Members Dan Magers Posted March 7, 2006 Members Share Posted March 7, 2006 Anyone know the average condenser mic current at 48volts? After struggling with making up a 14 pin AMP connector power supply cable (never again) for my old vintage console, I find the power supply is missing some supply rails. +48 is now +26.+18 is now +2.6. I checked the cable for shorts before connecting it up about three times to make sure. Power supply worked fine last week when I tested it on the bench. Haven't moved it, turned it on today and find this bull{censored}. +18vdc is a easy fix LM7818, The +48 circuit is the most complex crap for a simple low current supply I have ever seen. I may just simplify the regulator and RC filter with as much "C" as I can cram in the space. Mics don't need much current do they? I have 68VDC to start out with. I was thinking one series pass transistor, zener on its base, big cap, resistor, big cap... Link to comment Share on other sites More sharing options...
Members Scodiddly Posted March 7, 2006 Members Share Posted March 7, 2006 LM317HT is kind of nice for doing phantom regulation, though 68v may be a hair high for it. The standard is 14mA per channel, with some caveats. Link to comment Share on other sites More sharing options...
Members ashivraj Posted March 7, 2006 Members Share Posted March 7, 2006 This might have some useful info...http://sound.westhost.com/project96.htm AS Link to comment Share on other sites More sharing options...
Members Dan Magers Posted March 7, 2006 Author Members Share Posted March 7, 2006 I think the LM317 is good for 37 volts. Is the HT suffix a higher voltage part? Thoughts on lifting the gnd terminal of this device a few volts? Concern is noise more than anything else. Link to comment Share on other sites More sharing options...
Members Scodiddly Posted March 7, 2006 Members Share Posted March 7, 2006 Originally posted by Dan Magers I think the LM317 is good for 37 volts. Is the HT suffix a higher voltage part? Thoughts on lifting the gnd terminal of this device? Concern is noise more than anything else. The HT (um - HVT, perhaps) is for the higher-voltage part. I've done (and seen in other circuits) from ~55vdc or more down to 48vdc for phantom circuits. Link to comment Share on other sites More sharing options...
Members Dan Magers Posted March 7, 2006 Author Members Share Posted March 7, 2006 A three term device would be ideal. +48v always seemed like enough for a tube anode to me. I have a 317 based supply here I use on the bench. I may jumper it in and see what the S/N looks like. Thanks for the fast reply guys. Get back to work now. Link to comment Share on other sites More sharing options...
Members agedhorse Posted March 7, 2006 Members Share Posted March 7, 2006 Lifting the ground terminal by say 24 volts sould give good results w/ the LM317 set to 24 volts (24Vreg + 24Voffset = 48 volts) and (68V - 24V = 44 volts) which may still be high for a 317 but I can't say for sure. Some of this may be a spec. derating due to the SOA of the pass transistor in the 317, so at lower currents this may be acceptable. My book shows an input/output differential of 40 volts, and this appears to be independent of the ground terminal reference (within reason). Also note that you will need a minimum 10mA load current to be sure you stay in regulation. This is usually done via the LED and suitable (note power dissipation) dropping resistor. The circuit shown in the Rod Elliot link is also common, you do not (obviously) need the voltage doubler & input filtering circuit. You might also look again at the power supply to see if you missed something obvious... like a cold solder joint on the pass transistor OR possibly a floating ground terminal somewhere since a problem with 2 supply voltages is suspicous. Link to comment Share on other sites More sharing options...
Members Dan Magers Posted March 7, 2006 Author Members Share Posted March 7, 2006 Thanks Andy. I suspected the two problems were related but I found the voltages correct on the 7818 apart from the output pin. I noticed some of the complexity of the current design is a soft start/stop feature. Back to troubleshooting. Link to comment Share on other sites More sharing options...
Members boseengineer Posted March 7, 2006 Members Share Posted March 7, 2006 Keep in mind that the IEC 61938, section 7.4.5 proclaims the 48 V phantom version with a 6.8kOhm series resistor, i.e. the maximum current per leg is about 7mA and that's only if you short the darn thing. Every device that draws any significant current will have to able to cope with significant sag. If you are drawing 5mA you are left with only 14V at the terminals. That's one of the reasons why we went with the IEC "recommended" variant of 24V/1.2kOhm. With a 5mA current draw you are only down to 18V at the terminals and get actually MORE usable power than with a "nominal" 48V supply. Link to comment Share on other sites More sharing options...
Members agedhorse Posted March 7, 2006 Members Share Posted March 7, 2006 Originally posted by boseengineer That's one of the reasons why we went with the IEC "recommended" variant of 24V/1.2kOhm. With a 5mA current draw you are only down to 18V at the terminals and get actually MORE usable power than with a "nominal" 48V supply. Any problem with mic loading at 2.4k differential? Link to comment Share on other sites More sharing options...
Members boseengineer Posted March 7, 2006 Members Share Posted March 7, 2006 Originally posted by agedhorse Any problem with mic loading at 2.4k differential? Not that I'm aware off. I have yet to find the microphone that has any problems with 24V/1.2k.Actually I'm curious, do any potential "troublemakers" come to mind ? Link to comment Share on other sites More sharing options...
Members SoundMan Posted March 7, 2006 Members Share Posted March 7, 2006 I had a customer tell me his ovation guitar preamp draws 30mA off of the phantom supply. And he was using 2 of them and 2 SM87's on one of our powered mixers. SoundMan Link to comment Share on other sites More sharing options...
Members Scodiddly Posted March 7, 2006 Members Share Posted March 7, 2006 Originally posted by boseengineer Not that I'm aware off. I have yet to find the microphone that has any problems with 24V/1.2k.Actually I'm curious, do any potential "troublemakers" come to mind ? The Baggs Para-Acoustic DI is pretty power-hungry. I've read that the CAD M-179 is also power-hungry, but I've never had occasion to test that. Link to comment Share on other sites More sharing options...
Members boseengineer Posted March 7, 2006 Members Share Posted March 7, 2006 Originally posted by SoundMan I had a customer tell me his ovation guitar preamp draws 30mA off of the phantom supply. And he was using 2 of them and 2 SM87's on one of our powered mixers.SoundMan If the phantom supply is implemented accoding to IEC specifications, he wouldn't be able draw anywhere near 30 mA. We use SM87s all the time. No problems at all. Link to comment Share on other sites More sharing options...
Members boseengineer Posted March 7, 2006 Members Share Posted March 7, 2006 Originally posted by Scodiddly The Baggs Para-Acoustic DI is pretty power-hungry. I've read that the CAD M-179 is also power-hungry, but I've never had occasion to test that. Tried the LR Baggs. No problem at all. It sounds counterintuitive but you can actually draw more power from a 24V/1.2k supply than from a 48V/6.8k. Theoretical maxium for the 24V/1.2k supply is 120 mW. The 48/6.8k can only deliver 85 mW. Link to comment Share on other sites More sharing options...
Members agedhorse Posted March 7, 2006 Members Share Posted March 7, 2006 Originally posted by SoundMan I had a customer tell me his ovation guitar preamp draws 30mA off of the phantom supply. And he was using 2 of them and 2 SM87's on one of our powered mixers.SoundMan 30mA? That's double the short circuit current on your boards isn't it? I think the customer is mis-informed... According to my Ovation info, the Pro preamps draw 12mA max from phantom power with the tuner on. That leaves a nominal operating voltage of about 10 volts which is plenty for the Ovation preamp to operate. Link to comment Share on other sites More sharing options...
Members agedhorse Posted March 7, 2006 Members Share Posted March 7, 2006 Originally posted by boseengineer Not that I'm aware off. I have yet to find the microphone that has any problems with 24V/1.2k.Actually I'm curious, do any potential "troublemakers" come to mind ? Some of my products use the 15 volt (or is it really 13.6V???) standard which also works fine on all but non-electret style mics. Externally biased condensers are a likely troublemaker. Link to comment Share on other sites More sharing options...
Members boseengineer Posted March 7, 2006 Members Share Posted March 7, 2006 Originally posted by agedhorse Some of my products use the 15 volt (or is it really 13.6V???) standard which also works fine on all but non-electret style mics. Externally biased condensers are a likely troublemaker. Externally biased seems to quite rare these days. Some of the higher end Neumanns are not electret but as far as I know they use an internal DC-DC converter and polarize at 200 V. The DC-DC converter also seems happier with the higher current capabilities of the lower volt supplies. Link to comment Share on other sites More sharing options...
Members Scodiddly Posted March 8, 2006 Members Share Posted March 8, 2006 This is pretty interesting - a few weeks ago over on another (mostly DIY) forum, somebody mentioned a noticeably different frequency response from a mic when they'd changed the phantom power arrangements around a bit. Which makes me wonder about whether an adjustable-voltage phantom supply in a boutique mic preamp might yield interesting results. A high-end mic preamp is a couple projects down the line for me, I just need to remember to put that on my feature-creep list. Link to comment Share on other sites More sharing options...
Members agedhorse Posted March 8, 2006 Members Share Posted March 8, 2006 Originally posted by boseengineer Externally biased seems to quite rare these days. Some of the higher end Neumanns are not electret but as far as I know they use an internal DC-DC converter and polarize at 200 V. The DC-DC converter also seems happier with the higher current capabilities of the lower volt supplies. I know we had some issues with some of the specialty "botique" mics that I would have never considered using live. I forget whicg off the top of my head though. Link to comment Share on other sites More sharing options...
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