DIY portable amp project

Ruebezahl · Posted: Fri Jun 18, 2021 10:16 pm Post subject: DIY portable amp project

Hey everyone. So i have a little project i am working on and hope to get some help here, although it is not a synth. I hope i found the right section for this topic, please feel free to move it, if it doesn't belong here.

So while cleaning out the basement i found this speaker, without corresponding amp. It’s a nice size, and i thought to turn it into some kind of portable amp, kind of guitar-style amp, although i don’t have a guitar. But i would definitely like to include a preamp circuit strong enough to amplify contact mics and the like. Anyway, the preamp circuit i think i will not have too much trouble, for now there are a couple of fields where i have not much experience, so i will need some help:

Batteries: So i have this broken tablet, which batteries are still fine though. So i planned to use those for the amp. They are two packs and rated for 3,85v and 32Wh (each block 15.55Wh) and 8320mAh, however, fully charged i measured 4,35V, which seems normal for Lithium Batteries from what i read. With the battery pack there is a little circuit board with a few super small components, but it is very hard to make out what it is. I figure it is some protection circuit, but i don’t think i could connect a usb cable directly to there, since in the tablet it is also not like this. The usb socket for charging it is on the mainboard. Anyway from this little circuit board there are leading 8 cables to the mainboard. 3 red ones, three black ones, a green and a blue one. The red and black ones are all connected with each other, they are just the positive and the negative rail. Why they have three cables i am not sure. The blue and green i am not sure, i suppose they are for charging the batteries? But when i measured them while charging, they were giving some kind of low unstable voltages.

Well, anyway, i guess the easiest way would be to just replace this little circuit board with my own charging circuit. I found this cheap module in an electronic store here, it is rated for 3,7V, probably for those standard 18650 Lithium Ion Batteries. Perhaps not enough for the tablet battery, that is rated at 3,8 but goes until 4,35 when full. On the other hand apparently the 18650s will also go up to 4,2 when fully charged, so perhaps the difference is negligible. I would appreciate if somebody has some more information on that, since i am not so familiar with it. If it would just mean that the batteries wouldn’t charge to the absolute maximum, perhaps it would be okay. A more important question might also be if this module could handle those laptop batteries watt and ampere wise. The output of the batteries will not go through the module anyway i guess, so should be fine i suppose?!

Stepping up voltage: Another problem i will have to solve is how to step up the voltage. Since the laptop batteries only deliver 4,35V at the very max, that’s not suitable to power a 5-8W Amp. Probably i will need something like 12V. Again i found a cheap module that can step the voltage from anywhere between 3,5 and 18v up to 4 to 24v. is this what they call a buck converter? It is based around a chip XL6009E1. I heard before that buck converter are noisy or something, Would it be suitable to drive an amplifier circuit from it?

The last problem is the Amplifier itself. It is a bit more familiar terrain, but still i have a few open questions. I am tempted to go with a finished module here too, but i could also go with my own design, if the available one turns out to be not suitable. So i found one for around 3$, which sounds very cheap. It looks very simple, is based on a LA4422 Chip and is rated for 8W. Besides the sound quality, which i don’t expect magic for this price, but of course hope it’s not super noisy or something, i wonder about two major concerns: Watt and Impedance, bot of which i don’t totally grasp yet.

So for the Watt, in the speaker box there are two speakers, which have different Watt limits. The big one is nominal 5W, max 8W. The small one is nominal 2 W, max 5W. So does that mean i can only run maximum 5 W through them, before the small one burns? Or are they kind of just taking what they need? How to solve this, that the two speakers have so different requirements? They are obviously ment to be driven by the same amp, like you can see they are in series, only seperated by a capacitor, which does the frequency filtering i suppose.

Then for the Impedance, when i check the datasheet for the LA4422, the examples all have a 4 Ohm speaker. But the two speakers together, have an Impedance of 4+3,2=7.2 Ohm . Is that acceptable? Or is there a way to exchange some parts in the amp circuit in order to make it more suitable for higher impedance speakers?

battery with mainboard.jpeg

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batterypack open.jpeg

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Here i removed the tape, so you can see the small circuit board, which i suppose could be some kind of charger circuit...

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speakers.jpeg

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the speaker i found, with only a cap in between... not the most sophisticated frequency crossover i suppose?

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usb charger.jpeg

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PHOBoS · Posted: Sat Jun 19, 2021 6:36 am Post subject: Re: DIY portable amp project

Ruebezahl · Posted: Sat Jun 19, 2021 8:55 am Post subject:

Thanks a lot Phobos! And you are right indeed, they are in parallel, just like you said.
After your evaluation it sounds like i will buy this amp module. I think the risk is not too big, especially with this price. Perhaps i will keep the Watt in the lower registers, and try to not crank it up to the max, just to be safe.

I am still a bit unsure about the charging circuit and the step up module. Anyone, has some thoughts about it?

I tried to upload photos of the two modules, but somehow it just doesn't work. Is there some limit per user per day or something?

edit: hahaha actually the upload worked quite well, it uploaded the same picture 5 times, just forgot to tell me Laughing

I will take care of this!

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blue hell · Posted: Sat Jun 19, 2021 10:05 am Post subject:

PHOBoS · Posted: Sat Jun 19, 2021 12:02 pm Post subject:

I don't have any experience with using Li-Ion batteries so can't really comment on the use but be careful!
With 8Ah there's quite a lot of power packed into those when charged and you do indeed need some proper protective circuitry.
Not only for charging but also discharging in case of a short. Also something to protect it against any physical damage.

This video might be of interest: https://www.youtube.com/watch?v=LIWUYSDWjfk

edit: there is a difference between Li-Ion and LiPO (which is also a Li-Ion) but you do the research.

Sven · Joined: Mar 10, 2017 Posts: 51 Location: Norway

Ruebezahl · Posted: Sun Jun 20, 2021 4:16 am Post subject:

Sven · Joined: Mar 10, 2017 Posts: 51 Location: Norway

If you search Ebay for PAM8403 you find a lot modules shipped from China. Some have volume potentiometers. My link didn't work as intended Sad

Good luck

Ruebezahl · Posted: Mon Jun 21, 2021 4:20 am Post subject:

Ruebezahl · Posted: Fri Jun 25, 2021 8:16 am Post subject:

So i tried it now, and wow, for 4,3V and such a small amp, that's quite impressive! I think i will stick to this solution. However, i felt the sound was a bit too much highs, and not enough bass. I will add a little EQ circuit before the amp anyway, but i was thinking since the small speaker already has a highpassfilter though the one capacitor (i kept that feature), is it possible to give the big speaker a simple low pass filter with a resistor and a capacitor to ground? Like after the amp, and only for the big one. Or doesn't it work that way?

Another thing i spent way more time than i expected figuring out is the preamp. I will probably use the speaker for playing my synths, or just music from my phone or whatever, for that i obviously don't need any preamp. But i would also like to be able to amplify my xaphoon, which i pickup with a contact mic. Now i found some schematics for a special preamp for contact mics, but they call for 9V, and i wonder if it will work with this power supply as well, any thoughts on that? Would this preamp also be okay for magnifying guitar pickups, or microphones? Obviously there is always some drawback, and there is a reason why these different tools all have different preamps, but a preamp that works BEST for contact mics, but OKAY for other stuff that would be the greatest. Any thoughts about that are appreciated.

https://www.instructables.com/Guitar-Contact-Microphone-Preamp/

I would like to have two inputs, which i can choose for preamp or direct. And then mix those two channels, perhaps active with a 4069UB (because i am more familiar with cmos) or with opamps if that has any advantage(although i am a noob with opamps, and i think opamps like a big voltage range, usually even bipolar, are there opamps that work nicely with less than 5V?)

btw, this is the bass/treble EQ i want to include. I alreadz tested it on breadboard, and i am pretty happy with it, for such a simple passive circuit!

http://www.electronicecircuits.com/electronic-circuits/bass-treble-tone-control-circuit