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I am building a bluetooth speaker and am encountering two issues. Here is my setup so far: enter image description here

  1. When turned on, it gives two very loud POPS before working normally. Is there a simple way of delaying the audio for a few seconds when it is turned on? (other than adding a manual switch)
  2. By themselves, each component sound great but when they share power, there is a lot of hum. I've researched and I think I need a "ground loop isolater". Where should this be placed within the circuit for maximum effectiveness? (problem fixed by GLI)

Links: Amp Bluetooth Module

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  • \$\begingroup\$ The following is a bit dodgy, but should work due to the low current consumed by Bluetooth: try using just the +12V wire and leaving out the 0V/GND wire for the power connection (it should still get an adequate power GND out of the audio wires). If there is still hum, remove one of the (RCA, by the look of it) audio wires as well (yes that will make it mono, but just for testing...) \$\endgroup\$
    – markt
    Jan 10, 2014 at 3:12
  • \$\begingroup\$ Before I try that, is there any chance of damaging the bluetooth module by leaving the ground off? \$\endgroup\$ Jan 10, 2014 at 3:42
  • \$\begingroup\$ There is of course some risk (there always is when you're troubleshooting). You could minimise the risk by wiring it up first and verifying that the BT board and amp still share a ground through the audio wires by "buzzing" the connection with a multimeter. \$\endgroup\$
    – markt
    Jan 11, 2014 at 0:46

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Ah you didn't read the reviews for this amp - it seems that it does cause loud pops at power-up and one user has implemented a switch that disables the speakers but this could be done with a delaying relay circuit fairly simply.

As for the "hum" - if it is "hum" and not degradation of the signal or high frequency whistles (etc.) this is likely caused by your power supply having too much ripple on it. The amp is a class D type and these are notoriously poor at coping with power supply ripple. Power supply ripple will come from a standard transformer/bridge/smoothing capacitor and is inevitable in these configurations - you can try putting a 10,000 uF cap across the supply rails close to the power amp.

Why are class D amps poor at rejecting power ripple - the output transistors are basically used as switches and they alternate rapidly (above audio frequencies) and this means any power supply ripple is superimposed onto the speaker wires. A conventional amp can easily reject ripple because it operates linearly and will only push ripple to the output when driven at very high levels (because the transistors saturate when the ripple causes the power voltage to be cyclically at a minimum).

It could be wiring but it's easy just to put a big capacitor across the 12V power rails first then take it from there if that doesn't solve the problem to an acceptable level.

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  • \$\begingroup\$ Thanks for the reply. Anywhere I can find further info on the delay circuit? (I'm a bit of a novice at this) The hum only occurs when the bluetooth unit is sharing the same power supply. When the amp is on it's own, it sounds great. That's why I was suspecting a ground loop. \$\endgroup\$ Jan 10, 2014 at 13:38
  • \$\begingroup\$ Is it hum i.e. a bassy monotonous sound or is it different to that? Delay circuit uses a DPST relay. Each pole connects/disconnects a speaker. The relay is activated after power is applied and a delay times out. Probably the easiest circuit would use a mosfet with an RC driving the gate. The C (capacitor) delays the votlage rising on the gate and hence causes the relay to delay switching on. Relay coil would be 12V and in drain of FET. Let me know if more info needed. \$\endgroup\$
    – Andy aka
    Jan 10, 2014 at 14:10
  • \$\begingroup\$ Yeah, it's like a low hum with a bit of a buzz in there too. Only occurs when the two units share power. I hate to be a pain, but is there a more detailed schematic of a delay circuit? If I know exactly what to get and have a diagram, I can probably make it. I don't have a good enough understanding of electronics to make something up. \$\endgroup\$ Jan 10, 2014 at 14:19
  • \$\begingroup\$ You have to establish what's happening when the speakers pop in order to fix it. If you powered up with speakers diconnected then waited a couple of seconds and connected one speaker, does it pop? If so then you need to have a dummy load on the amp (maybe 22 ohms) and start the process again - does the speaker still pop when connected after a couple of seconds? Can't provide a circuit until this is established. \$\endgroup\$
    – Andy aka
    Jan 10, 2014 at 14:47
  • \$\begingroup\$ The speaker connections don't seem to change things. However, if I disconnect the audio input to the amp, the pop goes away on startup. I'm really interested in some sort of simple relay delay in between the amp and speakers. Any ideas on something like that? \$\endgroup\$ Jan 10, 2014 at 18:38

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