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Picture of two copper coils from PCB Main 1 of a Yamaha RX-V396RDS

This is an bit of an overly specific question, but I'm curious about the purpose of these coils inside a Yamaha RX-V396RDS amplifier and wondered if anyone could explain what they're for. They appear on PCB Main 1. Also, would having the resistor inside the coil touching the inside of the coil be likely to cause a channel to not work?

Background to the question: the front right output of the amp has been behaving a littler weirdly for a while, mostly being slightly quieter and less clear than the left, but I've lived with it. This evening I noticed that it had completely cut out, so went delving inside the amp to see if I could spot any loose connections or burnt out components. Nothing looked obviously out of place or broken, but I noticed that on one of the large boards connected to the output board the resistor in one of the copper coils was touching the inside of the coil.

I'm no expert, but I assumed that having two components directly touching like that probably wasn't ideal, so I carefully moved it to be more central. Having done that I thought I'd test it out and was surprised to find that the front right speaker was back. I tried to google to find an answer and looked at the schematics, but have no idea of what I did actually fixed things, or if unplugging the amp for the first time in a while may have reset something that had tripped or otherwise gone wonky.

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    \$\begingroup\$ If the coil turns touch then the coil is made of enameled wire, and having a resistor lead touch it shouldn't make much difference. Whatever the function of that coil is, it doesn't do much at all at audio frequencies. if it's not there for some sort of cable equalization on the digital signals coming in, I have no clue. \$\endgroup\$ – TimWescott Mar 3 at 20:21
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    \$\begingroup\$ My guess is that they serve some important purpose for the sales and marketing department at Yamaha. They would work even better if they had LEDs inside them for illumination. \$\endgroup\$ – Elliot Alderson Mar 3 at 20:40
  • \$\begingroup\$ Not a lot of inductance there, but could be to slow down the rectifier surge currents in the power supplies. If you have +-40 volt supplies, from a full wave rectifier, you'd need two. Fast current surges are bad interferers. And copper foil does not shield low-frequency magnetic fields. \$\endgroup\$ – analogsystemsrf Mar 3 at 20:48
  • \$\begingroup\$ Dunno if it's related, but I have an RXV595, and also sometimes have issues with one front channel being quieter / completely missing. I can usually fix it by turning the volume up - a bit before halfways to max the channel usually 'pops' back in, then everything works well again, and I can turn the volume back down... \$\endgroup\$ – sonicwave Mar 4 at 16:05
  • \$\begingroup\$ @sonicwave ahh that's interesting. I'll try that if it happens again, may have just been that I managed to do a similar thing while taking the case off or something like that \$\endgroup\$ – andyface Mar 4 at 19:02
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The coil can be found in service manual schematics. It is an RL filter to isolate the amplifier output circuitry from the speaker terminals.

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    \$\begingroup\$ So it's just a space saving measure to put the resistor in the middle? or does that do something else too because there looks to be plenty of PCB space. \$\endgroup\$ – Toor Mar 3 at 20:58
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    \$\begingroup\$ ..some PCB designer somewhere is loving this attention. \$\endgroup\$ – Tyler Stone Mar 3 at 21:32
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    \$\begingroup\$ They're listed as 1uH coils, and with a quick look at the schematic in the service manual, appear to be there to add stability to the output stage. \$\endgroup\$ – james Mar 3 at 22:12
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    \$\begingroup\$ @andyface I can't see how...the resistor and inductor are in parallel. But the question of whether or not the concentric placement on the PCB has any bearing on the electrical behavior is just burning in my little brain. \$\endgroup\$ – Tyler Stone Mar 4 at 5:25
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    \$\begingroup\$ @TylerStone It probably doesn't. The single-turn 'coil' made by the resistor is at right angles to the one made by the copper coil, so it's not going to have significant mutual inductance, and I can't see any other possible interaction outside of capacitive, which would be miniscule in comparison at the relevant frequencies. Perhaps an engineer somewhere just had a neat idea and wanted to use it even if it wasn't practical. \$\endgroup\$ – Hearth Mar 4 at 15:20
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Emitter Followers are prone to oscillation with capacitive loading with unity gain and so are complimentary Darlingtons emitter followers in a feedback loop used in PA's. Normally PA's use series with RC shunt snubbers to suppress absorb spurious RF oscillations > 100kHz to dampen them at the unity gain BW frequency of the power amp.

However, Yamaha, being the expert in Audio design that they are, prefer to raise the output impedance with leading current phase compensation using these series 1uH inductor coils shunted by a 10 Ohm resistor. This provides a breakpoint or HPF or phase compensator at 100kHz where the PA source impedance rises from milliohms to 10 Ohms and thus prevents the oscillation with phase margin.

The trick however, is to make them a high Q vertical axis low capacitance air coils so they have a very high self-resonant frequency and also by orientation, do not couple each other in the 5 power Amp channels on the same board. Only the best designers use this approach. enter image description here enter image description here

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