Okay, here's a write-up based on the notes I took while Mark Eldridge was setting gains on ItalynStalyn's car. I'm sure there are some errors here and there. Please point them out, so i can edit this. I plan on adding pics when I can get them.
By properly adjusting gain setting in an audio system, the user can maximize the amount of clean power available from their equipment. This reduces noise, adds headroom, and prolongs the life of the equipment being used.
The first thing to do prior to setting gains is to set all pre-amp levels and EQ’s to flat (zero) on HU and processors. Secondly, any electronic crossover settings should be adjusted to include the frequency of the test tones that will be used to set gain. For example, my subwoofer output is usually set to low-pass at 63 or 80Hz. For setting gains on the sub channel, I will use a 100Hz test tone, so I will adjust the low-pass for my sub output to 200Hz.
Once the adjustments have been made, you’re ready to pop in a gain setting CD such as Autosound 2000 Disc 104. Gain settings should be adjusted from the beginning of the signal chain to the end. So, you’ll want to start at the HU(s) and work your way through processor(s), then to the amplifier(s). The gain for each channel (or channel pair) should be set using an appropriate test tone. By appropriate, I mean the test tone should be a sine wave at a frequency within the usable range of a particular channel (see note on crossover settings above).
I use a 3-way active HU, so I have High, Mid, and Sub outputs. I will set the gains of these outputs using 100 Hz, 1 kHz, and 4 kHz respectively. I will also be using a test tones recorded at -5dB. This will allow some minor clipping when driven to maximum levels, but any distortion should still be inaudible. The Autosound 2000 Disc 104 includes multiple tracks that will allow for overlap. Gain overlap will allow the user to extract every bit of undistorted power out of their system. The table below illustrates the pros/cons of using overlap when setting gains.
Note: The mini amp can’t play the 100Hz tone with much output at all, so it’s likely necessary to place the speaker of the mini amp closely to your ear before setting the gain. The change in pitch that occurs when the signal begins clipping should be audible.
0 dB Overlap No-clipping, but good amount of power unused +/- 0.1% THD
5 dB Overlap Minor, inaudible clipping (Good compromise) +/- 0.3% THD
10 dB Overlap Clipping is audible (Max. overlap to set gain) +/- 1.0% THD
15 dB Overlap Noticeable clipping is present +/- 10% THD
With all of the preliminary stuff out of the way, it’s time to determine at what volume level our HU starts to distort. The mini amp is connected to one of the mid channel(s). The min amp is to be powered on, but the volume should be kept to it’s minimum (turned all the way down) to avoid hearing the internal amp clip, as opposed to the incoming signal. With a 1kHz test tone playing, the volume of the HU is increased until the pitch of the tone from the mini amp changes. If the HU reaches full volume without this occurring, that HU doesn’t clip. If there’s a slight change in pitch of the tone from the mini amp, the volume of the HU should be reduced to the point just prior to the change in pitch of the test tone. This is the HU’s maximum, unclipped output (MUO) setting.
Note: The change in pitch heard from the mini-amp, indicates the point at which the signal begins to clip. The observed change in pitch comes from second, third, and fourth-order harmonics which occur with clipping. So instead of hearing a single-frequency tone, we hear multiple-frequency tones playing together.
This process is to be repeated, in order, down the signal chain. So any processors between the HU and amplifier(s) are to be tested with the mini-amp and test tones. Remember to use an appropriate test tone for each channel being used (i.e. 100Hz for sub, 1kHz for mid, and 4kHz for the high channel(s)). When setting gains on processors in the signal chain, the HU’s volume should be set to the MUO, as determined previously. The gain controls of the processor being set should be set for the MUO. If the output of the processor clips even when its gain is set to the minimum, the HU’s volume may need to be reduced below the MUO.
Once the gains have been set to MUO on the HU and any processors in the signal chain, the amplifier is ready to be adjusted. Since the output of the amplifier is much greater than the pre-amplified signal, its voltage must be reduced prior to feeding it into the mini amp. This is accomplished by using a voltage divider. I am using a 10:1 voltage divider to set gains for my amplifiers. Depending on the output of the amplifiers being adjusted, a larger voltage divider might be necessary.
The voltage divider is placed inline, prior to the mini-amp. The inputs of the voltage divider are connected directly to the speaker outputs of the amplifier. The HU is once again, set to its MUO. The amplifiers gains are adjusted using the same test tones as before. Once the gain pots on the amp have all been adjusted for MUO, you’re done.
NOW, you can go set you EQ, but remember that any boosts will cause the signal in that range to exceed MUO earlier. To avoid this scenario, only use “cuts” in EQ, as opposed “boosts”. Good luck!