Bridging An Amp

[Ste69]

I keep hearing about Power Amplifiers being able to be bridged on their descriptions and stuff but what does this actually mean? What does it do? & how do you do it?

[Jivemaster]

Bridging is used to make both halves of a srereo amp drive one speaker linking the inputs carefully to give one side an inverted signal and linking one pair of op poles so the voltage out adds. It enables an amp to drive more power into a higher impedance speaker than normally. HOWEVER there are limitations, and not all amps are suitable, and you come closer to exceeding the safe operating area of the op deveces. Not often a good plan with pro kit. Not a good idea.

[james jag]

I have to say there are many pro amps out there that are fine in bridge mode, using amps in bridge is not that common place due to the fact if you need more power then there is generally a larger (more powerful) amp available on the market

[Jivemaster]

If you can find the "National Semiconductor Application Notes" books or the online version you will find in there all the details, circuits, calculations etc for bridging amplifiser.

 

With suitable amps it is fine, but needing to bridge may indicate that you are starting with less than optimal amplifiers.

[Ste69]

ah so basically bridging an amplifier is just using one amp on each speaker and plugging both outputs into the stero speaker which boosts the volume

[David Duffy]

ah so basically bridging an amplifier is just using one amp on each speaker and plugging both outputs into the stereo speaker which boosts the volume

Not really. In bridge mode, one side of a two channel amplifier drives in opposite phase to the other. The speaker is connected between the two positive terminals. (no connection to the negative speaker terminals) The way it works is that when one terminal is driving +ve, the other is driving -ve (and visa versa) so the speaker load sees the sum of the two channels.

 

In theory you get twice the output voltage, but as you also have to use a speaker with an impedance of twice what you'd normally have on each channel, you still only get twice the output power. (effectively the sum of both channels) It's handy if you need to power one high powered speaker and only have a lower powered stereo amplifier available.

 

Most pro amplifiers have a bridge option and it's quite safe to use it as long as you know what it does and the limitations of not using too low an impedance speaker in that mode. In bridge mode most amplifiers will only use the input signal from one of the channels. (usually channel A) I hope this helps explain bridging.

[Trunker]

Bridging an amplifier to a speaker has its uses though. If you put one amp to one speaker, if one amp blows you still have one amp working therefore one speaker still works and you have sound. If you use one amp per pair speakers (i.e. stereo) and the amp blows you have lost your sound. Running 'night-club' style I would recommend bridging amps on speakers (1 amp to one speaker). You will find a massive difference in volume as well as clarity.

[blackbird]

Hmmm...

 

Trunker, surely your point is based on the assumption that if a stereo amp blows then you will always lose both channels, which is not always true.

Also, ignoring the cost implications of doubling the number of amps needed, then if one channel of your bridged amp does fail, you are left with speakers which are probably of too high an impedance to run happily on the remaining half of your amp.

 

Of course I may have misunderstood.

[Simon Lewis]

Bridging an amplifier to a speaker has its uses though. If you put one amp to one speaker, if one amp blows you still have one amp working therefore one speaker still works and you have sound. If you use one amp per pair speakers (i.e. stereo) and the amp blows you have lost your sound. Running 'night-club' style I would recommend bridging amps on speakers (1 amp to one speaker). You will find a massive difference in volume as well as clarity.

 

 

You seem to be misunderstanding bridge mode operation. If a two channel amplifier is used in bridge mode, both amp channels must work. The number of speakers used is irrelevant, as long as the total load is equal to or higher than the minimum amp load when bridged. Therefore, the loss of one channel (or one channel's psu) will result in no sound at all. (Of course, you could revert back to single channel operation and hope that the remaining "working" channel hasn't been affected by the other channel's failure).

 

If an amp is run in two channel mode (stereo isn't really a helpful term) then one amp channel can go down, and leave the other working. However, the degree of independence hangs on whether the psu is made of two completely separate units or shares a transformer but has separate rectifier/smoothing boards (and indeed which component fails).

 

The additional power in bridge mode is theoretically four times than of dual channel mode (supply voltage is doubled, so P = [(Vx2)^2]/R ). However, this is rarely achieved.

 

If you were to consider a QSC PLX3002, a single channel can deliver 900W into 4 Ohms at 0.05% THD. In bridge mode, it delivers 3000W into 4Ohms, an increase of 5.2dB.

Therefore, it seems like a considerable advantage. However, when you look at the specification more closely, the bridgemode output power is a) at 1kHz only, and b) at 1% distortion.

 

The figures are a little more benign at 8 Ohms, but the overall issue is that you get more power but potentially degraded performance and greater amplifier stress. I would suggest that bridging does indeed increase output power put, but would query whether clarity (as linked to amplifier THD+N) is improved.

[WillW]

The additional power in bridge mode is theoretically four times than of dual channel mode (supply voltage is doubled, so P = [(Vx2)^2]/R ). However, this is rarely achieved.

 

While the maths is correct, you hit your head on the power limit of the amplifier. This is what always stops you going louder and is the barrier that makes the above irrelevant.

 

I actually use bridging for something much simpler. If you have a mono (i.e. single channel) signal into your amp, bridging has the effect of pairing that signal across both outputs. This allows you to trivially put one signal into two speakers.

[Simon Lewis]

I actually use bridging for something much simpler. If you have a mono (i.e. single channel) signal into your amp, bridging has the effect of pairing that signal across both outputs. This allows you to trivially put one signal into two speakers.

 

Isn't it better to either use the "parallel" switch (if fitted) or use a simple jumper lead or Y lead? If you do this trick in bridge mode, you have only one front panel attenuator working. In dual channel mode, both amplifiers retain their own attenuators and one amp can fail without affecting the other.

[basilbrush1982]

The additional power in bridge mode is theoretically four times than of dual channel mode (supply voltage is doubled, so P = [(Vx2)^2]/R ). However, this is rarely achieved.

 

While the maths is correct, you hit your head on the power limit of the amplifier. This is what always stops you going louder and is the barrier that makes the above irrelevant.

 

I actually use bridging for something much simpler. If you have a mono (i.e. single channel) signal into your amp, bridging has the effect of pairing that signal across both outputs. This allows you to trivially put one signal into two speakers.

 

Surely you can either link the outputs or run the amp in parallel mode. This seems the best way to mono the amp up in my opinion.

[MarkPAman]

I actually use bridging for something much simpler. If you have a mono (i.e. single channel) signal into your amp, bridging has the effect of pairing that signal across both outputs. This allows you to trivially put one signal into two speakers.

 

Thus doing something a bit like this.

[JimWebber]

Thus doing something a bit like this.

 

Is this true? It makes perfect sense, but it also sounds a bit too good to be true... - Wooden isolation blocks for your speaker cables anyone?

 

Jim - A lampie, so please be gentle with me!

[Simon Lewis]

Is this true?

 

Yes! The performance gains may be relatively small, and it does rely on a linear PSU ("refresh" rates for SMPS are much higher). Furthermore, although we do suffer variations, it seems that UK mains is a bit more robust than the US 110V supply....