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Old 14th January 2012, 12:51 PM   #1
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Default CMMR in bridged mode amplifier

Hi,
I was wondering if Bridged mode actually has better common mode rejection ration as compared to stereo therefore better S/N. The reason is assume is that noise unlike signal in Brdige mode travels in same path/direction thereofore when bridged one signal inverted it gets cancelled , i think the mcintosh uses the same technique with using an output transformer.

I am comparing Yamaha B5, 240 W into 8 ohm per channel with a pair of Focal car Amp FP 2.150 which is 400 Watt in bridged mode in 4 Ohm operated by 12 V, 120 Amp SMPS supply. I donot know i found Focal to be more clean and more transparent,. whereas specs ususally state an increased THD in bridged mode.

Any one else having similar experience with bridged mode ?
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Old 14th January 2012, 01:05 PM   #2
Bensen is offline Bensen  Belgium
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Indeed, I noticed this very clearly. Have several amps on my workbench in my basement, non of them are equiped with a mains filter.
My putting the lights on when the amplifiers are playing, nothing can be heard through the speaker with the bridged amp, the other (non bridged) amps giving a very notiable click sound through the speaker.

The same is noticed when switching the amps in and out when speaker is directly coupled on the amp's output. Nothing can be heard when doiing this with the bridged amp.

I can see the logic in this with a bridged amp.


Concerning the THD figures, in bridged mode, the amp "sees" half the output impedance. This explanes the higher THD.


Greetz
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Old 14th January 2012, 01:10 PM   #3
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Ok. What about SQ ?? do u see improvement by CMMR because by bridging the amp sees 8 ohm as 4 ohm raising THD, and possibly Halving the damping factor.
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Old 14th January 2012, 01:39 PM   #4
Bensen is offline Bensen  Belgium
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What do you mean with SQ?
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Old 15th January 2012, 03:15 AM   #5
system7 is offline system7  United Kingdom
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I've gotta admit, I like bridged mode. Rotel and NAD offer it on their power amps. One obvious way it works better is that no currents are dumped to ground, so ground will be reasonably stable. Not perfect because we don't use centre-tapped speakers and input transformers as a rule.

The classic unbalanced opamp configuration with good CMRR is this sort of thing:

Click the image to open in full size.

It relies on the 2.2K and 10K resistors ( and capacitors) being matched on both arms, and Va and Vb can be referenced to a different "earth" from the two other points with little problem. There is an issue with the different impedances on each arm which can cause some complications though. Fully balanced systems are more elegant, and approached in bridged mode. You don't imagine the cleaner sound of good bridged mode. But it's expensive, and the same trick can be done with unbalanced (live + earth rail) systems too with some thought.
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Old 15th January 2012, 08:14 AM   #6
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SQ=sound Quality.
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Old 15th January 2012, 10:56 AM   #7
system7 is offline system7  United Kingdom
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Quote:
Originally Posted by muddasirwaheedmalik View Post
SQ=sound Quality.
The perils of using abbreviations too much...

You can discount "Damping factor" straightaway. Many extremely musical amplifiers have very poor damping factor.

Click the image to open in full size.

This Classic Radford STA25 valve amp (a current amplifier with an element of voltage feedback) has a measurable output impedance that will give a woolly bass and exagerrated top-end with inappropriate speakers. It is my observation that a perfect voltage amplifier is indistinguishable from a current amplifier with the addition of resistors:

Click the image to open in full size.

I have yet to be disproved on this engineering fact, though I've been given some woolly waffle about device characteristics before the tube and SET fanatics quietly slink away!

You can design a speaker to work quite happily with a Low Damping Factor amplifier (high output impedance) as in this Arpeggio Loudspeaker Design.

Back on-topic, good CMRR (aka rejection of noise from the Earth rail) often gets thrown out of the window by sloppy design. Usually at the point of the volume potentiometer and tonal filtering and high frequency compensation for stability. For good CMRR on non-differential inputs, you want high input impedance, which is something that valves and FETs have in spades, and why they can work particularly well. Phew, that was a LONG answer, no?
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Old 15th January 2012, 12:08 PM   #8
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so what i get here is that there is no sonic benefit of bridged amplifier except for +3db head room gain.
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Old 15th January 2012, 12:17 PM   #9
system7 is offline system7  United Kingdom
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Quote:
Originally Posted by muddasirwaheedmalik View Post
so what i get here is that there is no sonic benefit of bridged amplifier except for +3db head room gain.
Not saying that at all. Bridged mode can hide a lot of CMRR problems on each single channel. It often works better. With bridged mode, you can get some ropey designs working a lot better. You follow?
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Old 15th January 2012, 12:44 PM   #10
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I tell you in my area here there are a lot of power breakdowns, loadshedding of electricity, whereas my krell FPB 600 , KRC HR preamp, Accustic Arts MK4, B&W 802 dedicated to critical listening 2 channel DOES gets thump noise on sudden power loss even when switiching to UPS!!!!! but in this HT setup dual Focals 2.150 car amp bridged to give 400W per channel 8 ohm-300 watts 8 ohm gives Zero thumo, noise , crackling , even on taking out or putting in RC cable. One thing very interesting is that due to increased gain +3db the sensitivity is also increased, so from L+R output from my processor i have to put its input level to its MINIMUM possible point to match with volume of subwoofer, perhaps that also increases the S/N ratio . BTW i believe the vocals are much clean and effortless.
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