Q for Nathan:
One of my gigs only requires a monitor wedge for the bass in the stage mix; and I’ve been using a QSC K10.2.
But, I’ve run into an issue trying to send a preamp signal to both the monitor wedge and the FoH board, either with the parallel thru port on the K10.2 or by putting an XLR Y cable on the Venus. I described the problem in the Live Sound forum:
Hello Sound Engineering TB’ers!
Got a Q about the QSC K10.2.
I’ve recently started using this fab cab in place of a backline amp for BOTH stage mix AND Vocals. It is truly amazing! I’m using CH1 for the vocal monitor feed and CH 2 for Line Level preamp DI feed.
The one issue I’m in the dark about is the CH2 signal chain. The cab is amazingly stout, when EQ’d properly, and fed from a pedal board preamp DI. BUT, if I try to connect the Through XLR on CH2 to a board for the FoH feed, the K10.2 CH2 volume drops to about 1/10th of the output before the Through connection. I’ve tried using an...
I originally thought I had some issue in the K10.2. But, now it seems there is something I am missing about the balanced XLR send to multiple loads. For the moment, I’ve been solving the issue by sending to the two target amps from two different preamps. Is there something about the Output Impedance of the XLR send on the Venus, which affects parallel sends? If so, can that limitation be addressed by design; or is there a fundamental problem with that kind of signal chain?
Thanks!
I'm not intimately familiar with the K10.2's technical specs, but I will say that in general passive splitters are not an ideal solution as they can set you up for impedance issues.
It looks like the balanced input on the K10.2 has an input impedance of 40kΩ, which is well above the 600Ω output impedance of the Venus Z XLR output, so there shouldn't be any issue there by itself. However I don't have any information on your FOH board, and that may be the missing piece to the puzzle. Mixers typically have a low input impedance for the balanced lines, so if the XLR input impedance on the board is 600Ω or close to it, that would mean that you're ending up with an equivalent impedance of less than 600Ω because of the parallel loads, and that would be too much for the Venus Z to supply by itself.
My best recommendation would be to use the 1/4" output from Venus to the K10.2 and the XLR to the FOH. If the K10.2 is being used as your "amp", there shouldn't be a real need to use a balanced line from the preamp to it. The 1/4" output has a buffer that isolates it from the XLR output, and this is the exact sort of scenario that made me want to add this buffer in.
[stop here if you're satisfied with that answer and don't need overly technical explanations of things]
Some basic impedance information that should help if any of this sounds weird:
Impedance is sometimes simplified as "resistance", which is partially true, but it's resistance that is variable depending on the frequency of the source signal. The part that seems a little backwards is that a lower impedance is electrically a "heavier" load. So a low input impedance means an electrically heavy load, and a low output impedance means an electrically "strong" signal that would be capable of driving this heavier load. A high input impedance is a light load, and a high output impedance is an electrically "weaker" signal.
In high power environments like amplifiers into speakers you want to match the output impedance of a device as closely as possible to the input impedance of the next device. With low power devices like preamps or pedals in general, you want the output impedance of a device to be significantly lower than the input impedance of the next device in line. For example, if the pickups of your bass have a 20kΩ impedance, it's perfectly reasonable to have a 1MΩ input impedance. If you're using a piezo pickup with a 2MΩ impedance, a preamp with a 1MΩ input impedance would be too "heavy" of a load for that pickup, and the tone would be impacted. This is why it is common for acoustic-oriented preamps to have 5.6MΩ or 10MΩ input impedance.
This is where things like buffers come in. A buffer will generally have a high input impedance and a low output impedance, which makes it the perfect tool to put between devices that otherwise would have impedances that don't go well together. If you're driving to drive a power amp that has a 20kΩ input impedance, something like an Underground Accelerator will struggle due to its 38kΩ output impedance. More, on the other hand, has an output impedance around 500Ω and is capable of driving much heavier loads. Since More has an input impedance of 5.6MΩ it's an easy load for Underground Accelerator to push, and the 500Ω output impedance of More can then do the heavy lifting into the power amp's input, so UA -> More -> amplifier would be a perfect chain to put together for that case.
So what happens when you have one source (like a Venus Z) hooked up to two loads? When two impedances are hooked up in parallel, you find the equivalent impedance with the equation 1/(1/X1 + 1/X2) with X1 being the first impedance and X2 being the second. So if we have a 40kΩ input impedance for X1 and a 600Ω input impedance for X2, then our total load is
1/(1/40000 + 1/600)
1/(0.000025 + 0.00166)
1/0.00169166 = 591Ω
It's not a lot lower than 600Ω, but we're definitely starting to get to the point where tone and signal strength will be impacted. It's also obvious here that the higher the impedance of a device, the less it's going to affect the overall equivalent impedance. 591Ω is a lot closer to 600Ω than it is to 40kΩ. So if we add another 40kΩ load in parallel it won't do much to the overall impedance, but if we add another 600Ω load it will drop the equivalent impedance significantly.
Thanks for coming to my lecture, make sure you read chapters 11-12 tonight and this will be on the quiz tomorrow morning.
