Looking for a simple circuit to amplify both line and microphone inputs

January 1, 2008 5:17 PM

I'm looking for the simplest circuit possible that can do two things: 1.Take a line output from a PC sound card and drive a small speaker, and 2.take the input from a microphone and amplify this to a line input level. What's the simplest circuit out there for this? Bonus points on offer if it can amplify up to 4 of each channel.

So I'm building a new system to test cell phones, which involves simulating a phone conversation. We play back speech over the speaker (which is placed near the cell phone microphone), and record the output from the speaker of the phone using the microphone. I'm looking to expand the system we use currently to test the phones by adding the ability to test multiple phones. I have a multiple channel sound card, but it only produces line outputs and can only record line inputs. So, I'm looking for the simplest possible circuit that can take 4 of these outputs and drive a small speaker on each, and take the microphone signal and amplify it enough to be picked up by the line input of the sound card.

I've had a look around, but haven't found any simple solutions that can handle this. Has anyone else?

Things to remember:

- Audio quality is not critical. As long as it can produce reasonable speech on the speaker and amplify the microphone sufficiently, that's enough.

- I'm looking to test a maximum of 4 phones at a time, so there would be 4 line>speaker channels and 4 microphone>line channels.

posted by baggers to technology (8 comments total)

Well, an op-amp should be able to do either.

A LM324 or LM248 is 4 741-type op amps and should be able to amplify the microphones to line level.

A LM386 op amp can drive a small speaker (with a capacitor coupler), but there's only one to a chip.

If you can find some of Forrest Mims' Engineering Notebooks, he has simple circuits to do these things. I don't think Radio Shack carries them anymore.

Here's a basic intro to the 386, but doesn't show a circuit diagram that you need. The 386 datasheet (p5) shows how to drive a speaker from a line input.

Here's a typical microphone amp, but it may be for a fancier mic than you're using.
posted by MtDewd at 6:52 PM on January 1, 2008

For microphone to line: use an LMC660 quad op amp. Repeat the following for each amp in the package: Connect your input to one end of a 0.1μF capacitor; connect the other side of the capacitor to one end of a 10k resistor. Connect the other end of the 10k resistor to the inverting input (marked - on the diagrams) of one of the op amps. Connect a 100k resistor from the output of the amp back to the same (-) input. Connect one end of another 0.1μF capacitor to the amp output, and connect the other end of the capacitor to your sound card's line input. Connect the + (non-inverting) input of the amp to the negative power supply rail. Connect a 9V alkaline battery to the supply rails. This circuit will consume so little power when not in use that there's no real need for a power switch.

If the level's not high enough, make the 100k resistor bigger (the voltage gain of the circuit is the ratio of the feedback resistor to the input resistor).

For driving speakers, it's hard to get simpler than the LM386.
posted by flabdablet at 7:03 PM on January 1, 2008

Actually, just reconsidering: for driving speakers from a PC sound card, it's hard to go past el cheapo powered PC speakers.
posted by flabdablet at 8:19 PM on January 1, 2008

Which probably have LM386's in them :-)
posted by flabdablet at 8:19 PM on January 1, 2008

An even simpler speaker amp is the Texas Instruments TPA2005. It is a class D amplier with an H-bridge output so that you don't need a big old coupling cap on the speaker output. You can wire it directly to your speakers. You need to hook it up to your sound card line out using the single-ended configuration shown in Fig. 28. 0.1uF input capacitors should be fine for your application and 150K ohm input resistors for a gain of 2.

For the microphone input you don't need an amplifier. You can just use an electret condenser mic cartridge which you can get for about $2 from Digikey. You can just hook it up directly to your line in as shown here. The bias voltage and line in are connected to the same terminal on the cartridge and the ground to the other. The ground terminal on the cartridge is the same one that is also connected to the cartridge housing. Look at the Soundblaster circuit for an example.
posted by JackFlash at 10:23 PM on January 1, 2008

This $15 amplifier from Radio Shack is no audiophile treasure but I've used it in a lot of projects to save the trouble of building something. It has an internal speaker and has a jack for an external speaker. It can also be used to amplify a microphone signal, but JackFlash is right: you should be able to plug a microphone directly into your sound card's line in.
posted by moonmilk at 10:44 PM on January 1, 2008

Microphones often need powering (or biasing if you prefer) - here is one of my previous answers with some links to reference information. And, another link on electret microphones.
posted by Chuckles at 1:37 AM on January 2, 2008

Thanks all: lots of good info there. Next stop is digikey and a soldering iron!
posted by baggers at 8:19 PM on January 2, 2008