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.diyaudio.com/forums/chip-amps/19896-gc-user-tool-nat-semi.html?highlight=spreadsheethttp://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.
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.diyaudio.com/forums/chip-amps/19896-gc-user-tool-nat-semi.html?highlight=spreadsheethttp://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.
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.
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.
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.
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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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
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
Perhaps they are simply being a little more honest than a lot of other manufacturers. We both know that there will be impedance dips due to the crossover network so i'm sure they were just being a little more conservative than those that quote 6ohms & fit a 4 ohm woofer
The crossover is actually one of the simplest i remember seeing on virtually any speaker (Sonus Faber Extrema excepted) so i should imagine 6ohms is a good call.
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B & W call my 703's an "8 ohm speaker" even though they dip to 3 ohms minimum. How they can do that is beyond me, even though the average impedance is way above that.
Bridging amps for more power usually means quality suffers.
You really need to be sure what is actually happening, and a 'scope across the speaker when playing loud might be revealing. If the clipping occurs as the output approaches the rails then paralleling isn't going to do much as you have hit the limits of the PSU. Even if you double the output available (watts RMS) the difference in sound level won't be as much as you expect... just noticeable but nothing more.
Thanks for the replys. I think I may have to put a bit more capacitance on the PSU. At the moment, the toroidal outputs to 2 bridge rectifiers and then straight to the amp boards. Each amp board has a 1500uF cap from V+ to GND and V- to GND.
Here is a link to the specs of the speakers I'm driving: Specification - BR6 | Monitor Audio
Here is a link to the specs of the speakers I'm driving: Specification - BR6 | Monitor Audio
I see it now.
They describe it as a 2 1/2 way.
The bass and mid are driven in parallel at the LF end.
This is effectively 4ohm at the Bass, 8ohm at the Mid and 8ohm at the treble.
I would call this a 4 to 8ohm speaker and not a 6ohm speaker.
The chipamp should be designed to drive the 4ohm combination of the 2B+1M i.e. 60W into 4ohms.
This will result in a 35W into 8ohm amplifier driving a substantially 8ohm speaker.
You need to Bi-amp to drive this speaker to more of what it is capable of.
If you do Bi-amp then the bass amp is designed as a 60W into 8ohm and the M+T amp is also designed as a 60W into 8ohm. This should be more capable of driving current into that combination of crossover and drivers.
The M+T amp might just about perform with 1500uF+1500uF smoothing. The Bass amp desperately needs >=10mF+10mF
They describe it as a 2 1/2 way.
The bass and mid are driven in parallel at the LF end.
This is effectively 4ohm at the Bass, 8ohm at the Mid and 8ohm at the treble.
I would call this a 4 to 8ohm speaker and not a 6ohm speaker.
The chipamp should be designed to drive the 4ohm combination of the 2B+1M i.e. 60W into 4ohms.
This will result in a 35W into 8ohm amplifier driving a substantially 8ohm speaker.
You need to Bi-amp to drive this speaker to more of what it is capable of.
If you do Bi-amp then the bass amp is designed as a 60W into 8ohm and the M+T amp is also designed as a 60W into 8ohm. This should be more capable of driving current into that combination of crossover and drivers.
The M+T amp might just about perform with 1500uF+1500uF smoothing. The Bass amp desperately needs >=10mF+10mF
Thanks AndrewT.
I think I may also have to put a bit more capacitance on the PSU. At the moment, the toroidal outputs to 2 bridge rectifiers and then straight to the amp boards. Each amp board has a 1500uF cap from V+ to GND and V- to GND. So obviously, when more power is required (the volume is turned up), the small 1500uF caps on the amp boards haven't got the capacity to keep the DC ripple small.
Here is a link to the specs of the speakers I'm driving if anyone is interested: Specification - BR6 | Monitor Audio
I think I may also have to put a bit more capacitance on the PSU. At the moment, the toroidal outputs to 2 bridge rectifiers and then straight to the amp boards. Each amp board has a 1500uF cap from V+ to GND and V- to GND. So obviously, when more power is required (the volume is turned up), the small 1500uF caps on the amp boards haven't got the capacity to keep the DC ripple small.
Here is a link to the specs of the speakers I'm driving if anyone is interested: Specification - BR6 | Monitor Audio
the clue is the description:
2 1/2 way tells us it is not a three way crossover.
It is a two way crossover that brings in the extra bass drivers at the low end of the frequency range.
In this LF range the 2B are 8ohm and the Mid is parallel and also 8ohms. The result is that at LF the speaker presents a 4ohm load. The amp must be capable of driving that.
2 1/2 way tells us it is not a three way crossover.
It is a two way crossover that brings in the extra bass drivers at the low end of the frequency range.
In this LF range the 2B are 8ohm and the Mid is parallel and also 8ohms. The result is that at LF the speaker presents a 4ohm load. The amp must be capable of driving that.
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With the midrange in parallel the impedance will half, so two x 8ohm drivers will give an impedance of 4ohms. The bass drivers are 4 ohms each & in series this will give 8ohm, the mid is also 8ohm & in parallel with the bass drivers you get 4ohm.
If you Bi amp then the amps will see only an 8ohm load as the bass drivers will never be in parallel with the mids due to seperate amps
Yes, you are set up for bi-amp. Many people would place an active crossover between the source and each amp. But your speakers have cross overs and will filter.
If you don't use a crossover and treat the drivers as if they were resistors.
The nominal impedance is the value that is not undercut by more than 20 %, e. g. a nominal 6 Ohm speaker should remain above 4,8 Ohm everywhere except at DC.
..which would make it a 4 Ohm speaker. The German manufacturer Canton started to use the description 4..8 Ohm speaker as a courtesy to layman customers. Many customers have no clue about impedances. When they buy 8 Ohm speakers and find their amplifier specified for 4 Ohm loads they become afraid to break something. So Canton writes 4..8 Ohm on 8 Ohm speakers to indicate their compatibility to all amplifiers, whether specified for 4, 6 or 8 Ohms.
Manufacturers sometimes do that, when the dips are in a region where only little energy is present in normal music programme.
In the case of the 703s however the dip is around 100 Hz where a lot of energy is present. Maybe they rely on the assumption that people who pay so much money for speakers spend a similar amount on the amplifers which should rule out that they surrender to low loads. Then B&W mention the dip to 3 Ohm in the specs to fend off claims in case somebody does connect an inadequate amplifier.
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