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Heresy Upgrade for Gravitas, Scale and Flat Response


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I have discovered an easy formula for addressing the two largest problems with the sound of my beloved 1981 Heresy 1.5's  (Heresy 1 drivers with the E2 balancing network).  The K22-E woofer is not efficient enough to balance the squawker, even with the 9 dB attenuation via the T2A autotransformer.  The enclosure contains only 1.6 cubic feet-before subtracting the drivers' and crossover's respective displacements.  These limiting factors are familiar to most of you.  I have achieved a vast improvement in frequency response, rhythmic drive, listenability and flexibility of room placement by changing-reversibly-only a few things. Bonus: 4dB increase in sensitivity over stock (96 dB/W @ 3 ft) to 100+ dB/W @ 1 m.


I sought long and hard a 12" woofer with an efficiency of at least 96 dB/W (2 dB/W more than the Klipsch woofer) that possessed Thiele-Small parameters appropriate for a small, ported enclosure. (Running computer simulations for the Heresy's 1.5 cu.ft.of useable volume, I quickly discovered that physics will limit the extension of unported cabinets to a 3 dB down point (F3) around 70 Hz,  regardless of any woofers I can fit.)  I found two worthy candidates in the professional sound reinforcement category.  The two readily and cheaply available candidates were the Peavey Sheffield Pro 1200+ and the Dayton Audio PA 310-8 12" woofers.  My free speaker box calculator app showed that the Dayton woofer ($60 each) yielded the better maximum flat bass amplitude of these two 96 dB/W-candidates.


Since I will not damage my one-owner vintage Klipsches (got them brand new in 1982), I bought a half-sheet of 1/2" thick MDF to make a new, port-friendly back panel for the Heresy cabinet and a short length of 3" diameter white PVC pipe along with a 3 1/2" hole saw at the hardware store.  I just traced the original back panel and transferred the screw holes onto the MDF. Good measurements and at least a circular saw will give you better results.  The box calculator app indicated that a 3" long tube of 3" inside diameter would tune the cabinet to 50 Hz to produce the lowest F3 possible with these woofers: 56.7 Hz.  I know that does not seem great, but keep in mind that the simulator app showed the factory woofer's unported F3 at above 70 Hz, plus it was too quiet to balance the squawker and cannot play as loud as the 450 W- capable PA 310-8.  You seeing where the "Scale and Gravitas" come in?  I installed with hot glue the 3" I.D. by 3"long tubes in the lower outer corners of the MDF panel where the pipe would not foul anything (e.g. wooden cleats) inside the box.


Since this new, much beefier woofer has a nominal 8 Ohm impedance instead of the Klipsch woofer's 11 Ohms, I modified the values of L2 and C2 in the Klipsch 'Balancing Network' (crossover) 12 dB/octave woofer low-pass section to achieve the smoothest transition to the squawker.  Measurements with pink noise, a calibrated measurement microphone and RTA program led me to a standard second order 600 Hz low pass circuit for 8 Ohms,.  That is, I merely changed the original woofer inductor L2 from 4 mH to 3.00 mH ( by unwinding some coils and measuring with an LC meter) and replaced C2, originally 33 muF, to 23.4 muF (a 22muF bipolar 100-Volt electrolytic in parallel with small value poly caps to sum to 23.4 muF).  I left the squawker section totally stock, except for replacing the spam can 2 muF squawker capacitor (connected to T2A terminal 5) with a polypropylene Jantzen of the same value.  The woofer now smoothly and powerfully balances that magic midrange horn. The speaker can now be placed a couple feet from the wall and still have much better bass than ever. 


With the Eminence ASD 1001 titanium tweeter driver screwed onto the factory tweeter horn [phase plug removed] with an adapter plate, described in my earlier posts, I measured very flat response from 50 Hz to 19 kHz +/- 2.8 dB ,with 1-octave smoothing; 101 dB SPL at 3 feet with 2.83 V pink noise in a medium sized, carpeted room.  The room actually created a modest bass hump to make 40 Hz quite audible ( with the speakers over two feet from the back wall! ).  I found that a third order 18 dB/octave high pass circuit at 6 kHz (8 Ohm), without going through the T2A at all, works wonderfully with the ASD 1001 and K52H squawker to prevent that deadly upper midrange hump, combined with the anemic bass, which made the stock Heresy's notorious, especially on denser sonic textures - and right hand piano notes should not honk.  Only a narrow 1.5 dB hump at 1.2 kHz and a shallow 1.6 dB depression between 400 and 800 Hz and no spikes anywhere.  This is the way I have always wished they had sounded. Bigger, smoother, sweeter. 

Edited by Antone
Correct erroneous hole dimension
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Sounds like you have made some nice upgrades without a lot of mods to the H1.5
Any pics if all the work?

Dollar for dollar Klipsch has no equals
Name one other speaker company that can build a speaker and keep working like new after 45 plus years of service. Answer NO ONE !!!!!!

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Thanks, carlthess40,  

I am no carpenter, just a professional symphony musician who likes to fix things and solder. The ports are not pretty,  but they work fabulously.  The xover I hardwired point to point with 18 gauge solid and stranded wires, keeping the lengths between the two xovers the same, so that both speakers’ signal paths were equal in length. 

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1 hour ago, Antone said:

... keeping the lengths between the two xovers the same, so that both speakers’ signal paths were equal in length. 


'Cause the wavelength "on the wire" of a 20kHz signal is nearly 50,000 feet long?



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Did some extended reading on a thread on the Heresy subject from 2011 here, you being the original poster. Found and am finding all this quite interesting as was just gifted a pair of the 1's. Suppose they could be 1.5 after reading and maybe I will be come curious enough one day to find out. Like or unlike some here, I am finding them to be adequate in all ways. Only 2 weeks into them now and starting to listen at moderate volume. The bass I have always wondered about for my self. Initial impression is flat tone or not using loudness is not enough bass for my taste. Will probably just see how I progress and will keep this pair stock for the moment. I am really impressed with these nevertheless...thanks for the read from the 2011 topic. Will continue to follow...thanks

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So, quite some time and water has come under the bridge since 2011.

Will go back to the original thread to finish.

The narrative on this page using similar language to explain the mod(s) except now your information is now more exact with numbers and values, essentially the same.

Have you made any more recent discovery since that time please?


Also, will presume there has been some adoptors to your work over time. It would be interesting to hear what their finding has been.

Also can presume the same mAy apply with Stan Man over at AK...


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Yes , billybob,

There are some major differences this time. I port -tuned the enclosure to 50 Hz, lowering F3 by around 20 Hz-no port previously. I chose this latest driver based on its compatibility with the Heresy cabinet, heeding T/S parameters. My previous woofer, Dayton series 2, was 4 dB less efficient and made for a much larger box. 

My graphs show flatness I could hardly achieve with a 31-band EQ before-no EQ needed now. Consider my posts from years ago superseded. This upgrade is based on much greater experience and measuring instrumentation I did not have before. 

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