A simple but effective device that enables you to match your pedals or budget rack effects to systems working at 0dBu or +4dBu operating levels.

A problem frequently encountered in studios is that of equipment incompatibility through the adoption of differing 'standard' operating levels. These are typically -10dBu (300mV sine RMS), 0dBu (775mV) and +4dBu (1.3V). Most home studio effects and semi-professional recorders operate at -10dBu, a level also accepted by any old guitar pedals you have taped to the equipment rack, whilst PA and professional studio machines generally run at +4dBu using balanced or unbalanced lines.

If you're one of those beleaguered souls whose desk inserts give out +23dBu and you rely exclusively on guitar chorus and flanger pedals to create your effects, this little project is for you; an in-line booster/attenuator to get your levels level. As an added bonus, this device may be used as a direct replacement for any of the circuit boards in the H&SR patchbay project.

The Circuit

The circuit comprises two sections; a simple resistive attenuator and a (barely) more complex amplifier. Component values have been chosen for conversion between +4dBu and -10dBu. Values given in parentheses apply to conversion from 0dBu to -10dBu. (You can replace R2 and R5 (Figure 1) with presets to obtain continuously variable gain and attenuation.)

As shown in Figure 1, the attenuator operates at a low impedance to minimise interference pickup and high-frequency loss through long cables. The amplifier circuit is an entirely standard non-inverting op-amp configuration. Use of a 5534 ensures negligible signal degradation but the input and output resistors prevent instability due to cable capacitance and should not be omitted. The same applies to the local supply decoupling capacitors C3 and 4 which should be fitted to each board even if several are sharing one supply. The 5534 is an irritable sort of op-amp and usually responds to unsatisfactory working conditions by oscillating at several MHz.

No power supply has been detailed. The circuit works from dual supplies up to +-15V, consuming about 4mA. The E&MM RackPack is entirely suitable though you could use a couple of nine volt batteries or run a couple of lines from your Tantek rack power supply.

Construction

The PCB has been designed to be used in two ways; either as a permanent patchbay installation or as a plug-in adaptor. In the first mode, it can be slotted into the HSR patchbay, using the phono and jack sockets from the original boards. As an in-line adaptor, PCB can be fitted with phono sockets and plugs, the plug centre pins soldered to the large copper lands provided. The phono plugs should be aligned by plugging them into the patchbay rear sockets before soldering them to the board and the link provided between the two jack socket locations is used for normalising. This is only functional when the board is a patchbay replacement, and can be omitted otherwise.

Alternative lands have been provided on the board to accept preset resistors for anyone who wants to interface between non-standard levels. As ever, a socket is recommended for IC1, and terminal pins or Molex connectors (preferably the polarised type) can be used for the power supply. Before applying power, check that all the capacitors and the ICs are the right way round, and of course, check that you are applying the power the right way round.

Testing

Well, what is there to go wrong? If you've installed presets then suitable relative levels can be established with the aid of the desk and effect level meters and a continuous tone from a keyboard or oscillator. If the board is connected into the insert point of a desk channel, then care should be taken to match the attenuation and gain ratios. The gain should be the same with the effects unit connected into the insert point as with nothing plugged into the insert point and you can tell how much level your pedal effect will take by winding up the input level until it distorts and them backing it off by a safe margin. With fixed resistors the circuit will work for the majority of equipment in common usage. Honest!