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DIY Small Portable Bass Amp

Discussion in 'Amps and Cabs [BG]' started by Caleb6543, Jul 5, 2014.

  1. Caleb6543


    Jul 4, 2014
    This is my first post; I hope this is the right place for this.
    I have a 120 Watt bass amp that was getting too heavy for me to lug around so I've been playing (small venue) with a small acoustic amp. That amp can't really hit the low notes and tends to get loud on other notes, so I thought I'd build a small amp with a woofer and electronics of my own making. The speaker is a Tang Band 6-1/2" woofer and the amplifier is based on a TPA3125 Class D amp chip. I'm hoping to add a preamp with tone controls in the future. I'm pretty happy with the sound, but there is a little bit of hiss and a high pitched tone in the output. Still tracking that down.

    This is where the project is at:
    CircuitBoard. CircuitDiagram. EnclosureView. FrontView.

    vmabus and vin*tone like this.
  2. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Cool, and welcome to Talkbass! I've asked the mods to move this thread to the Amps Forum, where more of the amp/preamp/cab DIY'ers will be likely to see it.

    You can find a few DIY bass preamp build threads in the Amps FAQ sticky at the top of the Amps Forum too.
  3. Caleb6543


    Jul 4, 2014
    I populated the new circuit board and dressed up the speaker a bit. So far, I like how it turned out.

    I shared the board at OSH Park (just search for "bass amp"), and there is a parts list there as well.

    Attached Files:

    vin*tone likes this.
  4. Hi.

    Yep, very cool, and welcome to TalkBass Caleb6543.

    I'm no class D expert by any stretch of imagination, but AFAIK most class D designers fight the hiss and the "whine" by tweaking the output LP filter if the sampling frequency can't be changed.

    IMLE and AFAIK only again, but the class D is said to be very unforgiving for the PS design as well, so make sure Your supply of juice is stout and steady :).

  5. Caleb6543


    Jul 4, 2014
    I played with the output filter on the previous amp by choosing some larger inductors. This time I just went with the recommended values in the data sheet. The high pitched noise may have been some ringing. This circuit also includes a small snubber capacitor and resistor which might help as well.

    Thanks for the advice on the power supply.
  6. coughiefiend


    Nov 12, 2013
    This is awesome!
  7. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    You must keep any of the output signal (including carrier) out of the input and PWM areas.

    If your are hearing hissing and noises then you are also most likely spewing RF all over the place too.

    Have you done any RF analysis?
    Last edited: Jul 24, 2014
  8. Caleb6543


    Jul 4, 2014
    How does one go about doing "RF analysis?" The best I could probably do is to place an AM radio near the speaker while it is on. :)

    As far as the wires going to the speaker itself, there are a couple inductors and filter caps that should knock out a lot of the RF. There are also small capacitors from the output to ground and RC snubbers on both legs as well to help. The input circuitry has its own ground plane and occupies one side of the board. The grounds are connected at the TPA3125 analog ground pins so the signal is referenced to the signal ground on the output amp. I tried to keep the traces short and wide, using copper pours when possible, for all the high current lines.

    The ground is connected at the voltage regulator on the signal side. Maybe I should have connected it at the ground on the opamp so any signal coming off the opamp is referenced to its ground. I've read that JFETs are quieter than MOSFETs. This amp uses a MOSFET on the input. It might be better to replace that with a JFET like most of the amp circuits I've seen on the net. One problem with that is most of those circuits use a J201 transistor and Digikey only carries a surface mount version of that. I think Mouser is out of stock on the through hole version. I haven't check Element14 or Jameco. eBay might have them from China.

    Attached Files:

    Last edited: Jul 24, 2014
  9. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    I can't comment in great detail because I don't have a lot of experience with that part, but I can make some comments in general which might help shed some light on the problem. Respectfully, without being critical of the work you have done...

    1. The noise is most likely a symptom of RF getting back into the preceding stages.

    2. Parts placement, spacing and orientation is critical to class D success.

    3. Larger components create spacing challenges.

    4. Longer traces create larger problems. Each trace can act like a transmitting or receiving antenna.

    5. There is a reason why most class D circuits use a combination of surface mount and small footprint parts like 1/8 watt resistors.

    6. I would recommend studying the layout, gut feeling is that you could improve it and decrease the overall PCB area by maybe 50% which would reduce a lot of possible causes

    7. A scope can be helpful BUT the very action of placing a scope probe on a sensitive node can cause or appear to cure the very problem you are chasing. Eliminating as many possible causes in the layout stages is the most produvctive approach.

    8. Grounds are not common at RF, inductances and capacitances can really mess with the notion of a signal common bus or ground.

    This is not easy stuff by any means.
    vmabus and T-Bird like this.
  10. Caleb6543


    Jul 4, 2014
    coughiefiend likes this.
  11. Woodstockz


    Sep 23, 2011
    San Diego, Ca
    Hi Caleb6543. Thanks for showing us your DIY project. It is a cool little practice amp project.

    I like your covering solution. I think that epoxy is a good solution.

    Since I haven't worked with the TPA3125, I don't have an answer for the hiss right now, but I have a couple of ideas otherwise. Just a couple of notes for reference, which are in no way criticisms.

    - If you look at speaker displacement charts, you can see that you can exceed Xmax at low frequencies approaching DC. A High Pass Filter (HPF/Rumble filter) on the input might help you get a little more out of your speaker, and protect your speaker at the same time.
    - Most people making small cabinets punt on the fundamental frequency, and tune to the first harmonic. For the most part, it works out OK. That said, mine have been tuned to the first harmonic, but I will eventually try one tuned to the fundamental.

    Francis Deck has done a lot of work researching small box design. Check out his paper on his "Compact 12" bass speaker". It is a great reference and has greatly influenced my thinking.

    Best Regards.
  12. Caleb6543


    Jul 4, 2014
    Thanks for the links. Maybe the next one will be even better.
    • :)

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