Amps go burrrrrweeeehp when turning off

underwurlde · 5th September 2009, 09:04 PM

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 · 5th September 2009, 09:15 PM

Hi,

Quote:

Originally Posted by underwurlde

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 · 9th September 2009, 08:21 AM

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

Any other insights please?

Cheers,

Andy

juma · 9th September 2009, 08:49 AM

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 · 9th September 2009, 09:12 AM

^ yes, good point.

underwurlde · 9th September 2009, 09:14 AM

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

Simple. (no regultors etc).

Andy

81bas · 9th September 2009, 09:25 AM

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 · 9th September 2009, 09:31 AM

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 · 9th September 2009, 09:43 AM

^ 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 · 9th September 2009, 10:02 AM

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.

5th September 2009, 09:04 PM	#1
underwurlde diyAudio Member Join Date: Feb 2005 Location: S.England	Amps go burrrrrweeeehp when turning off 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

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9th September 2009, 08:21 AM	#3
underwurlde diyAudio Member Join Date: Feb 2005 Location: S.England	OK I understand that. The relay idea is a cure but not a prevention. Any other insights please? Cheers, Andy

9th September 2009, 08:49 AM	#4
juma diyAudio Member Join Date: Nov 2005 Location: Zemun	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.

9th September 2009, 09:14 AM	#6
underwurlde diyAudio Member Join Date: Feb 2005 Location: S.England	Power supply is bog standard Torroid > Single full rectifier > 2x 6,800uF caps to give +33V, GND, -33V Simple. (no regultors etc). Andy

9th September 2009, 09:25 AM	#7
81bas diyAudio Member Join Date: Jan 2006 Location: Kiel	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

9th September 2009, 09:31 AM	#8
juma diyAudio Member Join Date: Nov 2005 Location: Zemun	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 !

9th September 2009, 09:43 AM	#9
underwurlde diyAudio Member Join Date: Feb 2005 Location: S.England	^ 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?

9th September 2009, 10:02 AM	#10
juma diyAudio Member Join Date: Nov 2005 Location: Zemun	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.