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Old 11th September 2012, 02:23 PM   #11
shanx is offline shanx  Canada
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That is the issue, here. Maybe different countries would have agencies that accept the open frame metal chassis like that. But that would be further subjected to extensive agency tests and there are plenty of companies willing to design commercial stuff that meets minimum electrical safety requirements (barely). Maybe adding $1 worth of sheet metal to enclose it properly is too much out of the profit margin. But if you are designing a DIY build, better to make sure you surpass the minimum safety standards by a wide margin..IMHO
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Old 11th September 2012, 05:48 PM   #12
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Except for tubes, as a rule of thumb, no electrical component should get too hot to touch. So I think you'll generally be far below the kindling point of the wood. For tubes, based on 50 years of observation, I'd a say a couple of inches will suffice (assuming adequate airflow), and to be really safe you could add a small metal foil reflector on the wood over the tubes.
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Old 11th September 2012, 06:07 PM   #13
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Quick point here. The staring point for paper to burn is 454 degrees Fahrenheit, just like the name of that movie.... Not much chance on a properly operating piece of electrical equipment, until the unit goes up in smoke, that should be long before the cabinet goes up in flames.
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Old 11th September 2012, 08:39 PM   #14
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Good point, Kindhornman, But when does it start smoking? I will have to look it up.

After all the input, I think the way to go is to build the electronics into a fully enclosed metal box that does not get too hot to touch. That is easy to test beforehand and wrapping it in wood should then be OK.

Here are two more drawings I have found. This one is from screen 17 of a Google search for "guitar amplifier service manual filetype:PDF" BTW. These things are not easy to come by.

It is a 60W amp with sophisticated digital processing, therefore it has a lot of circuit boards. There is a heatsink, but once again the top is left open.

Now to figure out how to build such a metal enclosure without a sheet metal bender. This might work:

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Making a simple sheet metal Bender - By Nev Sweeney
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Last edited by ingenieus; 11th September 2012 at 08:44 PM.
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Old 12th September 2012, 08:43 AM   #15
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Quote:
Originally Posted by ingenieus View Post
After all the input, I think the way to go is to build the electronics into a fully enclosed metal box that does not get too hot to touch. That is easy to test beforehand and wrapping it in wood should then be OK.
op is left open.
You're worrying far too much about absolutely nothing - there's no problems with wooden cabinets at all.
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Old 13th September 2012, 04:35 AM   #16
Struth is offline Struth  Canada
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Hi Guys

The CSA match test is clearly written and requires holding a flame to the material for a set amount of time. Particle board does not even warm up in the allotted time. Neither does most tolex or rubber coatings for amps. However, this did not stop the local CSA inspectorfrom stating "That test does not apply to wood, only to plastics" even though the test is written with no qualification as to its applicability.

The amp's electronics must be housed in a metal chassis for electrostatic shielding if nothing else. A metal box provides heat shielding and a path for fault currents to the safety ground. The chassis should NOT be used for random grounding of the audio circuit.

CSA tries to make it sound as if it is illegal to sell product without their label, but this is not the case. If you want third-party approval, the local hydro inspector can do this for a lot less money. A lot of equipment arrives here with just a Los Angeles County approval sticker. Electricity is the same everywhere so one agency approval is really as good as another.

Have fun
Kevin O'Connor
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Old 13th September 2012, 12:49 PM   #17
shanx is offline shanx  Canada
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It is nice to see Mr. O'Connor on DIY Audio, thank you for your clear and concise explanations.I deal quite regularly with CSA special inspection service, and found the interpretations of electrical safety code could sometimes vary from person to person. I do believe they do a good job of it, generally thorough in looking at basic safety. As you say there are other agency's that are equally good and competitive price wise. For example Intertek, UL can do inspections and their service is legally equal as far as acceptance by government bodies.
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Old 13th September 2012, 01:48 PM   #18
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Thanks to all the input received, I have now come to the conclusion that the wood is in very little danger of burning. The semiconductors will melt down long before that happens. Another variation of the old story of the fuse in the speaker line being protected by the output transistors.

If the metal enclosure and heatsink is sufficient the keep the electronics at a safe temperature, the wooden cabinet will therefore be in no danger. Doing that brings its own challenges, though. I was thinking of doing a 150W amp but I have now scaled that down to 70W. The 150W project can wait for next time.
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Old 13th September 2012, 02:51 PM   #19
Struth is offline Struth  Canada
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Hi Guys

I'm glad your dealings with CSA have been smooth, Shanx. My experience left me feeling that they are basically making whatever cash-grab they can interpreting "guidelines" in whatever way invokes further expense. In the 1990s their web site was fairly clear about what they do; today you cannot tell what business they are in.

The altruism of a self-regulating industry usually gets overtaken by a body that realizes money can be made. Fortunately that invokes competition by other parties. The fact is that it is insurance companies - again with their hand out - who want to reduce their own liability. Your insurance company wants to get out of paying claims, so if they can cast the blame onto another party they will. This is predominantly a USA problem where the country is run by litigators and corporations and everyone thinks that sueing someone is the answer to everything.

As far as 70W vs 150W for a solid-state output, the cost for both is about the same. What will matter is the entire package and how most players will use it. Where you might intend for the PA to be operated cleanly by fitting the preamp with a master volume, players will use it how they wish. What you might deem to be abuse is normal in the guitar world. Fortunately, robustness and functionality are the main criteria for solid-state PA design for pro use of any kind. The extra watts may require a heavier speaker, which is a nonissue if you build a head or rackmount rather than a combo amp.

Have fun
Kevin O'Connor
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Old 13th September 2012, 03:20 PM   #20
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Struth,
I think that the CSA situation is a bit more complex than how it seems to be presented. It seems that if you are to produce a product these days you will have to deal with more than one standards organization. If I get a UL label on a product here in the USA that is fine for here but if I plan on selling that product in Europe I don't think that label alone would fly. I am going to need a CSA or TUV or some other stamp of approval also for that market and then the Japanese or Asian markets will demand something else. Just meeting the connector standards in Europe are going to be very different than in the U.S. It would be nice if we had a worldwide standards body that we could all use and everybody could agree to, but I don't see that happening any time soon.

Steven
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