# What's that clunk when I turn on my receiver? **URL:** \ **Category:** Factual Questions\ **Created:** [March 10, 2003, 9:26pm UTC](https://boards.straightdope.com/t/whats-that-clunk-when-i-turn-on-my-receiver/160413 "2003-03-10T21:26:53Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![SmackFu](https://avatars.discourse-cdn.com/v4/letter/s/53a042/32.png) [@SmackFu](https://boards.straightdope.com/u/SmackFu)\ **Post date:** [March 10, 2003, 9:26pm UTC](https://boards.straightdope.com/t/whats-that-clunk-when-i-turn-on-my-receiver/160413/1 "2003-03-10T21:26:53Z")
When I power-up my home audio receiver, it takes a second or two to warm-up, then there’s a mechanical clunk/thunk noise. It’s not from the speakers, it’s from the actual unit. The same thing happens when I turn it off. Any ideas what this is? ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [March 10, 2003, 9:34pm UTC](https://boards.straightdope.com/t/whats-that-clunk-when-i-turn-on-my-receiver/160413/2 "2003-03-10T21:34:00Z")
Perhaps the transformer in the power supply? ---
**Author:** ![ski](https://avatars.discourse-cdn.com/v4/letter/s/bbe5ce/32.png) [@ski](https://boards.straightdope.com/u/ski)\ **Post date:** [March 10, 2003, 9:37pm UTC](https://boards.straightdope.com/t/whats-that-clunk-when-i-turn-on-my-receiver/160413/3 "2003-03-10T21:37:26Z")
It’s a relay inside the receiver. Mine is more of a click. ---
**Author:** ![Lagged2Death](https://avatars.discourse-cdn.com/v4/letter/l/ac8455/32.png) [@Lagged2Death](https://boards.straightdope.com/u/Lagged2Death)\ **Post date:** [March 10, 2003, 9:53pm UTC](https://boards.straightdope.com/t/whats-that-clunk-when-i-turn-on-my-receiver/160413/4 "2003-03-10T21:53:38Z")
Probably one or more relays. The amplifier’s outputs are usually connected to the speaker terminals through relays on modern equipment. This gives the receiver’s CPU control of the speaker connection, which allows the unit to do a few neat tricks. On some units, you can set speaker A/B/both/none options from the remote. The unit may be able to protect itself somewhat from accidental shorts by disconnecting the speaker terminals. And, as you have noticed, it can mute the outputs on power-up, preventing the annoying “ker-WHUMP” that would otherwise come through the speakers as the amp came on-line. This last item has become especially important with the recent proliferation of subwoofers. ---
**Author:** ![Trigonal\_Planar](https://avatars.discourse-cdn.com/v4/letter/t/0ea827/32.png) [@Trigonal\_Planar](https://boards.straightdope.com/u/Trigonal_Planar)\ **Post date:** [March 10, 2003, 10:06pm UTC](https://boards.straightdope.com/t/whats-that-clunk-when-i-turn-on-my-receiver/160413/5 "2003-03-10T22:06:32Z")
It actually is mechanical in origin. An electromagnet causes a metalic arm to actually strike a contact - that’s what you’re hearing. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [March 10, 2003, 10:35pm UTC](https://boards.straightdope.com/t/whats-that-clunk-when-i-turn-on-my-receiver/160413/6 "2003-03-10T22:35:53Z")
> [@](#): > > \*Originally posted by ski \* > **It’s a relay inside the receiver.** Yep, forgot that many receivers employ relays. ---
**Author:** ![TheeGrumpy](https://avatars.discourse-cdn.com/v4/letter/t/779978/32.png) [@TheeGrumpy](https://boards.straightdope.com/u/TheeGrumpy)\ **Post date:** [March 10, 2003, 11:28pm UTC](https://boards.straightdope.com/t/whats-that-clunk-when-i-turn-on-my-receiver/160413/7 "2003-03-10T23:28:48Z")
What I recall from my rudimentary electrical engineering course is that a change in flux results in an induced current. Turn on or turn off your stereo, and you get a “bump” of noise because of the momentary induction surge. Or perhaps I misunderstood the principle entirely. Just thought I’d point the thread in a different direction.