Many Part 15 radio stations utilize a mix of consumer and professional equipment. This opens some audio-line matching situations that many hobbyists need to know. These include
1. Level differences;
2. Patching of 2-wire (unbalanced) to 3-wire (balanced) lines;
3. Impedance matching.
Here is a home buildable circuit circuit that can solve part of the problem:
This subject opens up a problem I had using the Schlockwood at first for AM a few years ago and more recently for FM.
The output of the Schlockwood is a mono balanced TRS and all inputs to transmitters are an unbalanced two channel "stereo" input to accommodate typical outputs from computers or other audio sources.
Jim Wood, creator of the Schlockwood had to show me about changing the wiring in the cables so it would work and later I just did the mod on the circuit board. So now I can just use moulded cables.
I still to this day, do not know how all use it without doing mods to the cable wiring or on the circuit board of the Schlockwood. Even the Procaster and the Rangemaster....the AM transmitters the Schlockwood was made for, still have an unbalanced input to accommodate typical stereo audio sources. A indoor interface delivers audio and power to the transmitters mentioned so the wiring mods are still needed at the output of the Schlockwood. I use it now for FM with the MS-100 which takes a stereo input then combines the two and if the mono TRS 3 conductor signal is just fed to the transmitter it doesn't work! If anyone here uses the Schlockwood for processing please tell me...how come I have never heard about this from others? I even asked Jim Wood the question and he had no answer either. I suggested to him that this is the one big flaw in this processor and he could have a jumper on the board to change from balanced to unbalanced output, as he does for the input to accommodate typical audio sources.
The XLR input and output are the balanced. And the TRS is unbalanced. I just use XLR cable from my mixing board to input XLR and XLR output to a 3.5mm TRS cable to interface it to transmitter.
Roy, we need to talk.
You've got part of that formula right...
If you're starting out with a balanced output that means (of course) that there are 3-wires of the configuration: plus +; minus -; and shield/gnd. You only really need a TS connector and not a TRS.
But a further complication exists... a 3-wire XLR circuit most likely outputs at 0dB level, a professional standard. Whereas the TR or TRS unbalanced input is most probably designed to require -10 dB line level, the consumer standard. The resulting level mismatch could give severe clipping distortion. What is needed here is a resistive matching pad designed for -10 dB attenuation.
Carl, I haven't had no clipping distortion but have had a sporadic low frequency buzz/hum, that will just come and go...A matching pad is the part I left out. My mind drew a blank spot and couldn't think of what is was called...lol
Roy, that intermittent low frequency buzz/hum should go away if you solder a wire between the 'r' and ground 's'. The balanced line needs to be put into an unbalanced state. Doing this turns a 3-wire audio line into a 2-wire audio line.
Thanks Carl, I will do that.
Carl, That intermittent low frequency buzz/hum was reduced by 90-95% after your suggestion.
Your listeners will be happy, Roy!
I'm back. Roy, there might be a way to fix that last 5% of hum in your audio line.
If I am right, a -10 dB T-Pad Resistor Attenuator might do this.
Trouble is, it has been years since I dealt with designing such circuits and my data and charts are in storage somewhere, so we need someone fresh to show up with proper guidance.
I was looking matching devices online to convert/match XLR balance to TRS unbalanced. I've found a few. I might look into getting one of those.
BTW that buzz/hum is back to where it was sporadically. This hasn't been the whole time I've had the station up the past year. It didn't appear until I moved the studio into another room 2-3 months ago.
Lost in a maze of possibilities.
It is difficult to comment on equipment hookups since the variations found in the audio world open many possible ways of doing things.
As far as "Line Level" is concerned, here is some more confusion:
Simplified, The audio out jack from your computer for example or the headphone jack on a radio or MP3 player or hi fi receiver/integrated amp is *not* line level as it is controlled by a volume control and variable and is the output of a final amp.
Line level is a predetermined, fixed output not controlled by any control and taken from a point from before a final amp and feeds another device that will amplify it as desired. Example, the output of a turntable, or tape deck, or separate component tuner.
You're off the mark, Mark. You are referring in part to output levels that are not 'Line Levels' but are other types of levels, such as headphone levels and speaker levels. This is explained in the Wikipedia link given elsewhere in this thread about 'Line Levels'.
Even if an actual audio line has a volume control, as is the case with computer hardware audio outputs, the output can be set to official 'consumer Line Level' by using a reference tone at full modulation with a measured output of -10 dB.
I'll throw a few more confusing facts into this mix but I have reached my limit in terms of explaining it in a way that can be easily digested.
Did you know that a single XLR output connector can be either wired to deliver stereo left-channel or two unbalanced lines? The trick is to wire a common ground, which is one ground wire serving both the L + R channels, to pin 1 of the XLR. Wire the L+ wire to pin 2 of the XLR, and wire the R+ wire to pin 3 of the XLR.
Otherwise, for balanced stereo to be delivered by XLR connectors there needs to be two XLR connectors, one for L and the other for R.
Moreover, a single TRS connector can be wired to deliver stereo. But it will only work if it is plugged into a TRS female connector wired in the same configuration.
If you don't know how a female TRS connector is wired, it becomes a guessing game of chance.