A simple and inexpensive device which enables signal levels to be matched between mixing desk and multichannel tape recorder.

If it were not for our old friend non-standardisation, then this little box would not be necessary.

In the early days of the telephone, the decibel was introduced in order to permit acoustic or electrical signals and their ratios to be universally defined.

The industry defined 0dBm as one milliwatt of power into a 600 ohm load which works out at 0.775 volts. Most professional mixing desks are calibrated in this way and so, when the meters are reading 0VU (with a steady sinewave input), the output level will be 0.775 volts RMS.

Like most universal rules, this one has been broken, notably by the Japanese audio equipment manufacturers who decided that an antique telephone exchange was not a suitable standard against which to calibrate their equipment.

For reasons of simplicity, they changed their 0dB reference to one volt, exactly. (Apparently one early American President decreed that the value of pi be universally accepted as three, to simplify calculations. Students of mathematics will realise that this move also met with only limited success.) Having made this seemingly logical move to a simple one volt standard, they decided that their reference for budget and semi-pro equipment would be -10dB or 0.316 volts, which means that connecting a professional mixing desk to a Japanese budget recorder produces a noticeable metering discrepancy. In this case, we need to reduce the output level from the mixer until the two metering systems agree, and to this end, the piece of hi-tech circuitry described here was developed after many sleepless nights by our dedicated design team.

Method

Fortunately, the output impedance of a typical mixer is much lower than the input impedance of a typical tape recorder line input, and so a simple potentiometer circuit may be used.

The value of the preset pots was chosen to present a relatively high impedance load to the mixer whilst still being low compared to the line input impedance of the recorder. Four such presets are incorporated into one box, so that several units may be used together for systems having upward of four channels.

Construction

A metal box was chosen for its screening properties, and phono connectors are used for economy and their reliability in semipermanent installations. The presets are all mounted directly to a PCB such that they can be readily adjusted through four holes in the lid during calibration and cermet types were chosen for reliable operation.

Firstly drill the box lid as indicated in Figure 2 using either a power drill or a hand drill. If using a power drill, make sure that the lid is fixed down securely before you start.

Next, fit the presets and resistors to the PCB and solder them in place, but do not discard the cropped resistor leads. Solder these short leads to the phono sockets and fit these to the lid, placing a solder tag under one of them in order to make electrical connection to the case.

Solder a short piece of wire to the solder tag, and then fit the PCB to the back of the sockets making sure that all eight wires pass through their corresponding holes. Then solder the short lead from the case to the zero volts (0V) point on the PCB. Finally, screw the lid back onto the box and the unit is ready for calibration.

Calibration

Using a 1 kHz alignment oscillator or a steady note from a synthesiser, adjust the mixer gain until the output meters read 0VU, and then, selecting 'line in' on the tape machine (monitor source), adjust the presets until the corresponding meters on the tape recorder also read 0VU.

It is worth noting that transient sounds such as those produced by drum machines are interpreted differently by different meters, depending on their mechanical response time. In such cases it may be advisable to heed the fastest rising meter which will be the one reading highest when subjected to signals of a transient nature.

The unit is now ready for use and may be safely stuck to the back of the recorder using the sticky foam pads normally used for securing mirror tiles in place.

The Attenuator Box PCB is available at a cost of £2.95 (inc VAT/p&p) from: HSR, (Contact Details). Please allow 28 days for delivery. All payments to be made in pounds sterling only.