Zobels, bass shelves, oh my!

[CrowDaddy]

I notice that Morgan Jones, in his article on the Arpeggio full-range single driver speaker, uses a Zobel network to flatten the impedance curve at high freqs, particularly for use with SET amps.  He also adds bass shelf correction.  However, other full- range designs omit these., including , apparently, your Pipette and the big Voxatives.  What’s your take on impedance correction for SET amps?

 

[Paul Birkeland]

If you have a full range driver that rolls off a little bit on top, the low damping factor of a SET amp in conjunction with inductive impedance rise from the voice coil will tend to flatten that out, so this will be both dependent on the driver in question and the amplifier you intend to use. 

Bass shelf correction will reduce system sensitivity, so that's something to consider experimenting with, but you may end up not being too happy with the tradeoffs. 

 

[Paul Joppa]

I notice that Morgan Jones, in his article on the Arpeggio full-range single driver speaker, uses a Zobel network to flatten the impedance curve at high freqs, particularly for use with SET amps.  He also adds bass shelf correction.  However, other full- range designs omit these., including , apparently, your Pipette and the big Voxatives.  What’s your take on impedance correction for SET amps?

My take is that it depends on the speaker and the amplifier.

Most amps use feedback to increase the damping factor, and most speakers are designed to work with high-damping amps. Our speakers are designed to work with zero-feedback low-damping amps.

PB described the effect of voice-coil inductance at high frequencies. An RC Zobel will also make the high treble independent of damping factor. Many drivers, including the one used in the Pipette, have a conductive flux ring to reduce the inductance, making the difference small.

Low-frequency Zobels to do the same are theoretically possible but impractical - the necessary inductors and capacitors are huge and costly.

Baffle step correctors (shelving filters) are commonly used at speaker level when the cabinet is narrow and/or when the single driver has a rising mid-frequency rise. They work by attenuating the response above the upper bass/lower midrange - reducing the efficiency at those frequencies. A better option when using low-power amps is to do the attenuating at line level, before the power amp input. Efficiency is not reduced. We used to offer an adjustable passive filter called the "Fix" to do this, but sales were very few and it was discontinued more than a decade ago. It would be interesting to try it with the Pipette ...

 

[Altecboy]

Hello PJ.  Is there a schematic for the fix?
thanks
Les

 

[Doc B]

The Pipettes are interesting speakers in that the drivers have a somewhat scooped mid in open air. Putting the driver in a tapered pipe seems to achieve some of the same subjective effect as a baffle step correction filter - as does getting the speakers up against the wall. As for the top end, I did put Zobels on the drivers to see what they would do for the subjective high frequency performance. They made it rather harsh so they were abandoned after going back and forth with and without the Zobel for a while. In this particular application a diffraction ring of felt has more positive impact on subjective frequency response than the zobel - implying I suspect that there is more to be had working on top end smoothness than there is top end extension. They are teeny metal drivers and they go pretty high.

I have tried a Fix on them. Took it off. There seems more to be had in adding a Subette subwoofer for response below 70Hz than manipulating the frequency extremes of the Pipettes.