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Old 17th March 2005, 06:52 AM   #1
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Default need directions for 8 channel amp with LM3886's

OK, so I want to build an 8 channel amp for stereo quad-amping (4 amps per side ).

What I have now are 2 x 500VA(each can put out even higher than 700 ranges max) 25v-0v-25v transformers. 8 x LM3886T ICs. Also 16 x 6800uf Capacitors rated 63 Volts, would it be overkill/not enough ?. And last of all, 15AMP rectifiers with voltage rated way above application so it's not an issue.

Questions:

1. Since I'm doing the wirings P2P should I just follow the "Typical Application" circuit in the data sheet for every amp? It has so little parts which is a good thing but what has any of you guys done to make it more suitable for a specific tweeter/mid/woofer amplifications.

2. I don't have a preamp, so I might want to increase gain if it is not enough (I may be using line level passive crossovers). How do I do it ?

3. What is the minimum amount of heatsink needed (or what size ?), the tweeter amp my not even get warm, but the 4 woofer amps (2 a side) will definately be .

4. Do I leave pin 8 alone if I don't want the mute feature ?

5. pins 1 and 5 are BOTH connected to same V+ supply ?

That's about all the questions I can think of now, Ill be buying the amp components and other misc. stuff like switches, output terminals next weekend, any suggestions/comments welcomed.

Thanks
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Old 17th March 2005, 08:00 AM   #2
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8 channels P2P and one transformer => Not easy to get it working perfect due to the grounding. I'm just warning you.

Gain of 30 is probably sufficient and if not just change the feedback, Rf.

Mute pin: I strongly advise you to read the datasheet and AN-1192, yes it have to be connected.

Pin 1 and 5, yes, both have to be connected.

Heatsink, check the excel file below.

108 800 uF, yes, it's overkill. You will need at least 2200 uF/ per rail / channel (= 17000 uF per rail in total) if you would have separate power supplies and less if the amps share PS.

http://www.national.com/pf/LM/LM3886.html
http://www.national.com/ds.cgi/LM/LM3886.pdf
http://www.national.com/an/AN/AN-1192.pdf
http://www.national.com/an/AN/AN-898.pdf
http://www.national.com/appinfo/audi...gn_Guide13.xls
http://www.national.com/appinfo/audi...sign_Guide.pdf

If you want to make things easy for you, buy BrianGt's boards and consider to get more transformers.
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Old 17th March 2005, 09:20 AM   #3
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Default Good luck!

You'll need it.

You're gonna find it pretty darned hard to make an 8 channel P2P amp. look tidy.

Here's my 6 channel effort using BrianGT kits....I'm in the process of setting it up for a tri-amp configuration. - except I'll be using a different amp for the woofers.

If you are planning to quad amp, I would look at using less capacitance on the tweeter and mid channels. The whole strength of the Gainclone is based upon using low capacitance, which works better in the higer frequencies. Use as much as you like on the woofer though!

Your woofer amps will get warm, but the rest will probably hardly warm up at all. Obviously this depends on your speaker impedance, your heatsinks, and how loud you think you'll be playing it...

Good luck,

Steve
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Old 17th March 2005, 06:16 PM   #4
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Quote:
If you want to make things easy for you, buy BrianGt's boards and consider to get more transformers.
- Im in New Zealand now is it possible to get BrianGt's boards ? If so how much.

- I have two 500VA transformers, equals to 1000VA atleast, I'll be using one transformer for EACH side of 4.

Quote:
If you are planning to quad amp, I would look at using less capacitance on the tweeter and mid channels. The whole strength of the Gainclone is based upon using low capacitance, which works better in the higer frequencies. Use as much as you like on the woofer though!

Your woofer amps will get warm, but the rest will probably hardly warm up at all. Obviously this depends on your speaker impedance, your heatsinks, and how loud you think you'll be playing it...
- Ok, so I won't be using any of the 6800uf caps for mids and tweets just 1000uf's at the chip. But for the woofers go overkill with all caps I have (27200 per rail, thats 4 caps on each rail) ?

- I guess Im limited to 8 ohms speakers only with 35v rails.
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Old 17th March 2005, 07:20 PM   #5
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1) Goto http://www.chipamp.com to see what BrianGT is up to. He ships internationally.

2) It's probably not worth using more than two of your large caps on each rail. - You'll see little audible benefit, and the extra wiring complications probably won't help.

3) According to Nat Semi's overture design guide spreadsheet, you'll be OK at 4 ohms and +/-35V. You'll need more heatsinking though.

Bye the way, where did you score your caps from?

Steve
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Old 17th March 2005, 08:52 PM   #6
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You're not building a Linkwitz Orion system, are you? In any case, he has schematics for a multichannel amp to drive the Pheonix system which also has an 8 channel requirement. See this section on this page about amplifying the Phoenix. A link to the power supply schematic for 6-10 chip amps is in the third paragraph. The chipamps in this design are putting out 60w/1%THD into 6 ohm speakers with 30V supply rails and gain of 20x/26db.
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Old 18th March 2005, 02:29 AM   #7
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Quote:
Bye the way, where did you score your caps from? Steve
I got them for cheap from Vietnam, originally only intended to buy 8 but price calculated was only $1.55 NZD each (around $1.15 USD) . Much cheaper than $6 for a 4700uf here in NZ @ JayCar (much much cheaper). I went for 16 just in case

Quote:
You're not building a Linkwitz Orion system, are you? In any case, he has schematics for a multichannel amp to drive the Pheonix system which also has an 8 channel requirement. See this section on this page about amplifying the Phoenix. A link to the power supply schematic for 6-10 chip amps is in the third paragraph. The chipamps in this design are putting out 60w/1%THD into 6 ohm speakers with 30V supply rails and gain of 20x/26db.
No it is not a Linkwitz, but yes each cabinet will have a set up as Tweeter/mid-midbass/woofer/woofer.

Some more questions:

1. How should I fuse this thing, I need protection. each amp with 2 fuses ? thats alot of fuses and fuse holders to buy . Just a fuse at the AC mains to the transformers (I know it is important to fuse at mains but would I need fuses at amps also, I see you didn't have fuses there BlackDog) ?

2.Whats better. One fuse from AC mains to the two transformers or two fuses, one for each transformer.

3. should the mains fuse/s be slow or fast blow. The transformers are EI types.

I won't be ordering anything internationally cause Im only 15 and still relying on my parents to complete this project
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Old 18th March 2005, 07:04 AM   #8
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Quote:
Originally posted by Diy_Ambitious
I won't be ordering anything internationally cause Im only 15 and still relying on my parents to complete this project
You could ask your mum or dad for some contribution maybe?
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Old 18th March 2005, 07:42 AM   #9
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Quote:
Originally posted by Diy_Ambitious
1. How should I fuse this thing, I need protection. each amp with 2 fuses ? thats alot of fuses and fuse holders to buy . Just a fuse at the AC mains to the transformers (I know it is important to fuse at mains but would I need fuses at amps also, I see you didn't have fuses there BlackDog) ?
I don't know how important fuses in the supply rails are. I see lots of amps without them. The question is: what are the fuses protecting? The power supply can't fail and go over-voltage and fry the amp, can it? So the fuses are protecting the power supply from shorts in the amp (which would draw a lot of current ...) If the power supply draws too much current, then the mains fuse should blow.

Maybe if one rail goes down and there's lots of DC going through the speaker, the fuse in the rail that's still up will blow from the excess current draw, so the speaker will be protected. I don't know if this is valid reasoning, though.

OTOH, I built an AKSA amp with my daughter a few years ago. It has fuses in the power rails and a 100R resistor in parallel with each fuse. I think Hugh called them "rail resistors". You bring the amp up the first time with no fuses in place, check some voltages (esp across these 100R resistors to make sure the current draw is about right), set preliminary bias, etc, then try it again with fuses. The first time we powered it up (without fuses), one of the resistors fried due to some mistake we had made.

If you decide to fuse each rail to each amp module, and you're using circuit boards, you should use really cheap fuse holders that are made for through-hole mounting. They're just stamped metal, which is why they're so cheap.

Quote:
2.Whats better. One fuse from AC mains to the two transformers or two fuses, one for each transformer.
Again, what are the fuses protecting? In the event that the entire amp draws too much current from the wall, the fuse blows. How can that happen? Shorts anywhere. Or shorts between an exposed voltage in the amp and *you* to ground. So you have to have a fuse here. If only one half of the amp develops a fault, the extra current will be drawn through a single mains fuse and cause it to blow. I don't see any reason to fuse them separately.

Quote:
3. should the mains fuse/s be slow or fast blow. The transformers are EI types.
Slow blow. When you turn the amp on, there will be a momentarily large current draw unless you put in some sort of soft start circuit, an NTC resistor, etc. If the fuse is fast blow, it will have to be big enough to handle this turn-on surge, which will be much more than the amp will normally draw. For example, if the amp draws 2 amps max when playing loud music, the turn-on surge might be 4 amps. So you'd need a 4 amp fast blow fuse. This would allow a malfunction that draws 1.9 amps to not trip the fuse. Bad.

Quote:
I won't be ordering anything internationally cause Im only 15 and still relying on my parents to complete this project
I admire your ambition at 15. You picked a good screen name. Good luck with your project!
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Old 18th March 2005, 11:11 AM   #10
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From seeing the experience of others doing multichannel chip amps - think carefully about your circuit grounding scheme. Lots of people seem to have trouble with ground loop induced noise.

I recommend "local" star grounds near each chip / channel, including all signal returns (RCA input and speaker output grounds too - isolate these from the chassis.) From each of these "local" stars, return one fairly heavy ground wire to the power supply ground (which is also starred if there are separate power supply caps.) The two power supply grounds (because you have two transformers) would then be brought together at the chassis star ground, which is also where the mains safety earth is connected. You might choose to connect the two power supply grounds to the chassis through 15ohm NTC thermistors instead of direct connecting.

Have fun!
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