Parallels: It’s Not the Water that Kills You, It’s the Voltage.投稿日: 9月 30, 2014
My little Behringer XENYX 502 mixer surprised me last week: It passed an audio signal WITHOUT POWER!
Recently I decided to finally get a set of powered monitors after discovering the affordable Fostex Personal Monitor series. The white PM0.3 look like glorified PC speakers, but they fit perfectly on my shelf and are not over powered for my small space. They don’t have TRS inputs or a built-in power supply and heatsinks like the PM0.4s I initially spotted in a recycle (2nd hand) shop in Cheeba, but sound clear, balanced and hum-free driven by the Xenyx. What’s this have to do with passive mixing?
Having a pair of powered monitors seemed to finally fully actualize the function of this mixer that until now I’d mainly been using as a headphone amplifier. Now I can finally listen to records again while doing dishes and nap on the couch without cans pinching my ears or hair, or getting tangled up in the cable and yanking the mixer off the shelf and then breaking something. Best of all, I can plug in and power up much faster to jam with my music. They even did a good job monitoring the wet signal from my TC Electronic HOF Reverb in an LRC tri-amp setup with the Little Lanilei dry. What’s this got to do with electronics?
As a bonus, I discovered that they will also work just as well driven directly by the little Audio Technica AT-PEQ3 I got with my Technics 1200 from an old friend. This was a happy discovery resulting from a connection fail: I mistakenly plugged the output of the phono amp/eq into the main output of the mixer (next to channel 4/5 where it should have been). When I tried to adjust the levels and got no response. That got me thinking.
Then I noticed the blue LED was dim, yet didn’t assume it was broken because I heard the music coming out of the speakers clealy. Powering up the mixer stopped the music. However powering up requires me going across the hall into the bathroom to plug in my Furman power conditioner into the only grounded outlet in the apartment, normally reserved for the washing machine. So discovering that I could listen to music without this step was a joyful one.
According to the manual the main outs are in parallel with CD/TAPE which I have connected to the Fostex personal monitors. This means that they are directly hardwired together, and so actually completed the signal path between the audio source and the monitor inputs. So why did powering up STOP the music moving from one output to the other? Intuitively, I knew the answer. Academically, I think this fail helped me understand the difference between electrical voltage and current.
Electronics textbooks often compare voltage to water pressure. Maybe because of this, I pay attention when playing with water. For example, I was playing in the park with my kids over the weekend, and came across a water fountain/faucet. Our thermos was empty, so I tried to fill it up but the lower faucet (pointing to the ground/earth*) wasn’t working. Not to be denied, I put the thermos over the fountain, and tried to fill it upside down, figuring the water pressure coming out of the spout would be enough to fill it up. We’ve all seen this in a plastic water bottle somewhere. This didn’t happen though, and water started spraying everywhere but inside the thermos. Some water did make it into the narrow thermos, maybe about a 1/4 of its volume. Unfortunately this was just enough to stop anymore from getting in because the pressure from weight of the water inside was greater than the projectile force of the water coming out of the tiny hole.
This is exactly what I imagined was happening with my mixer. The AT PEQ is designed to put out a strong enough signal to drive powered power speakers, same as the mixer. So even if there are any other solid state components connected in parallel to the outputs, the load is not enough to impede the line-level signal. However powering up the mixer from the washing machine outlet in the bathroom, stronger voltages must now be present in the circuit which resist the audio signal (voltage) from the turntable preamp. The components in the (probably resistors) are designed so that the signal flows only one way: OUT. So in conclusion, solid state components like resistors, diodes and transistors can stop the flow signals, but so can other electrons with a more voltage flowing in the opposite direction or polarity.
I’ve been trying to get my head around electronics for three decades, but haven’t learnt much from textbooks because I don’t get dirty and dangerous experimenting. It’s just the fear of ruining something or hurting myself or just wasting time with things I don’t really understand, which is a stupid shame because this is really the only way I can learn how this stuff works.