A small bass guitar amp based on a TPA3125

[Caleb6543]

Here is a small class D amplifier based on a TPA3125 chip that I used with a bass guitar speaker. It might need a little work, but I was happy just to get some decent sound out of it. It is set up for 26 dB gain (although the schematic shows it set up for 20 dB; I had to hack the board a bit because the volume was too low.)

A video of the project can be seen here:
Small Base Guitar Amplifier and Speaker - YouTube

 

[xrk971]

Did you make that board yourself? Are you running it as a PBTL mono for 20 watts into 4 ohms? If you need more power look at the TPA3116d2 and you can get 50 watts/ch - prebuilt boards for $13 to $20. One thing about the 312x series is that they require a big electrolytic output cap whereas the newer 311x series do not. Cleaner sounding that way.

 

[29285]

TPA3125 contains a single full bridge, so there is no way in selecting PBTL. It is capable to drive 8Ohm speaker, and if you supply +24Vdc it delivers nearly the same power into the speaker than TPA3116, so using a single 8 Ohm speaker there is no point in upgrading the chip. I have used these in several battery driven guitar amps and they work nice. Meanwhile I swapped to TPA3118D2 simply to be capable to drive two 8R cabinets in parallel. The circuit shown should work fine. You could simplify even more by coupling the emitterfollower the direct way, without coupling caps and bias resistors.

 

[Caleb6543]

I looked at the TPA3116 chip but didn't go with it because the pin spacing was pretty small. I've soldered chips like that before but would rather not unless I had to. It is also a couple bucks more than the TPA3125; not that price is a huge issue. If it was, a board from China would be the way to go. But where is the fun in that? I think I can get a lot more sound out of the TPA3125 once I finish the pre-amp. The input signal is still quite small and the current draw according to the meter on the power supply was only peaking out at around 200 mA

 

[Caleb6543]

I'm in the process of having another circuit board made that includes a pre-amp with a volume control so I can set the volume at the amp and then just turn the bass guitar up to the max when I am ready to play. The schematics and board layout is below. I designed this one with just through-hole components to make it easier for someone else to solder it if it works and I make it public on OSH Park.

 

[Caleb6543]

I assembled the board and it seems to work quite well. There is a little hiss, but no high pitched noise as before. I "shared" the project on OSH Park, and there is a parts list if anyone is interested. Just search for "bass amp."

New Bass Amp Circuit - YouTube

 

[doctormord]

Splitted groundplanes are mostly a bad idea, even/also in mixed signal boards. Running traces over slots as well.

http://www.hottconsultants.com/pdf_files/june2001pcd_mixedsignal.pdf
http://www.hottconsultants.com/techtips/split-gnd-plane.html
http://www.hottconsultants.com/techtips/tips-slots.html
http://www.eetimes.com/document.asp?doc_id=1272321

 

[Caleb6543]

Thanks for the links. I'll have to remember that if there is another revision.

I bought a cheap Chinese power supply and it produced a ton of noise. So I pulled out an old linear power supply and the noise went down a lot. The power supply could be a culprit.

 

[doctormord]

You may use a common mode choke between the amp and the psu. You may also try to bridge your ground planes with 1-10pF on 3-4 places, ideally there, where you crossed the slot with traces.

 

[Caleb6543]

Here is a link to the final write-up on the project if anyone is interested.