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I've recently bought this little 2*3W USB-powered amp and it is working pretty well with my old Panasonic boombox speakers (rated 2.7 ohms, 15w peak input).

However when switching off the USB power i hear a "pop" noise from the speakers.

I've seen there are pop-suppression circuits to avoid that, but i'd like to go with a passive solution not requiring any extra power.

So can i just wire 2 slide switches like these in series with the speakers and manually flip them before turning off the amp?

Are the switches better wired on the positive or negative side of the speakers?

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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Simplified schematic of amplifier output and proposed switching arrangement.

The "pop" occurs because the amplifier is running on a single-ended supply. To provide drive for positive and negative excursions of the speaker the amplifier is biased to half-supply when quiet, so, on a 12 V amplifier you would expect to find 6 V on the left side of C1. On power-up the left side of C1 jumps up by 6 V and this is passed through to the speaker. On power-down the opposite happens.

So can i just wire 2 slide switches like these in series with the speakers and manually flip them before turning off the amp?

Yes. Be aware that 15 W in to 2.5 Ω will give \$ I = \sqrt {\frac P R } = \sqrt {\frac {15}{2.5}} = 2.4 \text A \$. Your switches are rated at 0.3 A.

Are the switches better wired on the positive or negative side of the speakers?

It doesn't matter which wire is cut.

I've added R1 so that on power-up C1 can discharge before you close the switch.

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  • \$\begingroup\$ The amp is only 3W per channel. The speakers were rated 15W. \$\endgroup\$ – Justme Mar 22 at 15:32
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    \$\begingroup\$ Yup, but that information wasn't in the question when I answered. I'll leave the re-calculation as an exercise for the reader. \$\endgroup\$ – Transistor Mar 22 at 16:05
  • \$\begingroup\$ can i omit C1 and the resistor R1 going to ground? i'd like to keep it as simple as possible... \$\endgroup\$ – eadmaster Mar 23 at 17:12
  • \$\begingroup\$ C1 is in the amplifier already. If it wasn't you would have DC running through your speaker. Yes, you can leave out R1 but you can expect a pop on switch on. \$\endgroup\$ – Transistor Mar 23 at 17:18
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Click and pop suppression is a standard feature in an amp driver these days.

Since the amp you have doesn’t handle it properly, consider a different amp. That would be less work than trying to retrofit this one.

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