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I’m a little confused as to how I should be matching my amplifier and speaker in bridged mono mode. Let’s say I have the following,

• Amplifier Power: 100W at 8 ohms • Amplifier Supported Output Impedance: 4-16 ohms • Speakers Rating: 8 ohms

Firstly, when we are in bridged mono, it would theoretically give 400W. But what happens to the output impedance: Does it remain the same, halve or double - 16, 4 or 8 ohms?

Secondly, is it safe to use my 8 ohm speakers if I keep the power low?

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  • \$\begingroup\$ Power P =V*V/R. A bridge amplifier has twice the voltage thus four times the power. (And I really have tho learn how to use the formula editor. Any links?) \$\endgroup\$
    – Oldfart
    Jan 18, 2018 at 12:09
  • \$\begingroup\$ @oldfart Oh yes sorry for that. Corrected. What about the impedance? \$\endgroup\$
    – Asad Moeen
    Jan 18, 2018 at 12:11
  • \$\begingroup\$ I am less savvy about impedance. That is why I used a comment and not give the answer. From what I see the amplifier has the same max current. So you should be able to still use 4-16 Ohm. But with four times the power you can easily blow up your speakers if they are not rated. \$\endgroup\$
    – Oldfart
    Jan 18, 2018 at 12:21
  • \$\begingroup\$ Amplifier Supported Output Impedance: 4-16 ohms Wording it like that is confusing! It would be much more clear to say: Amplifier is suitable for load Impedance of: 4-16 Ohms As stated in Andy's answer: the output impedance of the amplifier is generally very low and irrelevant. \$\endgroup\$ Jan 18, 2018 at 13:31

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For most (if not all) audio power amplifiers the output impedance is a fraction of an ohm and the speaker might be 4 ohms. The point I'm trying to make is that you don't match a speaker impedance to the audio amplifiers output impedance because the audio amplifier's output impedance is (usually if not always) much, much smaller than the impedance of the load.

Firstly, when we are in bridged mono, it would theoretically give 400W. But what happens to the output impedance: Does it remain the same, halve or double - 16, 4 or 8 ohms?

So, say the output impedance is 0.1 ohms (as per my earlier words), in bridged mode this would approximately double to 0.2 ohms but, as I previously said this is unrelated to the speaker impedance. The reason why amplifier manufacturers quote a speaker load range is to prevent overload and out-of-bounds distortion. Matching is not involved.

Secondly, is it safe to use my 8 ohm speakers if I keep the power low?

Yes it is. Reason: think about an 8 ohm speaker with a centre tap winding exactly halfway along the speaker coil. Now imagine what AC voltage you would see there when used in a balanced bridge. If you can't imagine it I'll tell you - it is 0 V AC and therefore two 4 ohm speakers with one on one amp and the other on the other amp is the same as an 8 ohm bridged amp.

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  • \$\begingroup\$ I basically asked about the output impedance to connect multiple speakers in series or parallel to eventually make use of the 4x amplified power and divide it based on speaker’s maximum power limits. How am I to make use of multiple speakers with various impedances to protect amplifier overload and speaker damage? Also, since the original rating of my amplifier is 100W@8ohms so in amplified state what is the new rating: 400W@what-ohms? \$\endgroup\$
    – Asad Moeen
    Jan 18, 2018 at 13:03
  • \$\begingroup\$ You said that the amplifier supported 4 ohms to 16 ohms. If the amplifier is only capable of producing 100 watts maximum then when bridged, the maximum output power is quite simply 200 watts. If the amplifier can support 200 watts into 4 ohms then a bridged version would give 400 watts into 8 ohms. \$\endgroup\$
    – Andy aka
    Jan 18, 2018 at 13:07
  • \$\begingroup\$ Thank you so much. And “into” I assume is the speaker’s impedance it is outputting to; making amplifier’s own output impedance irrelevant as you said? Cheers. \$\endgroup\$
    – Asad Moeen
    Jan 18, 2018 at 13:12
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    \$\begingroup\$ "200 watts into 4 ohms" means feeding 200 watts to a 4 ohm speaker - it's EE shorthand. Output impedance is irrelevant. \$\endgroup\$
    – Andy aka
    Jan 18, 2018 at 13:20
  • \$\begingroup\$ Thank you so much mate. The confusion was killing. Just a small little thing btw, do I also need to change the mode of operation of the Amp from stereo to mono to make it work? \$\endgroup\$
    – Asad Moeen
    Jan 18, 2018 at 13:46

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