The Audio Toaster

chicks

Lunatic Member
So, I've been thinking about a Class-D power amp build ever since I picked up the Luxman preamp. The Luxman is a showpiece, so I want the power amp to not be a distraction - completely hidden from view, if possible.

Been researching the available Class-D modules and the toroidal trannies to power them. Most of them are pretty compact; just need to find a box to put everything in.

Then I remembered that I had picked up this Netgear Storage Central SC-101 at the GW a couple of years back, thinking it would make a cool NAS. Well, it won't, it's highly proprietary, and only supports Windows clients - my clients are Linux. So, it's been gathering dust.

Opened it and ripped out the guts to find that it's basically a nice aluminum heat sink. Perfect for a power amp project. :banana: Internal dimensions are about 3-1/2" X 4" X 5-1/2".

Looking for ideas on which power module / transformer to use, keeping in mind that a heat sink will not be required in this box. Will be driving a pair of AR91's to moderate levels, so no need for massive amounts of power.

Any suggestions appreciated.

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I thought this would be related to the old Newtek Video Toaster for the Amiga computer systems...

But this is way cooler! Rock on!
 
Cool little enclosure! Kits from 41 HZ seem to be well regarded. Let me know if I can be of help with end panels for the toaster.
 
Narrowed down the amp board to the TA2022 from hifispot168 on feebay - $49.99 shipped. It's fully assembled, has the power supply components on-board, and a speaker protection relay. Tripath chip is mounted on the board's edge, making it simple to heat sink to the chassis. Dimensions are perfect for the toaster.

Now for the transformer(s). Looking at ApexJr, will have to call him tomorrow. Maybe two of his 80VA 42VCT's in parallel will work.

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I bought one of those poxy Netgear "NAS" things back in the day, found out about its limitations and returned it for a refund right away. It looked cool, pity about the rest of it. Now I'm thinking that maybe I should have kept it. This would be a great way to reuse it, albeit an expensive one. :D

Maybe one will show up around these parts at a reasonable price. I really like the idea behind this project and will be watching with great interest. As neat as that little Netgear enclosure looks, I'd almost hate to hide it. Maybe put it where someone can just see it and wonder what it is when they're done looking at the Luxman?

Distortion specs on that T-amp look suspiciously low. Usually the THD figure just skyrockets (10% or even more) when you reach the rated output power.
 
Ordered two of the 80VA, 44VCT toroidal trannies from Apex Jr. $10 ea. + shipping + CA tax. Pretty good deal.

I was dry-fitting a couple of 3" dia. particle board circles in the cabinet last night, not 100% certain that there will be room for both. However, after reading this article, I'm thinking just one will be adequate.

Per the article, speakers with 87db sensitivity (which just happens to be the spec for my AR91's) can be driven to a pretty good level with just 32 watts. I'm probably missing something, but an 80VA transformer powering an amp that's about 90% efficient, ought to produce the 32WPC, given enough reserve in the filter caps.

Right?

Edit: Calculator here says, for the Jazz & Trad Pop that I listen to, in-room, I'll need just 16WPC. I think I'm good - unless I ever switch to Rock or Metal; doubtful. ;)
 
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Chick:

For future reference, you can do these calculations yourself, its not hard. You just need a calculator or spreadsheet that takes the log of a number. Windows calculator was a scientific option that does it, but I'm not not knowledgeable enough about Linux to know if it comes with the distribution or if you need to go chase one.

Anyway, since the standard spec is at 1 watt the formula is really quite easy (note this works ONLY when you have the 1 watt sensitivity. If they pull the 2.83V at 6 or 4 ohms, you need to figure out the output at 1 watt and the power output at that impedance.)

What to do:

simply take the log of the power output, multiply by 10 and add it to the speaker sensitivity. This gives you the output level at clipping.

Formally it comes from the formula:

dB = 10 * log (Pout / Pin)

Pout is the Power output.

Pin is the Power input, in this case 1 watt.

Remember that this is the peak output, average is less then that, good rule of thumb is 10 dB. You may want to leave a bit of a safety margin as well, I like to use 3 dB.

So for 32 watts, you can get 15 dBw of amplification out of it.
10 * log (32) = 10 * 1.5 = 15​
87dB sensitivity + 15 dB amplification = 102 dB at clipping​
Subtract 3 dB headroom, 10 dB to get average level and you have 89 dB average level​

I hope this helps.

If you knew this stuff already, please excuse the post, I mean no disrespect.


Have fun with the build. And I do agree that your Luxman is quite the showpiece.

Shelly_D
 
Shelly_D, thanks for the math. Did more research on the transformer sizing, and found this on Elliott Sound Products site:
Transformer
VA Rating - Class-AB
The absolute minimum VA rating suggested is equal to the amplifier power. A 50W amp therefore needs a 50VA transformer, or 100VA for stereo 50W amps. Larger transformers (up to double the amp power rating) will provide a "stiffer" power supply, and this may be beneficial.

From WikiPedia:
Modern Class AB amplifiers commonly have peak efficiencies between 30–55% in audio systems

So, with an 80%-90% Class D amp, an 80VA transformer should be good for at least 40Wpc.

Another good SPL Calculator suggests that I'll have more than enough SPL for even the occasional Classical LP. ;)
 
Shelly, you "forgot" to subtract around 3dB for dispersion losses at listening position not made up by boundary reinforcement and the 2nd speaker ;). Chicks, I do like the looks of that. Please tell me you're going to cap the ends with wood?
 
Chicks, I do like the looks of that. Please tell me you're going to cap the ends with wood?

I had thought about doing that, I do have a couple of small bamboo "bar boards" in the garage. The existing caps are pretty nice, too, brushed aluminum surrounded by the same white plastic as the case, so we'll see what works. Again, I'm planning to hide the thing, so looks really aren't important.
 
Did more research on the transformer sizing for Class-D amps. Looks like the $10 80VA Apex Jr. transformer is sufficient for 40Wpc, so no need in this application for the second transformer.

From ZapPulse:
In a ZapPulse based amplifier almost no such loss should be encountered, and so you are not required to oversize the power supply, like would be normal when using a traditional class A/B amplifier. In fact you can mostly get away with under-sizing the power supply, because the music does not have 100% thermal power content. Measurements of practical music examples show that most types of music contain an average 30% thermal power measured over 30 minutes. In other words you can make a 300 Watts RMS amplifier with just a 100 VA mains transformer, if the amplifier is based on ZapPulse. This is a limit, and even if it will work in practical installations, we do not recommend this dimensioning. In practical amplifiers we recommend a 1:1 ratio of transformer size to output power. So a 2x200 W amplifier should use a transformer of 400 VA or more.
 
The last Class-D amps I owned used 400VA toroidals for mono 60-100W (depending on how generous you want to be with the rating) chips. Waste of money on the part of the mfg? Perhaps.
 
Perhaps, or perhaps they had a ton of them for other uses, or perhaps they just believe that a big-ass PS is the way to go. Such a thing seems to impress people regardless if it's necessary or not.
 
The last Class-D amps I owned used 400VA toroidals for mono 60-100W (depending on how generous you want to be with the rating) chips. Waste of money on the part of the mfg? Perhaps.

Based on the ZapPlus quote, overkill for sure. There could be any number of business reasons for overbuilding like that, including ignorance on the part of the design team (assuming that the ZapPlus quote is correct and that it applies to you amp as well).

Shelly_D
 
Another quote, from class D audio indicates that their supplied 400VA transformer is capable of powering two 500W modules:

Package deal includes our most popular 250W X 2, or 500W X 1 Class D Power Amplifier, Large Power Supply, 400VA AnTek Toroidal Transformer, and transformer mounting kit. This is a great combination of components for a complete audiophile quality power amplifier. The power supply included in this kit is capable or running two amplifier modules. I'm always trying to get good deals on the toroid transformers and pass it on to our customers... they can get expensive!
 
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