Calculate gain reduction on amplifier

Hi guys,

Edited by Lillike_natnek - 27 May 2021 at 2:53pm

Watch these...

Or, contact James @ Soundgear since you bought his amps and cabinets?

But in principle, all you need is a multimeter, preferably one with a true RMS readout, and some basic grasp of Ohm's law if you're wanting to get it approximately right. This disregards the impedance curve vs frequency and the simple fact that a driver's free-air AES rating no longer really applies once it's loaded into a box, but it's better than guesswork.

V = I*R

Use this to figure out the RMS voltage needed to obtain the rated power of the cabinet's passband at the nominal impedance. Note those down.

Drive your DSP with broadband pink noise (or better yet, Meyer's free M-Noise signal) at a similar level to your normal drive volume. Use the mixing console or DJ mixer if you can, and set the output Vu meters to the typical level you run at.

Compress it 2:1 if you want to be really safe, as this will give you a 6 dB crest factor which represents the most extreme forms of music. Most music sits closer to 12 to 16 dB crest factor, though.

Apply your required filters to the DSP outputs; high pass, low pass, EQ, the lot. Connect your multimeter's probes to the output of the amplifier channel that will be driving the speakers and set it to measure voltage, RMS.
If you can, plug one of each of the speakers in and use the link-through port on the cabinet as this will better represent real-world usage. You might want earplugs...

Then turn the level of the outputs up, one at a time with the others muted, until the previously calculated voltage level is reached. That gives you the dBU value to use for an RMS limiter - at least, based on nominal impedance and AES ratings...

Since you have an XTA, set the limit threshold there, and leave the rest on auto mode. It'll take care of peak, attack and release based on your filter settings for that passband.

For setting levels for playback, then yes, use your ears or add in an analyser. Start with the same pink noise signal for the latter, then move to some tunes you know well. Preferably something with clean, well recorded natural sounds like vocals, stringed instruments, pianos. You can do fun stuff later, but getting it right with the things that everyone should be able to tell sound 'right' first helps a lot.

You might then want to crank the system to max, and adjust the output limiters so one passband doesn't carry on getting louder when the others are in hard limit... Or add an input limiter to maintain dynamics.