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I WANT TO BELIEVE (in variable L-pads on mid drivers)

But I can't.



Variable L-pads ("attenuators") are largely out of favor in high-end audio circles even when power handling and reliability aren't such issues as they are for bass guitar enclosures. Many bass cab manufacturers and builders still seem to think a "knob feature" on the outside of the cab has higher cachet than actual audio performance. But designers elsewhere grok the downsides. So they are more than willing to remove a failure point and some incidental cost. The better bass cab designers are avoiding these attenuator toys except on tweeters. Even if reliability considerations are low in the docket, there are still plenty of issues warranting avoidance of variable attenuators on mid drivers.

  1. Variable L-pads are notoriously inaccurate at scaling faithfully because of their winding-and-contact scheme. For bass guitar, this twitchy scaling is not such an issue in the tweeter region where there is less important content. But for the all-important midrange I sure hate to introduce intermittent gackiness. I've had enough problems with crackling tweeter attenuators that jump levels erratically and sometimes burn out.

  2. A midrange driver is operating in an area of the spectrum where there is almost always appreciable content, and thus output. When one is driving a fEarful with a powerful amplifier that is really getting asked to make big SPL, they can be stressing even a 100-watt variable L-pad - especially if it is turned down. The mid driver can often see peaks of twice its rating and the common attenuators are not rated for that. This is not so different than selecting components for passive crossovers. More than a few times I've seen burned-out crossovers because the parts were not stout enough to handle enough current. Over the long term they fail.

    In the case of attenuators this weakness in the high-demand midrange is largely attributable to the winding and contact scheme, where the point of contact is bottle-necked by the inner resistor which is typically made of thin wire that cannot handle as much current. If the L-Pad fails it will usually burn out the inner resistor and the level will no longer be adjustable. If it fails it will also alter the impedance of the speaker and this will change the cross-over point of any passive crossover network that is driving this speaker. It can sometimes take out other components too.

  3. Passive crossovers are highly reactive when combined with drivers. When you change padding values on the fly other crossover attributes besides level-matching also get changed . Normally a crossover is designed with a fixed pad for accuracy and good off-axis response in the crossover region. The crossover region is generally at least an octave in each direction from the crossover point and sometimes considerably more.

    And there again, it's in the all-important midrange region. So changing the driver attenuation can also introduce crossover-area audio shifts that aren't desired. These may not be so noticeable to the player in the near field who's just trying to dial in a tone, but they can play havoc in the mid-field and beyond. It's tough for bass players to grasp perhaps that good tone should be more consistent for the audience and band-mates as well, but that's part of what I am shooting for.

  4. L-pads are a blunt-force global operation. Decent EQ facilities are capable of finer-grained control. Why move the entire midrange driver response up or down to attempt to respond to changes in preference or venue acoustics when an EQ cut or boost in a specific range or two would actually voice things better? EQ is more predictable as well. It doesn't shift the crossover point or play havoc with the polar response.

  5. If there is a tweeter, it is likely on a variable L-pad itself. Why confuse issues with another one? That can really skew imaging when there are already superior resources to change spectral balance - or should be anyway - on your bass, your head/preamp... even enclosure positioning should get some attention. It can also be counter-productive to have too many interacting adjustments to address.

  6. This last point is just my preference for rig configuration - by no means a universal approach or philosophy: I send identical signals from my preamp that are post-EQ and post-effects - one that goes to my power amp and enclosure(s), and one that serves as D.I. So I want my stage enclosures to have somewhat the same tonal profile as a P.A. stack. If I've got a variable attenuator on the midrange driver that's not rotated to roughly match the woofer (at parity), that scheme is just not going to give reliable indication - and it's going to make it tougher for the live sound engineer. I want my sound easily translatable to the big picture.

Anyway, some people are not going to get any of this. The above points might not yet seem to matter - the poor results have never been noticed in measurements or heard. Maybe some don't realize that the midrange imaging or reliabilty are not optimal... After all, bass players are used to seeing attenuators on tweeters and maybe don't know the difference between that and midrange drivers. And if a cab manufacturer is lackadaisical about crossovers in general anyway, what's one more shortcut? I think that mindset is absurd though, and I think many people will see some sense in at least a point or two here.

Those who do, repeat after me: Variable mid pad - BAD BAD BAD!

* including designers of quality home audio, studio monitors, sound reinforcement main and fold-back enclosures

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