Analysing signal unbalancing stage
A capacitive delta would be normal, but there's no reason not to use a Y as here. It's to help provide RF immunity. NOTE : Don't confuse &quot;Class Y&quot; a safety rating, with the Y (or star) topology seen here - 3 caps connected to a common point (as opposed to the more usual Delta : 3 capacitors in a triangle, one from IN+ to IN- and one from each input to GND. You're right the summing provides cancellation - but only to the extent that the inputs match perfectly, Which they never do at HF. Shorting the inputs to RF eliminates most of the problem before it reaches the inputs.

You don't dump the incoming RF noise from those capacitors onto the clean audio GND. Dump it to chassis GND (which will generally also be safety GND, especially in metal cased equipment where, if mains powered, exposed metalwork  must be earthed.

R1 provides the correct load for the transformer. Among other things, it damps the resonance of the inter-winding capacitance with the secondary inductance which could otherwise provide a peak in the high frequency response.


There isn't enough circuit to reveal the purpose of R2 but the RH signal might be a virtual earth mixing bus.