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I'm trying to sum two audio signals that zero at different voltages. One DAC outputs with a zero at GND (0V). The other (bluetooth module, Microchip BM62) has a virtual zero at 1.4V. I'm trying to sum them to drive a differential input audio amplifier (Maxim MAX98400AETX+). It's been quite a long while from my time with opamps so I tried passive summing with 4.7K resistors on each output and taking that output to the positive input on the amplifier then connecting the (-) input on the amplifier, DAC ground, and Bluetooth Line out Ground all to common ground and I get very low sound from the bluetooth module output. With each singularly connected to the amplifier, the sound is great from either device. Can someone point me in the right direction? Both signals are mono.

The signals both do not exceed 2.5-3V. I have 3.3V, 5V, and 12V at my disposal.

Thanks!

Edit: Here is a schematic of what I'm trying to do: enter image description here

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    \$\begingroup\$ DC blocking capacitors are your friend... \$\endgroup\$ – Ian Bland Dec 27 '17 at 23:19
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    \$\begingroup\$ "...a differential input audio amplifier" - which amplifier, and which Bluetooth module? \$\endgroup\$ – Bruce Abbott Dec 27 '17 at 23:47
  • \$\begingroup\$ @BruceAbbott, I added part numbers and schematic above. \$\endgroup\$ – Jeremy Dec 28 '17 at 15:33
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Audio signals only go down to 20 Hz. That means DC is irrelevant. A simple way to deal with audio signals with different DC offsets is therefore to capacitively couple the signals. That effectively high-pass filters them, which eliminates and DC offset.

For example, if the input of your summing amp has 1 kΩ impedance, then you would theoretically need 7.96 µF in series for a 20 Hz high-pass rolloff frequency. That would cause 3 dB attenuation at 20 kHz.

3 dB attenuation at the ends of the frequency range is acceptable in audio. However, there may be multiple DC blocking filters in the whole chain, so I usually set my rolloff frequencies lower. In this case, a 15 µF capacitor would be better. That allows for a few other DC blocking filters with the same rolloff in the whole eventual signal path, and still only attenuate by 3 dB at 20 Hz.

Be careful with the capacitor selection. Normally you'd use a multi-layer ceramic at this voltage and capacitance. However, those have two undesirable effects for audio.

First, the capacitance is somewhat dependent on voltage. At higher voltage, you get lower capacitance. This is a non-linearity that introduces distortion. One way to minimize this is to use capacitors rated to a much higher voltage. A 30 V 15 µF ceramic capacitor used in the ±5 V range will be pretty linear, for example.

Second, some ceramic materials exhibit significant piezo-electric effect. Unfortunately the ones that allow for higher capacitance do this more so. That means the capacitor acts a bit like a microphone. Shock and vibrations the board is subject to can end up in the audio signal.

In this case, electrolytic may be a reasonable option, especially if the polarity of the DC offset is known to always be one way. Otherwise, it gets more tricky.

Using a higher input impedance in the summing amp allows for lower capacitance. For example, with 10 kΩ instead of 1 kΩ, you can use 1.5 or even 2 µF instead of 15 µF. That opens up more possibilities in capacitor selection.

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  • \$\begingroup\$ That is a very through explaination, thanks! Size is going to be a factor so I think I'll try the 10K, 2uF route to keep the capacitor size down. \$\endgroup\$ – Jeremy Dec 28 '17 at 21:49
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The classic opamp summing circuit is based on the inverting amplifier configuration. If all you have are positive supply voltages things get a bit messy because you have to create an analog ground half way between the positive supply voltage and GND. There are tons of schematics on the web, search for summing amplifier, audio summing schematic, or things like that.

Separate from that, it is wise never to trust that your audio sources have zero volts DC offset, and use a coupling capacitor on each input. With 10K input resistors you can use a 1.0 uF ceramic capacitor and not have to worry about polarity issues.

Here is the basic idea:

enter image description here

Here is a link to the image:

http://www.wiringdiagrams21.com/2009/12/03/audio-mixer-inverting-summing-circuit-diagram/

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