Bridge amplifier on 8 om

matic · 15th September 2006, 03:58 PM

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, 05:50 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?

-Chris

MikeB · 15th September 2006, 05:53 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)

Mike

anatech · 15th September 2006, 06:01 PM

Hi Mike,

Quote:

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

Well, yeah. I didn't want to depress the guy though!

-Chris

MikeB · 15th September 2006, 06:05 PM

Ooops,

Mike

AndrewT · 16th September 2006, 08:49 AM

Hi,
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, 04:45 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, 05:02 PM

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

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.

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.

Mike

AndrewT · 16th September 2006, 09:44 PM

Hi,
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.

15th September 2006, 03:58 PM	#1
matic diyAudio Member Join Date: Apr 2005 Location: Subotica	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.

16th September 2006, 08:49 AM	#6
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, 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. __________________ regards Andrew T.

16th September 2006, 09:44 PM	#9
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, 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. __________________ regards Andrew T.

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15th September 2006, 05:50 PM	#2
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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? -Chris

15th September 2006, 05:53 PM	#3
MikeB diyAudio Member Join Date: Apr 2004 Location: Gütersloh	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) Mike

15th September 2006, 06:05 PM	#5
MikeB diyAudio Member Join Date: Apr 2004 Location: Gütersloh	Ooops, Mike

16th September 2006, 04:45 PM	#7
matic diyAudio Member Join Date: Apr 2005 Location: Subotica	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=RI2=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

16th September 2006, 05:02 PM	#8
MikeB diyAudio Member Join Date: Apr 2004 Location: Gütersloh	It's so much fun to throw vegetables after someone... 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. 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. Mike

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