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--- how_audio_output_works [2016/07/03 04:16]
schmidtleonard
+++ how_audio_output_works [2016/07/21 16:20] (current)
liz
@@ Line 1: / Line 1: @@
-Audio output behaves like a non-ideal voltage source. If the 3.5mm jack/plug has a light load across its terminals (e.g. air or a 1MΩ oscilloscope -- things with a high resistance/impedance) then you get wiggles of up to ~1V in amplitude at full volume. If the 3.5mm jack/plug has a slightly heavier load (e.g. headphones or a 50Ω oscilloscope input) then the voltage decreases a bit, and if it has a very heavy load (e.g. 8Ω speakers without an amplifier or a piece of tinfoil across the 3.5mm plug) then the voltage all but vanishes.
+Audio output behaves like a voltage source that droops as you draw more and more current. Output impedance, measured in ohms (Ω), characterizes the droop. More Ω more droop. Typically a BJT will require 1/100th to 1/20th the current it is driving at 0.6V, e.g. you need 1-5mA of control current at 0.6V to drive a 100mA load current. How do typical audio outputs measure up to this requirement?
-=== Dell Laptop 3.5mm Headphone Jack (2mA@0.6V)===
+=== Dell Laptop (174Ω, 0.89mA@0.6V)===
 {{ ::laptop_out.gif |}}
-We can't drive an estim circuit directly from audio output -- it can barely reach 1V at max volume let alone 100V+ -- but can we use audio output to drive an off-the-shelf amplifier which in turn drives an estim?
+=== Android Nexus 5 Phone (222Ω, 3.8mA@0.6V)===
+{{ ::nexus5.gif |}}
+=== 2013 MacBook (198Ω, 3.6mA@0.6V)===
+{{ ::macbook.gif |}}
+----
 === DROK TDA7297 12V amplifier ===
-True to its name, it can boost ~1V audio to 12V.
+We can't drive an estim circuit directly from audio output -- it can barely reach 1V at max volume let alone 100V+ -- but can we use audio output to drive an off-the-shelf amplifier which in turn drives an estim? True to its claim, it can boost ~1V audio to 12V.
 {{ ::laptop_pre_post_amplification.png |}}
+=== More reading ===
+Looks like will need a transformer as well...
+See [[https://kinkitech.wordpress.com/stereo-stim-usb/]] for more information.

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