HSR Stereo Autofader Project (Part 1)
An easy-to-build project that will take the difficulty out of producing a smooth fade on your stereo mixes.
Manual fading can be tricky to do correctly, but this simple design by Paul White gives smooth, controlled fades up to one minute long and only costs around £10 to build.
How many times have you had a manual stereo fade go wrong at the end of a complex mix? Either one channel goes first, or the fade is too long or too short and that means doing the whole mix again.
Professional studios use dedicated autofade units with variable fade times to take the guesswork out of this vital task, but these are far too expensive to rank high on the priority list of many home studios; we have come up with a build-it-yourself version that costs less to make than a reel of quarter inch tape.
The HSR autofader has unity gain and may be connected directly to the mixer's left/right outputs or via insert points if your mixer has them.
A single rotary control sets the fade time between one and sixty seconds, and it requires only the operation of a single switch to initiate the fade.
It is best to power the unit from a +/-12V supply such as the E&MM Rack-Pack, but two 9 volt batteries should provide satisfactory operation.
The autofader circuit is based around the readily obtainable LM13600 dual transconductance amplifier giving a tracking accuracy of better than 0.3dB between channels.
In this instance, the LM13600 is configured as a pair of current controlled amplifiers, the current through R9 defining the amplifier gain.
To produce a smooth fade, an exponential control law is needed, and this is provided by the discharge of capacitor C5 through R8 and VR1.
C5 is normally charged up to +12V as its positive end is connected via S1 to the positive power supply rail, but when S1 is opened, C5 discharges exponentially at a rate defined by the setting of VR1 until it is fully discharged, when the voltage at its positive end will be -12V.
This control voltage is buffered by TR1 and the resulting current flowing through R9 controls the gain of both halves of IC1. When the switch is closed, the circuit gain is set to unity and so no attenuation takes place, but opening the switch causes the gain to drop exponentially, thus generating the smooth fade-out.
A few of our readers have asked for projects that can be built on Veroboard and, as the circuit layout is fairly straightforward, Veroboard is probably the best thing to use in this case.
Figure 2 shows the layout used to build the prototype, but if you want to alter it, there should be no problem as the layout is not critical.
A screened box is essential to keep all that nasty mains hum at bay and the diecast variety is the easiest to work with as it drills easily without burring.
As always, check the orientation of the IC, transistor and capacitors before powering up and make sure that none of the Veroboard tracks are shorted together by blobs of solder.
To test the unit, connect up to the output of your mixer and feed in some music so that the meters read around 0VU at the output. If no sound is heard, flip the switch, and if all is well, the music will sound just as it did before the unit was connected.
Set the pot halfway and flip the switch again; you should get a good, clean fade-out, if not, check your handiwork.
The circuit is designed to work with semi-pro signal levels, higher levels may cause distortion though there is adequate headroom for normal use.
The inherent noise of the autofader is far below tape noise levels and decreases during the fade so it should never be a problem.
All the parts are available from Maplin and the autofader can be built in two or three hours so there's no need to wait for those long winter evenings.
Feature by Paul White
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