AUDIO CONTROL MIXER

AUDIO CONTROL MIXER PROJECT

In the next few weeks, work will start on an audio control center configured like a mixer. It will have up to 9 mixer inputs possible with one input a built-in turntable pre-amplifier. There will also be a master volume, bass and treeble controls and a balance control. Being configured like a mixer eliminates the need for an input selector switch. Input selector switches tend to get dirty and intermittent plus causes pops when you switch inputs. The mixer design also provides mixing of several inputs at the same time. At present, the design is going to have 6 tubes and use a cathode follower output to allow connection to solid state as well as vacuum tube equipment.
If there is no need for a turntable pre-amplifer or an external pre-amp will be used, the turntable pre-amp can be left off and then only 4 tubes are required. The only difference besides leaving the pre-amp stages off is a slight change in the power supply.

INPUT MIXER CIRCUIT
ALL POTS AUDIO TAPER

Input mixer circuit, input pots in circuit reversed from what would be considered normal to reduce mixer interaction between pots as each input level is adjusted. Instead of varying the load on the master pots mixing buss and cause unwanted level changes while mixing, a small load change is varied on each input instead. This arrangement results in better mixing levels. There can be up to 9 mixer inputs including the turntable (phono) input. Simply expand the mixer circuit out to as many mixer inputs (up to 9) you want. The more mixer inputs you add the lower the mix buss resistance gets. With 9 inputs the buss resistance will be 100K ohms (buss resistance includes the master pots). The mixer buss resistance should not be lower than 100K ohms. A balance control is used in another circuit of the unit. Depending on how many mixer inputs you plan on using, keep in mind you need to have a chassis large enough to accommodate all the mixing pots on the front side and input jacks on the back side.
The .1MFD capacitor at each input insures that any DC that may be present on a source does not get to the mixer pots. DC is a prime cause of pots getting scratchy.