1/19/2024 0 Comments Bias amp 2 pttI still have $3.50 of Radio Shack resistor-capacitor combos if only I could figure out a way to wire them in! I have poked and probed and asked around, but there seems to be no simple rewiring fix short of installing a separate intercom electronics system interfaced to the factory wiring on the R22 circuit diagram, the PTT relays connect to a single wire running to the radio audio, with no way to separate out the two microphones and install separate bias resistors for each. I delivered the David Clark documentation to Pat Cox in about March of 2005, and he put this issue in the problems-to-be-solved hopper for me (what, they didn't notice the problem for 27 years after initial certification?!?), but there must be lots of other more important issues, nothing has happened on it since. Robinson, for whatever reason, chose to imbed a Single Bias Resistor serving Both microphone leads-with the result that when you're on hot mic, or both IC switches are depressed, BOTH microphones use the SAME bias resistor-with the result that the microphone with the lower resistance of the two wins the battle, and takes all the current, and defeats the other microphone!! David Clark adheres to the idea that the airframe manufacturer provides the standard method of having a Separate Bias Resistor for each microphone outlet, so that each mic can get the right current. So each microphone requires a "bias" current to pass through the circuit. History: aviation microphones are wired to simulate the old carbon-cartridge mike of 60 years ago. It's a REAL PROBLEM if the two headsets in the aircraft do not have the same microphone impedance-and, worse yet, it's not a low-vs-high impedance problem, but an even closer match, so the year in which each microphone was manufactured strongly influences whether one mic defeats the other! I have documentation from David Clark on the R22 hot-mic or simultaneous PTT conflict. If it has a model number beginning with H, such as H-13, then it's made for civilian helicopters, and should work in anything other than a military helicopter. DC headsets with a U174 are probably not military - they sell more headsets to helicopter pilots than just about anyone. I could replace the GA plugs with a U174, but I just haven't gotten the ambition. I'm now using a Clarity Aloft headset, with an adapter, and I really like it. I've built my own headsets for about 20 years, and it's not rocket science. Or you can take the two plugs off and replace them with one U174. You can take your Cessna headset and plug it into an adapter and it will work fine. The last time I used on was in the early 80s. Carter Engineering sells one, or used to. You can use a military headset with a mike adapter, which plugs into the mike and increases the impedance, plus it probably has an amp in it also. You can hear ok, but the mike won't transmit well enough to be heard. Neither will harm the other, but the impedance difference is enough that military headsets don't work in civilian aircraft. It should be 'high' impedance, about 300 ohms. I don't fly Robbies, but I can't imagine that it's any different from any other modern aircraft. I take it since most people appear to be successfully taking their GA headsets and using them in R22s and other helicopters the R22 actually isn't weird that you can take your headset from the Cessna - plug it into the r22 with adapter (or vice-versa) and then into a 206 with no problems? In that case I would only want to beware of the headsets with u174s (Since theres a good chance they are miliary) and any GA two-plug headset will work with adapter or by soldering a u174 on? Any hard and fast rules for David Clark models? I'm looking to buy a used DC on ebay - most of them with a u174 appear to be military. If nothing else - is the R22 High or low impedance? I take it the mic is not a concern. A High impedance headset may not work particularly well with a low impedance amplifier.ĭo we have any numbers? This isn't voodoo. Obviously a low impedance headset could damage a high impedance radio. One manufacturer told me they would have to put a "Bridge Rectifier" in because the R22 had "Old radio technology." I said sure but to my mind a Full wave rectifier would be better suited to a D/C power supply and I can't really imagine how one would be of any use at all in a headset. So lets say you've got a headset with the U-174 - I take it GA is "High" impedance - Military is "Low." I'm getting the impression the R22 is odd (Only because this is what I keep hearing). There are a few posts on headsets but most are more concerned with whether or not you've got two plugs or one.
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