Well,
The answer is -it depends on lots of factors.
Most channel strips that are "raw"- i.e. not installed in a desk work at "internal" levels. This can vary depending on the make/type of desk. As a rule, most lower-to-mid spec desks have an unbalanced output to the stereo mix bus (presuming your channel has a pan and fader control) and will operate at anything from -6dB w.r.t 0VU (+4dBu) to anything lower, depending on the architecture of the original desk.
So the first thing to do is to hook it up, apply an input signal to the mic/line input, set the gain and fader controls to "0dB" to allow (hopefully) 0dB signal gain through the channel, make sure any EQ or filters/pads are "out", and then measure the signal voltage at the bus (or group!) output at the edge connector.
Check the schematic to see where the mix "build out" resistors are- depending on the channel strip, the output of the fader/pan buffer will hit the mix-bus buffer, and this will have a resistor which acts as the input to the mix bus. If there is a resistor here, replace it with a link to optimise output signal and to ensure good response (this normally hits the mix-bus rail, that then goes to the mix-bus buffer/amplifier) Sometimes this will be located on a motherboard, but often on the channel strip.
Now, you should have a "direct" output from the channel. This is usually an op-amp stage which is decoupled with an output capacitor. Depending on the circuit and the internal level of the strip, it may have enough level to provide a useful output (even though it is unbalanced)
An output transformer can be used to make a balanced output by connecting the primary "high side" of the transformer to the unbalanced signal output, and the 0V (Audio Gnd) of the channel to the "low side" of the transformer primary. The secondary terminals then become the +Ve and -Ve ("hot" and "cold") balanced audio output (XLR pins 2 and 3). The Audio Ground for the balanced output can be the chassis ground (or channel 0V if this is connected internally- be careful with your grounding, and watch out for "digital ground" which often is used for switching/relays/LED's in the channel and is inherently noisy.)
To be honest, don't get too tangled up with the impedance of the transformer you use- with an output transformer for a solid-state application, it's the "Voltage Ratio" (i.e. direct turns-ratio) that is more important. If the channel strip is working at -6dB relative to output line level, a 1:2 (stepup by +6dB) transformer will do. Make sure as well that it is an actual output transformer too- an output transformer is specially designed to handle the possible higher output currents required. Using an e.g. 10k:10k input transformer will compromise your output drive! Also, be aware that the mix bus buffer/driver stage you're tapping this signal from is not optimised for driving a line output to the outside world! You may be able to modify the final op-amp stage, but be careful here- it's best to have a look at the schematic if you have any doubts.
If your channel strip has a much lower bus output, you'll need to add a signal-booster stage (just an op-amp stage) plus a line-driver output- e.g. transformerless DRV134 IC, or an op-amp + transistor buffer (as discussed here countless times) driving your transformer. Either way, watch out for your global signal polarity- i.e. you may need to flip the transfomer windings to get +ve signal polarity from mic/line input to line output. This is because you don't know what happened after the mix-bus amp- it may have gone through patchbay insert buffers, or output routing, and don't presume there weren't subsequent inverting stages!
Best thing to do is just hook a transformer up to your channel and see what sort of levels you get. You may get mixed results, but experimenting is the best part of these projects!
Mark
September 07, 2004, 05:27:04 PM