SERIOUS ABOUT SOUND

Pro Interface PCB

Introduction

These boards are based on the tried and tested circuitry used in our Balanced Input and Balanced Output boards. Each one has two unbalanced inputs giving two balanced outputs, and two balanced inputs giving two unbalanced outputs. Connection is made via Molex type connectors.

Gain settings are variable. The gain for the 'unbalanced in to balanced out' elements is set using a cermet potentiometer, and is variable from unity gain to around +20dB. The attenuation on the 'balanced in to unbalanced out' elements is set via an optional cermet potentiometer or by changing two resistors per circuit. The potentiometer can be used when the board is close to the destination device. With long lines the use of the potentiometer may result in a small low frequency loss.

The "balance" between the two sides of the output is set using a cermet potentiometer, resulting in very high immunity from pickup. This is factory set and should not be adjusted.

The boards require ±15V to ±17V DC (±24V option available). They measure w100mm x d65mm x h18mm. There are 4 mounting holes.

Installation Service

We have designed a very high quality PSU for powering our equipment. Giving ±17V, it uses an adjustable regulator (LM317), giving better regulation than standard ICs (78/79 type), but with additional smoothing circuitry to enhance its performance still further.

We will house these units to your specification: in your equipment, in free standing cases, wall boxes, stage boxes or rack cases in any finish or configuration. Generating suitable artwork and having units silk screen printed or engraved is also part of our comprehensive customisation service.

Application Notes

These boards have found a number of applications in recording and sound studios. They have mostly been variations of its standard function of interfacing semi-professional and domestic equipment (nominally -10dBV unbalanced) to professional level balanced lines (nominally +4dBm). In many cases they can be built into the equipment in question.

In one studio we housed two of these units to interface two DAT recorders to the studio equipment, which included a valve equaliser with an input impedance of 600 ohms. The units were required to operate at +22dBm into this load. Using one of our 1A Dual Rail PSUs with an output voltage of ±20V the Pro Interface boards achieved this. After a week's rigorous testing, both electrically and aurally, the studio's Chief Engineer said: "They are now sitting in my rack doing nothing. By that I mean that they are acoustically transparent, like Quad Amplifiers aimed to be; a 'wire with gain'."

In another more unusual application, the user wanted potentiometer control of the levels of a stereo balanced line at a point between the source and destination, but wished the signal to remain balanced. To achieve this we used a Pro Interface board to de-balance the input and sent the unbalanced outputs to a dual gang pot. The output from the pot went to the unbalanced inputs set to maximum gain. This meant that the gain could be varied from minus infinity to around +20dB while maintaining the balanced line integrity within the system.

Specifications

Balanced Input:

• Distortion @ +22dB <.0025% (20Hz - 22kHz)

• CMRR @ 1kHz >58dB

Balanced Output:

• Distortion @ +22dB <.003% (20Hz - 22kHz)