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-   -   More newbie questions (http://www.diyaudio.com/forums/solid-state/29-more-newbie-questions.html)

Eric 29th November 2000 07:21 PM

Hi Everyone,

Just found this board the other day and have read everything with great interest. I am relatively new to the DIY area, so far the most complex thing I've designed and built is my subwoofer (based on a MASS 2012 driver).

I've seen the amp design by Pass Labs, Leech, ESP, etc. and am wondering just what I might be getting myself into. So I have a few questions:

1) How much money (on average) are we talking about to assemble something like the Pass a40 or Leech amps? Can someone ball park this for me $100-$200?, $400-$500? more?
It seems that the most expensive parts are the chasis, transformer, and heat sinks... Is this correct?

2) How much experience do I need to be able to successfully build one of these projects? I am pretty handy with my soldering iron and volt/ohm meter. Do I need any other type of test eqipment?

3) Are these projects that can be assembled in a weekend, or are we talking much longer time frames here (assuming all parts are already on hand)?

Thank for your feedback, I do appreciate it!
Eric

Jon T. 29th November 2000 10:28 PM

1) Figure $300-500 depending on which design, how much power, etc. A 60w amp will cost more than a 30w one, because of the bigger power supply. A Class A amp will cost more than a class AB one because of the larger heatsinks and larger numbers of output transistors. Heatsinks, chassis, transformers, and power supply capacitors will probably be over 80% of the total cost.

2) You probably want a function generator (or at least a CD of test tones). A scope would be handy, too, and can be picked up for about $100-200 at ham fests or $400-500 new. You don't need more than a 20MHz scope for most audio work.

3) Depends on how much concern you have for the final result. If you just want to slap together something that makes a sound, you can probably do it in a couple of days, especially if you use premade boards. If you want to do a quality job and make something that looks and sounds excellent, it will take some time.

-Jon

Eric 30th November 2000 02:36 PM

Thanks, Jon! Just looking to gather some basic information and see if this is something I can reasonably accomplish. I am really interested by the Pass a40 design. I figure I'll spend most of my time sourcing the parts for it.

I do have a test tone CD, but don't have a scope - nor do I know how to use one. Guess it's time to start learning!

I'd really like to take my time on this one and build it right. Sound quality is most important to me, but I also want the final product to be something visually appealing, too.

Guess I was hoping for a little less cost, but if I take my time and purchase parts little by little (more like 'as I find them') it won't be any big deal...

Jon T. 30th November 2000 08:25 PM

If you buy stuff surplus, you can usually get it a lot cheaper, especially heatsinks, powersupply capacitors, and transformers.

$500 might sound like a lot, but a design like the Pass A40 is probably in the same league as commercial amps costing 3 to 4 times that.

-Jon

Eric 30th November 2000 08:39 PM

Jon: Where can I find "surplus" sources for these types of parts? I have come across web pages for manufacturers of chasis, transformers, heat sink, etc. - I assume you are not referring to these?

Thanks, Eric

Jon T. 1st December 2000 02:33 AM

Here are a couple of links to surplus stores online:

http://www.halted.com/
http://www.mpja.com/
http://www.surplussales.com/SSIndex.html
http://www.meci.com/index.html
http://www.allcorp.com/

The only one I've dealt with is the first one, which is actually here in my neck of the woods (SF Bay Area), so I'll usually just go down there and poke around.

Most of the places have stock that they don't list on their webpages (i.e. Halted often has small quantities of great heatsinks), so you might try calling them, too.

Also, most reasonably sized metro areas will have a couple of electronic surplus dealers, more if your in an area with a lot of manufactures, military bases, etc. Try looking in your local yellow pages.

-Jon

Eric 6th December 2000 06:50 PM

Jon: Thanks for the links! I'm still tracking down these parts for the Pass a40 design. Seems like I may have to start making phone calls rather than surfing the web for the toroid & heatsinks.

Eric

Jon T. 7th December 2000 03:54 AM

Toroid of Maryland has all kinds of toroidal transformers available. I think Plitron may sell some direct, as well:

http://www.toroid.com
http://www.plitron.com

Heatsinks, you're probably better off getting surplus. Newark electronics sells a variety of the Wakefield heatsinks, including the popular 423, which is .67 C/W, for about $20. Unfortunately, it's double sided, which means you can only really use it with TO3 cased devices.

A site that someone posted in another thread here, http://www.sealelectronics.com , is offering two varieties of flat-back heatsinks, one at .80 and the other at .35 C/W. The .35 C/W one is only $35 which is a steal for something that size. Never dealt with them, though, so I don't know.

Finally, if you're good with metalwork, this guy has some fantastic looking DIY chassis/heatsink designs:

http://www.home.earthlink.net/~lotusblossom/index.html

A little out of my league, though.

-Jon

Eric 7th December 2000 04:59 PM

Heat sink calculations
 
Jon: Thanks for the toroid links, I think I've found what I need in that dept! I have a question for you about calculating heat sink values, though. Can you tell me if I have calculated this properly?

For the Pass Labs a40 design (100 wpc draw, 40 wpc output to speaker) we have a 40% efficiency. This means at idle, I need to dissipate 100wpc heat and at full load, I need to dissipate 60wpc heat. Assuming we have 25c ambient temp and wish to limit thermal rise to another 25c, we have the following formulas:

Idle: c/w = 25c/100w = .25c/w maximum dissipation
Full output: c/w = 25c/60w = .42c/w minimum dissipation

So, I should look for a heat sink with dissipation values somewhere between .25c/w and .42c/w - the closer to .25c/w the better, right?

Have I done this correctly?

Thanks!

Jon T. 7th December 2000 07:54 PM

For heatsinks, you always design for the worst case, so if the A40 is dissipating 100w at idle, that's what we need to design the heatsinks for.

If you want to keep the temperature rise to 25C, you'll need a junction/ambient resistance of 25/100 = 0.25. But it is not just the heatsink that factors into this figure. You also have to worry about junction/case resistance and case/heatsink resistance. The junction/case resistance is on the order of .5C/W (less if you use TO-3 transistors) and the case/heatsink resistance is about .1C/W. Since the A40 uses 6 output devices, you divide these figures by six.

So you can find the necessary heatsink by solving:

0.08 + 0.02 + X = 0.25

Which means you'd need a .15C/W heatsink, which is very big indeed. You could probably do this easily in a monoblock configuration, but it'd be a mighty big amp in stereo. Now you know why the big class A Krells and Levinsons are 10 inches tall and weigh 200lbs...

In reality, you'd probably just have to accept a 40-50C junction temperature rise, which would require a total junction/ambient resistance of 0.4-0.5, or a heatsink/ambient resistance of 0.3-0.35. That's a big heatsink, but in the realm of the possible. You could use one of those big .35 C/W heatsinks from Seal electronics, or a 2-3 Wakefield 423Ks (.67C/W each) and be ok.

Using this approach is probably overdesigning a bit but it's better than frying expensive output transistors.

BTW, if you are going to buy the heatsinks surplus, the ESP site (www.sound.au.com) has a nifty little heatsink thermal resistance spreadsheet you can use to figure out how efficient your latest surplus find is.

-Jon


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