Speaker impedance - Bass amplifier

[gapiro]

I'm a pragmatist. My bass amp is pretty powerful, and I work on the principle that it's designed to work down to 2 Ohms. I have a couple of bass cabinets - one is 4 Ohms and the other 8 Ohms. I cannot remember which one is which, and being very honest, even looking it up is pointless. They both work fine. The bigger one sounds louder - but maybe that's also because it's bigger. It also weighs over 60Kg - so maybe after wheeling up or down a ramp this is why I think it sounds louder.

 

(side note, I'm a bassist)

Sound volume is measured in SPL - Sound Pressure Level - it is basically down to how much air your speaker can shift. As a general rule, a larger cone has a larger surface area so would make sense to shift more air, however often a larger speaker has a less responsive cone- it can't move back and forward as fast/as far - thus some of the "loudest" cabinets are actually 10 or 12 inch speakers (such as http://barefacedbass.com/)

Incidentally, Barefaced do have a number of great technical articles explaining things - eg

http://barefacedbass...isplacement.htm

 

Also worth noting that power is on a log 10 scale - ie to double the voiume using "the same identical perfect speaker" would require 10 times the power - ie 100W to 1000W

 

Back to the OP - as most have said, as long as the resistance of the speakers is greater than the minimum output impedance of the amp, you are going to be fine.

Edited May 12, 2017 by gapiro

[Simon Lewis]

Technically (and pedantically) 'volume' or loudness is better expressed in Phons.

 

However, 'volume' does pyschoacoustically relate to sound amplitude, spectral content and duration.... Sound Pressure Level is used because it does a reasonable job of correlating with what we hear and because it is fairly easy to measure.

 

<pedant mode off> :-)

[Dave C]

Just to complicate things.... even when you know what the amplifier can deliver into a given load, bear in mind that an "8 ohm" speaker" is probably only 8 ohms over a very small part of its operating range...

Also, the impedance curve is greatly affected by the cabinet design and loading method.

 

 

http://machineryequipmentonline.com/electric-equipment/wp-content/uploads/2016/02/Loudspeaker-Enclosures-0543.jpg

 

 

Simon, which text is this from please?

 

thanks!

[Simon Lewis]

Dave,

 

I searched for "loudspeaker impedance vs frequency" in Google images until I found one that was usable and could be linked to. The page it is from is here.

[Jivemaster]

There is also the choice of speaker for the choice of the sound you want to produce. A 1 x 15" or 1 x 18" cab will offer a different sound from a 4 x 8" or 4 x 10".

 

Overall plugging an 8 ohm speaker into a 4 ohm capable amplifier will be fine.

[GussKolb]

Hi...What does the word "bridged" actually mean?I had a Carvin RC210 combo with head. In the back, the outputs were " 4ohms hi-freq, 4 ohms lo-freq" (I understand that these two are the bi-amped outputs) and then there were two separate outputs labeled "bridged only".

[DrV]

In a conventional unbridged amp, one leg of the speaker goes to ground and the other goes up and down. If the internal power rails are plus and minus 30V then in an ideal amplifier this leg can go up and down by 30V so the maximum voltage across the speaker at any instant can only ever be 30V (positive or negative)

 

Bridging is where you take two identical amplifiers and feed one with a signal exactly opposite to the other. You then connect the speaker between the two outputs instead of having one leg grounded. Now the maximum voltage is 60V (plus 30 on one leg and minus 30 on the other). As power is proportional to the square of the voltage, doubling the voltage quadruples the power. All this is assuming all sorts of things like ideal amplifiers etc but this is the principle.

[Just Some Bloke]

Bridging is where you take two identical amplifiers and feed one with a signal exactly opposite to the other. You then connect the speaker between the two outputs instead of having one leg grounded. Now the maximum voltage is 60V (plus 30 on one leg and minus 30 on the other). As power is proportional to the square of the voltage, doubling the voltage quadruples the power. All this is assuming all sorts of things like ideal amplifiers etc but this is the principle.

 

It should be pointed out that the most common use of bridging in an amp (generally, as opposed to bass amps) is when a stereo amp (i.e. 2 amps in one box to give you left and right) is bridged to send the same signal to all outputs. If you were using 4 front fills along the downstage edge of a stage and wanted to send them all the same mono feed running from a matrix output on the desk, then you could use a bridged stereo amp to feed 2 fills each per channel but all getting the same signal.

[DrV]

If you did that you'd end up with half the speakers out of phase with the other half. Edited March 27, 2020 by DrV

[Yorkie]

Bridging is where you take two identical amplifiers and feed one with a signal exactly opposite to the other. You then connect the speaker between the two outputs instead of having one leg grounded. Now the maximum voltage is 60V (plus 30 on one leg and minus 30 on the other). As power is proportional to the square of the voltage, doubling the voltage quadruples the power. All this is assuming all sorts of things like ideal amplifiers etc but this is the principle.

 

It should be pointed out that the most common use of bridging in an amp (generally, as opposed to bass amps) is when a stereo amp (i.e. 2 amps in one box to give you left and right) is bridged to send the same signal to all outputs. If you were using 4 front fills along the downstage edge of a stage and wanted to send them all the same mono feed running from a matrix output on the desk, then you could use a bridged stereo amp to feed 2 fills each per channel but all getting the same signal.

That would be bridging the inputs, which I have never heard of being called 'bridging'. DrV's description coherently states my understanding of bridged outputs

Edited March 27, 2020 by Yorkie

[Just Some Bloke]

If you did that you'd end up with half the speakers out of phase with the other half.

Oo. I didn't know that. Most stereo amps I have worked with have a "bridging" function to allow you to link the two channels.

[alistermorton]

Yes, but when you bridge a stereo amp, as Dave said, you don't connect to the +/- posts of each channel, the load is connected between the + posts of each channel, with the - posts left unconnected. If you connected a pair of speakers to each pair of posts, because the bridging switch exactly inverts one channel relative to the other (i.e. it doesn't parallel them), they'd be out of phase. So you could connect all four speakers, providing they were connected in such a way to be a) in phase and b) make a load that the amp can handle, across the left and right + connections when it as in bridged mode.

[Simon Lewis]

There's a difference between parallel and bridging modes...

 

In parallel mode, the same signal is applied to both channels of a two channel amp, but the amps remain independent. Typically, only one volume pot remains active. It's useful when you want lots of speakers all running at the same level etc.

 

In bridge mode, there is only one input which is split so that one amp channel is fed the signal in polarity and the other channel out of polarity. There is only one output, connected between the "hot" output terminal of each amp. It's useful when you want to try and maximise amp power, but comes with a bunch of caveats over load, distortion etc., etc.