Electronic – Help me hack the Yamaha Receiver

Don't worry about the input impedance of the stage. Even a low 1 kΩ impedance will only decrease the load from 8 Ω to 7.94 Ω. A more typical 10 kΩ will be invisible to the power amplifier.

Power amp for driving headphones is pretty easy. In fact, you could just do it all in one stage. See the CMoy amp for a popular and simple design:

• http://tangentsoft.net/audio/cmoy-tutorial/
• http://gilmore2.chem.northwestern.edu/projects/cmoy2_prj.htm

If your headphone amp will be battery-powered and mono, just connect the input of this schematic to the red wire of the speaker, and the ground to the black wire.

If you want to run stereo, or power it from the same supply so you don't need batteries, then you need to check whether the speakers are driven bridged or not. With a signal coming through them, you should always see an AC voltage between the two terminals of a speaker (to make sure you're measuring right). Then check if there's also a voltage between each terminal and ground. If there is, then it's bridged. If one wire has zero volts to ground, then it's ground. You can make sure with a resistance check.

If it's not bridged, then you can just do the same thing with two amps.

If it is bridged, you can't use that circuit. You don't want to short the active speaker outputs to ground or short the left and right outputs to each other. So you'd need to build a diff amp for each input stage, and connect ground to ground, V1 to one speaker wire (red), and V2 to the other (black):

Then Vout of this circuit connects to Vin of the previous headphone amplifier circuit.

If the speakers are driven class D, you might need additional filtering, but probably not.

Typical USB sound cards have an earphone jack and a microphone jack. Since it works with your microphone, it is set up to deliver power to the electret microphone - hence the 4.8 Volts when open. That is to say, the sound card is delivering DC Voltage to your microphone.

The microphone input of a sound card can't supply much current, but it is enough for a small transistor amplifier. If fact, electret mics all have a single FET transistor amplifier attached to them - the signal from the mic itself is very high impedance and requires that little amplifier to deliver low impedance signal. You might stretch a point and find a low power op-amp that will work with 3.8V and damn near zero current, but I bet your pocket book will say "ouch" if you go that way.

Picking up a voice from across a (normal sized) room shouldn't be a problem, though, even without a preamplifier. The mic can pick it up easily, and the sound card will digitize it. What you hear has got relatively little to do with it, however. It might well pickup and be capable of recording a voice that you can't hear when played back through the speakers. You can amplify the signal digitally, however.

The sound card samples at 16Bits, which gives you a lot of room to amplify things digitally. All you need to do is multiply the incoming samples with some number to amplify the signal. Need 20dB of gain? Multiply your samples by 10.

A better microphone shouldn't be neccessary, unless the one you've got is just utter crap. I use a 20 year old electret mic from an old Toshiba laptop for my experiments - it can pick up voices from all across the room, even if I can't hear them on playback through the speakers. A little digital gain, though, and all is good.

If you actually do go the preamp route, you will probably need to consider using an automatic gain control. Cranking up the amplification until a voice from across the room gives you a full scale recording means that noises closer to the microphone will overload it and you'll just get a distorted mess. Again, digital amiplification can save your bacon. An AGC in software isn't that hard, so just take advantage of the 96dB range of that sound card and do the amplification digitally.

Mmmf. Completely ignored the MEMs mic.

You might try connecting your MEMs mic like an electret with a simple transistor amp. It might work, since the data sheet say the module only needs 250µA at 3.3 Volts.

Like this:

^{simulate this circuit – Schematic created using CircuitLab}

You might have to diddle with R1 and C1 (different values) to make it go.
R1 might need to be bigger, maybe C1 could be smaller.

Looking at your linked picture of the Echo, it would seem that they are using multiple mics to eliminate noise and possibly track the voice source (for better noise reduction and to separate voices.)
The tear down shows a Texas Instruments DM3725 processor, which combines a DSP and an ARM core in one unit. I expect the Echo uses the DSP core for all of the audio processing, and the ARM does the internet and user interface parts.