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At the moment I'm using an amplifier based around a NPN transistor and some resistors. Its configuration is as an emitter follower. Pin 25 supplies internal audio from a DAC on the chip.

See this schematic, on the righthand side.

This handles DC offset (offsetting the AC signal into a DC signal), overvoltage input protection, switching between internal/external audio, and output. It is a bit distorted, but it works well.

Ideally, I'd like to integrate this on to one or two IC's with the minimum space usage. It'd be nice if cost was lower, but this is not critical. (Currently component cost is around $2.50 total for all components in the audio amp, in low quantities.) Also, it would be ideal if the IC did not require a dual rail power supply.

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  • \$\begingroup\$ So AUDIO_L_OUT is an output, and AUDIO_L_EXT is an input? But where are the signals coming and going? And R53 passes the output back to the input? Or it passes from input directly to output? I don't get what Q2 and Q3 are for, though. Can you explain the function with a little more detail? \$\endgroup\$ – endolith Nov 16 '10 at 15:19
  • \$\begingroup\$ AUDIO_L_EXT is the external audio channel input. Q2 and Q3 mux the amplifier's input between the external audio and the internally generated audio. \$\endgroup\$ – Thomas O Nov 16 '10 at 15:23
  • \$\begingroup\$ R53 is placed only when there is no audio amplifier (on some boards, it is an option) and acts as a pass-through between the input and output. \$\endgroup\$ – Thomas O Nov 16 '10 at 15:24
  • \$\begingroup\$ R56 is similar - it completely disables the input by grounding it. You'll notice "DNP" next to the resistors, this indicates "do not place" for the standard board. \$\endgroup\$ – Thomas O Nov 16 '10 at 15:26
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    \$\begingroup\$ Are you saying you want to make your own IC? \$\endgroup\$ – Kellenjb Nov 16 '10 at 15:45
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If you really need to mux the inputs, how about something like this?

mux and buffer

Since your DAC output is already DC biased, if the bias it uses is provided on another pin, you could eliminate the cap and resistor for the DAC output, and use that pin as the bias for the other input. Otherwise use one of these:

DC bias source

Depending on what your inputs and outputs are connected to, you might need grounded resistors on the other side of the coupling caps, RF bypass caps to ground, etc.

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  • \$\begingroup\$ The bias resistors are making the AC signal DC, right? \$\endgroup\$ – Thomas O Nov 16 '10 at 16:06
  • \$\begingroup\$ It DC offsets the audio signal, yes. \$\endgroup\$ – endolith Nov 16 '10 at 16:16
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You don't need to MUX audio. Just mix the two audio sources together. Maybe have the external audio muted.

Why are you using a discrete-transistor buffer? Use an op-amp for a buffer. Better sound quality, fewer components, smaller overall footprint.

Here's a design that does what you want. To amplify external audio, switch the micro pin to an analog input. Switch it to a DAC output to amplify internal audio.

  • Input voltage 1Vdc+1Vp-p
  • Output voltage 1Vp-p
  • Load impedance 600 ohms

The value of C1 depends on the lower limit of frequency you care to amplify.

alt text

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  • \$\begingroup\$ Input voltage 1Vp-p max (typ. 500mVp-p), 1V offset (or AC, if it can take AC in.) \$\endgroup\$ – Thomas O Nov 16 '10 at 15:54
  • \$\begingroup\$ Output voltage: line level (about 1Vp-p.) \$\endgroup\$ – Thomas O Nov 16 '10 at 15:55
  • \$\begingroup\$ Load impedence I don't know, max 600 ohms. \$\endgroup\$ – Thomas O Nov 16 '10 at 15:56
  • \$\begingroup\$ max 600 or minimum 600? \$\endgroup\$ – endolith Nov 16 '10 at 16:04
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    \$\begingroup\$ the DAC's impedance should be reasonably low because it has an internal buffer amp. \$\endgroup\$ – markrages Nov 16 '10 at 16:47

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