Amp powerdown sound

Discussion in 'AV Receivers' started by Jason GT, Sep 20, 2003.

  1. Jason GT

    Jason GT Second Unit

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    I have an odd situation with a 5 ch Power amp. Is the following abnormal?

    Currently I have the 3 front channels of this amp hooked up. It seems to play fine. However, when I power down the amp, source etc, I get a quiet but noticeable whine emanating from the center speaker. It seems to increase in pitch as it decreases in volume. It occurs maybe 10 sec after the amp is switched off. There are no sounds from the other 2 front channels, have not tried the surrounds.

    Is this a cause for concern?
     
  2. Wayne A. Pflughaupt

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    Wayne
    Generally, if you have outboard amps they should be powered up after the pre-pro and other gear is turned on, and on system turn-off, before the pre-pro and other gear is turned off. On power-down, wait 30 seconds or so for the amp’s capacitors to discharge before turning off the other gear. This process should eliminate the noise you’re hearing.

    You can get line conditioners with sequencing functions from Panamax and Adcom that will do this automatically.

    Regards,
    Wayne A. Pflughaupt
     
  3. LanceJ

    LanceJ Producer

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    Wayne: Why don't the amp designers use a relay (like most receivers) to immediately disconnect the output transistors from the speaker jacks when the power switch is pushed?

    And some amp trivia: a few years ago I was checking out some Adcom amps and the salesman gave me a demonstration of their heavily-built power supplies (he used a 200 watt per channel stereo model). While the music played at a pretty loud level through some floorstanders--you had to raise your voice to be heard, but didn't have to yell though--he switched off the amp........and the music kept going and going. It finally faded out after 10 seconds (I was counting) all the while sounding totally normal. I don't know if that meant anything technically speaking, but I still thought it was impressive!

    And if you want to easily see the capacitors which allowed the above demonstration (these components don't just filter the incoming power, but also supply the amp with extra power during loud musical passages when the 120 volt line can't) check out B&K's amps. Most B&K's have large open-mesh grills on top & you can see the enormous caps in there, looking like squat baby-blue soup cans. Large caps like that are very expensive (same story with large power supply transformers), which is why most mid-fi receivers don't use them and why such receivers don't sound as good as a properly made standalone power amplifier, particularly at high volume levels or with low impedance speakers.

    LJ
     
  4. Dave Milne

    Dave Milne Supporting Actor

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    Lance,
    Quality, high-current relays are expensive and can be unreliable. Most power amps don't use relays because the amp can be designed to shut down gracefully as the power supply caps bleed off. And as Wayne said... the power amps are turned off first so source component shutdown "funnies" are not an issue. With a receiver, the current demands are usually lower, and there's a bunch more "stuff" upstream that can cause shutdown funnies, so the designers often opt for a relay.

    BTW, I wouldn't worry about weird noises during shutdown... as long as they are low level and pose no threat to the speakers...
     
  5. Jason GT

    Jason GT Second Unit

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    Thanks Wayne, Dave, Lance for the replies.

    I tried shutting off the source first but it doesn't help - I still get the powerdown sound. But, I have it on Dave's word and professional reputation [​IMG] that this is nothing to worry about (it is at a low level).
     
  6. LanceJ

    LanceJ Producer

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    Dave, thanks for the info.

    BTW: Do you know why they don't put power meters on amps or receivers anymore (except McIntosh of course)? I thought they were pretty useful, even if they weren't 100% accurate. And I'll admit it, they looked kewl too!

    LJ
     

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