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Registered Member #902
Joined: Sun Jul 15 2007, 08:17PM
Location: North Texas
Posts: 1040
Hey all,
so I'm REALLY rusty on my circuit analysis (having switched majors to Computer Engineering and not taken the EE classes in a while), and I am a little fuzzy on my impedance matching with audio devices.
I am making a modification to a Yaesu FTM-350 mobile Ham Radio with a friend, and we may sell adapters for users of this radio which allow for people to tap into the line going from the main unit to the control head and break out the audio. While the radio has an external speaker jack, it only has one and most people with this radio use that jack for the PA or a room system. This has been done before, but not with proper matching of the output to Line Level for hookup into a system like a stereo (the line is amplified in the base unit before sending it over the cable to the control head).
I went through the schematic for the radio (won't post here since it is a paid for schematic), but the amplifier in question is this: datasheet for amplifier
I thought about doing a transformer configuration to match the outputs (which I had done with previous audio devices), but finding a fitting one might be problematic and we want to make this simple enough that anyone can build it. I think I might need to instead go with an LC matching circuit with the inductor on the output of the amplifier IC and the capacitor across the load. I have also read that since the load lines for line level devices are very small compared to the wavelength of an audio signal, that full matching is often ignored in exchange for "bridging," using a voltage divider to match the real resistance instead. Is this really effective or safe for general use on audio equipment?
The amplifier's datasheet shows the load resistances for low impedance loads like speakers, but line inputs generally have a very high input impedance. I thought at first that it would limit the power transfer, and therefore prevent a signal from getting too high into the line input (rational was that a car stereo having to amplify a weak signal was a better tradeoff than a strong signal damaging it), but I found that it indeed still transfers too strong a signal to the stereo.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I'm guessing that the audio for the FTM-350 is mono. and since the audio PA is stereo or bridge, I guess it's in 'bridge' configuration, giving plenty of audio at low supply voltages, with a 'balanced' load/speaker, and to ave the cost of a capacitor, the loudspeaker is probably directly across the two amplifier outputs, relying on low voltage offsets to give only a small 'standing' dc in the speaker. So, you should consider both 'speaker' terminals to be at about 1/2 supply volts dc
with a transformer and series capacitor in place of the loudspeaker you could have an isolated output or you could have a transformer and series capacitor from either output to 0v or you can have a resistive divider to 0v from either output, with or without capacitor from output
In any case, the amplifier load needs consideration;
with a very high load resistance (>>speaker resistance) the increased internal open loop gain of the amplifier may cause instability, or may not, I've not played with these so don't know.
some ic amplifiers output stages operate in Class-A/Linear/Emitter-follower mode with very light loads to 0v, so if you go for the non-isolated resistive divider you may get minimal/cheap with lower distortion BUT I'm not keen on using my amplifier or computer (and me) as an rf earth, which would happen with any un-balanced antenna (e.g. diople without BalUn) SO you should only consider electrically isolated solutions.
Small mains transformers tend to have thinner laminations than larger power transformers so make tolerable audio transformers for ssb, not sure about wfm.
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Matching impedance on an amplifier output
