# 5.15 Can I split the single pre-amp output from my head unit to drive two amplifiers with a Y-cable? [IDB]

[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)|       |    _-   |
\_____/                 |   |   |______|     __|  _-     |
|_______________________________|________|   -        |
|   |                            |
-----------------------------     ----------------------------
```

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 * ------------------
```

```           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).

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