I have my eye on a pair of Rotel RB06 power amps. They are stereo, but
by oving a few jumpers can be used in bridged mono configuration.

However, although in stereo mode they can drive 4 and 8 ohm speakers,
apparently in bridged mode they cannot be used on anything below 8
ohms, ruling out their use on my Dynaudios.

Can anyone briefly explain why this is the case? I have no reason to
disbelieve the Rotel manual and the nice bloke I spoke to on the phone
at Rotel, but would like someone to put into plain English the
technology behind it. Thanks.

wrote:

I have my eye on a pair of Rotel RB06 power amps. They are stereo, but
by oving a few jumpers can be used in bridged mono configuration.

However, although in stereo mode they can drive 4 and 8 ohm speakers,
apparently in bridged mode they cannot be used on anything below 8
ohms, ruling out their use on my Dynaudios.

Can anyone briefly explain why this is the case?

In bridged mode, with a 4 ohm speaker, each amp 'sees' a 2 ohm load.
One channel goes positive while the other goes negative and vice versa
so the voltage across the speaker is twice that in normal mode.
The current through the speaker is therefore doubled.
So if each channel produces its usual voltage but twice the current,
then the load impedance it sees is halved. Ohm's law....

--
Eiron.

May contain traces of irony.

Can anyone briefly explain why this is the case?

In order for a pair of stereo amplifiers to act as a single mono
amplifier (presumably with the intention of getting more sound out of a
single speaker than one of them could manage on its own) then the output
voltage from the pair of amps has to be added, effectively doubling the
voltage applied to the speaker.

If you double the voltage applied to the speaker, you also double the
current flowing through the speaker (and coming from the amplifier).

If you replace an 8 Ohm speaker with a 4 Ohm speaker, you again double
the current required from the amplifier.

At some point, this doubling current load exceeds what the amplifier is
capable of producing, perhaps risking damage.

--
Steve Swift
http://www.swiftys.org.uk/swifty.html
http://www.ringers.org.uk

On May 3, 9:15 am, Steve Swift wrote:
Can anyone briefly explain why this is the case?

In order for a pair of stereo amplifiers to act as a single mono
amplifier (presumably with the intention of getting more sound out of a
single speaker than one of them could manage on its own) then the output
voltage from the pair of amps has to be added, effectively doubling the
voltage applied to the speaker.

If you double the voltage applied to the speaker, you also double the
current flowing through the speaker (and coming from the amplifier).

If you replace an 8 Ohm speaker with a 4 Ohm speaker, you again double
the current required from the amplifier.

At some point, this doubling current load exceeds what the amplifier is
capable of producing, perhaps risking damage.

--
Steve Swifthttp://www.swiftys.org.uk/swifty.htmlhttp://www.ringers.org.uk

Thanks. It's a long time since I did physics O level....

Thanks. It's a long time since I did physics O level....

Well, I'm eternally grateful that someone has realised that I'm a
Physicist by training, and not an Audio Engineer. I've been meaning to
publish this fact for ages. I actually went over to the dark size, and
now I'm an IT professional in a large IT company. It pays better. The
Physics is somewhat rusty these days. You don't often need a high energy
particle physicist when playing with PCs.

--
Steve Swift
http://www.swiftys.org.uk/swifty.html
http://www.ringers.org.uk