Btukfyl

I built a prototype keyboard/sound synthesizer using a chain of 13 astable multivibrator circuits whose outputs are connector to an audio amplifier chip (LM386) and speaker, all powered off a 9V DC battery.

Each individual circuit gets tuned to one of the 13 frequencies in a musical octave (C5, C#, D, etc. up to C6) by varying a fine-tune trimpot in series with specific resistor values that get the oscillation into the ballpark frequency.

The oscillation is the classic BJT astable multivibrator you can see in Figure 1 here and is explained in this article.

The prototype stays correctly in tune for a short period (up to a day).

You can hear what it sounds like here.

What I can't figure out is why the circuit appears to get spontaneously detuned, i.e. one or more of the individual circuits end up with frequencies that are different from what they were tuned to (used an oscope, and then a reference piano).

The frequency deviation of the detuning is typically 2-5% which is audible noticeable (e.g. C5 at 523Hz might wander to 540Hz or 510Hz). Note - the detuning never occurs while playing. It is only several hours later, one returns and the keys no longer sound the same.

I had originally thought maybe the trimmer pots were mechanically relaxing by themselves. To eliminate this I replaced the trimmer pots to try to "lock in" the frequency based on resistor values alone since there's no variability left in the design.

But the de-tuning problem persists even after replacing the trimpots with fixed resistor values.

Any suggestions appreciated. Thanks.

You have clearly built a circuit that is fully analog in nature and produces a frequency in each oscillator that is dependent upon various factors such as:

1. Changes to the voltage level of the power supply to oscillator.


2. Changes of the Vbe level of the transistors with temperature.


3. Changes of the values of resistors over time and temperature.


4. Changes of the values of the capacitors over time and temperature.


5. Stray circuit behavior changes due to things nearby the prototype.


6. Position of the moon relative to the sun as viewed from earth.

There are ways to build circuits that do not have as much drift in operational frequency. They are designed to eliminate or cancel out the various effects enumerated above. One conventional way is to design a circuit that uses as single higher frequency oscillator based upon a close tolerance crystal. Then the use of digital counters are used to divide this frequency down to the desired frequency for each note in the scale.

To show the value of a digital circuit approach I created a small spreadsheet that shows the octave of musical notes from C5 through C6. (The nominal frequencies are integers taken from a chart found on Google and not the actual frequencies that you would get if you computed the notes with scale formulas from the A[440] reference).

Using a crystal frequency of 22.1184 MHz (which is a common MCU frequency used in the 8-bit embedded business) you can see that with an integer digital divide factor for each note that the generated frequency is very close to the desired nominal.