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 (174Ω, 0.89mA@0.6V)===
{{ ::laptop_out.gif |}} 

=== Android Nexus 5 Phone (222Ω, 3.8mA@0.6V)===
{{ ::nexus5.gif |}}

=== 2013 MacBook (198Ω, 3.6mA@0.6V)===
{{ ::macbook.gif |}}

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=== DROK TDA7297 12V amplifier ===

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.