Deep Dive into the Theremin v.2 Audio Amplifier

See YouTube video https://www.youtube.com/watch?v=90T0ZkN-oos&t=3s for further details.

audio_amp_schematic

The Theremin v2 audio amplifier and preview audio amplifier share the same schematic.

The input of the amplifier is on pins P1 and P2, where P2 is the ground connection. The input is supposed to be DC decoupled with a small electrolytic capacitor, as suggested by the data sheet of the integrated circuit TDA2003, which is the main component of this amplifier. However, the capacitor was not put in this schematic because it will find its place directly on the output of the audio generator stage. No reason to have two capacitors in series: one at the output of the audio generator and one at the input of the audio amplifier.

The signal coming from P1 goes to a voltage divider consisting of the resistor R1 and the potentiometer R2. When the potentiometer cursor is all the way toward ground, there will be no signal going to pin 1 of the IC and, therefore, the amplifier will be silent. When the potentiometer cursor is all the way toward resistor R1, the signal coming from P1 is divided exactly in half, since the value of the potentiometer R2 and of resistor R1 are exactly the same. This is the position where the volume will be the maximum possible.

I chose the value of 100K for both R1 and R2 so to have a relatively high impedance toward the audio generator, so its output won’t be affected by the movement of the potentiometer. At the same time, I wanted a value of R2 low enough to keep the input of the amplifier solidly on the ground when there is no input signal.

R2 is a logarithmic potentiometer, which means that its value changes according to a logarithmic scale. This is done to compensate for how we perceive the sound volume: the potentiometer increases the power of the sound wave in the opposite way as our ear perceives it. This way, we feel that the volume increases and decreases linearly when we turn the potentiometer.

The output from pin 4 of the IC goes to the loudspeaker through capacitor C3, which is used to prevent that a DC current goes all the way through. In addition, capacitor C5 and R6 provide a high pass filter that shorts to ground all the high frequencies that are not supposed to go to the loudspeaker. Values of capacitors C3 and C5 and the resistor R6 reflect the suggestion on the data sheet.

data_sheet_schematic

The power supply comes in through pins P5 and P6 and is filtered by capacitors C1 and C2.

C2 takes care of shunting toward ground any unwanted high frequency signal coming from/to the power supply. Capacitor C1, which is a high value electrolytic capacitor, is mostly used to boost the input current when the amplifier requires sudden increases due to peaks of the volume. These capacitors have also the values suggested by the data sheet.

The last part of the amplifier is the negative feedback loop, connected between pins 4 and 2 of the TDA2003. The loop is composed of capacitors C4 and C6, and the resistors R3, R4, and R5.

The negative feedback is used to limit the output power of the amplifier outside the region of frequencies where it is requested, thus preventing auto oscillations of the amplifier, which would otherwise behave like an oscillator.

The values of these components are again those suggested by the data sheet. However, C4 and R5, which correspond to Cx and Rx in the data sheet, are calculated based on the data sheet formulas visible in the above picture, where B is the maximum frequency of the signal that we want to amplify, which I set to 10kHz. I don’t believe we want to hear from the Theremin sounds that are above that frequency.

For further information on this amplifier and for a demonstration on how it works, please refer to my corresponding YouTube video.

Theremin v.2 Power Supply Design

theremin-v2-power-supply

For the new version of the Theremin, I have chosen to use a dual 12V power supply. This will have more flexibility because it will allow me to use more sophisticated units, possibly using op-amps.

The circuit is very basic: it uses a dual 14V transformer (not shown in the schematic) capable of providing 1.5A at its output.

A dual transformer is made up as in the following picture.

center-tapped-transformer

Is has a primary winding that is connected to the AC power supply outlet, and a secondary winding with a center tapped wire that is usually put to ground on the low voltage circuit side.

Voltage between either end wire of the secondary and the center tapped wire is usually the same (with the exception of specifically made transformers), which we call V.

The voltage measured between the two end wires of the winding is instead two times V or 2V.

Sometimes, instead of having a single secondary winding, we have two, carrying the exact same voltage. In this case, we can connect together the two closest wires and consider that as the center tapped wire. Then everything works as the first kind of transformer.

transformer-trans64

The AC current of the transformer is converted in to a DC current through the usage of a bridge rectifier and the capacitors C1 and C2.

The bridge rectifier converts the sine wave coming from the transformer into a fully rectified wave.

Full-wave_rectified_sine

Then, the capacitor that follows (in this case C1 and C2) starts charging over the ascending sides of the wave and discharging, partially, over the descending sides of the wave, basically filling the wave in between crests and making it look like more a straight horizontal line with some disturbance in it that we call ripple (the red line in the following picture).

ripple

In general, depending on the use of the power supply, we define a maximum value of the ripple that the circuit can handle.

In our case, we need to make sure that the voltage at the input of the regulators never goes below 14.5V, according to the data sheet, otherwise the regulator will not function properly.

The peak voltage provided by the transformer is its RMS value multiplied by the square root of 2, or:

peak_voltage

The minimum voltage we can have at the input of the regulator is:

regfulator_input_voltage

This is the max value of ripple that we can sustain.

To calculate the capacitor necessary to obtain this ripple, we use the following formula:

capacitance calculation

where f is the frequency of the alternate current which, in the USA, is 60Hz, and Ix is the maximum current that the power supply needs to provide.

So, we would need a capacitance value, for C1 and C2, of 2358uF.

However, the Theremin circuit will really not draw 1.5A from the power supply, so we can stay a little conservative, and use the closest value below the calculated one, which is 2200uF.

At this point we can safely say that the voltage on the output of the regulators will be exactly 12V (positive or negative, depending on the output side).

To further help the regulator, and preventing the current through it to go too close to the 1.5A threshold, where the regulator would not work anymore because the ripple becomes too high, we add to the output of each regulator another electrolytic capacitor, this one with a value at least equal to the capacitance value that we did not put at the input side. Since at the input side we put a capacitance of 2200uF instead of 2358uF, we will need a capacitor of at least 158uF.

However, to stay totally safe, I decided to use a capacitor at least 5 times higher, so I used the value of 1000uF for C3 and C4.

And finally, I added an extra capacitor (C5 and C6) to shunt toward ground any RF frequency that would travel back from the Theremin oscillators toward the power supply. A 0.1uF value is what is suggested by the data sheet of the regulator, so I used just that.

Why did I use this capacitor if there was already a 1000uF in there?

The reason hides in the way the electrolytic capacitors behave. In short, the electrolytic capacitors do not work well at high frequencies, so we need to add the extra 0.1uF capacitor, which is not an electrolytic one, to work in that range of frequencies. And since the range of frequencies is much higher than the one of the 110Vac outlet, a very small capacitance is enough to do the job.

 

Theremin v.1 Pitch Reference Oscillator Analysis

This is my first post of this kind, which I plan to use to give insights on how an electronic circuit is designed. In the future, I plan to post a similar article for every new circuit I design and present on my YouTube channel.

This way, people who are interested in such details can go to this site to get them, thus sparing all the others that don’t care from watching them on the original video.

Today, to get started with the new series of posts, I will go into the insights of the design of the Pitch Reference Oscillator used in the Theremin version 1.

I decided to use the Pitch Reference Oscillator because all the other Theremin’s oscillators are based on the same principle, although for the variable ones, an antenna is added to the resonant circuit to make its capacitance increase based on the position of the hand on the player with respect to the antenna. In fact, the antenna adds up to 8pF to the resonating tank, based on the position of the hand of the player.

For reference, you can watch any of the YouTube videos on my channel related to oscillators in the Theremin Project playlist.

Here is the link to the playlist:

https://www.youtube.com/watch?v=moT9iAaZU-I&list=PL3SNC7XyKklYZOs48Qfh16nTO4t8edDPU

I began the design of the oscillator starting from the base concept of positive feedback amplifier, which can be represented as follows:

feedback_amp

We have an amplifier with a positive gain, which should be 1 when there is an oscillation, and we have a feedback impedance Zf that takes part of the output signal and brings it back to the input of the amplifier. Zf is supposed to be chosen to obtain a positive feedback at the oscillation frequency.

This basic configuration needs now to be modified to insert in it an LC resonating tank. This should be on the output of the amplifier, so its oscillations can be fed back to the input and be sustained with no decay.

oscillator_block_diagram

The amplifier can now be replaced with a real circuit made with a transistor. I choose in this case a common base configuration, which gives me the required gain of almost 1, which is enough to sustain the oscillation.

The Zf will need to provide a phase shift of 90° to compensate for the internal capacitance of the transistor, which will add another 90° for a total of 180°, thus providing a positive feedback, considering that the transistor has actually a negative value of amplification (-1).

Therefore, Zf can be replaced with a simple capacitor that will bring the output signal back to the input with the desired 90° phase shift.

And once all the polarization resistors are added, the circuit looks like the following:

oscillator

In this circuit, R2, R3, R4 and R5 are the polarization resistors, which are calculated based on the specs of the transistor itself.

L1, C4, C6, C7 constitute the resonating tank. The Two variable capacitors are added to allow for a fine tune of the frequency.

C5 is the feedback impedance, or Zf.

The resonating frequency is calculated based on the following formula:

resonating_frequency

where L = L1, and C = C4//C6//C7//CBE = C4 + C6 + C7 + CBE

and CBE is the capacitance between emitter and base.

Temporarily excluding CBE from the calculations, the frequency therefore can vary between 258.2 kHz and 312 kHz. The CBE can be obtained from the transistor data sheet, adjusting for the polarization between base and emitter.

Also, we can slightly affect the resonating frequency of the oscillator by changing the polarization point of the transistor. This can be achieved by a potentiometer added on the base of the transistor itself, as per the following schematic.

final schematic

All the other capacitors in the schematic are used to make the polarization point of the transistor independent from the oscillation frequency, by shunting the oscillation signal toward ground. Thus, their value is such that they behave like conductors with respect to the high frequency of the oscillator.

At the end, considering the increase in capacity given by CBE, the frequency ends up to be centered around 400kHz, which was the required frequency for the pitch reference oscillator.