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On 4/19/2015 12:03 PM, Corey Bailey wrote:
>
> Let's start with the fact that active equalizers make better filters
> than they do gain amplifiers. I proved this on the bench years ago. In
> fact, it caused me to re-think the way I approached recording: listening
> for what I wanted to remove instead of boosting what I thought was lacking.
>
> When boosting frequencies with an active equalizer (API, MCI, Neve,
> Radial, etc.) you get unwanted phase distortion, not to mention running
> out of headroom if you are talking about the amounts required for record
> EQ. The reason is that, from a design standpoint, the vast majority of
> equalizers used in pro audio equipment (at least, all of those you
> mentioned) have the RC components in the feedback loop. Thus, the phase
> distortion when boosting beyond 3dB or so.
> In the case of record EQ, we are talking about only boosting the low
> frequencies for playback but, that is the area where phase distortion
> can be most difficult to hear. However, it is certainly measurable.
>
> My $0.02

Corey's right; equalizers indended for recording use usually have 
bell-shaped curves, and are 2nd-order. De-emphasis curves for phono 
discs are 1st order, and trying to approximate them with recording-type 
equalizers is *very* approximate and will indeed lead to phase issues.

Whereas record de-emphasis curves can be implemented accurately using 
simple RC filters. With appropriate buffering between them, they get the 
amplitude and phase right, to macvh the recorded curve.

Peace,
Paul

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