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Old 07-14-2002, 06:05 PM   #1
Lee

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What is "mixed-mono?"

Quote:
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):
Code:

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