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matic 15th September 2006 04:58 PM

Bridge amplifier on 8 om
I have stereo amplifier 2x100w rated 8 oms. Transformer is 300VA. Can i brigde this amplifier so i can have 200 w on 8 om?
I know for rule that if amplifier rated for 4 oms in bride must have 8 om speaker.

anatech 15th September 2006 06:50 PM

Hi matic,
It depends on the amplifer as to how it will react to this. If the amplifer is not designed to be bridgeable, assume that you may cause it to fail.

Question for you. Why do you want to bridge this amp?


MikeB 15th September 2006 06:53 PM

Matic, i think you gave yourself already the correct answer...
For bridging into 8ohm the amp must be capable of handling 4ohm.

So the answer is: NO

(btw, an amplifier rated 8ohms only is always suspect to me)


anatech 15th September 2006 07:01 PM

Hi Mike,

(btw, an amplifier rated 8ohms only is always suspect to me)
Well, yeah. I didn't want to depress the guy though! ;)


MikeB 15th September 2006 07:05 PM

Ooops, :ashamed:


AndrewT 16th September 2006 09:49 AM

an amp rated for 8ohm duty is no stigma.

Depends on the criteria adopted by the designer.

If I were rating an amp for 8ohm duty, my criteria would be.

Stable into 4r
Stable into 8r//test caps
Heatsink sufficient for continuous power at -10db for domestic and -3db for PA.
4r power rating >= 3 * 16r power rating. i.e. <=-1.2db on Load Voltage
Output stage rated to 8ohm and 60degree phase angle at typical maximum operating temperature on one second or longer SOAR.
Protection system to allow transient and long term overloads to be survived.

I suspect an 8ohm amp built that way may be more able than many 4ohm amplifiers out there in the market place.

matic 16th September 2006 05:45 PM

This is what i have:1 toroidal transformer 400VA, 2x39Volt which is, by some formula, enough for 2x120W at 8 oms. If i wanted to drive this amp at 4 oms (2x240W) then transformer would have to be at least double, 800VA.
Output for 120w at 8 om is 31volt which gives I=3,875. Because "I" is limited by transformer then, i think that teoretical, this amp can drive 4om only by 1/2 watt then for 8om? P=R*I2=4*(3,875*3,875)=60W. Based on this i think that in bridge this amp can drive 8 om. If not, please don't throw wegetables at me :)

MikeB 16th September 2006 06:02 PM

:bawling: It's so much fun to throw vegetables after someone... :D

You're right, if the power is limited by the power supplys ability to deliver current, the poweroutput would half for 4 ohms compared to 8ohms.
This means, if the current ability is limiting, you will not get any more amperes through your speakers resulting in having the same outputpower again when bridging. :cannotbe:

BTW, doubling the outputpower increases the volume by 3db, barely noticable. It's more interesting how effortless an amp can deliver the rated power without compressing and maintaining dampingfactor.


AndrewT 16th September 2006 10:44 PM

If I were designing an amp back to front and starting with the available transformer, then your 400VA can support about 260W of output.
130W+130W into 8r is about the maximum you can get from this transformer. The 39Vac just about matches the DC voltage (about 56Vdc to 57Vdc) requirement for 130W
Alternatively, you could do a 250W into 4r monoblock, but all parameters would need to be optimised to achieve this from 39Vac.
As I said back to front.

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