HELP PLEASE! Thump Sound through speakers

[Frankyc2003]

Hi everyone,

I am using an Icon Audio Stereo40i mki. And my speakers are Fostex 206 based horns.
Everytime I switch the amp off, after a good listening session, I am hit by a thump coming out of the speakers... It occurs exactly as the rocker power switch goes off...
:scratch::scratch::scratch::scratch:

So, first I am worried this will damage the speakers in the long run...
And secondly, is there anything that can be done to cure this?

For example, while using my 845pro from Ariand, I never, ever hear such thump...

All precious hints or tricks are much appreciated from all of you!
:cool:

[Puffin]

Hi Franky, hope you are still liking the horns. I understand that if there is any DC present you either get a switch-on thump, or a switch-off thump or both. I don't know anything about valve amps and how any DC is dealt with in the design. However I have built several solid state amps and DC can be dealt with in the circuit (see link) or by way of relay switching which take the capacitors out of circuit (I think)

http://www.zen22142.zen.co.uk/Design/dcthump.htm

Others may say differently.
Rob.

[Frankyc2003]

Hi Rob,


Hi Franky, hope you are still liking the horns.

yes I absolutely love them! They are great!
I am doing as they say on the webpage. I always turn volume full down before switching source or switching off the amp... Odd
The thump actually never happen when I switch source, only on power off!?!

My main worry is that it will damage the cones ultimately...

[Puffin]

I assume this is a new amp to you and you may have not had an amp before that does this. Older Creek amps did this and there are others (Older Naim amps?) that do this. There is a difference of opinion as to whether this damages speakers or not. I think it will depend on the level of DC present. You can check the DC offset of an amp by measuring with a multimeter across the speaker terminals (speakers disconnected) red probe to + and black probe to -, anything above 50mv would be considered a little high, but some say that anything under 100mv is not considered dangerous. What I understand happens is that the power supply capacitors discharge on power down and you may get a momentary hike of DC of say 2v or somesuch. I am sure others will be along to tell me if I am talking rubbish or not:D

[Frankyc2003]

Rob,
when you say + and - you mean one channel only terminals (red and black speakers terminals). I have 8 Ohm and 4 Ohm BTW...

[Puffin]

Yes, check one channel at a time red probe to red and black to black, I don't think it matters if you use the 4 or 8 ohm tap. You can switch it off with the probes attached and see what DC voltage you get.

[NRG]

Switch off thump can be caused by the primary inductance of the mains TX (back EMF), when the AC power is cut the magnetic field collapses producing a spike. You could try a 220nF cap X2 class wired across the TX primary or on/off switch maybe in parallel with a 250v VDR mains suppressor...

[Frankyc2003]

Cheers, I shall try it tonight

[Frankyc2003]

Switch off thump can be caused by the primary inductance of the mains TX (back EMF), when the AC power is cut the magnetic field collapses producing a spike. You could try a 220nF cap X2 class wired across the TX primary or on/off switch maybe in parallel with a 250v VDR mains suppressor...

How about something like this across the mains switch?
http://uk.farnell.com/epcos/b32921c3103m/capacitor-class-x2-10nf/dp/1200827
I have read that on another web...
Silly Idea?

[Marco]

Everytime I switch the amp off, after a good listening session, I am hit by a thump coming out of the speakers... It occurs exactly as the rocker power switch goes off...


It's quite simple: weevils have entered your speakers, through the bass port on the bottom of your Antals. It's their mating season, you see, and they like to find somewhere warm and dark to copulate. Your music is disturbing them, thus stalling the progress of nature and contributing to their possible future extinction. The thump you're hearing, therefore, is them telling you to STFU.

In order to rectify this sorry situation, you must alert your local RSPW, as soon as possible, where a nice man in black van will pop out with some special cheese to tempt them out.

Marco.

[Frankyc2003]

Ah! I see!!!
Bloody Weevils!

And now they must feel even better inside the cabs of Puffin Fostexs!

[Marco]

Unfortunately not, as he keeps his socks in there.

Marco.

[Frankyc2003]

http://4.bp.blogspot.com/__PceiJCsXuM/TGLw3MsfRdI/AAAAAAAAAr8/CirS-1m3x2I/s1600/loir+3.jpg

That's what I see looking inside my speakers!

[Marco]

Ah, you've unearthed the little buggers... Now get the cheese!

Marco.

P.S it's these in-depth technical discussions that I love most.

[Frankyc2003]

http://abeautifulmess.typepad.com/.a/6a00d8358081ff69e2017615efeb25970c-800wi

et voila!

[Mr Kipling]

The obvious answer (to my mind) would be - don't switch the amp on!

[Frankyc2003]

Yeah I know but it sounds so good guv'nor!

[Marco]

The obvious answer (to my mind) would be - don't switch the amp on!

It's so obvious, why didn't I think of that! :doh:

Marco.

[Mr Kipling]

The simple answers are always the best.

To be serious, as has been suggested, I'd try a snubber (capacitor and resistor network) which I imagine the Farnell item is (haven't looked) across the power switch.

[southall-1998]

The Exposure X has a switch on bump. Is a normal thing for Exposure amps though!!

[Frankyc2003]

I have actually read in loads of places that I basically shouldn't worry...
That's probably why I got worried...;)

One thing for sure, a Valve amp (integrated or power) should NEVER be switched on without a load presented to the speaker terminals...
Unless you want to destroy your output transformers...:doh:

[NRG]

How about something like this across the mains switch?
http://uk.farnell.com/epcos/b32921c3103m/capacitor-class-x2-10nf/dp/1200827
I have read that on another web...
Silly Idea?

No, but I'd go with the 220nF version...

[Frankyc2003]

Would you put in series with a Resistor? like a snubber RC? as suggested above.

[NRG]

No I wouldn't bother, you could also add one of these in parallel though...

http://uk.farnell.com/vishay-bc-components/vdrs05c250bse/metal-oxide-varistor-320v-disc/dp/1458582

[Stratmangler]

One thing for sure, a Valve amp (integrated or power) should NEVER be switched on without a load presented to the speaker terminals...
Unless you want to destroy your output transformers...:doh:

Not quite true, but if you're not sure about this then it's a safe rule of thumb to abide by ;)

[Frankyc2003]

Please tell me more...
I have always been told not to do it...
Where and when is it safe to do?

[Stratmangler]

SETs are not bothered about it.
Don't know the technical ins and outs of it, but I have it on good authority by someone wot knows :)

[Puffin]

Any up-date?

[Marco]

Yes, the weevils have moved out and are now heading towards Surrey.

Marco.

[Puffin]

Yes, the weevils have moved out and are now heading towards Surrey.

Marco.

Ah yes, they must have tracked north from Surrey to London after I left a scent when delivering the horns to Francky. I bet you that the one that reaches me first will be that little bugger weevil knievel who was the worst of the lot:eek: Hold up....what's.......... that noise........arrggh...he's only only here on a bloody motorbike!

[Frankyc2003]

:lol:
Well, after all this. It was the switch causing an arc. When I switch off now, I do it from the wall socket. When I get a chance, I will look into a fix but for now the weezils are gone!
Hopefully not back at your Rob...

[NRG]

The arc is caused by the magnetic field collapsing in the primary of the mains TX, you fit the X class cap to stop it! Just replacing the switch will not stop the arc if thats what you are thinking. Did you ever do any servicing of old cars with points? The points opening caused the magnetic field in the ignition coil to collapse and unless you had a suppressor fitted across them they would burn out very quickly...

[Puffin]

Use a 10nF /240Vac Class X or X2 Suppressor Cap across the power switch contacts.

[Frankyc2003]

:cool:OK I see.

I will indeed look into suppressors as soon as I get a minute.

Cheers everyone for your help!

[DSJR]

Have you thought of asking Icon Audio themselves?

[Frankyc2003]

Have you thought of asking Icon Audio themselves?

Quite right here!
That didn't even occur to me...
Probably because the amp is well out of warranty.
But well worth a shot, thanks!