Amps go burrrrrweeeehp when turning off

[underwurlde]

My amp upon turning off takes about 10 seconds to turn off (LED dims). During the final 1 second of power down there is a audiable whining, decreasing volume (obviously), increasing frequency, kind of like 'burrrrweeehp'.

Any ideas as to what may be causing this?

Cheers,

Andy

 

[Geek]

Hi,

Any ideas as to what may be causing this?

Bad design.

Amps as their supply bleeds off can get unstable and oscillate, or dump DC through the speaker.

A speaker delay relay can be a simple fix as not only does it add a turn-on delay, but cuts them off as soon as the power is cut.

Cheers!

 

[underwurlde]

OK I understand that. The relay idea is a cure but not a prevention.

Any other insights please?

Cheers,

Andy

 

[juma]

It's not very probable that you'll get a valid answer to your question if you don't provide more details on your amp and PSU.

 

[underwurlde]

^ yes, good point.

 

[underwurlde]

Power supply is bog standard Torroid > Single full rectifier > 2x 6,800uF caps to give +33V, GND, -33V

Simple. (no regultors etc).

Andy

 

[81bas]

Try to place a 0.1uf cap in parallel to R10
Basically, this cap is not intended to prevent burrrwwheeps, but who knows, which processes are starting in this schematic under low rail voltage

 

[juma]

Well, Geek was right (post#2) - this design is so bad that it's a small wander that it works at all.
And you got a funny case of two wrongs that make a right (and screwing everything else in the process): no Vbe multiplier bias temp. comp. but there are enormous output trans. Re (2R2) to make up for that.
If I were you I'd build any of good and proven amps from this site (symasym comes to my mind) but if you don't find it appealing, even a LM3886 would be a worthy upgrade (and you can use same heatsink and PSU).

Cheers !

 

[underwurlde]

^ Now that is interesting.

I am trying to learn about amps.... so I have started with an Cambridge Audio A1 (from the 80's / 90's) and am 'playing' with it.

This is my (poor) tinkering of it!

"no Vbe multiplier bias temp. comp."

How would one implement this?

 

[juma]

You can start from here:
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm

Final schematic is at the bottom of that page - Vbe multiplier is TR13 which is thermally connected to output transistors.

 

[underwurlde]

Thanks guys! Thanks for taking the time. I will have a look at that thread & 'play' & HOPEFULLY learn!

Andy