I was doing some noise troubleshooting in the studio today and obliquely discovered that in my current patchbay configuration, plugging a signal directly into the bottom input row of a 1/2 Normalled bay results in a signal that is roughly 6db hotter than if plugged into the top row. Why might this be? Anyone care to enlighten this patchbay noob? 🙃
In a typical 1/2 normal, you have the output of something like a preamp cabled to the top back connector, and the input of something like a compressor cabled to the bottom back connector. So with nothing plugged into the front, you have a signal from the preamp to the compressor.
The way this would be used is as follows:
Suppose you want to record both a compressed and a "safety" uncompressed signal. You have the output of the compressor cabled to the input of an ADC, perhaps even 1/2 normalled to it. And you connect a cable from the top front connector (output of preamp) to another ADC input. Note that the output of the preamp is seeing two parallel inputs, the compressor and the second ADC.
Suppose you want to use the compressor to process a recorded signal. Then you would take your DAC output and cable it to the bottom front connector (input of compressor). Note that unlike the top front connector, the bottom front connector disconnects the preamp output, so the compressor sees a single input, the DAC.
In your example, when you're connecting the output of something into the bottom front connector, it will only see the impedance of whatever is connected to the bottom back connector because the top connector is disconnected. But if you're connecting the output of something into the top front connector (which you shouldn't), it will see the paralled impedance of whatever is plugged into both back connectors. So less of the signal will go to the device connected to the bottom back connector.
I'm sorry, that seems very wordy, but I hope it makes sense.
Not wordy at all, thank you for the thorough explanation. I figured it was an impedance thing, I was just surprised that the signal loss would be that significant. It’s not how I would make a connection normally.
Now in your example of Multing the output of a preamp, say 1/2 normalled into DAC, with a patch from the top bay into a compressor, I can’t say I’ve noticed a significant loss of signal compared to the above example. Is the signal loss related to the source and destination impedance relationship? In both cases, the signal is being split, no?
Yes, output impedance is typically much lower than input impedance.
Think of it this way. When connecting a low impedance output to two high impedance inputs, the voltage varies very little from a single high impedance input.
But when connecting two low impedance outputs to a high impedance input, the paralleled outputs roughly cut the impedance (hence the voltage) in half, from E=IR. This is why we need mixers, which again have high impedance inputs, so the low impedance sources don't affect each other too much. I think.
Hopefully someone like Svart or EMRR will let us know how wrong I am and give us the real answer.
The "6db hotter" means you're getting full balanced/differential signal through. 6db less means you're only seeing one side of that. It's like The difference between TRS and creating TS by not connecting the ring from the TRS.