Another easy DIY module from Calsynth, the Stereo Total. It is a manual mono to stereo and stereo width controller.
The first block A is a Stereo Simulator that creates a stereo signal from an incoming mono signal. This is done by splitting the mono signal in two separate signals, passing the left through an R-C network to reduce mid range frequencies and the right through an operational amplifier in differential mode to obtain an output as a difference to L. Knob A STEREO DEPTH controls the depth of stereo, but also places the signal within the stereo field. Because the signal is filtered, mid range frequencies will be changed as you turn the knob.
The second building block B is a Stereo Width Control that allows you to change a stereo signal from mono to wide stereo. This is achieved by summing both signals L and R to create a mono signal when knob B STEREO WIDTH is turned fully counter-clockwise and by splitting them via complete negative crosstalk, when the knob is turned fully clockwise. Fully clockwise the stereo signal contains no centre image at all.
The Build
You get the panel and PC board with all surface mount components already soldered. Just add a 10-pin power header, two pots and knobs, and eight jacks. Takes about 30 minutes to build.
I used black and dark red Davies 1900 clone knobs. Also put black plastic nuts on the output jacks.
Testing and Calibration
The two trim pots on the back are labelled L GAIN and VOL. Leaving them in their center positions results in a good deal of distortion. Indeed, you could trim this little module to be a nice, nasty distorter. It took a while, scoping the input and outputs to figure out how I wanted it trimmed.
The trim pots affect only the top section, A, stereo depth. The Left output of A is affected by the trimmers, but not at all by the knob position! So to trim, you need to monitor the left output of A.
The gain is highly impacted by the frequency of the input. I looked at the gain at five different frequencies in order to set the trims. I didn’t want too much distortion, so I looked for the frequencies where the gain was the highest and trimmed to get those points to approximately 1X gain. These were at 50 Hz and 1.5 KHz, using a +/-5V triangle wave on the input. Below 50 Hz the gain increases and above 1.5 Khz, the gain decreases. There is also a dip in gain around 250 Hz to about 0.1, yes, one-tenth of the input. Thus the gain decreases from 50 to 250 Hz and then increases again, back to unity at 1.5 Khz.
What’s going on in the above scope shot is that the input is attenuated by about 50% and the shape is altered quite a bit.
Procedure for trim that I used
Patch a 1.5 KHz triangle wave to the input and monitor the Left output of section A. The input level is not crucial. The knob position is irrelevant. Start with the VOL trim pot and turn it all the way counterclockwise. The output level will go to zero. Then turn it up just slightly. Going higher will enter the distortion zone. Now tweak the L GAIN trim pot until the L output is about unity gain, i.e. the output level is the same as the input level. Done.
Results
The Right output from section A varies differently in amplitudes at different frequencies than the Left output. With the Depth pot full CW, Right will sometimes be lower and sometimes higher than the Left, depending on the frequency. With the Depth pot fully CCW (to the left), the Right output will be much lower than the Left output, at all frequencies except at about 250 Hz, where both reach a gain of 0.1.
So how does it sound?
Due to the frequency-sensitive gains, it’s going to work better with inputs having a wider spectrum of frequencies, such as a saw or pulse wave. Here are two recordings, the first using a sine wave input, and the second using a sine/saw wave from the Dannysound Cali Oscillator.
Both recordings were amplified to put the peak levels at -2.5dB.
Both recordings start with the sound of the input alone. Then the two pots are turned to minimum, Depth is then raised, then Depth to minimum and Width to maximum, then both maximum.
