Contest: Linear Power Amp in a mint tin (class Aa, class AB, or class B)

[danielwritesbac]

To promote efficient linear (non-switching) amplifiers, here's "Linear power amp in a mint tin" contest.
Transistors, power amp chips, and tubes are welcome in the contest.

Here's suggested amplifier parameters:
At least 9W@10%THD (7.5W@1%THD) per channel to 8 ohms stereo amplifier
OR at least 20W@10%THD (16W@1%THD) to 8 ohms monobloc Parallel amplifier
Not hot running (not hot enough to harm capacitors)
Power amp that has speaker jacks, and input jack
Amplifier small signal is analog (doesn't convert signal to digital)
Not Class D

External power supplies, SMPS and Linear are okay
Extras for "integrated power amp" functions are okay
Small rubber feet added to the bottom of the mint tin is okay
Reasonably decorous vent holes are okay--ventilation is good
Reduced linearity at pitches below 44Hz is okay if done for headroom.
Linearity is not required for power output above 9W@10%THD per channel in the stereo amplifier.
Linearity is not required for power output above 20W@10%THD in the monophonic parallel amplifier.

Prizes are announced at the end of the contest. There are multiple judges, not selected until the end of the contest. A possible alternative judging method is by poll. The goal of the contest is a useful efficient linear (non-switching) amplifier that works well inside a mint tin and promotes efficient linear audio. Advanced linear classes, like AaH, AH, ABH, AaG, AG, ABG, etc, must do sine, square, triangle and other common tests at least as nicely as Class AB.

For judging, I suggest this:
1). It is built (there are no votes for non-existent entries)
2). It is a linear (non-switching) power amp that doesn't overheat
3). Creativity is encouraged

[johnr66]

I think I could reconfigure this BTL 45 watt amp for stereo using horizontal mount ICs and lower profile components. The board would mount up to a heatsink with impeller type fan. The board in the pic is only 1-3/4" x 2-1/4".

I'm not sure if I could get it below 3/4" thickness (or even an 1") and not have thermal issues.

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Some of the car stereo power amp ICs need as few as three film caps to function, although I like to add RF filtering on the input. This may be another option. Power supply would be simpler as well.

[keantoken]

Power is limited not only by space but by choice of outputs. Powerful outputs are too slow to be used at low bias. Something with FT>20 at 10mA, and Hfe linear to over 1A, in a TO-220 or TO-126 package. EF output would cause massive crossover distortion unless there was some sort of low-bias non-switching scheme. Another possibility is common-collector outputs.

[Bigun]

Interesting challenge - looks like fun !

I believe we'll have to make the tin the basis of the heatsink and use an external PSU.

Big space users are capacitors so using few as possible is good.

I don't know anything about chip amps and I assume the solution is expected to be a chip amp given where you posted this challenge. But discrete amps can be pretty small.

Go Class AB for best tradeoff of sound quality against power dissipation. TO-220 LAT FETs for outputs - eliminates need for drivers and bias generator which cuts further down on parts. Single device input, feeds single device VAS. Load the VAS with a CCS (not a bootstrap that needs a large cap but a single JEFT CCS perhaps).

[qusp]

do I read you right, you see using SMD as cheating? its against the rules to use SMD? can I use a fan? =)

[danielwritesbac]

Originally Posted by Bigun

Interesting challenge - looks like fun! I believe we'll have to make the tin the basis of the heatsink and use an external PSU. . . .
I don't know anything about chip amps and I assume the solution is expected to be a chip amp given where you posted this challenge. But discrete amps can be pretty small.

Thank you. Discrete amplifiers are welcome! Thanks for your input. Handmade amplifiers can be fine tuned for low heat. Tubes, Transistors, Chips and Hybrids are all welcome! A chip amp has the bias sealed inside and can be selected at time of purchase by either choosing a cool running chip or undervolting. And all of these options do exist.

So, I can't imagine any way to predict what or who will win.
But, here's some ideas. . .

The following seem good for useful and efficient:
Vent holes through the bottom and sides are okay, but not required. Drillings in the vertical portion of the lid are okay, but not required. Heat spreader bar is okay if inside the mint tin. Rubber feet and rubber-coated feet are okay and good for functional air intake vents in the bottom of the mint tin, and you're sure to notice that some input and output jacks have insulators that exceed by about 1/8"; so, some little feet on the bottom of the mint tin can help resolve that problem too. External power supply is okay. Headroom and current management accessory circuits are okay but not required. Integrated amplifier accessory parts, such as volume knob, tuner, etc. . . are okay but not required since the contest is for a linear power amp. Input jack(s), power jack, and speaker jacks seem to be required for function.

What is unlikely or less likely to win the contest:
Too hot for longevity
External fans/heatsink sticking out
Non-decorous vents in the top of the mint tin and/or extreme mutilation.
*The point and the fun challenge of the contest is efficient linear design that actually runs cool enough to work within the constraints of a mint tin.

Originally Posted by keantoken

Powerful outputs are too slow to be used at low bias.

Too much bias = swamping clumsy with bias = inefficient = too hot? Is that why this (attachment #2) is terrible? Apparently, some chips aren't suited for mint tins.

The chip shown in the attachment wasn't "plan a" anyway.

Since I can't vote for myself, I decided to try a few different chip options to learn what works and it wasn't this:

[danielwritesbac]

Originally Posted by qusp

do I read you right, you see using SMD as cheating? its against the rules to use SMD?

My apologies for the language error. That part was worded badly, but the idea of it was to block prefabs hastily shoved into mint tins. The #1 goal is promoting efficient linear design. We wouldn't want to hinder creativity and therefore SMD discrete parts are okay to use. I expect that any judges will be looking for successful efficiency as a primary concern. The size of the parts is not important if you can close the lid.

Originally Posted by qusp

Can I use a fan? =)

Of course. However, some of your competition is probably going to be efficient design that doesn't require a non-decorous fan sticking out. So, if you want to win, try making a cool running amplifier. Just remember that the target goal is an efficient linear power amp.

[keantoken]

Originally Posted by danielwritesbac

Too much bias = swamping clumsy with bias = inefficient = too hot? Is that why this (attachment #2) is terrible? Apparently, some chips aren't suited for mint tins.

That's basically it. Our outputs need to be Hfe linear to 2A to drive a 6R load to 9W. What outputs are sufficient? Where is a 2A transistor with Ft>20 at 10mA... Perhaps we could find something in the <80V Vcemax region...

If we knew the thermal resistance to ambient of the altoids tin we could calculate our minimum bias. I would stand the tin up on its side with vertical vents, this way convection is maximized, and more surface area is in contact with the air.

How about someone rig up a 1W CCS with a DN2540 and test the temperature of the altoids tin after 30 minutes? This would get us the thermal info we need. Does an altoids tin have enough heatsinking power to run 9W?

I think it needs at least a +-15V PSU (30V single rail). It looks like an external PSU is mandatory unless there is magic 10W joule-thief, and a way to filter it well enough for audio.

[bobodioulasso]

I fear a micro fan to be compulsary.

[danielwritesbac]

Originally Posted by bobodioulasso

I fear a micro fan to be compulsory.

Can you do an internal fan with decorous vents and thermo speed control?

Originally Posted by johnr66

Some of the car stereo power amp ICs need as few as three film caps to function, although I like to add RF filtering on the input. This may be another option. Power supply would be simpler as well.

That's a good idea. I don't actually want to win, but do want to raise the bar slightly, so perhaps I'll do something like a TDA1554Q with a $9 laptop cord for power. Those parts are somewhere in the desk and might as well be employed to make a nifty little Christmas present.

I'm thinking of trying a heat spreader bar thermo-pasted and pop-riveted to the bottom (inside) of the mint tin and vent holes drilled through the bottom of the mint tin. And, then little rubber coated feet elevate and allow those vents to work. Well, that should let some cool air in, and then the remaining task seems to be determining some fairly decorous way to let the warm air out.

Originally Posted by keantoken

Does an altoids tin have enough heatsinking power to run 9W?

A 9w+9w (10%thd, 8R load) stereo amp of Class D or hyper-efficient Class B will run with a mint tin as the heatsink. After all, some Linear plug packs compete with switchmode plug packs for Class 5 efficiency.
So, it is possible.
If the efficiency percentage of 18 watts total system power goes into the speakers, the remaining percentage heats the mint tin. I think that the main problem is ventilation versus capacitor longevity.

Originally Posted by keantoken

I think it needs at least a +-15V PSU (30V single rail). It looks like an external PSU. . .

External PSU looks good. If you could arrange for lower loss at the amplifier then it would run cooler and could use lower voltage.

Originally Posted by keantoken

Where is a 2A transistor with Ft>20 at 10mA... Perhaps we could find something in the <80V Vcemax region...

Possibly Toshiba BJT? Maybe a Sanyo or Fairchild BJT? But, have you considered a FET design?