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There are a lot of great resources here for people who want to mix together multiple outputs of the same or similar load impedance. However, in my case, I want to mix a mono, amplified, ground-referenced output designed to drive 2.5W into 4 Ohms with a mono line level output (unknown output impedance).

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https://www.epanorama.net/circuits/speaker_to_line.html suggests

Speaker level signals are usually in 3..20V range (amplifiers up to 50W output power). The line level signals should be in 0.3..2V range to be suitable for amplifier's line level input.

By my calculations, the 2.5W amp outputs a max $$\sqrt{2.5\rm{W}*4Ohm} = 3.16\rm{V}$$ signal, which is close to the desired <2V range.

Knowing the above, and assuming I cannot adjust the amplifier's volume, would the posted schematic and values still be generally appropriate?

Kenn Sebesta
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  • Bad idea, the speaker outputs of the PAM8302 look like they're floating, connecting one to GND will probably destroy it – Finbarr Nov 27 '23 at 13:39
  • @Finbarr Great point, let me remove that part of the question so that the original intent stands. – Kenn Sebesta Nov 27 '23 at 14:40

1 Answers1

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PAM8302 is a Class D amp with H bridge outputs for actively driving both the positive and negative terminal of a speaker.

So the outputs are not referenced to ground, and at least the offset should be removed.

Technically you don't know if the positive wire alone resembles an audio signal and should not be used for line level conversion without a signal transformer to convert the differential output bacl to sigle-ended.

Since you have an amp chip which takes line level audio in, it might be best to use the chip input, not the chip output.

Justme
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  • Thanks a lot for catching that. I have used your answer to update my question so that the general idea still stands, even if in my specific application it is unworkable. Even though I will have to figure something else out for my specific chip, I'm still curious to learn about resistor choices for passively mixing speaker-level and logic-level signals. – Kenn Sebesta Nov 27 '23 at 14:44
  • @KennSebesta I can update my answer, but as there are no standardized "speaker level" and "line level" signals, and their voltage depends on volume setting anyway, it might be impossible to determine a general case how much you need to divide the speaker voltage signal down to line level voltage. A generic shelf speaker emits about 80 dB SPL at 1 meter when fed with 1W, which for an 8 ohm speaker is 2.828VRMS. Some line level devices are 2VRMS but some are less. Maybe divide by 2 with two 1kohm resistors; you get 500 ohm output impedance which should be enough for generic line inputs. – Justme Nov 27 '23 at 16:19