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Old 16th September 2004, 01:14 AM   #1
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Default Hopefully not too much to ask

What I want to do is build a subwoofer as a present for my dad for christmas. He is planning to get a surround setup, and will need a sub. I already have the speaker and box, but now need the amp.

I am going to be using this amp circuit:
http://www.mif.pg.gda.pl/homepages/t...s/simp200W.gif

With lm3875 chips and MJ15003/4 transistors.

Like I said, hopefully this isn't too much to be asking for for free, but would anybody be able to help me design a PCB for this project?

I am not in a big hurry since christmas is still over 3 mo away, so if somebody is willing to spend some free time doing this I would really appreciate it.

Thank you so much!
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Old 16th September 2004, 01:38 AM   #2
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not worth it...
if its a 4 ohm driver, just use a single LM3886 to get about 80 watts..

if its a 8 ohm driver, bridge two LM3886's for about 120watts....

80 watts is PLENTY, and you don't even need a PCB for a simple LM3886 circuit
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Old 16th September 2004, 07:52 PM   #3
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No, 120W is no where near enough. I have tried bridge 3886, not at all impressive.
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Old 16th September 2004, 08:18 PM   #4
cjd is offline cjd  United States
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Quote:
Originally posted by soundNERD
No, 120W is no where near enough. I have tried bridge 3886, not at all impressive.
Must be the driver/enclosure combo you're using then.

I have 200W driving a pair of 15's in an IB configuration (in a not too huge room) and I can watch the wall oppposite the driver manifold flex sometimes. Playing back T-rex footsteps will cause the "ripples in the water glass" thing that tips the people in the movie off to there being something scary coming, even when the stuff is too low to hear (though, I do get a tingle on my spine...). and the glass is sitting on a table that's on carpeted concrete basement floor. And that 200W is more than I need (the drivers run out of excursion before the amp clips, and my ears are already bleeding at this point)

But, given that I spent $100 on the 200W amp (which also includes phase control, crossover, line level or speaker level inputs, speaker passthrough, etc.) I still question the value in trying to use one of these chip-amps in this situation.

I should add, I'm still working on PCB layouts of boards (buffers, NI and I 3886 for regulated power, regulated power boards) that will lend themselves to parallel configuration well. Doing a BPA wouldn't take much effort either. Though, the LM338 reg shouldn't go beyond supplying two chips... Progress is ongoing, but it's at a very leisurely pace. I'm at the point where I think I need to switch programs so I can actually send the design out for proto boards. It's not a custom layout though.

There are layouts out there for various paralleled, briged, etc. setups as well already.

C
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Old 17th September 2004, 12:26 AM   #5
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Default 200watts?

i think your being optomistic with 200watts. i could be wrong, ive built a large array with 6 x tda2030 driver and bd907 bd908 using roughly the circuit in the data sheet and tda2030 chip runs best at about 36v. what about adapting the low pass circuit in figure 20 and running 4 amps with 4x6 inch drivers. or use two of your circuit with paired 8ohm drivers. c ya soon, steve.. ..
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Old 17th September 2004, 12:39 AM   #6
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Default p.s.

nice layout though, what driver/s are you using to need more than 120w? i like the use of array drivers, more surface area!
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Old 17th September 2004, 01:09 PM   #7
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what layout are you talking about?

Would I even notice much of a difference with 400W using this circuit than a bridged chip? I would use the LM3875 so I don't lose sound quality with the mute.

The speaker is actually a car speaker I couldn't resist on clearance for $20. It was a $100 12" Rockford Fosgate Punch Z that somebody had bought then returned because it didn't fit. It was still brand new and works perfectly. I know it is no where near the best, but well, its all I can afford, and still sounds decent.

Anyway, I believe the SPL is somewhere around 86-88. Would a bridged 3875 be enough? Wish it was a DVC so I could bridge 2 amps for each VC. Then I'd have at least around 300W.Now that I think when I tried the LM3886 bridged, I didn't do too well (just soldered some wires on 2 seperate boards to make one invert). Then I was trying it with a cheapy speaker (Some no-name 10" speaker). Then I realized the trace that kept the mute off had broken, so only one amp was running. I would think 2 3875 bridged would sound pretty good would it?

Or would the other circuit put out much more volume before distortion? Is it really worth the extra time trying to design a PCB for it then figuring out how to mount 4 TO-3 chips plus 2 to220 chips to a heatsink and having to mess with insulators for the to-3 chips?

When I design this one, I want to have phase control built into it. Anybody know of a good circuit for this?

Finally, would I be better off running both chips in either iverted or non-inverted (which one is better for this?) and using a DRV134 to split the input, invert one side then balance them?

Thank you all so much.
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Old 17th September 2004, 03:24 PM   #8
cjd is offline cjd  United States
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Um...

you could only afford a $20 driver, but you want to spend how much putting together a GC to power it?!

Take a look at the BrianGT 4870 board group buy (not quite available yet, but very soon it seems). It sounds to me like this is the perfect solution for you here.

Also, the subtle SQ things you might find with a GC are largely going to be moot driving a subwoofer. You mostly need power. You may want to increase capacitance significantly as well.
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Old 17th September 2004, 06:31 PM   #9
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Quote:
Originally posted by soundNERD
what layout are you talking about?

Would I even notice much of a difference with 400W using this circuit than a bridged chip? I would use the LM3875 so I don't lose sound quality with the mute.

The speaker is actually a car speaker I couldn't resist on clearance for $20. It was a $100 12" Rockford Fosgate Punch Z that somebody had bought then returned because it didn't fit. It was still brand new and works perfectly. I know it is no where near the best, but well, its all I can afford, and still sounds decent.

Anyway, I believe the SPL is somewhere around 86-88. Would a bridged 3875 be enough? Wish it was a DVC so I could bridge 2 amps for each VC. Then I'd have at least around 300W.Now that I think when I tried the LM3886 bridged, I didn't do too well (just soldered some wires on 2 seperate boards to make one invert). Then I was trying it with a cheapy speaker (Some no-name 10" speaker). Then I realized the trace that kept the mute off had broken, so only one amp was running. I would think 2 3875 bridged would sound pretty good would it?

Or would the other circuit put out much more volume before distortion? Is it really worth the extra time trying to design a PCB for it then figuring out how to mount 4 TO-3 chips plus 2 to220 chips to a heatsink and having to mess with insulators for the to-3 chips?

When I design this one, I want to have phase control built into it. Anybody know of a good circuit for this?

Finally, would I be better off running both chips in either iverted or non-inverted (which one is better for this?) and using a DRV134 to split the input, invert one side then balance them?

I seriously doubt, to the point of saying is non-existent, that the mute on the LM3886 and LM4780 will affect the sound quality. That is starting to become an urban legend that should be vanished from our DIY scene. If the project is well implemented there won't be any difference between "mutable" and "non-mutable" chips, or if there is they might favour the 3886 and 4780.

If you want to make a high power project, forget about the 3875 and go the 3886 or 4780 way. End of the story.

To how many SPL a bridged amp using these chips will take your speaker will depend on its impedance and other specs.

I would think of a 2 + 2 chip amp, parallel/bridged and using a summing circuit for both channels. Or use separate subs that may be better. That is another urban legend: that you don't need separate subwoofers. Two subs may sound better than one according to what you listen to.

But building a project with these chips will be simpler than a regular one using TO3 and TO220 parts.



Carlos
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Old 17th September 2004, 07:27 PM   #10
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Quote:
Originally posted by cjd
Um...

you could only afford a $20 driver, but you want to spend how much putting together a GC to power it?!

Take a look at the BrianGT 4870 board group buy (not quite available yet, but very soon it seems). It sounds to me like this is the perfect solution for you here.

Also, the subtle SQ things you might find with a GC are largely going to be moot driving a subwoofer. You mostly need power. You may want to increase capacitance significantly as well.
yes, I do because once I get some money down the road, I will invest in a better driver. Good present for next christmas, by then I'll be old enough to get a job to afford it.

I never said I want to build a great amp, just want the most bang for the buck.

Another thing im thinking about is using mosfets. Would that be an easy and affordable way of building a high power amp?

And, another reason I do not want to mess with mute is the increased complexity. Less parts = less traces = larger traces in the same spot for what really needs them = better sound. Am I not correct here?

About the room, this is a huge room, about 40-45 ft by 25ft, with no walls between rooms, so bascially it would have to make enough sound to cover the entire room while losing some to the other rooms. Would a bridged 3875 be good for this?

Finally, cjd, are you saying a phase control would be expensive? Do you think it would make a large difference? All I know is that most expensive units have them, so if they are as simple as using a potentiometer to control how much of the sound goes to the inverted and non inverted inputs of an opamp, I might as well throw it in.

Thank you all again for your input on this project!
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