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Old 9th July 2011, 04:12 PM   #1
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Default Amp attenuation scheme discussion

Mods, I thought about putting this in the construction thread and if that is a better place for this then please move it there but it seemed to be more about enclosures there. I've searched before posting this. There were no threads that specifically addressed the issue of where to attenuate. I don't know very much about audio design so please correct me where you can. Ideally we would like to design a system where the noise of any audio component is not amplified. An extreme case would be to have the only attenuation on the outputs of the amp. This is where the noise of the pot would be of the smallest relative value to the signal, right? What are the issues that make this practice undesirable?

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Old 9th July 2011, 07:36 PM   #2
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The main issue is probably finding and paying for a 1/2 watt attenuator versus a 100 watt attenuator. It's impossible to build an amp that doesn't amplify some noise. The goal is to have such low noise that even amplified it is inconsequential.
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Old 9th July 2011, 07:52 PM   #3
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sofaspud
Noted, but I was thinking more along the lines of a switch or relay system simply for the convenience of layout. Even if we go with pots, wouldn't two 100W ceramic pots cost less than a line stage? Has any manufacturer done this with success (ie: make an integrated with pots on the outputs)?
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Old 9th July 2011, 08:38 PM   #4
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Your idea isn't totally unheard of. Maybe these guys can help you decide. If I were contemplating the issue I'd have to run some numbers. I'm not aware of a commercial hi-fi amp that uses output attenuation (but they probably exist). We're talking another heat sink so its practicality is questionable, at least on the surface. Without numbers it's hard to get deeper.
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Old 9th July 2011, 11:33 PM   #5
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sofaspud
I emailed the guys at the link you provided so we'll see what they say. Why would you need an additonal heatsink for the output signal control? Isn't the output power 0 Watts when fully attenuated or fully on and only half of the amps rated power when at half power output?
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Old 10th July 2011, 12:18 AM   #6
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I'm not really sure what you're trying to accomplish, but it sounds like you want to attenuate the high level signal level between the amp and speakers to reduce percieved noise level? If so, that's the hard way to do it. Any decent amp won't generate any audible noise, so it would be a lot easier to do it at the low level input of the amp.

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Old 10th July 2011, 02:13 AM   #7
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Michael Bean
Thanks for joining this discussion. You have hit upon the point of this discussion for me. What exactly makes it harder to attenuate the signal at the output given the advances in source selection and optical coupling that we have available to us nowadays. Is it only a matter of power handling? Given the non-linearities and noise level of most stereo pots, I would like to understand what keeps designers from attenuating at the amp output.
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Old 10th July 2011, 02:35 AM   #8
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Well, I don't know what power level you talking about, but to attenuate appearant loudness level by one half would require dissapating 90% of the output power as wasted heat. So, for example, if your amp is putting out 100 watts, you would need to turn 90 of those watts into heat, not a trivial matter, and also very wasteful. That's why it's usually done on the input to the amp where the power levels are much easier to deal with.

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Old 10th July 2011, 02:41 AM   #9
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Another question, are you talking about hi-fi or musical instruments?

Mike
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Old 10th July 2011, 02:47 AM   #10
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Michael Bean
Oh, if that is true then that is a VERY good reason not to attenuate at the outputs but...if the amp is unattenuated there is no power dissipated in the attenuator and if output signal is 0 or completely attenuated then no current is flowing and again, no power is dissipated in the attenuator, correct? Please explain how 90W gets dissipated in the attenuator on a partial signal output? Strictly hi-fi at this point, clean signals and no designed phasing or distortion.
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