Newsgroups: sci.electronics.misc,rec.audio.misc Subject: Re: Help with impedance matching.. References: <[email protected]> [email protected] writes: > Hi all, > > I am in the middle of a project involving I/O and realized it's > probably not gonna work the way i want it to. > > I am a hardware h*ck*r, in the definition that I customize my personal > electronics to do what i feel they should do. In the words of the book > by that name: I have fun while voiding my warranty. > > So, I am trying to input audio directly from the PC, to a device that > is manufactured with an electret mic for vioce input. > > I wired in a phone jack w/switch to cutoff the mic when a plug is > inserted. > I used a direct double ended 3.5mm stereo patch cable from the SPKR OUT > port to the jack i wired in the device. For a few test runs, it worked > briefly, with some distortion, then the audio section of my PC mobo > died. I asked around, and found it was probably due to the impedance > difference of what the PC expected to drive, and the resistance of the > circuit that originally used an electret mic. > > What I'm looking for is: *If* impedance is the problem, In your case the impedance levels do not sound to be the problem. The problem are the signal levels. The microphone input need much lower signal volume than line output of soundcad normally give. This cause overload of the mic input that is heard as distortion. > what kind of > circuit do I need to introduce between the output of a sound card and > the input to a device that uses an electret mic in order to avoid > burning out one or the other? I was told by a local shopkeeper to do > something with transformers, but after that, he wasn't sure. > > Equations, schematics or other such advice is appreciated... You can find circuits for this here: http://www.epanorama.net/circuits/line_to_mic.html Here is the best match: 20 dB PAD for line to electret microphone input This circuit is designed to interface consumer (-10 dBu) signals to microphone input that is designed for two-were electret microphone capsules. This circuit provides around 20 dB of signal attenuation (typically enough to make thigns work, if more attenuation is needed use higher R1 value) and blocks the DC bias used in the electret microphone input to get to the line signal side (C1 does this). This circuit look like around 1 kohm signal source to at the mic side, whic matches pretty well to electret capsule characteristics (similar impedance and provides DC path for bias to go to ground). C1 +Line level in --||----R1----+-- +Mic level output + | | +----R2----+ | Ground (input)----+--------------- Ground (output) R2 = 1 kohm R1 = 10 kohm C1 = 10 uF NOTE: The polarity of C1 is marked to the circuit in case you use an electrolytic capacitor. A "dry" plastic or ceramic capacitor is preferred in this circuit. > > Regards, > --Electro-- > aka The Other David > www.dprg.org > -- Tomi Engdahl (http://www.iki.fi/then/) Take a look at my electronics web links and documents at http://www.epanorama.net/
