Matt Morris Posted May 19, 2012 Report Share Posted May 19, 2012 Hi all, I'm currently in the process of building a manual switch box so that I can switch between 2 sources into 1 input on my 552. The two sources are a fig8 mic and a line level out radio receiver. I'd switch to the fig8 mic when recording ambiences and switch back to the radio receiver at all other times. I did have a thought of making the switch box a bit more elegant by having it automatically switch when phantom power was applied. I would only need phantom power for the mic so I could maybe set up a relay energise via phantom power and switch contacts over to the mic. It would save having to dig into my bag and also avoid accidentally knocking the switch at the wrong occasion. I'm not entirely sure how I would split off the phantom power though, or if the switching is asking for trouble. I suspect it is. Has anyone else ever thought of doing something like this? Quote Link to comment Share on other sites More sharing options...
carbonhobbit Posted May 19, 2012 Report Share Posted May 19, 2012 The 552 does this already. Read the manual. Line level can't send phantom and mic level can be toggled from powered to un-powered and is all done from the front of the mixer. Quote Link to comment Share on other sites More sharing options...
Matt Morris Posted May 19, 2012 Author Report Share Posted May 19, 2012 Yes, I understand that and don't need to read the manual. I would like to build an external switch box that switches automatically when 48v is applied down the cable, i.e. when I switch on the phantom power on the 552's front panel. I think this could be achieved with a relay but not 100 sure. Quote Link to comment Share on other sites More sharing options...
Wandering Ear Posted May 19, 2012 Report Share Posted May 19, 2012 The 552 does this already. Read the manual. Line level can't send phantom and mic level can be toggled from powered to un-powered and is all done from the front of the mixer. The 552 is capable if sending phantom power to the inputs when set to line level if that feature is turned on in the menu system. I've used this feature before for recording high spl content like gun shots and fighter jets with cindensor mics. Wandering Ear Quote Link to comment Share on other sites More sharing options...
studiomprd Posted May 19, 2012 Report Share Posted May 19, 2012 Phantom power does not supply much current, probably not enough for as simple a solution as you envision. Quote Link to comment Share on other sites More sharing options...
Matt Morris Posted May 19, 2012 Author Report Share Posted May 19, 2012 Phantom power does not supply much current, probably not enough for as simple a solution as you envision. Thanks for the reply. Yes I did wonder about the current draw when using a relay. Then I thought maybe I could build a simple circuit with some kind of a switching IC. The current draw would be less for sure, but then the whole thing would probably get more complicated than I'd hoped for just a simple switch. Quote Link to comment Share on other sites More sharing options...
Mark LeBlanc Posted May 19, 2012 Report Share Posted May 19, 2012 If you don't mind a small manual switch http://www.sescom.com/product.asp?item=SES-XLR-AB Quote Link to comment Share on other sites More sharing options...
Jay Rose Posted May 19, 2012 Report Share Posted May 19, 2012 It might actually work. There's no particular standard for what a phantom input will supply, but a Schoeps eats only about 5 mA at 48v, and that's one of the thirstier mics. Some other good mics eat only half that. But let's say you've got 5 mil to play with Digikey sells a bunch of small 48v DPDT relays (I assume you want to switch balanced) that want 5 mil or less. Panasonic DS2E-S-DC48V-TB is typical, about $5. So it's worth trying... Just A. A coil inrush might make the power source momentarily noisy. So be careful about switching while there's signal in the system. B. Don't try to fancy things up with an indicator LED... unless you're sure you've got the energy. Quote Link to comment Share on other sites More sharing options...
Matt Morris Posted May 19, 2012 Author Report Share Posted May 19, 2012 It might actually work. There's no particular standard for what a phantom input will supply, but a Schoeps eats only about 5 mA at 48v, and that's one of the thirstier mics. Some other good mics eat only half that. But let's say you've got 5 mil to play with Digikey sells a bunch of small 48v DPDT relays (I assume you want to switch balanced) that want 5 mil or less. Panasonic DS2E-S-DC48V-TB is typical, about $5. So it's worth trying... Just A. A coil inrush might make the power source momentarily noisy. So be careful about switching while there's signal in the system. B. Don't try to fancy things up with an indicator LED... unless you're sure you've got the energy. Yes! I knew there was some legs in this idea. Maybe I could just use the phantom power as the base current for a transistor, then a 9v battery at the collector to energise the coil. My only concern would be splitting the cable and its noise implications on the signal. If I did just split the cable into the relay coil terminals, wouldn't that just introduce the relay into the audio signal? I think I should be using a capacitor somewhere here but not sure where or why. I think I need to dome some reading up on my electronics! Quote Link to comment Share on other sites More sharing options...
Jay Rose Posted May 20, 2012 Report Share Posted May 20, 2012 I wasn't asking enough questions, sorry. You'd turn on phantom when you wanted to use the figure-8, which I have to assume is a condenser (rather than a ribbon, the only native figure-8 mix but kind of heavy and delicate). So the question is, are you using a phantom mic or one with a battery? If it's a phantom mic, you're got two loads, the relay coil AND the mic. Gotta figure for at least 10 ma. Do some tests with a resistor and voltmeter to make sure your mixer/recorder will supply it. Sorting them out at the mic end is easier. Use one winding of a center-tapped transformer, in parallel with the cable. Ignore the other winding. Pick one with an audio impedance at least 5k so it doesn't load the signal, and just figure for its DC resistance in series with your relay. Or if you're willing to put a battery out there, you could power the mic with it! Or... thinking out of the box... how about two 48v relays, one coil between each leg of the balanced signal and shield. The two of them would use a bit more current than one, but might still fit in your budget. Wire their switches in parallel - they'll both energize at the same time - and you've lowered the contact resistance and/or increased signal reliability. Just consider the effects of the relay coil across your signal. Digikey part C93404-ND, a telco relay, draws 3.1 mA per. So its coil resistance is 16k. A 600 ohm nominal mic would never even know it's there. Quote Link to comment Share on other sites More sharing options...
studiomprd Posted May 20, 2012 Report Share Posted May 20, 2012 " Yes! I knew there was some legs in this idea. " " make the power source momentarily noisy. " and you are switching the mic power on and off as well... Quote Link to comment Share on other sites More sharing options...
Jay Rose Posted May 21, 2012 Report Share Posted May 21, 2012 " make the power source momentarily noisy. " and you are switching the mic power on and off as well... Well, yah. I assumed the OP would know enough to pot down when changing phantom settings on the mic. My point is that you might not want anything important going through the mixer - even on other channels - when you activate the relay(s). Quote Link to comment Share on other sites More sharing options...
LarryF Posted May 21, 2012 Report Share Posted May 21, 2012 Lectrosonics used to have a patented system called Plus Power, that turned on remote speakers by grounding the center tap of a transformer across a balanced line which then pulled current from another remote balanced transformer across the line energizing a relay in series with the remote center tap and a 24 Volt battery and it worked very well. I would recommend using a line level transformer with a center tapped winding or two windings wired tied together to create a center tap. The ends of the center tapped transformer go to pins 2 and 3 of the balanced audio line and the center tap goes to a low current 48 Volt relay and the other side of the relay coil to pin 1. Typical relays will operate at much less than their rated voltage, i.e., 24 Volts. The pin 2 & 3 audio signal is isolated from the relay by the transformer. When you apply phantom power, the double pole relay switches to the microphone. Removing power lets the relay switch back to the receiver output. You will want a shielded transformer to keep from inducing hum into the mic level signal. It also must have high reactance at low frequencies to keep from rolling off the low end. If the transformer has good low frequency specs, this is covered. Note that the DC current through the windings of the transformer are in opposition, which keeps from magnetizing the transformer. The transformer should also have DC resistance under 1000 Ohms per coil, which shouldn't be a problem. You can even put an LED in series with the relay coil if you want a phantom power indicator. If the device you are going to has a pin 2 or 3 low resistance DC path to ground, you may not be able to develop enough voltage across the transformer to pull the relay to the mic position. 50 Volt blocking caps in the receiver side of the connector but not the mic side will prevent the loading. Best Regards, Larry Fisher Lectrosonics Hi all, I'm currently in the process of building a manual switch box so that I can switch between 2 sources into 1 input on my 552. The two sources are a fig8 mic and a line level out radio receiver. I'd switch to the fig8 mic when recording ambiences and switch back to the radio receiver at all other times. I did have a thought of making the switch box a bit more elegant by having it automatically switch when phantom power was applied. I would only need phantom power for the mic so I could maybe set up a relay energise via phantom power and switch contacts over to the mic. It would save having to dig into my bag and also avoid accidentally knocking the switch at the wrong occasion. I'm not entirely sure how I would split off the phantom power though, or if the switching is asking for trouble. I suspect it is. Has anyone else ever thought of doing something like this? Quote Link to comment Share on other sites More sharing options...
Matt Morris Posted May 21, 2012 Author Report Share Posted May 21, 2012 Apologies for not responding sooner. Things have been a bit mad with work and family over the past few days. Thanks for all your suggestions gents, all of your advice is very much appreciated. I'm going to spend the first bit of spare time I get digesting it all. I did foresee problems by just putting a relay in parallel with the mic, and had thought about an isolation transformer but my experience with them is pretty limited. I'll look into your suggestions Larry, thanks again. Quote Link to comment Share on other sites More sharing options...
Matt Morris Posted June 8, 2012 Author Report Share Posted June 8, 2012 Lectrosonics used to have a patented system called Plus Power, that turned on remote speakers by grounding the center tap of a transformer across a balanced line which then pulled current from another remote balanced transformer across the line energizing a relay in series with the remote center tap and a 24 Volt battery and it worked very well. I would recommend using a line level transformer with a center tapped winding or two windings wired tied together to create a center tap. The ends of the center tapped transformer go to pins 2 and 3 of the balanced audio line and the center tap goes to a low current 48 Volt relay and the other side of the relay coil to pin 1. Typical relays will operate at much less than their rated voltage, i.e., 24 Volts. The pin 2 & 3 audio signal is isolated from the relay by the transformer. When you apply phantom power, the double pole relay switches to the microphone. Removing power lets the relay switch back to the receiver output. You will want a shielded transformer to keep from inducing hum into the mic level signal. It also must have high reactance at low frequencies to keep from rolling off the low end. If the transformer has good low frequency specs, this is covered. Note that the DC current through the windings of the transformer are in opposition, which keeps from magnetizing the transformer. The transformer should also have DC resistance under 1000 Ohms per coil, which shouldn't be a problem. You can even put an LED in series with the relay coil if you want a phantom power indicator. If the device you are going to has a pin 2 or 3 low resistance DC path to ground, you may not be able to develop enough voltage across the transformer to pull the relay to the mic position. 50 Volt blocking caps in the receiver side of the connector but not the mic side will prevent the loading. Best Regards, Larry Fisher Lectrosonics Hi Larry, Do you think you could explain why I would be using a centre-tapped transformer to isolate the audio? I'm a bit confused, is it to energise a 24v relay instead of a 48v? Only I'm having difficulty in finding a line-level isolated 1:1 transformer with a centre tap. The ones I was looking at are for sale at Farnell.com in the UK and made by OEP. There are also some made by Neutrik which appeal as they have a great rep. Thanks, Matt. Quote Link to comment Share on other sites More sharing options...
rofin Posted July 26, 2012 Report Share Posted July 26, 2012 I don't know if you're still up for this Matt, but ... Place a couple of diodes (glass passivated for low leakage preferred) back to back across pins 2 and 3 of an XLR-3M. Anode of one to pin 2 Anode of the other to pin 3. Join the Cathodes together. This will keep the whole thing balanced and any noise (unlikely) will remain common mode and hopefully knocked out by the mixer input stage. From the Cathodes feed via resistor to a 12V low power DPCO relay coil and the other end of the coil to pin 1 of the XLR. A bit difficult to know the resistor value 'cause it depends on mic. load and relay coil load but somewhere around 1K is likely. Suck it and see with a trimpot perhaps. Connect pin 2 of the XLR to one of the relay commons and pin 3 to the other. Connect your condenser mic to the normally open N/O contacts of the relay and your Line to the normally closed N/C. Rob Quote Link to comment Share on other sites More sharing options...
Matt Morris Posted July 26, 2012 Author Report Share Posted July 26, 2012 Hi Rob, Cheers for the reply. I did actually build the switch box but made it a manual latching push switch instead of automatic. I'm not 100% happy with it though so I'll definitely try your suggestion with my mk2 box. Sounds straightforward enough. Thanks again, Matt. Quote Link to comment Share on other sites More sharing options...
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