I've bought a subwoofer that works great sonically, but it has a relay which clicks from time to time and is driving me nuts. It seems most like some kind of auto-standby function: it clicks half a second after a moderately loud signal starts playing through the sub, and it clicks again some time after there's been nothing playing. But here are the weird things:
My questions are these: what do you think the relay is for? Can I just remove it from the circuit, or is it useful enough to keep (in which case I'll probably want to wrap in some sound and vibration dampening stuff so that it doesn't click as loudly)? Why does it make the speaker hum as it turns on?
- It doesn't click as long as the input level is low enough (or, more likely, the output level of the amp). I mean, the sub is playing very well, it's doing its job, and as long as some threshold is not met, the relay does not click on.
- I can't hear any difference in the sound produced by the sub when it clicks on.
- But I can hear what seems to be 50 Hz hum coming out the subwoofer driver when the relay clicks on, and this hum ceases when it clicks back off.
- The time for it to disengage seems to vary, it's around a minute but it's not always the same. Maybe just an imperfection of an analogue timing circuit, temperature-related, perhaps? The amp does get warm the longer it runs - not super hot, but it warms up a fair bit.
- The power draw from the all with the relay off is 24-25 W, with the relay on it's +2.5 W - around 27.5 W.
My questions are these: what do you think the relay is for? Can I just remove it from the circuit, or is it useful enough to keep (in which case I'll probably want to wrap in some sound and vibration dampening stuff so that it doesn't click as loudly)? Why does it make the speaker hum as it turns on?
Last edited:
It is an ordinary enough relay, 2 pole change over type. Not weird at all.
See what it switches in and out of the circuit, so you can find the reason for the hum.
Is the unit designed for the supply in your area?
See what it switches in and out of the circuit, so you can find the reason for the hum.
Is the unit designed for the supply in your area?
Yes, it is a 220 V mains unit for my 220 V AC grid. The power supply is as simple as it can be - a toroidal transformer, and one can see the 4 separate main rectifier diodes on the board.
What's weird about this relay is I can't understand what its function might be, given the 5 observations I've made about it. Particularly, the fact that I couldn't hear any actual change in the sub's output - other than the added hum - as it turns on and off. The hum is nothing dramatic, mind you, but audible in a quiet room and when listening to something at lower volumes.
What's weird about this relay is I can't understand what its function might be, given the 5 observations I've made about it. Particularly, the fact that I couldn't hear any actual change in the sub's output - other than the added hum - as it turns on and off. The hum is nothing dramatic, mind you, but audible in a quiet room and when listening to something at lower volumes.
Last edited:
Thanks for the ideas, but there is definitely no soft-start in this unit. Not protection either, at least not in the sense of my understanding of this feature. As I said, the subwoofer works - plays - in both positions of the relay, and it is only the sound levels above certain threshold that makes it kick in (and do nothing, seemingly).
Kicking in different (higher) secondaries from the transformer is my guess, poor man's class D 😀
See if the suppy to the output devices changes
Last edited:
Or a voltage controller, changing in some particular predictable way, like a tap changer in some mains voltage controllers.
I thought so as well! My AVR has an "Eco" mode which does the same thing, switches the transformer windings with a click of a relay (and saves a ton of power, as I seem to never need the non-eco mode), and I immediately thought about this when I heard it click with no change to the sound accompanying the clicking. And upon looking at the board I decided this is not it, but I don't know why - you could very well be right because there are two sets of 3 wires coming from the transformer into two separate connectors, one with thinner wires and one with thick ones. I will investigate further with a DMM, thanks for the pointers!
And there are two pairs of caps that stand out from the rest, 50 V / 4700 uF and 25 V / 470 uF. I wonder if humming occurs because the bigger caps are dead or halfway dead due to age.
And there are two pairs of caps that stand out from the rest, 50 V / 4700 uF and 25 V / 470 uF. I wonder if humming occurs because the bigger caps are dead or halfway dead due to age.
Last edited: