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Old 10th July 2004, 05:11 PM   #1
BAM is offline BAM
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Default I want to build a GC, where to start?

Hi. After much thought about Gainclone chip amps, I came into the possession of an all-aluminum chassis for an old ethernet switch. It is about 1.5" tall, 8" deep, and about 14-18" wide. It is a very open design, and it is all aluminum. It has lots of large (2mm) drilled holes at each end of the case and I think I can stand this up vertically and let heat rise through the case. I know of a thermal control circuit that I will use to power a pair of DC blowers at the top end of the case which will kick into high speed if the amp heats up too much. I want to build a very simple design, with only three channels. One channel should be able to be powered on and off separately from the other two and used for a little subwoofer if I want to later on. I'd like to have 50W RMS into 4 ohms from two channels and then 100w RMS into 4 ohms from the third channel, so if I hook up 8 ohm speakers it will be 25x25x50 instead of 50x50x100. This case has a power cable connector and a power switch on the back, which will be great for my application. I have some specific questions as well:

1. Am I totally crazy? I have soldering experience from soldering together crossovers. I have thermal management experience from my exploits in trying to build one of the most silent PC computers that I have ever heard (or not heard). I do well solving engineering-type problems.

2. I expect I will need three big transformers and one little one: three toroids for the amp channels and one to supply 12v DC to the thermal control circuit for the fans. Will I be able to fit a toroidal transformer in the 1.5" inside height of this casing?

3. What is the chance that I could pay someone to make me PCBs for the complicated stuff?

4. How much of this stuff do you think I could get from Parts Express as far as circuitry components goes?
http://www.partsexpress.com

5. I know I will need some major heatsinking for this much real power. The only problem is, I have no idea where to get the heatsinks from.

6. Is there a web site that can guide me through the build process?

While I've been doing speakers for a long time now, I haven't ventured into amplifiers at all. I think this should make a good starting point for me.

Addendum:
7. Inverted or non-inverted? Is one simpler than the other?

8. How many watts of power should I expect to blow away as heat?
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Old 10th July 2004, 07:09 PM   #2
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Addendum: How much should I expect to spend? I probably won't do this if I'm going to lay out more than $150. I read BrianGT's nice manual, how can I get the kit?
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Old 10th July 2004, 09:46 PM   #3
Nuuk is offline Nuuk  United Kingdom
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Quote:
How much should I expect to spend? I probably won't do this if I'm going to lay out more than $150.
That's probably why nobody else has replied!

Get real. If this is your first build, make a simple two channel amp first. Get that working OK and then, having learned the ropes, you can try something more ambitious.

This will help you to get started but as I am based in the UK, I cannot really help you with where to buy stuff.

That case sounds a little small for all three channels and the heatsinks that you would need for those output specs. And you certainly won't get your torroid transformers in there (or any other type).

One safety tip I will give you is not to run before you have learned to walk!
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Old 10th July 2004, 10:04 PM   #4
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What sort of moneys are we looking at for this? I think a two-channel Gainclone like Brian's Kit will work well enough for me. How many watts will it put out though? The case is actually 2" deep and is 16"x8" on the bottom. It is actually aluminum, so the whole thing could theoretically be a heatsink. I'd need a dielectric thermal compound. The stuff I use on my computer is too conductive. According to my Parts Express catalog, my enclosure can fit an Avel Lindberg Toroidal Transformer with 160VA load rating without any problem. Even two of them if necessary. They are 1.7" high, so I could even put the top on without creating a shorted turn. There are two versions offered. One has 18V+18V secondary outputs, and another has 22V+22V secondary outputs. What sort of output could I get from that transformer combined with the BrianGT kit, and would it be better than the home theater reciever I am using now? Also, there will be active (fan-forced) cooling in the case, so that means that I can dissipate a good deal of heat. The active cooling, as I said before, would be provided by a DC fan of some sort that is controlled by a thermal probe. For fan power, I could just cannibalize an old 12-volt adaptor.

Avel Lingberg 160VA 22V+22V transformer -> $38.79
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Old 10th July 2004, 10:53 PM   #5
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One last thing:

I've been reading some other people's project Gainclone pages and I see no mention of output power. Now while I don't think I still hold the belief that more watts leads to better quality sound, I still think that one needs at least 1 watts per channel for good listening. Suppose I use the LM3875 chip and the 22V+22V 160VA transformer. How do I know how many watts my finished amplifier will be able to put out?
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Old 10th July 2004, 11:43 PM   #6
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Okay...I've been an idiot and not read enough material before I began posting. The Gainclone is like 25W RMS. Good for some smaller speakers. The transformer I was looking at will be enough. The chassis will be the right size, and I can get the insulated version of the amplifier chip and just bend an aluminum piece to interface with the case. I can do both channels from a single transformer without spending a fortune or buying a huge transformer. It is extremely important to get the right component values, and use brand name components. And so on. I'll post back tomorrow or some time later when I've absorbed enough info.

Final question: How much did you spend on your Gainclone?
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Old 11th July 2004, 12:06 AM   #7
cjd is offline cjd  United States
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You have more reading to do. Especially regarding power output, but I suspect there are other areas you'll discover much to learn. (This has been one way I pass the time this last month while sitting at home looking for work, so I've come a long way on the knowledge and was where you were when I started - so I just say this as a reflection of my own experience - there are many things I've learned that are far from obvious). The LM3875 is "capable of delivering 56W of continuous average power to an 8ohm load with 0.1% THD+N from 20Hz to 20kHz." (quoting the spec sheet) Though I have no idea why you're even worried about that as a number. FWIW, I have a LM3875 powering a pair of older 3-ways with a 12" woofer and they get FAR louder than I need them to.

160va 22v would probably work well enough on a pair of 3875's if you went with a regulated power supply - or, one per channel otherwise (though it may be a bit overkill on the VA in this case, it wouldn't hurt) 22v seems to be an ideal balance (unregulated) for a range of speakers - if you KNOW you'll be running 4ohm speakers, the 18v may be the better option.

You probably would want to go with a parallel setup for your sub amp dreams, though I'll also assume you'll want to pack in an active crossover here as well to split the low frequency signal out.

Having picked up some of BrianGT's basic kits ($35usd + shipping) to start with, the transformer (330va 25v, $48), rca jacks and binding posts, plus enclosure and volume control potentiometer, I'm looking at something in the ballpark of $120 for two channels. And this is basic components to get my feet wet.

I'll second Nuuk's motion that you start with a basic kit (grab a bigger torroid and save your box for later) and grow from there. It's addicting enough fun anyhow.

C
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Old 11th July 2004, 12:18 AM   #8
ble0t is offline ble0t  United States
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All the info I think you are looking for is in:

1) the LM3875 datasheet (link)

2) BrianGT's build guide (link)

Enjoy
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Old 11th July 2004, 03:26 AM   #9
BrianGT is offline BrianGT  United States
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Quote:
Originally posted by BAM
Addendum: How much should I expect to spend? I probably won't do this if I'm going to lay out more than $150. I read BrianGT's nice manual, how can I get the kit?
I am still selling kits:
http://www.briangt.com/order

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Brian
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Old 11th July 2004, 04:32 PM   #10
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I want to use this amp to run whatever speakers I happen to build, but it will be in the range between 4-8 ohms for all speakers. I don't understand why you'd want a lower voltage secondary winding if the amp chip is going to be running at 4 ohms if I can provide proper heatsinking. Are you able to draw more current if the voltage on the secondaries is lower, or are we worried about frying the chip? This case is all-aluminum, and I can even stick an old PC heatsink on the inside to get air naturally rising through the case. Do the rectifier diodes get hot? I may try to heatsink those as well into a piece of steel coming off each one individually and not touching any case metal. (Another crazy idea I just had was to use molex 4-pin connectors like you'd find in a PC's case to carry the power for the two channels so I can disconnect stuff easily and safely. Like the transformer's secondary windings and the power from the rectifiers to the amplifiers.

Like I said, I have more reading to do. I'm glad to hear that the chip puts out 56W RMS into 8 ohms. I wonder if I can manage to get twice that at 4 ohms if I double up on the transformers to feed the power supplies in Brian's GC Kit. So I'd better go read.
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