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Posts by robrob

 Steve, you may not be aware of this but other people will read my post.
 Steve, see the output transformer secondary in the schematic below? That's a balanced output. The coil, speaker wire and speaker voice coil make up the loop for electron flow. It doesn't need a ground connection. That's how professional audio equipment send out their balanced 3-pin XLR signal, with a transformer made up of a single wire connected to two output terminals (hot, cold and a separate ground wire). A balanced line rejects RFI and radio magnetic interference...
This output transformer's secondary puts out a balanced signal (equal impedance) and is not grounded:     But most speaker amps today tie one of the secondary wires to ground for safety reasons and this makes the impedance between the + and - wires unequal so you lose the benefits of balanced output:  
 This is only true if your amplifier does not have common ground output. It seems that most speaker amps today have their negative speaker terminals tied to ground for safety and liability purposes. It may even be a UL listing requirement.
In theory there would be a benefit to keeping the two channel cables separate but Canare Star Quad is used for very long microphone (very sensitive to noise) runs and does a fantastic job of suppressing noise.
Very cool test rig!
The headphone power calculator spreadsheet is here: http://www.head-fi.org/a/headphone-power-calculator-spreadsheet   There's also an Android version that's free, just search for rob robinette at the Play Store.
Both tables are for 32 ohm headphones but going up to 600 ohms makes little difference in speaker load due to the 2 resistor L-pad attenuator. You need to vary both R2 and R3 to keep the speaker load near 8 ohms. You can download my 'Headphone Power Calculator' spreadsheet to play around with.
No, they're pretty much for DIY'ers.
Do the easy thing and pick a pair of resistors to give you the attenuation you want:  
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