[Elecraft] K3 - Splitting main/sub audio to 2 headphones

[Jim Brown]

On 6/2/2014 11:03 AM, Bill Frantz wrote:
> I had this problem when installing a FTM-10 in my Mazda Miata. I 
> placed the speakers in series so as not to overload the audio 
> amplifiers. The output level is somewhat marginal when traveling at 
> freeway speeds with the top down, but so is conversation with a 
> passenger.

That was a mistake. Audio power amps have very low source Z, typically 
<0.05 ohms, and most are designed to drive a 4 ohm load. In other words, 
they are a constant voltage source. That means they will easily drive 
two 8 ohm loudspeakers. For all practical purposes, the power delivered 
to each loudspeaker is the same as if it were the only one connected. 
The limit on this is the power supply -- as you approach the minimum 
rated impedance and high signal levels, the power amp may clip a dB or 
so lower.

When you wire the loudspeakers in series, that constant output voltage 
divides between them as a function of their impedance AT EVERY 
FREQUENCY, and the impedance of ANY dynamic loudspeaker varies over a 
range of at least 50:1 with frequency. The rated impedance of a 
loudspeaker is the minimum value of the impedance, which occurs for most 
loudspeakers in its lower midrange. Which brings us to the second 
problem with your hookup -- unless the two loudspeakers are identical, 
and identically installed, the ratio between the two impedances at each 
frequency will modify (distort) the frequency response of the other!

73, Jim K9YC
Retired Audio Engineer