Building Mixers

A much more flexible option is to use a mixer, a device which allows the outputs of multiple players to be summed together. With this technique, no input switching is required and the advanced crossfading features of the Sony CD changers can be taken advantage of. I am placing to sample mixer designs on this page which you can build. The first, a passive mixer, can be made with only resistors and RCA jacks and give reasonable performance (depending on you degree of audiophility). The second circuit is a active mixer which gives very good performance and is very low noise and distortion for an active circuit. If you are capable of building it, this is a far superior circuit. Mixers are also commercially available. Most have features which are unnecessary in this application (e.g. adjustable gain on each input).

simple passive mixer which is inexpensive and easy to make. It is easily expanded to any number of players at the expense of lost gain. Only 1/2 of the circuit is shown (i.e. one for the left, one for the right). This circuit is relatively low noise and distortion, however it has the following disadvantages over an active circuit:

*The volume of each CD player drops to 1/3 it's original value (-9.5dB) with 3 players.

*You'll begin to see high frequency roll off if you have more than a few nF of cable capacitance.

The 2k resistors should be replaced with larger values if your CD player outputs are not rated for such a low impedance (each player sees 3k ohms each when all 3 players are connected). Increasing the resistance may reduce distortion from you CD player output stage, but you will begin to pay the penalty of increased noise and high frequency roll-off.

simple active mixer which is inexpensive and easy to make. It is easily expanded to any number of players. Only 1/2 of the circuit is shown (i.e. one for the left, one for the right). This circuit is very low noise and distortion. No gain is lost when connecting multiple players. Overall gain can be adjusted by changing R4. The 2k resistors should be replaced with larger values if your CD player outputs are not rated for such a low impedance (each player sees 2k ohms). Increasing the resistance may reduce distortion from you CD player output stage, but you will begin to pay the penalty of increased noise.

Luis Fernando Cantor B.

Electronica de Estados Solidos

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