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Old 24th November 2003, 06:59 PM   #1
amt is offline amt  United States
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Default Parallel or Bridged 100watter

Hi group,
Ive just aquired some very inefficient 5ohm speakers and would like to build a simple 100watt amp. I hooked up the biggest amp I have, a MC modded Hafler and the mids and highs were horrid compared to my 3875 IGC. But of course, I have no volumn or bass.

After checking all the Bridgeclone threads, Im still confused since there never appreared to be a conclusive design. I checked the National apps and both the 100watt bridged and 100watt parallel designs look very simple. The bridged looks to have lower distortion but the parallel will drive a 4ohm load easily.

http://www.national.com/an/AN/AN-1192.pdf

What should I attempt? Im not comfortable with too much engineering but can assemble anything. Ive got 6 3386s on the way.

Insights please, and any comments on the sound of either.

amt
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Old 24th November 2003, 07:59 PM   #2
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If it's a four ohm load, go with the simple paralel set up, as if you go to a birisged configuration, you will have to parallel at least four, probably six devices for each channel (if you only used two per speeker for bridging, the thermal protection would almost certinaly cut in at quite low power levels and sound nasty).
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Old 24th November 2003, 09:25 PM   #3
amt is offline amt  United States
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So a parallel configuration is manditory for a 4 0hm load regardless of whether its the beginning of a bridged circuit or just a standalone. Upon re-examination of the National app, it appears that the biggest difficulty with the parallel circuit is that the feedback resistors need to be matched or as close as possible. Never bought 0.5 tolerence resistors before. That should be fun.

thx

amt
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Old 24th November 2003, 09:42 PM   #4
sam9 is offline sam9  United States
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Just to complicate things: If the speakers are two way, and you want to extend your DIY experiences, using the two ICs to bi-amp your speakers might be an alternative.
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Old 24th November 2003, 11:54 PM   #5
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Hi, just to try and clear things up a little. If you were to run a single chip by itself, you would be fine with around 50W into an 8 ohm load, and the chip could happily drive a four ohm load to a power level of perhaps 60-75W. However, due to teh thermal limitations of the package, and things like internal resistance you are not able to get your desired 100W amp.

So, if you were to parallel two chips, each chip would only be supplying half the current to the speaker at any one time (this has the effect of each chip seeing the load as being twice the actual impedance, 16ohms for an eight ohm speaker, or eight for a four ohm speaker). So, you can have it so that each chip is supplying the same 50W of power you get into an 8 ohm load when you are in fact only driving four (which is absorbing all 100W).

The chips could drive a still lower impedance, however, the effects mention above about the limited increase in power will come into play, so you won't get double the power into a two ohm load even though connecting it to the chips won't damage them in any way, and they will be happy to give out over 100W to it.

The Bridge & Parallel option is another option for you here, although, it would probably be even more complex and you would end up needing alt least six, probable eight chips to drive each speaker which would be quite a big first project.

Andrew.
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Old 25th November 2003, 12:20 AM   #6
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Good explaination. I think that even a dependable 75 watts would be sufficient since I will be eventually using a sub and active XO and be eliminating 80hz and below. This should lessen the demand for wattage in the speakers.

Is there a reason that I havent seen paralleling discussed and the focal seems to be on bridging? Is paralleling inherently inferiour or are low ohm loads uncommon?

I guess I will build one per NSCs' diagram unless someone has a successfull GC version I can use. I looked pretty closely at past thread and dont remember one but if it was discussed over several thread w/o a schematic, I may have overlooked it.

amt
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Old 25th November 2003, 04:41 AM   #7
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I looked over National's paper on their bridge/parallell configuration, and it looks pretty good. First, do your speakers have a 5 ohm impedence or do they have a 5 ohm DC resistance? If they have a 5 ohm imedence, you will probably want to use the parallel set-up. This will get you a little under 100 watts for a 5 ohm impedence. If your speaker has a 5 ohm DC resistance, it is probably an 8 ohm impedence and you will want to use the bridged layout to get the power you are looking for.

One thing I noticed between the two layouts is the differene in distortion figures. The bridged configuration has a much lower distortion due to cancellation of harmonics inherent to bridged designs. The parallel layout can deliver more current for a given heat dissipation, but doesn't have the distortion cancelling figures that the bridged configuration has. To get the best of both worlds, go with the bridged/paralleled configuration. The configuration isn't much more complicated than a bridged or paralleled. It just has more parts. This will give you more than enough power, and it will be able to drive a 4 ohm load just fine.

Cheers,
Zach
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Old 25th November 2003, 10:19 PM   #8
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Default but which one?

I looked again at the NSC distortion plots and see that the parallel version is higher but it drops w/ output. If have the amps working at 40+ watts most of the time, will I notice the difference?

And there are three parallel/bridged schematics and Im not sure which is which. Fig 13 looks WAY too difficult w/o a PCB. Fig 14/15 looks doable P2P but doesnt appear to be 200watts. Are those variations of the 100 watt amp or merely examples of parts of Fig 13? Lastly, Fig. 17 looks like it could be handled P2P and may be the best way to go. Ive not made a PCB before so P2P would be convenient. I did see a board for sale from Europe at $45ea but since Ive not seen anyone complete one and report back, Im not sure thats a good bet yet.

amt
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Old 26th November 2003, 01:33 PM   #9
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I'm planning on building a 400W into 4 ohms subwoofer amp following the figure 13 scheme, although I think I'm going to need six chips. I chose the servo architecture insead of the "basic" precision resistior one, figure 17, becaure it has better low end response. When using precision resistors only, National claims that dc blocking caps are necessary and they cause a low freq rolloff. National omitted including a graph to illustrate this fact.

One the other hand, I have heard at least one claim the the servos are audible. So, if you are going to be using a seperate subwoofer, maybe the basic scheme would be good for you.

What I am wondering is what if any changes to the values listed figure 13 would be necessay to add two more chips.

Cheers.
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Old 26th November 2003, 11:48 PM   #10
usekgb is offline usekgb  United States
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You shouldn't need blocking caps in the fig 17 schematic because the bridged architecture cancels out any DC that might be at the outputs of your op-amps. It's a very elegant solution, and is basically just four gainclones.........two inverting and two non-inverting. Also, there is a blocking cap at the input of the figure 17 schematic that will eliminate DC at the input. Just make this value at 0.47 uF and you will have an LF f3 point unter 10 Hz. This won't hurt your bass response, and will actually give the chip a little more headroom due to the fact that it won't have to produce subsonic frequencies (usually a good thing).

As far as using servos, I don't think they're necessary. They seem to take a little life away from the sound. They don't hurt the frequency response, but they just seem to take some of the "oomph" out. Just my opinion.

If you want to add two more chips, you shouldn't have to change any values. Just add one to each side of the balanced schematic. BTW, I don't think you'll be able to get 400 watts in to a four ohm load with this layout. The data sheets says the continuous power for the bridged/paralleled layout is 335 watts, with a peak of 450 watts. You can parallel as many more chips as you wnt, but this won't increase the output power. If you want to get more power, you will have to go down to a 2 ohm load, and double the number of chips in the bridged/paralleled layout. That should give you somewhere around 450 watts continuous in to 2 ohms.

Cheers,
Zach
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