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Old 7th May 2017, 11:48 AM   #1
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Default Proper attenuator

Hi

I use a DAC connected directly to a power amp to drive my speakers. The DAC has digital attenuator which has to be set to ~-40db for comfortable listening volume. It is said to be a bit too much of digital attenuation (low SNR), and I was suggested to use additional attenuation.

I checked in-line attenuators, which is the cheapest and simplest approach, but some folks here say they're not good. I could also buy a passive preamp, but then again, it adds the distortion of it's own.

What is the recommended approach in my situation?

Last edited by dimedrol2; 7th May 2017 at 11:50 AM.
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Old 7th May 2017, 12:10 PM   #2
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Make a "passive preamp" with a potentiometer, simple. Much better than using the DAC's digital attenuator
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Old 7th May 2017, 12:49 PM   #3
DUG is offline DUG  Canada
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Look here:

Help in choosing a potentiometer as a "Passive preamplifier"
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Old 7th May 2017, 01:12 PM   #4
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I think I'd be just fine to have a fixed attenuation of -20db. And I believe such a device would be simpler and cheaper too, comparing to passive preamps. But then I read folks on this forum saying that in-line attenuators suck. Are they really that bad, it doesn't make sense to look in their direction? If so, what makes passive pre's a better choice, provided I am ok with fixed attenuation? Or are those folks basically saying that all types of passive attenuation suck?
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Old 7th May 2017, 01:19 PM   #5
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It sounds like you want good quality. Then the best way is no digital attenuation, then the only way is analogue, and that is achieved with a potentiometer. Like I said, simple
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Old 7th May 2017, 01:20 PM   #6
AndrewT is offline AndrewT  Scotland
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you can buy fixed attenuators. They are not cheap.
You can build your own fixed attenuator and it will fit inside the RCA plug.
A -20dB attenuator can be created from a 10k and 1k0 pair of resistors. (not exactly -20dB but probably close enough).
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Old 7th May 2017, 01:36 PM   #7
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do passive preamps have any advantage over in-line attenuators at a fixed attenuation level?
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Old 7th May 2017, 01:44 PM   #8
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No, the advantage would be that you can leave the digital attenuation at zero and adjust your listening level with the potentiometer (volume control) and achieve maximum sound quality at all levels.
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Old 7th May 2017, 01:48 PM   #9
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Here's what John Westlake (audiolab MDAC designer) says about passive preamps:

Typical passive pots will introduce there OWN thermal noise source – a 50K resistor has a 107dBV noise contribution at 25Deg.C 20K BW… this needs to be added to the systems noise figure (but bare in mind the absolute valve changes depending upon the volume position – thermal noise is determined by system Impedances)… the advantage of direct connection between DAC output and Power amplifier is you don’t introduce additional noise sources… Off the top of my head, MDAC’s EIN (Equivalent Input Noise) is about that of a single 2.5K resistor.

As you can see, there’s no point worrying about Digital verses Analogue gain control, you will ultimately face the same limitations of thermal noise – however its somewhat easier with a Digital preamplifier to reduce the systems total thermal noise levels by maintaining a lower impedance signal path to the amplifier (Higher circuit impedance = higher thermal noise levels).

A Passive pot just attenuates the signal into the system noise floor (while contributing a significant noise source of is own). The Digital system attenuates the signal into the amplifiers noise floor – as the passive pot, but WITHOUT the extra noise contribution of higher circuit impedances a passive pot introduces).

Only by having ultra low impedance levels can a passive pot result in improved SNR - however you would need a power amplifier to drive these circuit impedances without introducing distortion due to the current requirements to drive these impedances. Feasible, but not easy!!! and not with a typical Pot - also, Pots become very non linear at higher currents - you would need to use a stepped attenuator... say 50 Ohm environment, 2V RMS... distortion free is going to require current and result in significant power dissipation... one things for certain a typical 50K or even 10K passive pot is not going to cut the mustard here...
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Old 7th May 2017, 01:53 PM   #10
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What is meant by digital attenuation as he is applying it here? He uses the term "Digital verses Analogue gain control" but we are talking about attenuation not gain.
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