Here is a switching boost converter that would fit the bill from a quick eBay search, $14 including shipping (and a 5 week postal wait from China):
DC Converter Boost Module Power Supply 80W Out 35~60V | eBay
Vin = 10 - 32vdc (the car goes to 14 - 15vdc when running/charging)
Vout = 35 - 60vdc (can set it anywhere in that range)
Iout max = 5A
Pmax = 120w, but 80w without a fan. Your TDA 2050 does about 32w rms max.
* 90% efficient conversion
* 100kHz switching frequency, way above audio range so you are not likely to hear switching noise
* no short circuit protection, so be careful not to short the output leads hooking it up
The one thing to be a little careful with here is current specification for your power supply. That 32w is an average, of course. Do the math on 32w and 40v and you get about 800mA. But look at the TDA2050 spec sheet and you see the output current is actually thermally limited at 5A. In other words, that chip can handle much larger peak current loads and you should be careful not to starve it out with a small supply. The one I've spec'ed here also does 5A max, unlike some cheaper boost converters you'll find on eBay and elsewhere that have lower max currents.
Looking at that another way, say you have 8 ohm speakers, and say the TDA2050 will do about 35v on 40v rails (they never go all the way to the rail). That is 35v/8R = 4.4A, or (4.4A)(35v) = 153w instantaneous, quite a bit over the 32w average.
Also keep thermal in mind. This one is rated at 80w without a fan (and up to 85c ambient which is 185 deg F, good for autos), but that is likely in a room temperature ambient and not a 110+ degree car sitting in the summer sun. That 80w ability will probably be down somewhere around 40 - 50w when it gets in a hot ambient like that, if sitting in a trunk or until the car cools off. Many of the smaller boost converters are not good for these higher temps, either.
Have fun! Blow that trunk lid right off the car with some good bass.
DC Converter Boost Module Power Supply 80W Out 35~60V | eBay
Vin = 10 - 32vdc (the car goes to 14 - 15vdc when running/charging)
Vout = 35 - 60vdc (can set it anywhere in that range)
Iout max = 5A
Pmax = 120w, but 80w without a fan. Your TDA 2050 does about 32w rms max.
* 90% efficient conversion
* 100kHz switching frequency, way above audio range so you are not likely to hear switching noise
* no short circuit protection, so be careful not to short the output leads hooking it up
The one thing to be a little careful with here is current specification for your power supply. That 32w is an average, of course. Do the math on 32w and 40v and you get about 800mA. But look at the TDA2050 spec sheet and you see the output current is actually thermally limited at 5A. In other words, that chip can handle much larger peak current loads and you should be careful not to starve it out with a small supply. The one I've spec'ed here also does 5A max, unlike some cheaper boost converters you'll find on eBay and elsewhere that have lower max currents.
Looking at that another way, say you have 8 ohm speakers, and say the TDA2050 will do about 35v on 40v rails (they never go all the way to the rail). That is 35v/8R = 4.4A, or (4.4A)(35v) = 153w instantaneous, quite a bit over the 32w average.
Also keep thermal in mind. This one is rated at 80w without a fan (and up to 85c ambient which is 185 deg F, good for autos), but that is likely in a room temperature ambient and not a 110+ degree car sitting in the summer sun. That 80w ability will probably be down somewhere around 40 - 50w when it gets in a hot ambient like that, if sitting in a trunk or until the car cools off. Many of the smaller boost converters are not good for these higher temps, either.
Have fun! Blow that trunk lid right off the car with some good bass.
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I'm glad that helped! One more thing, thinking about it a bit more. Do you really need 40V? I was assuming you must be using a high impedance speaker (32ohm) or several lower impedance speakers in series.
If your speaker is 8 ohms then that is a drive voltage of sqrt[(32w)(8ohms)] = 16v. If the TDA2050 chip needs about 5v overhead to the rails then that would give you a rail voltage of around 21v. For a 4 ohm speaker that same math comes up with a 16.5v rail voltage.
If you do need a lower voltage, here is the similar 12 - 35vdc output version of that same power supply from an eBay vendor, still $14 total from China. The output current and power capabilities go up at the lower output voltage, so this one is now in the "overkill" specification range, but for $14 it is hard to beat:
DC IN 10-32 OUT 12-35V 150W adjustable boost converter | eBay
If your speaker is 8 ohms then that is a drive voltage of sqrt[(32w)(8ohms)] = 16v. If the TDA2050 chip needs about 5v overhead to the rails then that would give you a rail voltage of around 21v. For a 4 ohm speaker that same math comes up with a 16.5v rail voltage.
If you do need a lower voltage, here is the similar 12 - 35vdc output version of that same power supply from an eBay vendor, still $14 total from China. The output current and power capabilities go up at the lower output voltage, so this one is now in the "overkill" specification range, but for $14 it is hard to beat:
DC IN 10-32 OUT 12-35V 150W adjustable boost converter | eBay
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nevermind...
Ignore my post above. That is what I get for typing and talking on the phone at the same time.
For 8 ohms that is 16v rms, which is about (1.414)(16v) = 22v peak, or 44v peak-to-peak, meaning a 44v power supply rail. That matches up with the datasheet's listed +/-22v swing (dual rail, 44v single rail) for 32w into 8 ohms. Note from the sheet that driving it so close to the power supply rail results in fairly high distortion, that voltage headroom issue. For lower distortion the data sheet shows driving it at a level of around 25w into 8 ohms, which works out to be around a 40v swing if using a 44v power supply rail. 4v of chip headroom rather than my guess of 5v above.
datasheet:
http://www.st.com/stonline/books/pdf/docs/1461.pdf
Make sure that your amp has the 1000uF-or-so output coupling capacitor they show, since you are running it single rail, to block the dc from the speaker.
Ignore my post above. That is what I get for typing and talking on the phone at the same time.
For 8 ohms that is 16v rms, which is about (1.414)(16v) = 22v peak, or 44v peak-to-peak, meaning a 44v power supply rail. That matches up with the datasheet's listed +/-22v swing (dual rail, 44v single rail) for 32w into 8 ohms. Note from the sheet that driving it so close to the power supply rail results in fairly high distortion, that voltage headroom issue. For lower distortion the data sheet shows driving it at a level of around 25w into 8 ohms, which works out to be around a 40v swing if using a 44v power supply rail. 4v of chip headroom rather than my guess of 5v above.
datasheet:
http://www.st.com/stonline/books/pdf/docs/1461.pdf
Make sure that your amp has the 1000uF-or-so output coupling capacitor they show, since you are running it single rail, to block the dc from the speaker.
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