Description

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

3.6 What is "mixed-mono?" Can my amp do it? [JSC, IDB]

Some amplifiers which are both bridgeable and able to drive low
impedance loads also allow you to use "mixed-mono" mode. This involves
driving a pair of speakers in stereo mode as well as simultaneously
driving a single speaker in bridged mono mode off of ONE pair of the
amp's channels.

To do this, you connect the mono speaker (typically a subwoofer) to the
amp as you normally would in bridged mode, and then connect the left
and right stereo speakers to the left and right stereo channels,
respectively.

However, for this to work, the amplifier must actually use both input
channels in bridged mode. Many amplifiers, when placed in bridged
mode, will simply "copy" and invert either the left or the right
channel. This practice ensures high output to the mono speaker, but
eliminates the possibility of mixed mono since you lose one channel.

It is VERY important to use passive crossovers when configuring your
amplifier in mixed-mono mode in order to keep from overloading the amp.
The reason almost all new amplifiers are able to run in mixed-mono
mode (even if they are only 2-ohm stable) is that the impedance seen by
each channel of the amplifier is the same across the entire frequency
spectrum when using passive crossovers. Here's how it works: Take a
typical 2-channel amplifier that is stable to 2 ohms (stereo) or 4 ohms
(mono). When the subwoofer is connected with a low-pass crossover (at
100Hz, for example) then the amplifier "sees" a 2 ohm load on each of
its channels (see 3.5) from 100Hz and down. When the full range
speakers are connected with a high-pass crossover (at 125Hz, for
example), the amplifier "sees" a 4 ohm load on each of its channels from
125Hz and up. The passive crossovers prevent the amplifier from seeing
more than one speaker on either channel at any given frequency. Of
course, between the two crossover points the amp DOES see more than one
speaker (and therefore the load on the amp dips to 1.33 ohms when using
4 ohms speakers).

A graph of impedance vs frequency for ONE channel of an amplifier would
look similar to this when using 3 4-ohm speakers and crossover points
at 100Hz(LP) and 200Hz (HP):

       +-----------------------------------------------------------+
       |                   ****************************************| 4
       |                  *                                        |
       |                 *                                         |
       |*************   *                                          | 2
       |             * *                                           |
       |              *                                            |
       |                                                           | 1
       |                                                           |
       |                                                           |
       +^-----^-----^-----^-----^-----^-----^-----^-----^-----^---^+ 0
        25    50   100   200   400   800  1.6K  3.15K 6.3K 12.5K 20K

Continue to:

• prev: 3.5.5 Can I bridge my 4 channel head unit?
• Index
• next: 3.7 What does "two ohm stable" mean? What is a "high-current" amplifier? [JSC]