L-pad versus Autoformer for Tweeter Attenuation
Tweeter attenuation
In a two-way speaker system, it is common for the tweeter to have higher sensitivity than the woofer. This allows the tweeter sensitivity to be dialed back for preferred integration with the woofer. Dialing back the tweeter is most frequently done with an adjustable two-resistor control called an Lpad, but is sometimes done with a switch controlled multi-tapped coil called an autoformer. To my knowledge, all original Altec systems were designed with resistor or L-pad control of tweeter sensitivity.
About L-pad level control for tweeters
My crossover designs typically use an L-pad to adjust the sensitivity of the tweeter. If a supertweeter circuit is added, a separate L-pad is used to control the supertweeter sensitivity. My experience is that the L-pads I use are robust, reliable, and impart no noticeable degradation to music reproduction. In my own system I use L-pads I've owned for many years. They continue to provide beautiful and reliable sound reproduction. High-quality L-pads are relatively inexpensive, $32 per pair in 2023, helping to keep crossover costs under control. Here is a photo of a high-quality 100 watt rated L-pad available from Parts Express. These are available in 8 and 16 Ohm impedances.
In a two-way speaker system, it is common for the tweeter to have higher sensitivity than the woofer. This allows the tweeter sensitivity to be dialed back for preferred integration with the woofer. Dialing back the tweeter is most frequently done with an adjustable two-resistor control called an Lpad, but is sometimes done with a switch controlled multi-tapped coil called an autoformer. To my knowledge, all original Altec systems were designed with resistor or L-pad control of tweeter sensitivity.
About L-pad level control for tweeters
My crossover designs typically use an L-pad to adjust the sensitivity of the tweeter. If a supertweeter circuit is added, a separate L-pad is used to control the supertweeter sensitivity. My experience is that the L-pads I use are robust, reliable, and impart no noticeable degradation to music reproduction. In my own system I use L-pads I've owned for many years. They continue to provide beautiful and reliable sound reproduction. High-quality L-pads are relatively inexpensive, $32 per pair in 2023, helping to keep crossover costs under control. Here is a photo of a high-quality 100 watt rated L-pad available from Parts Express. These are available in 8 and 16 Ohm impedances.
Anatomy of an L-pad
An L-pad placed between a crossover and a tweeter uses two adjustable resistors to maintain a somewhat constant impedance load on the crossover as the tweeter volume is adjusted to sound in balance with the woofer. Each resistor is a tightly wirewound nichrome (nickel-chromium alloy) element housed in a circular slot of a ceramic housing. A conductive radial wiper contacts the resistors and is rotated to change the active length of each resistor, thereby changing resistor values. A schematic diagram of an L-pad is shown below.
Notice that as we rotate the wiper to the left, the active portion of the series resistor becomes longer, increasing the resistance, and the active portion of the parallel resistor becomes shorter, decreasing the parallel resistance. The combination of these resistance changes results in modest impedance changes with tweeter level setting. The table below shows the calculated change of impedance seen by the crossover network at various knob rotary positions and corresponding attenuation levels.
Knob Rotation, % Lpad Input Impedance dB Attenuation Lpad Input impedance dB Attenuation
100% is full right for 8 Ohm Lpad for 8 Ohm Lpad for 16 Ohm Lpad for 16 Ohm Lpad
0% is full left Ohms dB Ohms dB
100 6.66 0 11.42 0
75 8.32 2.38 14.43 2.82
70 8.62 2.83 14.98 3.35
50 9.71 4.61 16.89 5.58
25 10.37 7.42 18.15 9.39
12.5 10.08 1o.3 17.8 13.40
It is surprising how little difference these impedance shifts make to overall response curves calculated using the SPICE electrical circuit analysis program. There is nothing here to cause concern.
Knob Rotation, % Lpad Input Impedance dB Attenuation Lpad Input impedance dB Attenuation
100% is full right for 8 Ohm Lpad for 8 Ohm Lpad for 16 Ohm Lpad for 16 Ohm Lpad
0% is full left Ohms dB Ohms dB
100 6.66 0 11.42 0
75 8.32 2.38 14.43 2.82
70 8.62 2.83 14.98 3.35
50 9.71 4.61 16.89 5.58
25 10.37 7.42 18.15 9.39
12.5 10.08 1o.3 17.8 13.40
It is surprising how little difference these impedance shifts make to overall response curves calculated using the SPICE electrical circuit analysis program. There is nothing here to cause concern.
L-pad reputation
I have experienced heavenly music playback using L-pads for tweeter and supertweeter sensitivty control. That said, L-pads seem to have a poor reputation with some audiophiles. I wonder if hearsay, common in discussions of audio, may underlie this perception, or whether there is something to it.
What is an autoformer?
An autoformer (also known as a tapped coil) is an electrical transformer which uses only one coil. The signal voltage is applied to one end of the coil. The other end of the coil connects to ground. The coil can be tapped at one or more points to pick off a fraction of a signal. See the autoformer concept figure below. The large rectangle represents a coil of many turns of enameled copper wire wrapped onto a former. The coil will have a magnetic core made of laminations of silicon steel or nickel, or a mixture of these laminations. I have shown an open magnetic circuit, but the circuit can and most likely will be a closed loop of magnetic material. We are showing 10 volts AC applied one end of the coil. The other end of the coil is grounded (0 volts). We are showing the coil tapped at 10% intervals. Each interval contains 10% of the total number of turns. This gives us access to 10 different voltages relative to ground. We can use a 10-terminal switch to access each of these points. This allows us to control volume. There is more to the theory than this, but this gives the idea.
I have experienced heavenly music playback using L-pads for tweeter and supertweeter sensitivty control. That said, L-pads seem to have a poor reputation with some audiophiles. I wonder if hearsay, common in discussions of audio, may underlie this perception, or whether there is something to it.
What is an autoformer?
An autoformer (also known as a tapped coil) is an electrical transformer which uses only one coil. The signal voltage is applied to one end of the coil. The other end of the coil connects to ground. The coil can be tapped at one or more points to pick off a fraction of a signal. See the autoformer concept figure below. The large rectangle represents a coil of many turns of enameled copper wire wrapped onto a former. The coil will have a magnetic core made of laminations of silicon steel or nickel, or a mixture of these laminations. I have shown an open magnetic circuit, but the circuit can and most likely will be a closed loop of magnetic material. We are showing 10 volts AC applied one end of the coil. The other end of the coil is grounded (0 volts). We are showing the coil tapped at 10% intervals. Each interval contains 10% of the total number of turns. This gives us access to 10 different voltages relative to ground. We can use a 10-terminal switch to access each of these points. This allows us to control volume. There is more to the theory than this, but this gives the idea.
About autoformer level control for tweeters
Dave Slagle of Intact Audio is the only purveyor I know who designs, manufactures and sells integrated autoformer level controls for tweeters. Dave has named these Speakerformers. The Speakerformer has multiple electrical taps, each providing a given amount of attenuation. A multi-position switch accesses these taps. A toggle switch provides attenuation levels halfway between the levels corresponding to the taps. Speakerformers are expensive at $350 per pair, purchased directly from Dave. The cost impact of Speakerformers is doubled if they are required for tweeters and supertweeters. However, some audiophiles will not flinch at this cost if there is a possibility of audibly improved performance. Speakerformers are available in different attenuation ranges and intervals. Below is a photo of Dave's beautifully made products. Like L-pads, the impedance load on the crossover high-pass filter will shift a bit with attenuation. A shunt resistor is soldered by the crossover builder onto the high-quality printed circuit board to adjust for the correct input impedance in the vicinity of the required attenuation setting. As I discussed with the impedance changes associated with L-pads, I think impedance shifts with level setting of the Speakerformers will not significantly affect the response curves calculated for the crossover networks.
Dave Slagle of Intact Audio is the only purveyor I know who designs, manufactures and sells integrated autoformer level controls for tweeters. Dave has named these Speakerformers. The Speakerformer has multiple electrical taps, each providing a given amount of attenuation. A multi-position switch accesses these taps. A toggle switch provides attenuation levels halfway between the levels corresponding to the taps. Speakerformers are expensive at $350 per pair, purchased directly from Dave. The cost impact of Speakerformers is doubled if they are required for tweeters and supertweeters. However, some audiophiles will not flinch at this cost if there is a possibility of audibly improved performance. Speakerformers are available in different attenuation ranges and intervals. Below is a photo of Dave's beautifully made products. Like L-pads, the impedance load on the crossover high-pass filter will shift a bit with attenuation. A shunt resistor is soldered by the crossover builder onto the high-quality printed circuit board to adjust for the correct input impedance in the vicinity of the required attenuation setting. As I discussed with the impedance changes associated with L-pads, I think impedance shifts with level setting of the Speakerformers will not significantly affect the response curves calculated for the crossover networks.
Why use a Speakerformer?
A reasonable argument favoring autoformer-based tweeter level controls has to do with electrodynamic damping of the tweeter. In principle, an autoformer based control with sufficient attenuation will reduce the source resistance in series with the voice coil resistance, thereby improving electrodynamic damping of the tweeter piston mode, which, in principle, can reduce ringing of the diaphragm at the driver resonant frequency. The resistance of an L-pad reduces tweeter electrodynamic damping, the advantage going to the Speakerformer. I looked online for discussion of the benefits of Speakerformers and found several users who are quite impressed by them. One user's listening experience lead him to believe that the Speakerformers are better than fixed resistor L-pads, which are slightly better than variable L-pads. Therefore, I was eager to find out for myself if I would hear an improvement of reproduced sound through the use of Speakerformers.
A listening session comparing L-pad tweeter control with Speakerformer based tweeter control in my old breadboarded Hiraga-circuit crossovers
I recently had an opportunity to find out first-hand about sonic benefits of the Speakerformers. A customer purchased a pair to be used in a pair of Hiraga circuit crossovers I am building for him. With my old breadboarded Hiraga-circuit crossover networks I use with my Altec A7 speakers, I need only 3 dB attenuation of the Radian 475PB compression drivers I prefer for my the Altec 511B tweeter horns. I set the L-pads and the autoformer controls for 3 dB of attenuation and alternately played good sounding tracks, first using L-pads for tweeter control, then using the switched Speakerformers. I repeated this comparison a number of times. The conclusion I reached is that the music produced by both the Speakerformers and the L-pads was lovely. To my (not too bad) ears, the Speakerformers and the L-pads were indistinguishable from each other.
A different listening session with Speakerformers
Joined by my audio wingman, Stephaen, I had a nice long listening session with Speakersformers for tweeter level control in brand-new 8-Ohm Hiraga-circuit crossovers I built for the customer who specified Speakerformers. This was in my system with Altec A7-500-8 speakers. We played a long playlist of wonderful sounding tracks. All the tracks sounded better than I have heard them before. I heard lyrics remarkably well. Transients were more dynamic. Generally, there was greater clarity. The word "drive" comes to mind. Stephaen prefers music played at low levels. Despite the low levels I heard a remarkable amount of inner detail, intelligibility, and space. I have since listened by myself at higher levels and find the results to be comparable to the lower-level results, but, of course, louder. Stephaen is reluctant to make pronouncements. He wasn't certain to what he should attribute the change, but he did say that the system was sounding unusually good, which fit with my perception.
I can't be certain yet, but all I have to attribute the sonic improvement to is use of Speakerformers in place of high-quality L-pads for adjustment of tweeter level. I have been using excellent sounding crossovers that I breadboarded years ago, using the same capacitor families used in all of the crossovers I sell. Admittedly, I have a silicon-steel-core inductor in the primary filter of the high pass circuit of my old crossover, versus air core inductors in the same location of my currently manufactured crossovers. Also, some of the chokes of my personal crossovers were home-made, but of high quality. That said, the crossovers I sell have not sounded significantly different from the breadboarded crossovers I've been playing in my system.
I am currently a bit surprised that I heard such sonic improvement in the brand-new crossovers, but missed the improvment listening th
I expect that some of my customers will be interested in specifying Speakerformers for control of tweeters and supertweeters. I look forward to building a pair of crossovers for my system using your Speakerformers.
Conclusion
As discussed above, in my system, with my ears, I did not find the Speakerformer controls sonically superior to L-pad controls. Perhaps with different tweeters, a crossover configuration presenting lower source resistance to the tweeter, a need for greater attenuation levels, and younger ears, a user could find a benefit from the use of Speakerformers. If the tweeter is sufficiently damped at its resonant frequency, additional damping may not be provide an improvement. Possible Speakerformer benefits from increased tweeter piston mode damping will not be as great as for a low attenuation as for high attenuation. If cost is not a major consideration, a user would not go wrong to use the beautifully made Speakerformers. It might be like the story about a person finding his friend clicking his fingers repeatedly. When asked why, the friend said it was to keep tigers away. When probed further the friend said, "You don't see any tigers, do you?"
A reasonable argument favoring autoformer-based tweeter level controls has to do with electrodynamic damping of the tweeter. In principle, an autoformer based control with sufficient attenuation will reduce the source resistance in series with the voice coil resistance, thereby improving electrodynamic damping of the tweeter piston mode, which, in principle, can reduce ringing of the diaphragm at the driver resonant frequency. The resistance of an L-pad reduces tweeter electrodynamic damping, the advantage going to the Speakerformer. I looked online for discussion of the benefits of Speakerformers and found several users who are quite impressed by them. One user's listening experience lead him to believe that the Speakerformers are better than fixed resistor L-pads, which are slightly better than variable L-pads. Therefore, I was eager to find out for myself if I would hear an improvement of reproduced sound through the use of Speakerformers.
A listening session comparing L-pad tweeter control with Speakerformer based tweeter control in my old breadboarded Hiraga-circuit crossovers
I recently had an opportunity to find out first-hand about sonic benefits of the Speakerformers. A customer purchased a pair to be used in a pair of Hiraga circuit crossovers I am building for him. With my old breadboarded Hiraga-circuit crossover networks I use with my Altec A7 speakers, I need only 3 dB attenuation of the Radian 475PB compression drivers I prefer for my the Altec 511B tweeter horns. I set the L-pads and the autoformer controls for 3 dB of attenuation and alternately played good sounding tracks, first using L-pads for tweeter control, then using the switched Speakerformers. I repeated this comparison a number of times. The conclusion I reached is that the music produced by both the Speakerformers and the L-pads was lovely. To my (not too bad) ears, the Speakerformers and the L-pads were indistinguishable from each other.
A different listening session with Speakerformers
Joined by my audio wingman, Stephaen, I had a nice long listening session with Speakersformers for tweeter level control in brand-new 8-Ohm Hiraga-circuit crossovers I built for the customer who specified Speakerformers. This was in my system with Altec A7-500-8 speakers. We played a long playlist of wonderful sounding tracks. All the tracks sounded better than I have heard them before. I heard lyrics remarkably well. Transients were more dynamic. Generally, there was greater clarity. The word "drive" comes to mind. Stephaen prefers music played at low levels. Despite the low levels I heard a remarkable amount of inner detail, intelligibility, and space. I have since listened by myself at higher levels and find the results to be comparable to the lower-level results, but, of course, louder. Stephaen is reluctant to make pronouncements. He wasn't certain to what he should attribute the change, but he did say that the system was sounding unusually good, which fit with my perception.
I can't be certain yet, but all I have to attribute the sonic improvement to is use of Speakerformers in place of high-quality L-pads for adjustment of tweeter level. I have been using excellent sounding crossovers that I breadboarded years ago, using the same capacitor families used in all of the crossovers I sell. Admittedly, I have a silicon-steel-core inductor in the primary filter of the high pass circuit of my old crossover, versus air core inductors in the same location of my currently manufactured crossovers. Also, some of the chokes of my personal crossovers were home-made, but of high quality. That said, the crossovers I sell have not sounded significantly different from the breadboarded crossovers I've been playing in my system.
I am currently a bit surprised that I heard such sonic improvement in the brand-new crossovers, but missed the improvment listening th
I expect that some of my customers will be interested in specifying Speakerformers for control of tweeters and supertweeters. I look forward to building a pair of crossovers for my system using your Speakerformers.
Conclusion
As discussed above, in my system, with my ears, I did not find the Speakerformer controls sonically superior to L-pad controls. Perhaps with different tweeters, a crossover configuration presenting lower source resistance to the tweeter, a need for greater attenuation levels, and younger ears, a user could find a benefit from the use of Speakerformers. If the tweeter is sufficiently damped at its resonant frequency, additional damping may not be provide an improvement. Possible Speakerformer benefits from increased tweeter piston mode damping will not be as great as for a low attenuation as for high attenuation. If cost is not a major consideration, a user would not go wrong to use the beautifully made Speakerformers. It might be like the story about a person finding his friend clicking his fingers repeatedly. When asked why, the friend said it was to keep tigers away. When probed further the friend said, "You don't see any tigers, do you?"
Pete Riggle Audio
2112 S. Olympia Street, Kennewick WA 99337, USA
shop phone: 509 582 4548 email: peteriggle@msn.com
VTAF™ Trademarked. U.S.Patent No. 7630288.
Website content Copyright © 2023 Pete Riggle Audio, All Rights Reserved.
2112 S. Olympia Street, Kennewick WA 99337, USA
shop phone: 509 582 4548 email: peteriggle@msn.com
VTAF™ Trademarked. U.S.Patent No. 7630288.
Website content Copyright © 2023 Pete Riggle Audio, All Rights Reserved.