What is a preamp actually doing? - diyAudio
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Old 9th July 2013, 11:46 PM   #1
thommy is offline thommy  United Kingdom
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Default What is a preamp actually doing?

I'm building a single source system in one box. i have the dac as a source and two amps with power supply to drive the speakers. I need to be able to control the volume as i don't want the amps running at full whack all the time.

Normally there would be a preamp in-between to select and then control the signal sent to the amp. As I don't require channel selection, what is this stage doing other than attenuating the signal, and is there any reason why i can't just just put a stereo stepped attenuator between the output of the dac and the input of the amps?
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Old 9th July 2013, 11:49 PM   #2
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It is an amplifier of low voltage and sometimes contour and "tone" control that prepares the "signal" for a Power Amplifier.
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Old 9th July 2013, 11:52 PM   #3
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Yes, an attenuator will do exactly as you describe but there will be no modification of the signal ... coping for loudspeakers that have a non linear response, hence tone controls etc.
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Old 10th July 2013, 12:14 AM   #4
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Thankyou. Would the possibility of doing this depend on the output level of the dac and the input impedance of the amps? Or is that not an issue. Tone controls are of little interest to me so it seems like a winner if it will really work.
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Old 10th July 2013, 12:21 AM   #5
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As long as the DAC has enough drive capability for the amps to reach the desired output, then a nice attenuator is perfect.

"Drive capability" includes voltage swing and current capability. If the amps have reasonably high impedance and the attenuator impedance is high enough, most DACs will drive it without running out of current capability. I'd stay at least 20K.

Voltage swing is potentially an issue. Industry standard is 2VRMS, is that enough to drive your amps to full output? It should be, at least with a fairly typical amp with 26dB of gain up to a couple hundred watts. It may feel odd that you have to turn the knob up a lot higher than you are used to, though.

One other concern is cable capacitance. If you intend to put your amps in the same case as your DAC or within a few feet, that's not an issue. If you are going to drive 12 foot long cables, you should consider a buffer after the volume pot. As cable length increases, so does capacitance and the importance of a low impedance source.
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Old 10th July 2013, 12:44 AM   #6
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I think you have the jist. A power amp is just that, it amplifies what is fed to it. The pre-amplifier just gets everything to the same level for power amplification.
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Old 10th July 2013, 12:51 AM   #7
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Thanks guys. The amps I'm looking at have input impedance listed as 2x10Kr, and I can assume my DAC board has standard output levels as the volume knob wasn't touched when i swapped it over from a retail unit.
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Old 11th July 2013, 10:45 AM   #8
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10k for input impedance is quite low.
You may need to buffer a vol pot to drop the output impedance sufficiently to drive that 10k.

Take care with DC levels.
Is your power amp AC coupled or DC coupled?
Is the output from the DAC suitable for a vol pot?
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Old 11th July 2013, 11:06 AM   #9
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Quote:
Originally Posted by thommy View Post
Thanks guys. The amps I'm looking at have input impedance listed as 2x10Kr, and I can assume my DAC board has standard output levels as the volume knob wasn't touched when i swapped it over from a retail unit.

what is the output impedance of DAC board ? if less than 1k then you are fine...
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Old 11th July 2013, 04:32 PM   #10
qusp is offline qusp  Australia
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what is the DAC? digital volume may be doable, that can be part of the DAC functionality, or many of the decent players like audirvana, puremusic etc, have rather good volume controls now. if you have sorted out your gain structure well enough, so that for normal listening you have the volume up a reasonable way, then you have no problem. the poweramps are on 'full whack', whether you have a volume control there or not.
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