Did a search, couldn’t find much on this. I just got some M700s yesterday and figured I’d try some of my rhodium shorting covers for RCA inputs. Given that it had been a while since I had listened, the equipment was off, I had a slight sense that volume levels for equivalent settings on the pre-amp seemed lower once the caps were in place and the system was powered back on. But the sound seemed maybe “smoother” and with a lower noise floor (?) hard to say since I took breaks between.
But anyway today I listened to a few songs, then decided to remove the covers from the RCA inputs and yep, i get an extra 2-3dB in volume without the shorting covers on the RCA inputs. Again, I’m using the XLR, so I figured the unit would just sense or default to that, but apparently this is firmware/programming related? Does the unit automatically lower speaker connection output, even via balanced inputs, when there is something connected to the RCA unbalanced inputs?
It appears that the RCA input is in parallel with 1/2 of the balanced XLR input.
Therefore, if you short the RCA you are shorting one leg of the XLR input resulting in reduced level.
Definitely do not use RCA shorting plugs. The XLR and RCA inputs are tied together so when you do that you’ll get 6dB lower gain and lose any advantage balance brings to the table.
I guess that’s gotta be the case…just given how these amps operate otherwise, I initially assumed it was some kind of software thing - kind of like the protection circuitry - whereby if it sensed a connection on the RCA outs, it automatically lowered output power by a certain amount. But your theory makes sense electrically too, I think. I think pin 2 on the XLR is “hot” so I guess it would be pin 3 - but I thought you’d also get some kind of polarity inversion with that kind of situation.
Why is the difference 6dB, exactly? Is that the normal gain you get with balanced as compared to unbalanced RCA? For some reason I always thought it was 3dB.
Interestingly enough, I do still stand by my initial perception of a “smoother” more “refined” sound in the upper mids and treble when the shorting plugs were in, but I guess that could just have been the lower volume.
Paul has an excellent series on YouTube where takes opamp theory and “simplifies” things for mere mortals (I’ll never use the words “Dumbed down” because “most” people ain’t Dumb).
Back in my RF Tech days we used PECL then later LVDS circuits that fed mux/demux chips that the Company I worked for designed. Without going into too much detail here. The +6dB gain in the audio world comes from the voltage swing from the -ve voltage rail up to the +ve voltage rail.
I’m not sure there are any op-amps in the output circuit. Possibly in the class-A stage, but since they’re switching amps, I would have guessed it’s computer controlled rather than a simple case of being wired in parallel. It’s been pretty widely discussed that these amps (M700, S300) will shut down if they sense any possible issues. So I assumed that the unit is sensing a short on the RCA input and thereby telling the computer to revert to single ended/unbalanced operation - only powering one of the pins on the XLR outputs. But what do I know? I’m obviously mixing up a bunch of concepts here - the PWM output (prior to LP filter) of class D amps is generally MOSFET or other discrete transistors, not op-amps, IIRC.
You are over thinking it. Paul already said you are shorting out one of the input legs. remember balanced has two signal leads and one ground. No magic going on here just basic electricity.