Hi all, First I'll start with the concept. I wish to be able to put an aux. input into my comms headset, mix it with the comms traffic but so that only I can hear it. It needs to be passive, discrete and preferably integrated into the belt pack. The aux input will either be a PFL cue from a sound desk or a 3.5mm headphone input from my ipod or laptop, so some form of isolating transformer would be a good idea to avoid putting hum (or in a lot of cases worsen the already existing hum) on the comms system. The signal from any stereo input needs to be summed to mono as I hate wearing double muff headsets. This is to use with a party line system that is Clearcom/tecpro/asl/metro/stonewood compatible (as most of my clients use) So, firstly I went looking for a system that had this feature already built into it's belt packs. Clearcom's encore system has an aux headset connector, which I am lead to believe is an output only. Dead end. Secondly I found a product from interspace called the CiO which rang almost every bell, but the price (£275 + vat list!!!) and the fact that this is an extra box (which requires a separate power supply) kind of ruled this option out. I've got a load of the interspace PC-Di boxes, and they are fantastic, but the CiO sadly did not quite fit the bill. So to look down the home brew route. Unfortunately my electronics knowledge is sadly lacking, so I thought about bastardising an existing unit. I looked at the canford tecpro units, wishing they had an extra input channel. Lightening struck the old grey matter, and with a flash of inspiration I reasoned that if I got a dual channel unit, the audio from the second ring could be my extra input. Not so I hear you cry! On the Tecpro BP 113 units the audio from ch 1 is routed to the left side of a double muff headset, and ch.2 to the right. So a dual channel design and no mixer element involved. But there is also the BP115 I cry back! The monaural version! I seam to recall using a metro audio unit that was dual channel, but fed both signals into a single sided headset. Could it have been a DC2 I was using? Could I achieve the mono sum of a headphone level signal by using the mono summing circuit from the rane notes? If so then a small circuit with a few resistors and a isolating transformer could do the trick, but the output of a laptop, ipod or headphone out on a desk is effectively a very small power amp, and I know better than to try to combine the outputs of a power amp together, damage could ensue. Rather than alter the beltpack in any way I could make adaptor cables that have a 3 pin XLR for input to ring 1, then a 3.5mm minijack to input into ring 2. If the power supply is shared for both rings then I might have to isolate them in the comms box, to avoid sending 24v dc to the audio input on ring 2, but that is a bit beyond my means. So to re-cap (just in case I've lost you all...) 1) Do any other possible solutions exist? 2) Would my <monaural dual channel belt pack as a mixer> idea work? 3) Is the metro/stonewood DC2 like the techpro BP 113 or the monaural BP115? (Might as well admit I'm bidding on a DC2 for £9.99 on ebay at the mo, worth a punt to see if it might work) The Stonewood website is greatly lacking in detail... 4) Can you safely mono sum a headphone signal via the method described for mic / line signals in the rane notes? Thanks in advance, James