sounds will be rolled off. This is called a Filter. It passes low-frequency signals, but rolls off the fast ones, a LOT.

If you add a capacitor at Z1, it will cause a high-pass filter. The gain for high frequencies will be increased. Be careful as a large capacitor might cause very loud noise and even oscillation! (a constant loud noise and tone, even if there is no incoming signal.)

What if you add BOTH an input capacitance across Rin and a similar feedback capacitor across Rf? Do the effects cancel?

One of the reasons these operational amplifiers are popular is that their gain depends on the RATIO of feedback and input resistors or capacitors. The actual resistor or capacitor is not important, just the ratio. Not all audio circuits use Operational Amplifiers, but some do.

DISTORTION. Turn off power and use a small knife and pry the TL082 out of its socket. Ease an LM358 into that socket. Check carefully to see where its pins are located, for pins 4 and 8. Apply power. See Fig. 5. Apply signals. The LM358 is usually considered a LOUSY (that's a technical term) high-distortion amplifier. If you put in a sine or triangle wave, the distortion will be obvious.

Now if you connect a 2-kilohm resistor from the output to the - 9-V supply, (you can do this with your fingers - nothing will bite!) - the distortion will go away. If you connect it from the output to the + 9V supply, the distortion will also go away. The LM358 is one of a very few amplifiers that will distort like that. It's very efficient, and low-power, and inexpensive, but it does not have low distortion for audio signals unless you put an extra "pre-load" on its output.

Part TWO: Filters

There are many kinds of filters. They change the Frequency Response. They add or cut, the Treble (high frequencies) or the Bass (low Frequencies).

Figure 2. shows a basic BAND-pass filter. It passes signals in the MIDDLE of the signal bandwidth. Also, this is an ADJUSTABLE filter. You can fool around with it, and see what interesting effects you hear. Adjust the 1-turn pot at R202. THIS makes it easy to adjust the center frequency and the Quality (Q) of the circuit. The gain is always 1, due to the ratio of R203/(2 x R201). The bandwidth is always 1/(3.14 x R203 x C), or about 160 Hz. That's a pretty narrow bandpass! (Considering that audio frequencies usually go down past 100 Hz and go up above 10,000 Hz!) This is a very versatile bandpass circuit because trimming R202 does not change the bandwidth, but does change the center frequency. You can trim R202 and move the center frequency around, for interesting effects.

*** If there is any adult who can help you understand the shapes of the signals, that might be nice. If you know an electrical engineer, engineer, or electronics technician - or even a science major, they might help you interpret what you are hearing.

*** If you can find an engineer or technician who can loan you an oscilloscope, for a few hours, that might help you understand what you are looking at. Do fast wiggly signals sound high-pitched? Do slow wobbly signal sound "low-pitched"? This is likely to make sense if you think about it.

If you compare Vin versus Vout on the 2 inputs of a scope, you can see the phase shifts between the input and the output. Or if you can borrow a dual-trace scope, you can see the input and the output at the same time. An engineer, technician, or scientist can help you interpret what you are looking at.

Figure 3. If we add the BAND-pass filter to a subtractor,this this can be a BAND-reject filter, often called a NOTCH filter, as it rejects mid-frequencies. Play it by ear, as you adjust the R202. Which settings sound the most interesting?

Figure 4A. shows an ALL-PASS filter. It passes low frequencies, high frequencies, and mid frequencies, but it screws up the phase response. When you put the signal through 2 in series - does it sound weird? Try different pot settings.

Figure 4B shows a similar ALL-pass filter. It likewise passes all frequencies, but with a leading phase response, different from the circuit of 4A. Can you hear a difference?

Sometimes people make a cascade of several all-pass filters in series, one after the other adding their phase shift. Have you got several pots? I gotta see if I can hear all those effects.

If this is for a YOUNG student, he/she should request MORE advice from a parent, or older friend, or engineer or technician. Older students may need less supervision. They may be able to explain why things sound "weird"!

NOTE to the student. Please make NOTES on each circuit, each experiment. Mostly so you can go back to where you were, if you find something interesting.

Figure 5. just shows how to connect the + and - 9 volt power to all circuits. Be sure you don't put in the amplifier backwards, or it will probably be damaged, and would hurt the batteries.

Construction ideas: If you have a soldering iron, you can build these circuits on a "breadboard" - or on a copper ground plane. If you have clip leads or a "solderless breadboard", you can use that, as these circuits are not critical about big or small signals, or high impedances. When you think you have finished a circuit, check to make sure you have made it right, before you turn on the power. Make sure the power supplies do not get shorted out, as the batteries can be ruined quickly by that.

Best wishes! Have fun!
Robert A. Pease, Electronics Engineer
December 2004
Posted ~ January 2005.