The musical signal's form does never resemble a square wave. The frequency range perceived by an average adult hardly goes above 17KHz. Hence I do not give a dime to those heated discussions whether it's appropriate to test audio amplifiers using a 100KHz meander signal. But being an electronic engineer in my heart as well as by education and passion - I can assure you that giving an audio amplifier a try with 100KHz square wave test signal can reveal quite some technical qualities (or lack of those) in the design. Overshots caused by negative feedback loops or signal slopes formed by input/Miller capacitances are amongst things that are easier to observe with meander than when using sound-frequency test signal.
Wasn't it too complicated?
Before turning to the true and tried CMOS 555 I tested several oscillators based on old soviet К561ЛА7, К561ЛН2, as well as 74HC04 and 74HCT04; the last unsuccessful attempt was made using LM555. All the attempts listed above have failed due to the horrible ringing the chips were generating at its outputs. The current spikes in the power line were also quite hard to tolerate. Therefore I came up with only two budget solutions:
Use a low-bandwidth scope so one does not see high-frequency oscillations (it's a joke)
Utilize CMOS version of 555
Important: in this design only a quality CMOS variants of 555 timer should be utilized. For example: TLC555 datasheet by TI.
To my taste here is one of the most clear representations of what ticks inside 555:
People who love micro-controllers may start laughing at me here. Honestly I was also tempted to build a universal square wave generator based on ATmega-8 that I had at hands. But somehow I am too lazy to program when the same result can be achieved by a comparable amount of soldering without turning on that annoying thing (computer ;). And to tell you the truth I strived to have as minimal of those high bandwidth current spikes as possible - in order to test quality audio equipment I wanted to have a quality test signal as well. Regardless the fact that the generator's bill of materials amounted for just a few bucks.
The simpler the better
While publishing this pretty trivial design I still hope that it may help some good people to save time on experimenting with more noisy alternatives.
Square wave test signal generator
C1 = 1 nF
R1 = 6.2 kOhm
R2 = 1 kOhm
R3 = 300 Ohm
R4 = 5 kOhm
C2 = 1 uF
C3 = 10 uF 25 V
C4, C5 = 0.1 uF
Obviously testing audio amplifiers with the test signal well centered relative to the ground is very convenient. On the other hand adding DC shift helps to qualify how well the servo-circuitry works - the one that is responsible for assuring the steady zero DC at the output. Thus having a choice between "open" and "closed" output was a good thing to build in the test signal's generator: just install a switch or a "jumper" that would short C2.
Why restricting ourselves to single frequency?
In theory the generator's frequency can be calculated as follows:
f = 0.72 / (R1 * C1)
However in the reality the obtained frequency tends to be lower, especially at the higher end of the generator's range.
Once I heated up my soldering iron I did not want to stop with a single 100KHz frequency. By adding a simple line of DIP-switches and few extra capacitors and resistors we are able to cover pretty wide frequency range.
I settled on the following set of R1 and C1 switchable substitutes:
That gives us a convenient choice of generated frequencies:
250 kHz - 1 nF 2.2 kOhm
100 kHz - 1 nF 6.2 kOhm
30 kHz - 10 nF 2,2 kOhm
10 kHz - 10 nF 6,2 kOhm
3.1 kHz - 0.1 uF 2.2 kOhm
1.1 kHz - 0.1 uF 6.2 kOhm
465 Hz - 10 nF 150 kOhm
46 Hz - 0.1 uF 150 kOhm
4.5 Hz - 1 uF 150 kOhm
Of course the frequency values are very approximate, it depends on the used components variations.
The finished generator circuit can be fed from 4 AA batteries or a simple wall-brick. I strongly suggest you staying away from switching power supplies here and use an old good low frequency transformer. Since I like good CMOS 555 a lot - I added the reverse polarity protection to this design.
Square wave generator with selectable frequency
Kynar wire "web" routing - inexpensive and quick
Left mouse click on pictures will show them full-size