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I'm an engineering student (ChemE), just starting to get in to electronics and electrical engineering. Next semester I'm planning on adding an electrical engineering minor, but I wanted to get some hands-on experience first, so I'm about to build a bluetooth speaker, as shown below (sorry if anything about that diagram is unconventional):

enter image description here

Before I start soldering, I'd like to know if I've made any egregious/obvious errors. I have all of the components for this build already, but I'm fine buying new components if I've messed things up.

On the bottom left I've shown a battery and charging port, and I just sort of connected them on the diagram, but this is all one piece in reality and already wired, and as I've charged it up and it hasn't burned my house down I assume it works fine. Then I've got a step down (12V to 5V) to power the bluetooth card and some indicator LEDs. There's a toggle switch right before the volume potentiometer to switch between that and a headphone port as input. Then I've got a ground-loop isolator, followed by my amp, and then the crossover circuit and the speakers.

I think I made sure that all of my parts are rated correctly for the current/voltage they'll need to handle.

If there are problems somewhere I don't know where, so instead of listing the specs for every part in here, if anyone wants to know the specs for a specific part just comment and I'll edit them in.

Thanks much for any help you all can give!

Requested information:

  • I can't find any technical specs for the bluetooth card, as I'm ripping it out of this USB receiver
  • The potentiometer I bought (here) has a resistance of 10k Ohms
  • This is the ground loop isolator I'm using. Again, unfortunately I can't find any technical specs, as this product is intended for consumer end-use in audio systems.
  • Here is the full spec sheet for the amplifier I'm using
  • Both LEDs have the same specs, here is the data sheet, it lists Vf as 3.2V when If = 20mA

Sorry I couldn't find some of the information you wanted, I'll let you know if I can get my hands on it.

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  • \$\begingroup\$ Looks like the volume pot is shown wrong (the wiper is the terminal on the arrow, you'll see x ohms from one side the wiper and total resistance minus x from the other side to the wiper. Those two terminals have a constant resistance. Also, usually there's some other components to add to a buck converter, but I'm guessing that's an all inclusive module. I'm not sure about the isolation transformer, but I'm not too familiar with that sort of thing so I won't say more about that. \$\endgroup\$
    – michaelyoyo
    Commented Oct 14, 2015 at 19:11
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    \$\begingroup\$ I'd put a smattering of operational amplifiers into the mix. At the very least I'd use one to buffer the transformer from the audio sources, but I'd probably use one for each of the audio sources (after they've been mixed from stereo) to give some isolation. I'd also reconsider the LEDs; rather than powering them directly from the audio sources (probably a bad idea) I would either control them with a MOSFET directly, or use another op-amp (for each source) as a comparator with some manner of delay such that the LED doesn't flicker. \$\endgroup\$
    – CharlieHanson
    Commented Oct 14, 2015 at 19:59
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    \$\begingroup\$ Could you add the following information to the question: 1. Bluetooth module output voltage/load information; 2. Resistance of LOGPOT; 3. Model/specs of GLI; 4. Model/specs of T-AMP; 5. Vf and I of both LEDs. \$\endgroup\$
    – CharlieHanson
    Commented Oct 14, 2015 at 20:39
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    \$\begingroup\$ What do you actually intend the LEDs to indicate? \$\endgroup\$
    – got trolled too much this week
    Commented Oct 14, 2015 at 23:30
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    \$\begingroup\$ If you just want the LEDs to show which position the switch is in, use a two pole switch: one pole for the audio, the second to apply +5 V to the appropriate LED/resistor circuit. \$\endgroup\$
    – Peter Bennett
    Commented Oct 15, 2015 at 6:07

1 Answer 1

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You should not be loading the analog signal lines with anything besides the input that the signal line is meant to drive, and in this case, a signal attenuator (the potentiometer). In other words, take those LEDs off those audio lines!

Not that it will work anyway. For example, the audio line out from a PC or laptop or tat BLE adapter (the green 3.5mm jack) reaches at most 2Vpp. So the signal won't even overcome the LED's voltage drop to turn them on. But you should NOT have LEDs or anything else connected to a relatively high impedance analog signal line, and especially something non-linear like an LED. And those audio signals you could plug into the jack or generated by the bluetooth adapter are intended to drive very high impedance (~10KΩ+) inputs, the typical input impedance for an amplifier. For example, the amplifier you've picked has 22kΩ input impedance.

If you want to indicate switch position, just get a DPDT switch and wire one pole to the 5V (or 12V) power line, and the LEDs with series resistors to ground on the two of that pole's throws. Then of course wire your audio input to the other two throws and the output to the potentiometer to the second pole.

Also, your potentiometer is connected incorrectly for volume control. As it is wired, it is just a fixed resistor in series. The resistance of between pins 1 and 3 never changes, if it's a 10kΩ pot, then you've wired it to just be a 10kΩ resistor in series with the audio signal. Another tip with signals: you generally want the load being driven by them to be as fixed and unchanging as possible while you're actually playing audio. Load changes will cause distortion. In fact, that's how volume control works - by varying the load, but in a small and controlled fashion.

With that in mind, the correct way to wire the potentiometer is with pin 1 to the audio signal, pin 2 out to the amplifier, and pin 3 to ground. This is seen as a 10kΩ resistor to ground by the audio signal. Depending on wiper position, the signal amplitude will vary.

enter image description here

Also, I'd suggest powering the bluetooth dongle from a 5V linear regulator. I doubt it uses more than 100mA, and it's possible the switching regulator could inject noise into the audio signal path.

Finally, given the simplicity of this circuit, it is an ideal candidate for "star grounding". Simply put, every component that has a terminal going to the - terminal of the battery ('ground') should have it's own wire just for that component that is connected at the battery - terminal. All ground connections are made in this way, so all grounds connect at a single point (and it might even look kinda like a star). This would remove the need for a ground loop isolator completely.

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  • \$\begingroup\$ "If you want to indicate switch position, just get a DPDT..." That is what I did, after some discussion in the comments on the question. "Also, your potentiometer is connected incorrectly..." That was also mentioned in the comments. I appreciate your work in making the comments discussion into an actual answer though, hopefully it will help people out in the future. Also, I've not made any of the ground connections yet, so I'll look into the star grounding method you mentioned, it sounds promising (although the ground loop isolator is already soldered into the circuit). \$\endgroup\$
    – realityChemist
    Commented Dec 14, 2015 at 18:57
  • \$\begingroup\$ I didn't read the comments. I also didn't realize how old this question was. Sorry, if I'd noticed sooner, it might have been more helpful. \$\endgroup\$
    – metacollin
    Commented Dec 15, 2015 at 13:32
  • \$\begingroup\$ That's OK, I'm taking my time building this circuit (mostly because I'm not good at soldering and I have to take my time with it). I appreciate the answer. \$\endgroup\$
    – realityChemist
    Commented Dec 15, 2015 at 14:04

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