This article is from the Car Audio FAQ, by Ian D. Bjorhovde (ianbjor@mobileaudio.com) with numerous contributions by others.

[This section was written by someone who wishes to remain anonymous,

but I will field any questions on the subject -IDB]

Yes. When two loads are connected in parallel (such as with a Y-cable)

they get the same voltage as each other. They do NOT get the same

voltage as if only one load was connected because the head-unit has an

internal resistance (typically around 600 ohms). So, given that the

amp has a typical input impedance of around 10k ohms then we get

something like this:

----------------------------- ---------------------------- HEAD UNIT ________ | | AMP | ______| |_________Vamp___________ | | | R(head)| | | | | _ | __|__ |________| | | __|___ |__| - _ | / \ | | | | | -___|__ | Vi | | | |R(amp)| | _- | \_____/ | | |______| __| _- | |_______________________________|________| - | | | | ----------------------------- ----------------------------

for the single amp situation. Please realize that the R(head) and

R(amp) are internal to the head unit and amplifier and in fact are not

deliberately added resistors but are characteristic of the real world

circuits (non-ideal) in the head-unit and amplifier (and eq's, etc.).

These numbers are typical, check your specific equipment for its

particular specs. the worst case situation is a high source output

impedance and low load input impedance.

So, assuming a typical head unit and single amp the voltage seen at the

amp (Vamp) is given by (Ohms law/Kirkov's law/1st year EE/high school

electronics technology class/etc.):

R(amp) Vamp1 = Vi * ------------------ R(amp) + R(head)

Vamp1 = Vi * 0.94

Now, putting two amps in parallel from the original signal, R(amp) is

effectively halved while R(head) is unchanged. Using the same voltage

divider formula we get:

10000/2 Vamp2 = Vi * --------------------- 10000/2 + 600 Vamp2 = Vi * 0.89

So, for an Alpine 4V preout, Vi in the diagram (the open circuit head

unit line level output) is 4V. Thus Vamp1 = 3.76V and Vamp2 = 3.56V.

With two amplifiers' inputs connected in parallel, the voltage is

reduced from 3.76V to 3.56V or approximately 5%, not a big deal.

If you had a more typical 1V preout you would get Vamp1 = 0.95V and

Vamp2 = .89V, also not a noticeable drop.

This is also why this is slightly more susceptible to noise than a

direct one-to-one connection. If the noise level inserted due to

cabling was 0.1V per cable then the noise level in the signal reaching

each of the two amps would be a slightly higher percent of the signal

level but not doubled. (this is also why the 4V head unit is favored

over the 1V unit for noise immunity: 0.1V noise / 3.76V or 3% is much

less than 0.1V noise / 0.95V or 10% even in a one to one connection).

Continue to: