There is a misconception that noise pickup is reduced by placing the input RCA connectors (and similarly XLR input connectors in a fully balanced system) on opposite sides of the amplifier rear panel and then running short wires to the amplifier modules. This arrangement is the worst of all worlds. The internal loop areas available for the power transformer’s magnetic field with which to interact with are huge, even in transformers fitted with a GOSS band. Secondly, to reduce common mode RFI pickup, good practice dictates that the incoming interconnect screen be tied right at the input connectors directly to the chassis. At RF, this makes the source, the interconnect shield and the receiving power amplifier a single enclosure and offers good protection from external radio frequency interference. However, if there are any HF noise sources inside the amplifier – like an SMPSU or, more commonly, diode turn-off ringing, the internal HF loop area between the source and separated input connectors is very large, leading to noise pickup. By keeping the input connectors co-located with the signal returns bonded together and using a single RFI cap (2-5 nF disc ceramic or if SMD, an 0805 NPO/COG type – do not use a film capacitor) to the chassis right at the input, the loop area for HF pickup is greatly reduced.
These two slides show how to considerably reduce the internal loop area wiring in an audio power amplifier to better achieve low noise.
Nice slides as usual.
It seems to me that it is easier to keep a small grounding loop surface area inside the amp by putting the main transformer in the front side of the amplifier (we see this on many commercial design, Pass for exemple).
And with this, the main transformer is also outside the grounding area. Of course main AC wire are crossing the whole amplifier. But is it a big problem ?
Thanks for your comment Philippe. You are correct, there is no fixed way to do this and placing the transformer towards the front can offer benefits. With respect to the mains supply crossing the other cabling, just make sure the mains wires are twisted or closely coupled (as in using sheathed mains cable for example) and that there is a good gap to the other cabling.