Sound Performance Lab Analog Mixer Operation, Volume Control, Calibrating the Monitoring System

Operation

Volume Control

The single volume potentiometer controls volume for both channels. The audio signal is controlled directly by the potentiometer—thus no VCA or DAC regulators in the paths which tend to produce bigger tolerances and distor- tions.

The scale of this control has been purposely marked in percentage rather than in dB values. A dB scale would not make too much sense because we cannot plan for absolute values in signal level reproduced by individual systems and in playback rooms.

Calibrating the Monitoring System

Both the signal level which is send to the Volume 2 controller as well as the input sensitivity of the power amps or active speakers should be matched to ensure a proper overall gaining. An inappropriate adjustment might happen when, for example, a (fairly small) 10% volume level setting might already result in an extremely high playback level.

Important: You can encounter very loud signals during calibration, so be sure to keep your ear protection on. For calibration we recommend measurement with a Real Time Analyzer (RTA) or a SPL Meter (in this case, SPL translates to “Sound Pressure Level“). First the measuring machine and microphone are set up at the listening position. Then one produces in the DAW a sound file with “pink noise”, which is played back and measured.

Each measurement should be done through one channel and loudspeaker at a time. A recommendable calibration is the playback of a 83db SPL refer- ence signal at the playback location—a common reference volume level. The DAW output level should be adjusted to 0 dB, and next, the power amp‘s or active speaker‘s input level should also be set to 0 dB. Now the Volume 2 level control is adjusted until the RTA or SPL Meter measures 83 dB with the pink noise playback. For a perfectly matched gaining the Volume Control would now be set above a 50% scale position. At this point one can record or note the exact value for 83 dB. Should this 83 dB SPL occur markedly above a 60- 80% scale position, one should lower the power amp’s/active speaker’s input sensitivity (=higher dB value). On the other hand, the power amp’s/active speaker’s input sensitivity should be raised if the 83 dB SPL point is reached far before the 60% control level. In cases where regulating power amp/active speaker inputs is not enough, one may reach good values by changing the D/A converter output level (for example from +4 dB to 0 or -10 dB in cases where this switch option may exist). In any case, the converters should always be driven at full scale from the DAW.

Mute

You guessed it ... this switch mutes all speaker outputs. It is particularly valuable in panic situations, for example, if the computer crashes and its converters suddenly output uncontrolled high level noise. It is also very useful in any case where one simply wants silence without changing the Volume settings—e. g. to answer the telephone.

IMPORTANT: When making any cable connection changes, you should not only activate the mute switch, but follow the on/off powering procedures noted on p. 6.

Volume 2 7