This Months Mod
With that example in mind, heres a fairly simple mod. In most budget mixers, the power and ground connections are distributed to each module via a motherboard or ribbon connector. This is fine for an 8-channel, rackmount devicethe original Soundcraft 200 Series, for examplebut when the frame is extended to accommodate 24 additional modules, a ribbon cable is not capable of doing the job. Not knowing the original wire dimensions, I measured one conductor of an 80-inch section of 28-gauge ribbon cable. The resistance was 0.5 ohms or 0.075 ohms per foot, certainly not an effective ground. Each module was at a slightly different voltage and ground potential, creating the internal hum.
The solution was to route a ground wire to each moduledetails at www.tangible-technology.coma fix that not only lowered the hum but resulted in many users remarking about improved low-end punch and a better stereo image. Each console is different, so theres no way to tackle your specific problem here, but I will show how to interrogate a console. Plus, well examine a more recent upgrade.
Bright Lights, Big Torture
In order to determine whether the source of the hum is internal or external, everything should be disconnected from the mixer, except for a pair of headphones or some sort of monitoring system. Knowing how hard this can be, the following procedure assumes everything is connected. Note that muting a channel module does not disconnect it from the busing systemonly de-assigning the mix and/or groups will effectively take a module out of the system without actually removing it. (This doesnt apply to aux sends, which are typically hard-wired to their respective buses.) Be sure that no video monitors, power supplies and wall warts, or power amps are anywhere near the console and its cabling.
To begin, monitor the mix bus for all noise, and then mute all effects returns. Expect the hiss to be reduced, because effects are notoriously noisy without any help from the aux sends. If hum is present on all buses, then its probably worse in the aux sends and should diminish when thereturns are muted. Now, mute all channel strips. What remains should be a minimal amount of hiss. If background hum is present, now is the time to de-assign all channels and returns (if possible). If the hum goes away, your console may be a candidate for improved grounding. But to fully qualify, disconnect all external wiring and repeat the test.