Amps go burrrrrweeeehp when turning off - diyAudio
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Old 5th September 2009, 09:04 PM   #1
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Default 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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Old 5th September 2009, 09:15 PM   #2
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Hi,

Quote:
Originally Posted by underwurlde View Post
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!
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Old 9th September 2009, 08:21 AM   #3
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OK I understand that. The relay idea is a cure but not a prevention.

Any other insights please?

Cheers,

Andy
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Old 9th September 2009, 08:49 AM   #4
juma is offline juma  Germany
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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.
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Old 9th September 2009, 09:12 AM   #5
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^ yes, good point.
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Old 9th September 2009, 09:14 AM   #6
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Power supply is bog standard Torroid > Single full rectifier > 2x 6,800uF caps to give +33V, GND, -33V

Simple. (no regultors etc).

Andy
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Old 9th September 2009, 09:25 AM   #7
81bas is offline 81bas  Germany
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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
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Old 9th September 2009, 09:31 AM   #8
juma is offline juma  Germany
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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 !
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Old 9th September 2009, 09:43 AM   #9
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^ 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?
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Old 9th September 2009, 10:02 AM   #10
juma is offline juma  Germany
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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.
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