Members H.R. Shove and Stuff Posted January 18, 2015 Members Share Posted January 18, 2015 There's a thrift shop next to my gym that I occasionally stop by - today I saw the long brachiosaurus neck of one of those old intercom mics that could have been used at bowling alleys or drive through's at a fast food joint. Took it apart - some useful stuff for other projects, but I really like the buttons So I thought I'd re-purpose the mic and enclosure for a lo-fi recording mic and maybe put a few effects or filters in the base to make use of the buttons. Anyway, the point of this thread was when I finally turned all the innards into outards, I couldn't figure out how to remove the neck of the mic to get to the mic itself so I couldn't get to it. There were two wires coming from the mic that were connected to the PCB, but there was also a very short wire that was stuffed up inside the housing that was not connected to anything. From the PCB connections, it looks like one wire was ground, the red wire was connected to the PCB and a short stub of a black wire was unconnected. Is it safe to assume that the red is the +/pin 2 and the short black is -/pin 3? Is there a way to check using a multimeter? Does it matter as long as it's consistent? Is there a reason they only used one of the two outputs for the system? As a side questions: any fun ideas for effects/filters/etc that are useful in real time that aren't too easy to implement on the DAW? There are 4 buttons and a 7 segment single digit display. I'll house a simple preamp inside, but besides that, the skies the limit. Thanks for your advice. Matt Link to comment Share on other sites More sharing options...
Members WRGKMC Posted January 19, 2015 Members Share Posted January 19, 2015 The mic element itself is a two wire high impedance coil. With a dynamic mic, the mic element connects to a transformer which has a balanced output 600 ohms. The element connects to the primary of the transformer, the secondary connects to your XLR jack. The short black wire is likely ground that connects to the mic casing (and does not ground to either leg of the element) and connects to the XLR ground. The Wires connected to the PCB are your positive and negative balanced signals. I'm not sure what the PCB did. It may have taken the place of the transformer a dynamic mic needs to run low impedance. It may be a preamp making it a condenser mic, or it may just be a switching thing and the mic was run at high impedance. If you can take a pic and post it I can give you better tips on what you got, but to make this a low impedance dynamic mic there has to be a transformer inside the mic casing. The Chain is this Mic element Hi z > Transformer converts high to low impedance balanced so there's high voltage low current and less loss over long cable run. > Mic cable > Transformer (or circuit) Converts the signal from Low Z balanced to High Z for preamplification in mixer/preamp. The circuit may take the place of the low to high Z transformer but it essentially does the same thing. Link to comment Share on other sites More sharing options...
Members H.R. Shove and Stuff Posted January 20, 2015 Author Members Share Posted January 20, 2015 Hey WRGKMC, thanks for the reply. The reason I think that the short wire is one of the sides of the mic and not ground is because on the PCB, the one black wire that was connected to the PCB was connected to pin 4 of an LM381N, which is ground, according to the data sheet. I can't tell where the other red wire goes on the PCB at the moment, but it's near the Lm381N, so perhaps that is acting as a preamp? There are a handful of chips and transistors, regulators, etc on the PCB, not too intuitive, but I can tell a decent amount are for the electronic switching. There is also a preamp and amp for the little speaker in the unit. I measured the resistance across the two wires that were connected to the PCB and got 658 ohms. When I touched the leads with the probes, a scratching noise came out of the mic as I moved a probe up and down. I guess that means a transformers up there somewhere? I can get some pictures tomorrow, but thanks for the info! I think we're on the right track now Link to comment Share on other sites More sharing options...
Members WRGKMC Posted January 20, 2015 Members Share Posted January 20, 2015 Yea, that sounds like its a preamp. Condenser mics use a preamp to boost the voltage up so it will travel long distance, just like they do for your AC voltage coming into your house. They use big transformers to convert the power at the station to very high voltages. High Voltage/Low current will travel over long distances. Its the high current low voltage that has an issue with wire resistance because current is lost through heat.Super high voltages don't even need wires to travel. Lightening for example can be in the mega volts yet the current in comparison can be very low. Once the high voltage transmission lines get to the neighborhoods and business where its needed, they revert the high voltage back to high current. This allows the electricity to "Work" driving motors, heaters etc. Its also safer stepping the voltage down because its less likely to jump across an insulator and cause fire or death. Your mic does basically the same thing. The diaphragm has a coil attached that moves in a magnetic field. It produces a fairly high current with low voltage. They step that voltage up and current down for transmission purposes. This way the frequency content isn't eaten up by the cable resistance and capacitance. This step up is either a transformer OR a board (never both) That board likely requires phantom power to operate. Maybe a battery 1.5V, or 5, 12 48V phantom through the cable from the mixer/preamp depending on the type of mic. You could substitute the board for a transformer. You can scavenge one from a bad mic or buy one. The better ones like Jensen are not cheap. How good it needs to be depends on how good the cartridge diaphragm is. Or you can just use the board as is. If I knew what model mic it is would be helpful. Link to comment Share on other sites More sharing options...
Members H.R. Shove and Stuff Posted January 21, 2015 Author Members Share Posted January 21, 2015 Thanks again for all the info! I pulled back the casing around the three wires to see if I could figure out what was going on, and it turns out that one of the two wires that were connected to the PCB was ground/shield/whatever. One of the two wires has a red casing that I'm assuming is part of the signal. The other is bare wire, the same wire that was connected to ground of the LM381N. The short wire that was not connected is in a black casing. Why would they only use one half of the mic? Here are some pictures. Probably used at a school or bowling alley or something. Mic Base. Besides what's written on there, there aren't any more identifiers for what it is. There was a sticker/metal plate on the bottom but all the writing has rubbed off. Top PCB mounted to base. Has the chips for the buttons. Main PCB. Didn't take too great of a picture as there's a lot going on and it would be hard to figure anything about with tracing it. Some notable chips are the LM381N, LM384N, DM74123N (multivibrator) and DM7410N (CMOS stuff). There are a handful of 2N3567 transistors and a power supply section. Here are the wires coming out of the microphone neck after they were stripped back a bit. The two long ones were connected to the PCB while the shorter black one was tucking inside unconnected. There is some black insulation on the longer wire, but that is actually a smaller piece of wire connected to the longer non-insulated section. They must have cut it too short and needed to make it a bit longer to reach the PCB. I guess I could just wire up the three wires to an XLR and plug it into a mic preamp and see what happens? Link to comment Share on other sites More sharing options...
Members WRGKMC Posted January 21, 2015 Members Share Posted January 21, 2015 I'd connect it up to the XLR jack as you suggested. The black and red are the two sides of the mic element. Connect the shield wire to pin #1 and the ground casing (if it has one) The Red to Pin #2 and the Black to Pin#3 If that doesn't work Jump pins #1&3 black and shield pins together. This will make it unbalanced but it may work as well. The board looks to be an intercom system, likely 70V. The mic should work on its own. What quality its able to produce is questionable. Bogen makes allot of that 70V PA stuff for factories and businesses. It doesn't reproduce music very well. Its mainly for Voice and possibly have a Muzak system connected in like you have in your grocery store. You hear music then have the mic cut in with horrible sounding sales announcements. That's about the quality of that mic fully functional. With an XLR connected up to a good system you may get a frequency response between 200~10K. Maybe less with a megaphone quality. Goose necks are commonly used in Studios for talk back and other places like Courts and conferences, podiums etc. Most of these are for Talk frequencies and even a $10 Budge Samson or Nady mic can usually blow them away for musical quality. I'd likely build a desk stand for it. You may get some cool tones micing a guitar amp or something. If you get a clip for the thing it may be useful on a drum set. It all comes down to how well the cartridge works. If it sucks, sell it on EBay and get something better to play with. Mics are so cheap now its not worth spending much time on them for anything other then educational purposes. Link to comment Share on other sites More sharing options...
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