What exactly do you mean by "the voltage drop of the LM386"?
The LM386 is an audio power amplifier, and I can't think how the term
"voltage drop" would apply to it.
Measuring between pins 4 and 6 will give you the power supply voltage,
which is determined by whatever is supplying power to the circuit, not
by the LM386.
I suspect that Shelton was inquiring about how far short of the supply
rails can the load input voltage be.
The datasheet is at:
http://www.national.com/ds/LM/LM386.pdf
One little bit of relevance here that I see in the datasheet:
The graph in the right column, middle row, Page 4, "Distortion Vs.
Output Power"
Conditions: 6 volt supply, 8 ohm load
There is a "base/root end of takeoff" of distortion upticking with
output power at close enough to .2 watt, arguably as low as .18 watt.
The worse distortion one of these figures has THD roughly .3%, and the
better of these is for THD upticking just a little at about .2% or so.
These wattages for sinewave applied to an 8 ohm load are 3.58 and 3.39
volts peak-to-peak respectively. Subtract these from 6 volts and divide
by 2, and respectively for these different argued degrees of distortion
threshold 1.21 volts and 1.305 volts respectively is average of the 2
directions; negative/positive; for LM386 to need to have a sinewave to
have peaks "short of the supply rails" in order to have distortion "not
significantly upticking" or "barely upticking".
In usual "music duty" and "voice duty", I seem to think that peaks
getting distorted by the amplifier to 2% are very acceptable. In the
above graph, the curve is sharply taking off through the 2% point at about
1/4 watt. With an 8 ohm load, a 1/4 watt sinewave has peak-to-peak
voltage 4 volts - with 6 volt supply this has average of the 2 shortfalls
from the rails being exactly 1 volt.
I would like to comment that the distortion will probably be in the form
of flattening the peaks - so I suspect that for good lower distortion the
output signal needs to have peak-to-peak level more than 2 volts less than
the voltage across the supply rails - I would say about 2.5 volts.
- Don Klipstein (don@misty.com)
