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Old 3rd April 2010, 01:26 PM   #1
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Default Bridged or Parallel LM3875 for best music quality

I have built a Gainclone amp using LM3875 chips, which sounds very nice. If I turn the volume up loud, eventually it starts to clip, or it could be the current limiter of the chip.

Anyway, I wanted to know what the best configuration would be (Bridged or Parallel) for an amp driving 6 ohm speakers, and if this would cure the problem I have at very loud volumes.

I check the Overture design guide and also http://www.national.com/an/AN/AN-1192.pdf but would like to know other peoples thoughts on the sound quality of bridged compared to paralleled. I am very happy with the sound it produces at the moment and don't want to increase the power to find that I have lost some of the musical quality of the amp.
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Old 3rd April 2010, 01:49 PM   #2
AndrewT is online now AndrewT  Scotland
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I have not tested a parallel nor a bridged implementation, but reading quite a number of reviews I get the feeling that both topologies require a lot of builder/designer skill to match the performance of a single chip.

What is your speaker sensitivity and is it really a 6ohm design or more likely a 4 to 8ohm design using a 4ohm driver and an 8ohm driver with an appropriate crossover?

If it's a 4 to 8ohm, the majority of the load is the 4ohm bass/mid and the chipamp should be designed for that load.

Separating the two halves of the crossover will reduce the loading on the respective dual chipamps and may result in improved performance, i.e. passive Bi-amping.
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Old 3rd April 2010, 03:26 PM   #3
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I am driving Monitor Audio BR6 speakers. Each floor stander has a tweater and 3 x 6.5 inch mid range drivers in.

They have two sets of binding posts on the back, which are connected together at the moment. I'm not sure about the internal wiring.

Maybe my best option will be to make another amp and Bi-amp the speakers.
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Last edited by portreathbeach; 3rd April 2010 at 03:39 PM.
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Old 3rd April 2010, 03:40 PM   #4
ra7 is offline ra7  United States
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The two sets of binding posts are misleading. Usually, they are fir bi-wiring not bi-amping... Meaning that no matter how you connect to the binding posts, it ultimately goes through the built in crossover anyway.
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Old 3rd April 2010, 03:48 PM   #5
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Ah, I'll have to open them up and have a look inside to see how they are connected.
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Old 3rd April 2010, 04:24 PM   #6
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Here is the connections from the binding posts to the speakers:

Click the image to open in full size.

As the 2 bottom drivers are connected only to the LF binding posts and the tweater and mid range are connected to the LF binding posts, does this mean I would have no problem bi-amping these speakers?
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Old 3rd April 2010, 04:35 PM   #7
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No problem at all

If you find a set of speakers with jumpers on the back to connect two positives together & another to connect two negatives it stands to reason that if you remove the jumpers then the crossover will be broken into two seperate parts.

From what little i have read on the LM3875 it needs good PCB layout to prevent problems. I think you'll be better off Bi amping, feeding the same signal into both with one driving the woofers & the other the mid & treble drivers.

Not much could be simpler in all honesty
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Old 3rd April 2010, 04:47 PM   #8
AndrewT is online now AndrewT  Scotland
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measure the DC resistance at the LF terminals and also at the HF terminals.
This will roughly guide you to what impedances of drivers have been used.

The HF/Mid could be split with a little rewiring and a third pair of terminals, if you want to try Tri-amping.
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Old 3rd April 2010, 05:07 PM   #9
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Measuring across HF terminals I get 7.2 ohms and across LF terminals 7.3 ohms
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Old 3rd April 2010, 05:12 PM   #10
AndrewT is online now AndrewT  Scotland
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That seems to indicate that a pair of 4ohm bass drivers are used in series to create an 8ohm bass load.
The Mid driver must also be an 8ohm Mid load.

Is it a 4driver speaker, 2B, 1M & 1T?

I wonder why they call it a 6ohm speaker?
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