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Old 22nd September 2006, 08:31 PM   #1
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Default Building a 5 channel GainClone

Hey Everyone,

Well, my Sony ES-333 died, and it was time for me to buy a new amp! I had built a gainclone in the past that was Bi-amped that I use in my office, and I like it so much, I decided to build one for my surround speakers.

The amp I am thinking about building would be 5 lm3886's and I am likely going to use some chipamp boards when they become available again.

The 5 channels do not include the mains, I am going to bite the bullet and upgrade to 7.1, so this amp should never have much current draw, except maybe for the center channel.

I have a few questions about the power supply, and what I should do. I have two exact 200va 48vct transformers that I was thinking about running in parallel to up the amperage. The main reason I wanted to take this approach is so I have full current available to all chips at all times, also, it seems like it would be a little cheaper for the rectifiers/caps. (I'm doing this as cheaply as I can get by with.)

So my question is, how much do I need for the filter caps(assuming 400va and 5 lm3886's), and is this setup advisable? I found a previous post on parallel transformers, but with 2 amps instead of 5, so I thought I would repose the question. Also, I am not using torridals, so do I need to shield them massively from each other?

One other quick question. I use a Pioneer SX-1280 for my main amp which will pump a lot of power. I am worried the lm3886's may not keep up (especially on the center channel) without me dropping the gain on the mains quite abit. As such, I thought about bridging two 3886's for the center, since I suspect it will be the one doing the most work. However, since I only have about 400 watts, I suspect the current pull will be a little high and I may not really need to add more power to the center anyway. My center is a homemade sealed 3-way sealed box, and is fairly ineffcient. I have never calculated the baffles in the crossover, so there are probably a few frequecies which will require MUCH more power. I know, fix the speaker! :-)

Long story short, would 400watts be enough for 4 surrounds at fairly low draw, and one bridge setup?


Thanks!
-Kevin
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Old 22nd September 2006, 09:36 PM   #2
owen is offline owen  United Kingdom
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It would be fine for a class T set-up, but in this configuration, I'd go for a 600 or 800 VA transformer, to provide plenty of current reserve for the centre - yes the peaks may *only* be 200w, but, the energy transfer of the chips is nowhere near the 90% or so a class T amp has.

That means heat. Lots of it, at high levels, but not unmanagable. It also means that for your 200 w output centre, you should allow 400 W (or VA, they're pretty much the same thing), just for that channel, or risk diminished dynamics etc that an undersized transformer would bring.

But, if you've found a deal on 400 VA transformers, and can get 2, then you'll be in hog heaven, with a complete overkill PSU!

This would be the route I'd take...

Owen
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Old 23rd September 2006, 01:05 AM   #3
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Thanks Owen,

It sounds like I will just try this out with no bridge and run 5 strainght 3886's. So how much smoothing should I do? 10,000-20,000mf?

I usually just put about 10,000mf in there or so (depending on what I've gutted laterly). Right now, I only have 2 6800mf caps that are even at the proper voltage rating, but I suspect with 5 3886's that may be not enough.

Any thoughts, and I will start soldering. :-)

Thanks!
-Kevin
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Old 23rd September 2006, 07:51 AM   #4
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Default hi kevin, calculus

hi,
is not too difficult to calculate.
each ch. will be able of 60watts@8omhs and about 100watts @4omhs. that gets you to peak current 100=i^2*4, i=5amp rms 5*1.4=7amp peak for max pwr ever from each ch.
7*5=35amp peak for 5 ch amp; 400va=i*v, 400/35=9.8v for max peak current.
so calculating this way you wouldn't be able to drive none the voltage will drop to low, but
in home cinema audio programs i think there is not such a sound track that will drive all ch to max pwr at one long time??
a nominal calculation is depending of time to drive the 5 ch.
if you are interested i can help you with the calculus, write to my email directly.
best regards
Williams
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Old 23rd September 2006, 11:31 PM   #5
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You want a movie that will drive your amps to peek? Watch "Twister"

That will do it.
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Old 27th September 2006, 05:24 PM   #6
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Thanks everyone. I found some 80v 47,000mf caps for $17.50 each on apexjr.com, and I think I will go that route. I will try out the xformers I have for now, and I can always upgrade if they don't work out. :-)

My budget was unexpectedly increased thanks to the Goodwill store! My favorite place to buy vintage audio. I found a Sony TA-N731 amp for $10 that I think I can get 75-80$ for on Ebay! ;-)

I know it's off subject, but I wanted to share some of my finds @ thirift stores over the last few years.

1. AR5 pair in AWESOME condition - $10 for the pair. I wired around the brass wirewound LPads, and they work like new. Someone had even repaired the surround on the woofer relatively recently. I will never sell these speakers as they simply have the best midrange I have ever heard. (Now in my bedroom with a 280watt Yamaha Natural Sound Amp. This is also where my laserdisc, CED, and beta player are hooked up!)

2. B&O Beovox M100 - $24 for the pair. Recent purchase. Not my favorite speakers, but they do sound really good. They have a wider sound stage than the AR-5's so they fill my living room better. They also have really good bass, so I quit using my homemade sub for now. But I honestly think the mids are a little lacking. One tweeter is gone, which is a problem. I had to resurround the mids, but the woofer (Seas I think) has a rubber surround.

3. Pioneer SX-1280 high-powered receiver, Teac AG-6300 10.5 in reel to reel, and a Highend Pioneer turntable whose model aludes me - $599.00 for the three. A bit higher than I would normally spend, but I really wanted the Teac reel to reel and I was planning on selling the receiver to recoup costs. Now, this amp is driving my mains, the B&O's, WELL over their clippling limits. (Fortunately, they have protection circuits which kick in somewhere between 75 and 100watts. ) Now I'm looking to sell the reel to reel to buy a new preamp. Anyone interested? :-)

I have found many other misc amps/speakers, etc which for the very least are good for parts and project boxes. My Gainclone will use a Nakamichi tape deck box, and an old Sansui receiver box more than likely.

Long story short, hit the thrift stores people! Unless you live in Texas, then I want you to stay out so I can make all of the finds. ;-) I visit a circuit of stores a few days a week on my lunch break and I am simply amazed at the stuff I find. I tend to buy quite a few records while I'm out as well! I suspect it won't be too many more years when you won't find records anywhere.

Ok, I've diverged from the topic quite enough, thanks again everyone for your help. I am ready to go electrocute myself! :-)

Thanks!
-Kevin
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Old 5th January 2007, 04:03 AM   #7
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is this amp completed? how did it turn out?
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