Hello everyone


I try to build my first synthesizer from a tutorial. i expanded the circuit to some more oscilators (The schema is in the atached files).
i want that i can control the volume from every oscilator with a seperate potentiometer. The Problem is if i turn one of the potentiometers (R6 / R15) the volume from every oscilator lowers and not just from the one i want.
can someone explain me why and how to fix it?


Thanks

 

Because you cannot just wire signals together to combine them. Look at what happens if one pot is turned down to zero. Then all of the signals are shorted to ground.

You need a mixer circuit to combine the signals.

For a simple fix, connect each signal to the output through a 10K resistor. But this will reduce the output level. You really want a mixer based on an opamp adder.

 

Are Q1 and Q2 upside-down?

http://www.cappels.org/dproj/simplest_LED_flasher/Simplest_LED_Flasher_Circuit.html

ak

 

Are Q1 and Q2 upside-down?

http://www.cappels.org/dproj/simplest_LED_flasher/Simplest_LED_Flasher_Circuit.html

ak

Ah yes they are upside down. But i soldert them the correct way on my stripboard.

 

Because you cannot just wire signals together to combine them. Look at what happens if one pot is turned down to zero. Then all of the signals are shorted to ground.

You need a mixer circuit to combine the signals.

For a simple fix, connect each signal to the output through a 10K resistor. But this will reduce the output level. You really want a mixer based on an opamp adder.

thank you. This is working.
but sholuden't it not also be working if i use a diode instead of the 10k resistor in the output? i think with the diode the volume doesn't get reduced but the effect is the same that the other audio signal isn't shorted to ground?

 

A diode will change the output waveform. Look up audio mixer circuits. That is what you need. The simplest is my suggested method with resistors. If you really want a make an analog synthesizer, you will need opamp mixers.

 

but sholuden't it not also be working if i use a diode instead of the 10k resistor in the output? i think with the diode the volume doesn't get reduced

Yes, it will. A typical small signal diode requires q 0.7 V drop across. A single diode in series with audio will affect the positive and negative half-cycles differently depending on which way the diode is inserted into the circuit.

ak

 

Yes, it will. A typical small signal diode requires q 0.7 V drop across. A single diode in series with audio will affect the positive and negative half-cycles differently depending on which way the diode is inserted into the circuit.

ak

I thinkk in this circuit i just have a alternating DC so i dont have a negative half cicle. Or am i wrong?

 

Amplitude peaks can occur on either half cycle. For the same smoothing filter, full-wave rectifying the audio yields a more accurate equivalent DC output with less audio-frequency ripple on it.

ak

 

When you use diodes to combine DC voltages, the higher voltage is passed and the lower one is suppressed. That is not what you want here. To combine two two audio waveforms, you want to produce the sum of the two, which is what the resistors or an opamp summing circuit does.