5.1 Home Theater Questons (Gainclone) - diyAudio
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Old 25th April 2007, 04:05 AM   #1
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Default 5.1 Home Theater Questons (Gainclone)

Hello,

I have been looking into making a 5 channel gainclone for home theater use. I have a few questions before i get too far into this project.

First i was hoping to design my own PCB for this setup and i was wondering what the approximate cost would be to have a company make a board for me if i made the layout. This leads me to my next question....

Is there any software available on the web for circuit board layout? I used a program years ago called something....trace. It was very user friendly. I am looking for a board similar in quality to BrianGT's boards which i own and love!

Next i was wondering about the power supply. Would it be best to use multiple transformers and rectifiers or would one transformer be sufficient? If it is ok to use a single transformer what size would i need and how would i then feed all of the channels.

I was also wondering how an amplifier such as this would compare to a decent store bought receiver with built in amplifier. I am wondering what the cost benefit ratio would be between something like a Denon vs. a Gainclone.

My last question is what kind of output can i expect from some of the commonly available chips. I was looking at the 3886's as this is what my current gainclone is but i would be open to any suggestions.

I know this is a lot to go over and i have just started my research into the topic. I really appreciate any advice anyone can give.

For reference, the only speakers i currently own are a pair of Shielded Dayton BR-1's which will eventually be used for surround speakers. Everything else will be constructed later.

Do you think that this would be sufficient for a decent Home theater setup?

Also, are there any other types of amplifiers i should consider?


Thanks!

Ryan Smith
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Old 25th April 2007, 02:33 PM   #2
kscharf is offline kscharf  United States
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generalguitargadgets.com has a nice pc layout you can use for the 3886/3875 chips. The circuit is fairly typical for this and except for parts values the circuits do not vary much. You can add an rc zobel network to the output if you want (104 cap and 10 ohm 1w in series to ground)

The 3886T is rated at 68W output to 4 ohms with a +/- 28v supply.
It is also rated at 38W output to 8 ohms with a +/- 28v supply. If you want to be able to use either 4 or 8 ohm speakers (or anything inbetween) the +/- 28v level is the best. However the chip will handle up to +/- 42v and is rated at 50w output to 8 ohms at +/- 35 volts. If you will use ONLY 8 ohm speakers an want the extra power use the +/- 35 volt supply. For the 28 volt level a transformer with 21-23 vac is called for, for the 35 volt level use a 25-28 volt transformer. The actual transfomer voltage will vary since transformers are rated at a specific line voltage and current output so your mileage (and voltage) will vary!
I'm getting 37 volts with no load from a transformer that gives a measured 56 volts AC center tapped (with 47000uf and a 25A rectifier).

I am building a three channel amp and will post my results when it is finished. I can tell you that Niles Audio sells a few receivers and an amplifier using the '3886 and they sound very good. We only rate them at 30w (8 ohm) and 38w(4 ohm) because that is with ALL channels (12 of them) running at less than .1% THD. They will put out more power, but NOT with all channels at once at .1% THD. With only a single channel driven, the output at .1% THD is what you would expect from the National spec's. (IOW, just how heavy a transformer do you put in the ps to handle the rare case of all 12 channels driven full bore at the same time?)
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Old 25th April 2007, 05:20 PM   #3
Fenris is offline Fenris  United States
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I've built a 5 channel amp for a friend and am building one for myself as well. Actually, it's a 15 channel amp with active crossovers with a single LM3886 for each driver. My friend's was built using P2P and cheap components (probably less than $200 overall in parts!) and uses the Dayton 6.5" and silkie drivers. The transformers (7 of them) were all buyout EI cores rated at about 100w each, and put in parallel. There's a little hum, but it's the cleanest and loudest setup I've ever heard.

I'm using 15 Brian GT amp boards, 5 of the power supply boards (with 50% more capacitance) and my own crossover PCB with Dayton RS180 woofers and RS25 tweeters. I've got an Avel Linberg 625VA (approx 35v DC, 20A) torroid that runs the amps. Since the drivers are all 8 ohms, I'm not too worried about the voltage being a bit high. According to most general practices, you should have twice the VA rating as maximum planned output watts, but in a HT, you're not driving all the channels continuously, so I'm not worried. Plus you generally have a subwoofer, so that takes away some of the power requirement of the system. If you do the SPL calculations, you're well into the hearing loss danger zone before you run out of power.

If you're just going to run 5 straightforward channels, I'd say you would be fine with a 625VA. If you need more power, I'd look at a bridge/parallel arrangement with a separate 200VA transformer for each channel (monoblocks). There are tons of PCB designs out there, but it might be cheapest to use Brian GTs (THEY'RE ONLY $4 RIGHT NOW!). The cost of low-volume custom PCBs is pretty high. If you want to design your own, the standard is Eagle. You can get the free version, but it limits the size of your PCB to about 4" x 6". Another company, ExpressPCB, has it's own software, but it's propritary to them (you can't use it with any other company). Personally, I make my own PCBs at home - it's faster and cheaper, but not as pretty.
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Old 25th April 2007, 07:59 PM   #4
kscharf is offline kscharf  United States
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Actually many commerical amps have transformers rated at 150-180% of the full power output rating. (so a 100w amp would have 150-180va transformers).
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Old 25th April 2007, 09:26 PM   #5
Fenris is offline Fenris  United States
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And many more advertise "7x100 watts" and have a less than 500VA transformer. Each channel may be capable of 100 watts, but not all can be driven at the same time. They just figure that surround sound program material won't drive all channels continuously at once, which is a reasonable assumption.
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Old 25th April 2007, 09:40 PM   #6
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Great info! This could end up being a decent recourse for future noobs like me.

I was wondering....when using multiple chips is there a way to run them in parallel or bridge them in such a way that I could use multiple chips per channel? My thoughts where that I could run two chips for each main speaker and the center but I would only use one chip for each surround speaker. I saw the deal for Brianís boards and I think I am just going to pick up 12 or so for this project and possible future use. It would be really cool if there was a way to hook a couple together to make a single channel.

As for the power supply...how many of his boards do you think I will be needing...I have lots more research to do but I would like to order the boards before the end of the month while they are still on sale!

Thanks again,

Ryan Smith
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Old 25th April 2007, 11:27 PM   #7
Fenris is offline Fenris  United States
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What you are talking about is well documented by National here: http://www.national.com/an/AN/AN-1192.pdf

Look on the forum for BPA 200 or 300, we've been discussing a "challenged" attempt for the past month. Not the fault of the theory, just a bad implementation.
An extreme example here: http://www.shine7.com/audio/bpa300.htm

It's actually pretty easy to bridge two amp boards to give about 100 watts, but only with 8 ohm speakers. You can use the Brian GT boards, but will need an inverter (fairly simple). For four ohm speakers, you need to bridge/parallel four amp modules and ensure tight tolerances on the resistors that set the amp gain. I've done it for a friend's bass guitar amp and it turned out quite well.
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Old 26th April 2007, 11:34 AM   #8
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I'm running Klipsch Heresy speakers in my theater along with SAE 2200 amplifiers. The Klipsch I believe are 96-97 db efficient and the SAE's put out 100 watts per channel. The room is 12 X 18 roughly and I run out of steam quickly with only 100 watts. It doesn't take much with an action movie to run out of power.

Are you sure you will be happy with the 68 watt capability of the gain clone?
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Old 26th April 2007, 02:31 PM   #9
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After reading the PDF that was posted above I have changed my plans. I am thinking of running a bridged/parallel setup for the left, center, and right speakers which should provide around 200 watts. Then for the rear speakers I will run a bridged or parallel setup which should provide around 100w. This would require 16 boards but I think it is a better fit for my needs.


Am i on the right track here?

I am thinking of running 2 approx. 600VA transformers to feed the left, center and front and 1 approx. 400VA transformer to feed the surround speakers.

I am still wondering how the sound quality of something like this would compare to a regular store bought 5.1 receiver.

Thanks!

Ryan Smith
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Old 26th April 2007, 03:36 PM   #10
Fenris is offline Fenris  United States
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Personally, I would make each of the center, right, and left its own monoblock. It will simplify construction, reduce chances of ground loops, and be more flexible for the future. A 325-400VA transformer would be fine for each. The surrounds could be monoblock or a stereo pair with another of the same transformer.

Sound quality will be a step up from a store bought 5.1, but the most influential thing will be the speakers you choose.
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