Implementation of Direct Output Buffers

Posted on Feb 7, 2014

A total of six buffer circuits (using three dual op-amp packages), five for the input channels and one for the master output channel. (2) High input impedance (particularly for the five buffers tapping channel faders). (3) Low output impedance (<600 ohm), and stability when driving shielded cables of moderate length. (4) Use of op amps with low noise and distortion and wide

Implementation of Direct Output Buffers
Click here to download the full size of the above Circuit.

bandwidth; use of low-noise metal film resistors throughout. (5) Nominal audio output levels compatible with today`s standard for ground-referenced audio lines, i. e. , -10 dBV (as typified by the Alesis ADAT recorder`s "unbalanced" inputs). (6) Unity voltage gain, and pads (voltage dividers) on inputs where appropriate. (7) Immunity of solid-state devices from damage caused by excessive input voltages. (8) Maintenance of consistent signal polarity from inputs to outputs. (9) All buffers must fit on a perf-board 2. 3 x 3. 7 inches in size, to be located above the chassis plane in a space formerly occupied by two filter capacitor cans. Figure 10 shows schematics of the four different buffer circuits that I settled on, and photos of their perf-board assembly installed in the MXM are shown in Figure 4. For polarity consistency (specification #8 above), all mic inputs must use today`s standard of XLR pin 2 as "hot" or "+" (where positive voltage swings are analogous to the compression part of a sound wave), with XLR pin 3 as the inverse; on the direct outputs, the signal polarity on the "tip" contact of the 1/4-inch jack should match that of pin 2 on the input XLR. With two stages of triode amplification, the Channel 1 preamp is a non-inverting amplifier, while each of the other input channels are inverting stages with their single pentodes. Thus, I chose a non-inverting output buffer design for Channel 1 and an inverting...

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