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Old 13th April 2007, 07:16 PM   #1
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Default LM3875 / T-Amp 5066 / 41Hz Amp 32 for THIS project?

Okay, some of you may have read my enclosure questions in the loudspeakers forum. For those who haven't, I'm building a loudspeaker enclosure out of acrylic. It's a sealed design with 8" woofers, 5" mids, and 1" tweets. Crossover at 600Hz/3.5kHz. (I'm reusing the drivers and crossovers from a set of JBL G300s. Crossover points aren't really negotiable for this project due to budget constraints.)

Anyway, because of the limited size of the drivers and my stubborn insistence on a sealed enclosure, I assume this is going to be a rather inefficient arrangement. Is that a safe conclusion? So as much as I like the idea of the T-Amp 5066 (couldn't be easier to modify and the price is definitely right) I wonder if it's going to be enough power. Seems like it might turn out to be disappointing.

Oh, by the way, the amplifier is going to be IN the sealed enclosure. As I mentioned elsewhere I'm a graphic designer so sometimes I let form overrule function.

Because I was skeptical I went to the National Semiconductor site a week or so ago and requested samples of the LM3875 chips. Sure enough there were two of 'em on my doorstep a couple days later. Crazy. But now that I'm actually looking around it seems it's more affordable to just buy a kit that includes the chips than to piece one together.

I'm also interested in the Amp 32 from 41Hz. Seems fairly straightforward.

So here are my requirements. Please share your opinion (or facts if you have them?) about each of the above amps based on these needs:

1) Sound good. I think I've read enough reviews of each to be confident that any of the 3 will meet this goal. Especially for a guy (me) who usually listens to a home Sony system and a computer Altec Lansing "system".

2) Enough power to get reasonably loud reproduction from my above setup in a rather large space. I'm not trying to blow anyone away or win any awards. Just sound good and not be disappointed that it doesn't go louder.

3) Run on a 12v SLA battery. This is clear to me on the 5066, but less so on the other two. I've soldered plenty in my life, but I can't read a schematic any better than I can read Korean. (I can read Japanese, if that helps...)

4) Affordable. The T-Amp can be powered by a supply I already have (12v 2amp), so there's not extra expense for me there. I'm really trying to keep the budget down. How much am I really looking at with the LM3875 kits or the Amp 32 kit?

Thanks!


- Jon
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Old 13th April 2007, 08:33 PM   #2
Fenris is offline Fenris  United States
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LM3875 won't run on a single +12v supply. It should have at least a +/- 18 v supply. The T-amp 5066 and Amp 32 use different packaging versions of the same chip, so power should be the same. The Amp 32 uses higher quality components, but requires assembly of SMT components, not for the faint of heart. There's really no way to get high power, high quality and cheap in the same package. Pick two. I'd recommend the T-amp for its simplicity. If you're going to put the amp in the speaker, I'd definitely recommend the T-amp or Amp32 because of their high effiency.
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Old 13th April 2007, 08:52 PM   #3
phn is offline phn  Sweden
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It's hard to say what you need without knowing the SPL (sensitivity) of your speakers. But if the 5066 isn't enough, the AMP32 won't be enough. We are talking 2db difference.

Some sort of T-amp definitively has advantages: small and effecient.

One thing you should look into is active chip crossovers. These are relatively cheap, draw little power (<50W +/-12V trafo) and have no power loss.
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Old 18th April 2007, 09:56 AM   #4
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Default Re: LM3875 / T-Amp 5066 / 41Hz Amp 32 for THIS project?

First I will say, "you can't have that". Decide which of your desires are less important as it is not just a matter of making a declaration and having the pre-designed ICs bend to your will. I apologize if that seems rude, it is only an intention to state things plainly.

Quote:
Originally posted by jonkun227 Oh, by the way, the amplifier is going to be IN the sealed enclosure. As I mentioned elsewhere I'm a graphic designer so sometimes I let form overrule function.
Forget sealed enclosure unless you can accept having a large heatsink strapped onto the back. Any power amp creates heat that can't be left unchecked, even with a 'sink on the back the speaker cabinet will heat up. You should also insulate the inner side of the 'sink to keep as much of the heat as possible radiating external to the cabinet. It would also be good to isolate the amp from vibration by using a flexible coupling of some sort - but since it's a clear speaker you'll have to decide what looks best, or perhaps paint the inside of the cabinet a color so the guts aren't visible.

Quote:
Because I was skeptical I went to the National Semiconductor site a week or so ago and requested samples of the LM3875 chips. Sure enough there were two of 'em on my doorstep a couple days later. Crazy. But now that I'm actually looking around it seems it's more affordable to just buy a kit that includes the chips than to piece one together.
I suggest that you now give the chips to someone who can use them, as ordering samples without a clear intention to, and actually, using them is an abuse of their generosity.

Quote:
I'm also interested in the Amp 32 from 41Hz. Seems fairly straightforward.
and unsuited to your goal, output power is not enough.

Quote:
So here are my requirements. Please share your opinion (or facts if you have them?) about each of the above amps based on these needs:

1) Sound good. I think I've read enough reviews of each to be confident that any of the 3 will meet this goal. Especially for a guy (me) who usually listens to a home Sony system and a computer Altec Lansing "system".
This is unnecessary. Who builds amps to not sound good?

Quote:
2) Enough power to get reasonably loud reproduction from my above setup in a rather large space. I'm not trying to blow anyone away or win any awards. Just sound good and not be disappointed that it doesn't go louder.
Pick between this or #3. You can't have both.

Quote:
3) Run on a 12v SLA battery. This is clear to me on the 5066, but less so on the other two. I've soldered plenty in my life, but I can't read a schematic any better than I can read Korean. (I can read Japanese, if that helps...)
The current requirement is too high for any lengthly listening at a volume level "reasonably loud". You could design or buy a separate step-up boost power regulator board, but even then the current draw is too high unless your 12V SLA is more like a 100A car battery size. You can't just take the rated capacity of a SLA for a runtime estimate either as they aren't designed to be ran down to nothing, it will wear out the plates even on a deep cycle type.

Quote:
4) Affordable. The T-Amp can be powered by a supply I already have (12v 2amp), so there's not extra expense for me there. I'm really trying to keep the budget down. How much am I really looking at with the LM3875 kits or the Amp 32 kit?
Not possible. If the cost of a 12V, 2A SLA battery is really a factor at all, keep using your present stereo. You will need to spend more than 2 x 5066 amps cost, plus at least $30 for a suitable sized pair of series SLA batteries for 24V rail to rail and that $30 is for short runtime only.

Be more realistic, you want a special result but aren't willing to spend even enough for an ordinary result. I'm not trying to be an *** about it, this is just the way the evidence played out to a conclusion.

Either forget SLA power or allocate at least $50 for a pair of 12Ah, 12V cells minimum. If you can instead settle for it not playing loud, I mean wishing it could play louder contrary to what you wrote above, then the AMP32 kit is the best bet. 5066 or another amp that could use 12V is the next best, and with only 12V power and a Class-D (or "T" as they call it) digital amp, it is more power conservative so your SLA cells need not be so large for decent runtime.

Even then, your present 2A SLA is just too anemic for regular use, you could listen for a few hours at a time a few times but then the battery plates are wearing again. In the 2A range, you'd be better off using cells that can withstand more deep discharge and recharge cycles like 10 x AA NiMH. Don't use more than 10 series with the amp chips spec'd for 12V though, as their peak charged voltage (roughly 1.35-1.45) would result in higher series voltage than the low end Sonic Impact chips can handle.

"Loud" = voltage * current = expense˛


Your project seems interesting but something has to give. I don't think you will be satisfied with the output of any amp running off 12V, especially not when it's something you're putting this much time into custom building.
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Old 18th April 2007, 10:47 AM   #5
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The closest thing that I feel would suit as many of your goals as possible is to use 1 x AMP11 ( http://www.41hz.com/main.aspx?pageID=131 ) per speaker, powered by 2 x 12V (in series as 24V total), 7Ah SLA batteries per speaker.

This is roughly

$72 - 2 x AMP11
$10 - shipping
$60 - 4 x 12V/7Ah SLA batteries @ $15 ea. Cost will rise if not available locally at that price
------
$142

+ misc addt'l costs like heatsinks, battery charging system, hardware to mount, anything listed on the AMP11 page you don't have, etc.

If you cannot spend that much, get the T-amp and one 7Ah battery and be satisfied with lower power. I did not look at your speaker project at all, was assuming these are two separate mono speakers (for stereo). One AMP11 mono per speaker, or one T-Amp for both since you can't bridge the outputs on a T-amp (5066) to use one per speaker. The 5066 is not at all a bad amp, but it is underpowered for going above medium loudness with average efficiency speakers.
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Old 18th April 2007, 06:08 PM   #6
judtoff is offline judtoff  Canada
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Why is porting not an option? You could go ported and gain 3db...effectively needing half the power.

About the SLA, I have done this, using LM1875s, it wasn't the best set up but it still got loud enough.

My suggestion would be getting a inverter and modifying its output transformer to give you +/- 25 volt rails(you'll need to center tap it yourself)

Add a rectifier and 1000uF of caps and you have a +/-34 volt supply. Perfect for those LM3875's

I think you should bi amp those speakers and use active crossovers. use LM1875s for the tweets, and the 3875s for the mids and woofers.

In my experience you'll get about an hour to two hours of run time doing this.

Check the parts express article on podzuma iirc it was similar to your requirements.
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Old 2nd May 2007, 04:24 AM   #7
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Sorry I never got around to actually responding to the great input here. Thanks, everyone, for your comments.

In the end I decided to try the T-Amp because A) it's cheap; B) it's so simple to incorporate; and C) I've heard so many great things about it.

I built a prototype out of scrap maple veneer MDF. It sounds FANTASTIC. I don't have any equipment to actually measure anything, but the most important test, do listeners enjoy it, has passed with flying colors. Nobody believes it's only 15 watts. I work in a production print shop the size of a Target or Walmart or whatever. The cubicles where I work in prepress are on one end and the production is on the other end. They can hear it clearly down there and it isn't distorted. (Of course it depends on the quality of the track from my iPod... Some are compressed so poorly that no audio processing could ever hope to make it sound good. Now that I have a system where I can tell the difference between the crappy tracks and the crappy speakers I'll have to clean up my playlists a bit.)

Now I'm really anxious to do the acrylic. We have a couple pieces of 1/2" acrylic left over from a job that I can use for the front and back surfaces. I just need a few more (I'm too cheap to go out and buy the stuff; about $100 for a full sheet) and I can build it the way it was meant to be done.

If I get time I'll post pictures of what I have so far.

Thanks again!


- Jon
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Old 2nd May 2007, 04:39 AM   #8
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Quote:
Originally posted by Fenris
There's really no way to get high power, high quality and cheap in the same package. Pick two. I'd recommend the T-amp for its simplicity. If you're going to put the amp in the speaker, I'd definitely recommend the T-amp or Amp32 because of their high effiency.
Asking for all three was mostly a joke. In my line of work (printing) my philosophy is that a business can choose to be up to two out of three things to their clients: The Fastest, The Highest Quality, or The Cheapest. Clients need to understand that one of those three areas MUST be sacrificed if the business intends to succeed. High quality is possible at low rates if speed is not important; the printer has to gang similar jobs to cut material and labor costs, but this can be done without sacrificing quality. However, jobs have to sit on hold until enough have come in to make it worth running. Or the job can be run immediately for a premium because of the tremendous waste involved in one-off jobs.

Anyway, the point is I understand that concept. It's true in most areas, just different criteria.

Quote:
Originally posted by judtoff
Why is porting not an option? You could go ported and gain 3db...effectively needing half the power.

Check the parts express article on podzuma iirc it was similar to your requirements.
Because I am very stubborn. I really enjoy working with acrylic. I think it's beautiful. And it just seems silly to me to make something out of acrylic that isn't airtight. (Obviously I want a slight leak for changing atmospheric pressure over time.) Like I said before, I'm not trying to win any awards with this project. And I don't expect the response to ever be properly measured. I'm just having fun with a project for my desk at work.

The Podzuma project was cool. Similar enough goals to give me a few more ideas, but different enough that I'm glad mine will still be unique. Thanks for the heads-up!


- Jon
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Old 2nd May 2007, 12:15 PM   #9
judtoff is offline judtoff  Canada
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Ok porting is out, what about using a passive radiator?(a cone with a mass attatched that acts the same as a port essentially) The gain in efficiency will give you nearly twice the play time at the same volume!

Anywho, I'd just get 2 T-Amps and call it a day, if its not loud enough, then look to the LM3875.

BTW I just finished making a amp for my girlfriends car using a LM4780, 4 4700uF caps(gotta filter the hell outta it, stupid inverter is noisy), a $10 inverter, a 13-0-13 transformer. It sounds great, and gets loud with her infinity referance speakers. The Kicker...its only putting out 13 watts per channel.


good luck
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Old 2nd May 2007, 07:34 PM   #10
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I thought about a passive reflex. I was shopping at Parts Express for different sizes. But their smallest radiator is 8" and really raised the budget for the project. It also requires far more knowledge and care to tune it than I feel I'm ready to do this time. But as far as appearance, efficiency, and general impressions I really think that a passive radiator setup in an acrylic enclosure would be gorgeous. If anyone knows some actual figures for volume and whatnot to get me going with any given driver/radiator set, please let me know. I think that would get enough attention at work that I could probably sell a couple to the suits and pay for my own setup. I left the system at work overnight, so I expect to get a few comments when I get in to work this afternoon. I've already had 2 people ask me to build them similar systems.

Won't ever be a full-time job, but if it pays for my habit (er, hobby) then I'll be in good shape.


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