Sony STR-DA7100ES Technical Background page 33

Identical aluminum heat sink fins resonate at the same frequency, subtly
degrading output transistor performance. That's why the STR-DA3100ES uses a
series of vertical ribs, spaced at staggered intervals. This diffuses resonance to
maintain sonic integrity. Because the ribs from the letter S, we call this design
the "S-form" or "sF" Fin design.
The inside of the STR-DA3100 ES, showing the two sets of massive
aluminum heat sinks. At this angle, the s-Form ribs appear as circles on
the white aluminum heat sink fins.
The engineers of Sony's ES Series try to look at everything that influences
the sound quality, however subtle. Their examinations have prompted us to
reconstitute the chemical composition of the speaker terminal lugs. We
discovered that a resin with reduced fillers and pigments helps suppress
resonance, especially when the audio program gets loud. You can actually see
the result of this effort on the back panel, where the speaker terminals no longer
have their customary bright red and deep black pigment!
ES Receivers v1.0
sF Fin Heat Sinks
Low-filler speaker terminals
Page 33