Friday, June 5, 2015

Adjusting level for ABX

My QSC ABX box does have a built-in potentiometer system, though I'm not sure I want to use it for a whole host of audio perfectionist reasons.

Instead, I want to use a small passive attenuator box right before the power amp.  This will allow the high impedance output of the attenuator to have as short a cable as possible, perhaps 1f or 0.5M.  That's important because with a given amount of source resistance added by the attenuator, an RC network is created, and the longer the cable the more the C and the lower the frequency at which rolloff or phase shift of a particular magnitude appears.

I also want the passive attenuator impedance to be as low as possible.  5k sounds good.  It shouldn't be too low, such as 1k or lower, because that often causes more distortion in the source that feeds the attenuator.

The potentiometer should NOT be stepped as it will be necessary to adjust to 0.05dB or better.  Fake steps might be OK but I'd rather avoid them.  Wirewound may add inductance (but might actually be OK).  Two mono pots and not stereo (though two stereo pots would also work).

I've seen rankings like this from cheapest to best.

0) Fake Blue Velvet pots from China (this may be what I got on ebay a few years ago).  Some say these aren't too bad, if not as good as the genuine article.  Others say nothing like the real thing.  Said to be counterfeited more than Gucci handbags.

(Note: Even real Blue Velvet may have fake "steps," so I'm not sure I'd want it at all.)

1) Real Alps RK27 Blue Velvet from trusted source (e.g. PartsConnection)

(Note: Black Velvet is a real stepped attenuator.  I think Blue is fake steps.  I'd rather have no steps whatever.)

2) Nobel AP25

2.5) Bourns NOS, Allen Bradley NOS

3) TKD

Here's a Vishay 0.75W 26 turn bulk foil 5k trimmer.

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