hi everyone, it's me again - after building myself a "real" 3886 amplifier with the real power supply, I accidentally laid my hands on a 18.5V, 3.5A SMPS from a broken laptop. I figured I could use it to build a small chip power amp in the 5W-10W range so I have a few questions:
- What chip would be the best for this application? I have or a LM1875 somewhere on my shelf, but while looking at the datasheet I found that this chip operating on a single 18V supply would provide even less than 5W into 8ohms, so it's not ideal. Maybe using a chip designed for a low voltage single supply power supply would yield better results? Like one of those car chipamps (for example TDA2003) - do you have any suggestions which chip would work best in this situation?
- What about the class D amps? Take for example the TDA7492 - it should be able to provide more than 15W into 8ohms on 18V PS and 1%THD. Does anyone have any experience with them - are they any good?
- And speaking of the laptop SMPS - I've read a lot about how noisy the switching power supplies are. Would it really be a big problem in this build or will it work just fine?
Please look at your specific situation. SMPS are not generic, so to say "SMPS ARE ALL NOISY" is not fair. How noisy is YOUR SMPS. FInd out, you have it in front of you.
Class D is a digital amp, the TDA7294 is a plain old bipolar amp in a chip.
A single 18v supply is the same as +9 and -9 supplies. SO at best about 6vAC into a load. If I grant it 6v optimistically, then I get about 4.5 watts into 8 ohms. The difference between 4.5w and your 5w is unhearable. The difference between 5w and 10w is only 3 decibels, this half watt would not make a detectable difference from 5w. Using an IC that is capable of more than that will not overcome that limit. The power amp does not create power, it only controls power. The power comes from the power supply.
Quote from: Enzo on June 28, 2016, 10:29:21 PM
Please look at your specific situation. SMPS are not generic, so to say "SMPS ARE ALL NOISY" is not fair. How noisy is YOUR SMPS. FInd out, you have it in front of you.
That's true, so I looked at it on my oscilloscope and it looked alright on a quick glance. Guess it will work fine.
Quote from: Enzo on June 28, 2016, 10:29:21 PM
Class D is a digital amp, the TDA7294 is a plain old bipolar amp in a chip.
Not TDA7294, but TDA7492 - confusing, I know, but those are two different chips. The datasheet claims it should provide ~15W into 8ohms or 25W into 4ohms with 1% THD. If true then I guess it would make an audible difference from 4W @ 8ohms or 9W into 8ohms for TDA2030 (and those are with 10%THD).
Quote from: Enzo on June 28, 2016, 10:29:21 PM
A single 18v supply is the same as +9 and -9 supplies. SO at best about 6vAC into a load. If I grant it 6v optimistically, then I get about 4.5 watts into 8 ohms. The difference between 4.5w and your 5w is unhearable. The difference between 5w and 10w is only 3 decibels, this half watt would not make a detectable difference from 5w. Using an IC that is capable of more than that will not overcome that limit. The power amp does not create power, it only controls power. The power comes from the power supply.
Yeah, I was just wondering how much clean output I can squeeze out of that power supply, but if it's not worth the hassle then I guess I'll just build a Tiny Giany amp and call it a day (oh, this TDA2040 chip specs look fine!). Thanks for the reply!
A 15 watt over a 7 watt would be an audible difference...barely. That would be 3db. Without getting out my calculator 4.5w to 15w might be 4db. 3db is just enough to hear, but you would have to compare them side by side. It would be like the difference between 4.5 and 5 on the volume knob.
45 watts would deliver twice the sound level as 4.5 watts, if all else is equal.
Check the data sheet on your digital IC. Are you prepared to layout a board for those surface mount pins? And solder the tiny leads? Not to mention all the extra circuitry, like the 3.3v regulator for the digital circuit, and the low pass filter on the outputs to carve off the digital noise.
Just my opinion, but I think it wise not to focus on watts.
The LM1875 or TDA20x0 series are simple five leg things with only a few external common passive components.
Regarding the TDA7492 - there are non-expensive complete boards out there that are actually just the application circuit from the datasheet, but I guess it's the not-cool, non-diy way to do it - considering the small difference in wattage between this chip and TDA2040 I guess I'll just settle for the latter and look for a high-efficiency speaker to go with it!
Thanks again.
You can always use two 18 volt psu giving a dual rail supply?? I think my little Casino amp runs on 20VDC rails.
With that supply the LM1875 chip is loud through 2 x 10 speakers. 8|
Where I live computer PSU's are cheap as always plenty in the bins at the Salvo stores.
The local council has a recycle store and again you can pick them up for ~$5 each.
I run some of my pedals on recycled SMPS,,, Yes you need to watch out as some are noisy for audio application.
Phil.
Ed; Oh yeah just make sure the secondary is isolated from the earth pin on the primary,, some carry the earth across to the negitive on secondary which will not work. Find the ones that only have a 2 pin primary,, they are likely ok.