Attenuators

[jksuperstar]

There seems to be sporadic comments about attenuators (be it passive or active) around the forum. It seems to be a very good idea, and deserves it's own coherent thread. I'd like this thread to be used for describing the pros & cons of various types of attenuators, plus describing products available.

The idea behind an attenuator is that the volume of the digital outputs of your DAC are typically also digitally controlled (ie-pre-DAC). SO if you turn down the volume, you are actually reducing the number of bits used to generate audio, and so you are loosing resolution. A 24 bit output, when turned to low volumes, can result in resolutions at less than 1/2 the available bits! So, the fix is to turn the DAC all the way up, then insert an attenuator on the signal before it reachs your monitors. This controls the volume in the analog domain, so there's no loss in bits.

Attenuators come in 2 flavors: Passive or Active. Passive uses no powered components (ie- typically only resistors), while Active can use gain stages. There are pros & cons to using either one, and may depend on your situation.

Passive:

PROS - Does not use active electronics, so there is no gain stage. This means it can't amplify noise (but doesn't mean it can't introduce noise (thermal, etc).) Simple construction, tends to be very durable, and does not require power.
CONS - Because it only uses passive components, the load on your DAC output may not be ideal, and this *can* effect the frequency response of your *system*. Yes, it's just a resistor (or resistor divider), but once you include the capacitance of your cables, the input to your amp, and the output of your DAC, there can be side effects. Probably only audible if long cables were used. Also, more difficult to implement accurately if balanced lines are used.

Active:
PROS - Can also act as impedence matching, so your DAC is driving a near ideal load. Active components can drive multiple outputs simultaneously, or can incorporate filters or other sound treatment to accomodate for rooms (such as an RTA tuner).
CONS - Active gain stages can insert noise, or act differently from your DAC, so the linearity of your signal is effected. A well designed unit shouldn't be effected by these short comings, however.

[jksuperstar]

First up, a cheap DIY kit from the guy at TubeCAD:

http://glass-ware.stores.yahoo.net/aikampstepat.html

Passive, uses a stepped-attenuator, so it doesn't have ill effects of potentiometers, like dirty contacts that add "scratching" sounds as you adjust the level. Won't have smooth volume sweeps, but WHO CARES? It's not a performance volume.

[elektro80]

A most excellent thread!

I`m all for handling monitoring gain in the analog domain, but I guess you all know that already.



I should add that most semipro / pro monitors have balanced inputs and most semipro/pro audio interfaces have balanced outputs. An MGC should be able to handle a balanced signal.

[elektro80]