Power off thump

[Digsy 1971]

I have a power-off thump on my Dension DH-201 / DMP3A (hard disk based MP3 player head unit). I've heard of power-on thumps on amps before but until Googling I hadn't haer of thumps on power-off but it look slike it is just as much of a common problem.

It doesn't always happen and ususlly turning the vlume to zero before switching off seems to prevent it, but I'm looking for (a) a good explanation of what causes it and (b) a cure.

So far I've seen people talking on teh net about using relays to isolate the speakers during power up and power down, which would be relatively easy to do using the electric aerial signal wire driving a pair of DPDT relays through a transistor fed from the +12v line. Another possible cause for power on thump I've read about is that the amp has been built without blocking capacitors in series but I'm not 100% sure whether this wouls also stop the thump on power off.

Any thoughts?

 

[Digsy 1971]

Bump.

I could use an answer to this guys, as it seems to be getting worse If it carries on I'm going to have to make up a relay box controlled by the aerial wire that disconnects the speakers at shut down.

 

[jtf]

are you using a seperate amp to power spekers?

 

[Digsy 1971]

Hi there. Nope the head unit is wired directly to the speakers. No external amp.

 

[richjthorpe]

The only time I've heard something like this is on an old external amp of mine. When I turned the stereo off, the amp didn't have a soft turn off and shut off immediately meaning my sub gave off a thud.

If the amp in the head unit hasn't got soft turn off, then this would again create a thud. I don't know if there is a way round it unless there is a capacity that you can put in line maybe. Only thing is to get an external amp with soft turn on/off and run the speakers from the amp.

Richie.

 

[Digsy 1971]

Hi theer Richie. Thanks for the reply.

By "soft turn off" do you mean a delay on switching off the amp until the head unit had shut down? The odd thinkg is that the thump happens a good few seconds after the head unit has apparently shut down. The sequence is:

1) Press "off", or turn the ignition off.
2) Music stops. Shut down logo displays on the head unit.
3) Aerial retracts.
4) Shut down logo dissappears.
5) Wait a couple of seconds...
6) THUMP.

If it doesn't thump, then I can hear a faint click (which might just be a very soft thump).

Does that give you any ideas? What do you think of using a relay box to physically disconnect the speakers on shut down?

 

[Ayub]

the "thump" is a relay/diode in the amp that is faulty or the amp is a "crap" make/model" and it did not come with it.


I cant see you mention a amp make or model so i cant really say for sure. However its one of these. Has this just stared to happen? In that case its more a faulty one. If its a new amp get another to try it ( same make) or it may be simply down to " you get what you paid for" .


Get amp repaired ( about £30/40)
change amp
While you at it check ure ground i'm sure its not that but you never know

 

[simplyincaraudi]

Hi guys,

The most frequent cause for this i've come accross has been the connections!
The best thing to do BEFORE spending money for repairs is check ALL connections. Make sure the earth on the headunit is at least the same size as the main power lead, ensure all wires are crimped OR soldered, check your speaker wiring as i have seen this be the cause once before! If you have an amp check the earth and make sure it's bolted/screwed to shiny metal, "if its dull its dead".

These are the only real problems i've come accross in my audio carrer! Only seen a couple of knackered amps be the cause BUT that amp breaking caused by a bad earth anyway.

Hope this helps!

 

[Digsy 1971]

the "thump" is a relay/diode in the amp that is faulty or the amp is a "crap" make/model" and it did not come with it.


I cant see you mention a amp make or model so i cant really say for sure. However its one of these. Has this just stared to happen? In that case its more a faulty one. If its a new amp get another to try it ( same make) or it may be simply down to " you get what you paid for" .


Get amp repaired ( about £30/40)
change amp
While you at it check ure ground i'm sure its not that but you never know

Thanks for the reply but I'm afraid you havn't read my posts properly. The head unit is a Dension DH-201 coupled to a Dension DMP3A MP3 player - about £500 quid's worth of kit so I doubt "you get what you pay for" would come into it. There is NO extenal amp. I've just installed it with all soldered connections. I upgradded the power and earth wires to something capable of taking 10A (the car stereo fuse is only 7.5A) and the chassis is bolted straight into the car frame. It's not the earth.

 

[Digsy 1971]

Hi guys,

The most frequent cause for this i've come accross has been the connections!
The best thing to do BEFORE spending money for repairs is check ALL connections. Make sure the earth on the headunit is at least the same size as the main power lead, ensure all wires are crimped OR soldered, check your speaker wiring as i have seen this be the cause once before! If you have an amp check the earth and make sure it's bolted/screwed to shiny metal, "if its dull its dead".

These are the only real problems i've come accross in my audio carrer! Only seen a couple of knackered amps be the cause BUT that amp breaking caused by a bad earth anyway.

Hope this helps!

Thanks for the reply. As above, I don't think any of the connections are suspect. As for the speakers, they worked fine with the steror I took out before fitting the Dension.

I'm surprised that no-one can realy tell me what is causing this. Power on and off thumps are common in home audio setups (yes, even expensive ones) especially if you don't turn your components on and off in the right order. If I ever have to switch my home kit off when I go on holiday my sub ALWAYS thumps when I turn it on no matter what I do.

I've bought the bits I need to make the relay box now.

 

[richjthorpe]

Darren,

Sorry I haven't replied earlier.

The thump is due to a switch in the Amp shutting of while the volume is still turned up. Soft Turn Off basically turns down the volume on the amp before switching off completely. Try it with yours, before turning the ignition off, turn the head unit to zero volume and see if there is a thump.

Richie.

 

[Digsy 1971]

Hi Richie,

I did do some experiments with this back around the time of the first post, and I did think that it was helping, but its not 100% successful. I still get the odd thump when the volume is down. As the amp is built into the head unit I have no control over when it switches off, but as the thump occurs well after the head unit kills the "aerial up" signal I figure I can use this to control my relay box. If all goes to plan the feed to all four speakers will be cut several seconds before whatever causes the thump happens.

 

[StuD19]

Hi mate, i had a similar problem with a head unit of mine and basically what had happened was somebody had the music turned up loud and pulled the RCAs out the back of the head unit. I sent it off to get fixed and it only cost me about £30. can't remember exactly what happened inside the head unit as it was a long time ago. I would advise to take it to an ice shop but preferably one you know as some can try to feed you all sorts of rubbish. hope you get it sorted mate.

 

[Digsy 1971]

Well, I've fixed it using a 4-pole relay with a 75mA coil and 5A contacts, powered from the permanent live wire (yellow) and triggered via a 1A NPN transistor using the aerial signal (blue wire). The whole thing lives in a smal box and I've put male and female ISO blocks on the flying leads so its just plug and plau into the existing loom. Works a treat