Mono to Stereo Effect

Sometimes it’s nice to be able to take a mono track and spread the sound in the sound stage without having to introduce other effects like reverb or chorus. Although it’s always nice to have more tools at one’s disposal I also have the frustration of having an Alesis mixer whose aux send is mono (it mixes stereo pairs down to the send) but a fully stereo effect chain. My solution is this mono to stereo effect box. This was inspired by a circuit given by R. A. Penfold in Practical Electronic Music Projects (Babani, 1994). The idea is to introduce a frequency-dependent phase shift in one channel. At low and high frequencies the right and left channels are identical, but in between one channel is out of phase with the other, peaking at 180 degrees at around 1kHz. The effect is similar to reversing the phase of one of two identical channels but because the effect cancels at low and high frequencies it is rather milder and avoids the sensation of having water in one’s ear. I adapted the circuit to be fully discrete, using a pair of transistors instead of op-amps. That makes it more hovercraft. Here is the schematic:


In practice R11 is a trimmer used to take a slightly attenuated signal to one channel while the other channel is taken from the output of the phase shift circuit. There is small loss associated with the phase shift circuitry and this allows the balance to be adjusted. The circuit is powered by a 9V battery and draws just under 3mA. Here is the circuit under construction and finally mounted inside a small plastic project box from Maplin:

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These are the phase shift and gain characteristics from a QUCS simulation (the phase shift is actually continuous, the jump seems to be an artefact of QUCS’s trig calculations):


And here is the transient response showing it comfortably handling a 1V input signal.


Finally of course the most important question is: “how does it sound?”. With a simple solo instrument with little in the way of harmonics such as a flute there is not much discernible effect. With a more complex sound such as a piano there is a mild but useful “spreading” effect most obvious when switching between the dry and wet signals. Here is the Roland Sound Canvas piano first without the effect then with it:

With a full song the effect is quite dramatic. Here is a snatch of Suvarna first in mono without the effect then with the mono version fed through the effect:


5 responses to “Mono to Stereo Effect

  1. Pingback: Mono to Stereo: Digital Implementation | The Ramsgate Hovercraft

  2. I was wondering where the output wire in the third picture connects to in the circuit. In the photo, the lead to the output is not placed. Thanks for your help.

    • I ended up fitting a preset potentiometer across the output, replacing R12 and connecting the output wire to the wiper. This allowed me to do fine adjustment on the balance.

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