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, 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 don't say if your pc motherboard is designed to drive a pair of speakers directly or if it requires amplified speakers (usually called "line out")
I don't think anything should have burned out. Or shouldn't have due to a impedance mismatch anyhow . . . . In the case of speakers directly driven by the PC there's a low impedance power driver that wants to "see" some particular load impedance - usually 8 ohms or 4 ohms - minimum. Try to drive a lower than normal impedance and it would burn out - but something with a microphone input is a high impedance.
Electret mikes typically have a small voltage impressed on the line - usually that is via a 4,000 to 20,000 ohm resistor - for two wire devices - a three wire mike would be a different story but unless your talking about broadcast quality equipment most of the inexpensive stuff it two wire. The voltage powers a small amplifier built into the electret mike - a single FET, usually, that takes the extremely high impedance of the electret and outputs a lower impedance (that being whatever the resistor in the amplifier the electret is feeding in the range of 4K-20K.
So other than overdriving the piss out of the amp you were plugged into, and a remote possibility of damaging its input stage, there should have been no damage - if the electret mike was on the line along with the PC amp there's a good chance it was destroyed.
My PC with on board sound has three jacks on the back - mic(rophone) spk (speaker) and line - line is a high impedance output and is designed to go into an amplifier - what you were doing. (amplified speakers used in PCs) In the case of an amp designed to work a two wire electret mike I would add a capacitor to keep any DC out of the line output of the PC. (point one microfarad 100 volt high quality non polar, polyester, polystyrene, or polypropylene type) It is unlikely that the voltage would kill the line out stage but I wouldn't want DC there.
If by some chance your PC sound output uses a "bridged amplifier" to drive speakers and you put a ground that was common to the computer and amplifier to the output - it would destroy the bridged amplifier in the computer. Because there is no ground on a bridged amp both wires are driven by separate amplifiers (total of four amps for stereo). That is done to allow more power out with a low voltage supply - and I don't know if any mobo manufacturer does it - but a sound card might do something like that. You work around that kind of problem by only using one wire and the chassis or using a resistor divider or cap to isolate the PC amp from ground - I'm guessing you don't have a bridged amp though.
So now the amp in your computer is dead? A transformer wouldn't fix that . . . A transformer is a work around for bridged amps - but it costs more and works no better than the other methods I suggested.
You don't need to match impedances in audio work. They only have to be reasonably close to work or in the case of a low impedance output driving a high impedance input they don't have to be close at all.
Think about how you did things - I'm guessing you grounded the amp in the PC inadvertently/accidentally and blew it that way. For instance if you are plugging and unplugging amps while it is running or your switching the output you might kill it, or if your layout is messy and the output was grounded by a loose wire floating around.
The distortion was probably due to too high a signal level into the amp you were driving. That can be cut down with resistors or just using the line output into an amp with a line input. (line is typically set at one volt of signal measured peak to peak or .707 volts of AC signal) It isn't carved in stone, to my knowledge, but most amps are rated that way.
BTW electret mikes are actually capacitor mikes of old - the electret (an insulating polymer that was impressed with a high voltage as it cured and becomes permanently electro statically charged) is used in place of the high voltage bias supply that capacitor mikes of old required. The output impedance of a capacitor is infinite, or damn close to infinitely high - an FET is tailor made for working with high input impedances since it draws no current and works with voltage.
(close) Impedance matching in audio is only of a concern with driving loudspeakers - for power transfer. Any standing waves that may be generated by mismatches would take instruments to measure - you won't hear them. Inside an amp - you care that one amplifier stage matches the next so power/signal isn't wasted unnecessarily, or you end up using more stages than it should take.
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Radio shack has an "impedance matching cable" meant to go from a line out to a mic in. I'd give that a try. My guess is that it's just a resistor in a cable.
Jeff
--
"They that can give up essential liberty to obtain a
little temporary safety deserve neither liberty nor
safety"
- B. Franklin, Bartlett's Familiar Quotations (1919)
--
No, it wasn't.
You probably wired the jack wrong on the mic side and connected the
mic power supply into whatever audio port you were trying to drive
the mic amp with.
That probably is what happened. I was trying to get the timing right, and was plugging in the cable as the PC was playing. I hoped it would be able to handle a momentary shorting, but apparently not...
I'll try to draw up a diagram for others of how i did things....
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.
You can find circuits for this here:
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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).
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.
--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at
http://www.epanorama.net/
Another question, what if I want to use a source that doesn't offer a 'line out' port? Is there anyway to simulate line level outs from, say, a headphone jack on an MP3 player?
the cap (for DC isolation) and resistor divider (lowering the output into the range of a line out). The ratio of the resistors determines the signal attenuation.
Want to cut down the signal more? Increase the 10K or decrease the
1K.
Some home stereo systems can put out over 200 volts peak to peak - but a computer works from a 12 volt supply so the voltage swing is proportionately lower.
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I've uploaded a schematic for the circuit I've made incorporating Tomi's line level/electret circuit. This isn't the phone circuit I mentioned, rather, the reason I tested my circuit on the phone first.
here's the link:
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It works, but I seem to be getting some interferance, so my next steps are to try a metal sheilded enclosure, a sheilded connecting cable, and keeping the mic/speaker grounds separated until their return to the device.
A big thanks to y'all for your help. I appreciate it.
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