I ordered the Intact Audio speaker autoformer with a -6dB to -15.5 dB/0.5dB attenuation range based on the fact that my Altec 802 drivers are about 12dB more sensitive than my 99 dB/1W/1M Altec 414A woofers. Other than that, I went into this project hoping that Google would be my friend, only to find out it wasn't!😉
Since I use external crossovers for my Altec 753C, disconnecting (and reconnecting for comparison purposes) the Altec 32C/802 horn/compression driver is a breeze. The 753C-style cabinets essentially serve as bass bins in which the 414A woofers operating in wide range mode (unfiltered) + the 20275 diaphragmed 802 compression drivers mounted on the Lycan3R WP12024 horns are perched on top!
I was having a hard time fully grasping the technical notes Dave Slagle provided in his website, so I decided to wing it - a term we used at Juilliard when we're forced to find a workaround (be creative😉) when confronted with a technically complicated (or almost impossible to play) lick and make it sound convincingly musical. I came up with the above concoction using a Sprague 2uf Paper In Oil cap in series at the primary input. Instead of experimenting with various fixed value shunt resistors, I connected two terminals of a 16 Ohm L-pad via alligator clip leads. Now, I could hear how varying the shunt resistor across the autoformer alters the sound in real time.
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| Altec 414A (wide range mode) + WP12024 + 802 + 20275 |
After many hours of playing several familiar tunes and tweaking the L-pads, I took an RTA trace with my octave iPhone app and was surprised to see that aside from musically satisfying, my ear-tuned crossover also looked quite decent!😊 IME, that's not always the case...😉
Nevertheless, I was frustrated because I couldn't explain exactly what I came up with, so I emailed Dave Slagle and Joe Roberts requesting for technical assistance.
While waiting for their respective replies, I decided to sweep my Altec 802/20275 compression drivers so I'd have an idea of their impedance characteristics between 800 Hz - 5 kHz instead of merely relying on their 16 Ohm nominal rating. The above YouTube video shows how to do it the old school way!
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| Crossover @ 2500 Hz attenuation = -12dB |
According to Dave, the above schematic is the simplest way to use the autoformer. In this configuration, it's a direct substitute for an L-pad. Use your favorite online crossover calculator to plug in your crossover point and nominal (or measured) driver impedance and you're good to go.
Lesson learned: don't try Western Electric capacitors!🤣
Seriously though, the main reason for using a speaker autoformer is to take advantage of its ability to transform impedance (in tandem with the shunt resistor aka swamping resistor) when the crossover capacitor is installed at the input of the primary.
My buddy Joe Roberts patiently guided me through the process of understanding and reverse engineering my concocted crossover (first schematic with L-pad Rshunt) in conjunction with the Intact Audio speaker autoformer. In the interest of brevity and simplicity, I'm omitting the technical details, which I honestly don’t feel competent enough to discuss. So let’s proceed on how to use these handy online calculators.
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Joe referred me to this capacitive-reactance calculator (pictured above). It turned out that my chosen 2uf capacitor was cutting in at around 2500 Hz.
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On Dave Slagle's Rshunt calculator, I input my measured driver impedance at the crossover point = 11. For the crossover impedance, I plug in the reactance value from the capacitive-reactance calculator = 31.83, I set the attenuation slider to 12dB, shunt R = 39 Ohms!
My ears weren't too far off when I measured 34 and 36 Ohms respectively at the terminals of each L-Pad.😊
Another example - let's say someone got very lucky and found a pair of priceless PIO Western Electric 2.5 uf caps and wanted to use it at a 900 Hz crossover point. The measured driver impedance is 10 Ohms, reactance = 70.74 and a -10dB attenuation was required, Rshunt = 241.8 Ohms or build up a shunt resistor to the closest value.
After decades of playing around with crossovers, IME, smaller value PIO capacitors (less than 4uf) sound better. I also prefer the sound of a built up 4uf (1uf +1uf + 2uf in parallel) over a single 4uf, unless it's a Western Electric PIO.😆 I have reason to believe Joe Roberts' suggestion that the quality and type of Rshunt also has an effect on sound. Hello Allen-Bradley!👋
But don't take my word for it because what sounds good to me may not necessarily agree with your taste and requirements. Nonetheless, this type of crossover is one of the easiest and safest projects to get your feet wet if you're interested in indulging in DIY.
I've learned more from listening to music rather than endless tweaking and flavor of the month upgrades. So listen a lot and have fun experimenting!
I've been enjoying this crossover + autoformer concoction exclusively with the Altec 414A/802/20275/WP12024 horn for more than six months. However, the magic disappeared when I used it in the context of my Altec 753C speaker system (414A/802/20275/32C horn). That pairing sounds dark and slow!
To give the speaker autoformer arrangement a fair shot, I went as far as rigging a simple 4uf cap + L-pad XO sans top end EQ boosting network for the 753C. Alas, the 4uf + L-pad still yielded better sound from the Altec 753C than the autoformer-based version.
This Westrex crossover for the L8/L9 theater speaker system, which used the KS12024 or KS12025 theater horns, was posted by Joe Roberts in this HiFi Haven thread. It was my inspiration for this project. Given that there are plenty of Altec theater type horn users who are very satisfied customers of Werner Jagusch's autoformer based crossovers, could it be that this approach to crossover building isn't as well suited for monitor type horn speaker systems?
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| Altec 32B + 806A |
Ever since I incorporated a top end EQ boost in the crossover for the 32C/802/20275 in the Altec 753C and 32C/806/27344 in the Altec 756B, I found the use of my precious Altec 3000H as super tweeters redundant. JP's WE757A remake and J-Rob's pair of Lycan3R WP12027s, which spent quite some time in my man cave, benefitted from this high frequency EQ as well. In sharp contrast, this crossover + HF EQ was totally incompatible with the Lycan3R WP12024 because it made the horn sound too bright!
It might be possible to integrate an HF EQ boost in an autoformer-based crossover for my Altec 753C speakers. But I've been enjoying its current configuration for more than a decade, so why fix something that ain't broke?
The point is, for my purposes, I optimized that crossover network for the 32C/80/20275 whereas I was able to coax out the utmost potential of the Lycan3R WP12024 horn + 802/20275 using the Intact Audio speaker autoformer-based crossover discussed above. Again, keep in mind that what sounds optimized to my ears may not be to yours.
As mentioned in my previous Lycan3R WP12024 upload, the Western Electric derived 90° bent horns (WE/Altec 32 and KS12027) were designed for professional monitoring applications, not for the theater. Thus, comparing the bent horns to the WP12024 or other theater type horns is like comparing apples to oranges.
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| Lycan3R WP12024 + Altec 802 |
I'd like to reiterate that earlier this year, I was fortunate to hear two hifi systems based around G.I.P. Laboratory Monitor 1 speakers, which use the compact Klangfilm theater horn. Whether I'm sitting close to the speakers or farther back in my man cave, the front row and intimate perspective portrayed by the 414A/802/20275/WP12024 was very similar to my G.I.P. Monitor 1 experience. In the same way, when I switched to my Altec 753Cs, the mid to back row presentation persisted regardless of where I was situated in the room.
Perhaps it's safe to conclude that the design brief for a theater horn/speaker system is to maximize the dynamic impact and realism of a live amplified musical event or movie soundtrack to every individual in a theater. On the other hand, a studio monitor horn/speaker system is expected to reproduce as faithfully as possible what was captured by a mic feed from a concert hall, jazz bar or recording studio in a near-field environment.
The Lycan3R WP12024 is a fine example of a 1" format horn with a dominant theater DNA!
Excellent info as I am looking to implement this with my Bolero/802/Horn setup. Are you using a stunt resistor too or just the Autoformer and Capacitor?
ReplyDeleteThanks!
DeleteIf you review the article above, you'll find the answer to your question. I wrote the segment about speaker autoformers to serve as a starting point or template for those who are interested in this technology. I've also provided my personal opinion/experience about capacitors and types of horns I've listened to.
I've stressed above that people have different tastes and listening requirements, so I'd like my readers to experiment and "hear for themselves" what enhances their musical experience!