Found in a Fender Stage 160 DSP. The outout transistor emitters are returned to true ground via 0.47R resistors. The speaker 'groundy' side is returned to true ground via 4x paralleled 0.22R. The 'hot' side of the speaker goes, via a hefty choke, to the centre tap of the transformer winding which feeds the bridge from which are derived the +/- 45v rails for the output stage. These rails go directly to the output transistor collectors.
The service manual has this to say about it -
' "Flying Rail" power amp topology.The output stage is referenced to ground, and the output signal is derived from the centre tap of the high voltage power supply (C86 & C87), which is floating. Diff amp U7B amplifies voltage developed at the load sense resistors (R120 - 123) ((these are the aforementioned 0.22R's)) to provide negative current feedback. Inserting an
8ohm load at Ext Spkr (J7) turns on FET Q20 creating an attenuation network (R117-119) to compensate for the increased current at 4 ohms total load impedance. This allows the amplifier to have a similar frequency response running either an 8 ohm or 4 ohm speaker load '.That FET that the description mentions, is basically slung across a couple of resistors in the differential input network to U7B, so just serves to scale the voltage derived from the current sense resistors. It is switched by an additional contact on the "Ext Spkr" jack, so it's a stereo type, as opposed to the mono type on the internal speaker jack.
So, has anyone come across this topology before. Any opinions as to what the thinking is behind it ? Could it possibly be just to allow incorporation of this load compensation scheme, making it easier to configure the opamp current sense stage ?
Arfa