Second approach to maximizing bass output in Iron Lawbreaker - diyAudio
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Old 10th December 2008, 04:33 AM   #1
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Default Second approach to maximizing bass output in Iron Lawbreaker

I verified the efficacy of and then provisionally discarded my initial approach, which turned out to be, in effect, adding an all pass filter to the woofer with the corner frequency at the upper impedance peak because of what I felt was a less than optimal transient response.

I'm about to try a second approach which uses an air core transformer. Instead of using a JBL 2220B for the nominal 16 ohm system, this uses a 2220A (a newly remagged, reconed 8 ohm version of the same) with the air core transformer operating common mode on the input signal to reduce the upper bass and midband sensitivity approximately 3db. At the in system BR upper impedance maximum (at about 70 hz), a series capacitor will resonate with the 'primary' winding to produce an amplitude peak that is expected to be on the order of 3 - 4 db at 70 hz, which, according to Basta speaker modeling software, will also be a reflection null of several decibels from the back wall in the speaker's expected spatial orientation. (Even in free space, minimal if any peaking is expected from this approach, btw.) As a potential side benefit, the air core transformer will provide some series inductance low pass filtering, not incidentally without any effects from saturable core materials.

I am waiting for some magnet wire to arrive tomorrow or the day after to begin the final series of tests with this approach, since I am winding the air core transformer.
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Old 17th December 2008, 12:04 AM   #2
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I've wound the air core transformer secondary (took several hours) and applied it, and I can say it works. It brings up the 50-150 hz range about 2.5db while still providing most of the series inductance I need for a quasi first-order low pass filter at 500hz, (although 1-2 mH extra seems to be needed to fully match its response to the HF section) and even provides a db or two attenuation at 32hz where the BR tuning impedance minimum of about 8 ohms is. In this case, the latter would be an advantage, since I'm using a nominal 8 ohm woofer with a 6 ohm Rdc in a 16 ohm system and the attenuation here works out to boosting the impedance minimum by two or three ohms which would put it more in the comfort zone of a 16 ohm output tap or an OTL amplifier like I'm using.

With this latest change, the free space response of the 16 ohm JBL 2220A/Altec 288G system is maintained flat out to about 60hz while maintaining virtually full 2220B midband sensitivity, and has useable output to 30hz (down about 8db) while not providing an unduly punishing load at any frequency. Not too bad for a 3.5cu ft system.

I like the sound balance better too, with improved inner detail, but I may have discovered a distortion problem with my HF driver on that channel which I feel I need to resolve ASAP.
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Old 21st December 2008, 04:41 AM   #3
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I've just finished winding the secondary on the second air core transformer, which takes about six hours to do to get something significantly better than a scatter wind (we're talking about 1700 turns here), so sometime next week after I finish potting the air core transformers, I'll be able to get both channels going with the LF xover that includes the air core transformer.

The apparent efficiency is very high for the Iron Lawbreakers with this approach (along with a couple other tricks I'm using), which extends the midband efficiency to about an octave and a half above 20 hz. Only a physically very large fully horn loaded system would be significantly more efficient than this 3.5 cu ft speaker, and a decent sound stage from a fully horn loaded system (with bass extension that could be considered hi-fidelity quality) would only be practical at a listening distance greater than one attainable in a room the size of a typical living room.

The Iron Lawbreakers are easily capable of '3d imaging' that fills the entire room with the appropriate recordings. My vacuum tube Sonic Hologram processor also is helpful in this regard. Interestingly enough, I still have a working version of an earlier 'solid state' version of a sonic hologram device that I built (slightly different processing, but the active device SQ is the issue here, and the solid state version is actually simpler), and, by direct comparison, it sounds very much dirtier with a much greater loss of inner detail than the vacuum tube one. The solid state version uses two RF JFET buffer stages only for active devices in the signal path for each channel.
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Old 31st December 2008, 05:02 AM   #4
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I found a limitation in my approach regarding combining using activated charcoal in the enclosure and the air core transformer. It turns out that the activated charcoal significantly dampens the BR impedance peaks so that they do not exceed about 25 ohms, thus reducing most of the impedance transforming latitude of the air core transformer. So, without employing core materials (or using a yet larger air core device which already weighs about 10 lb) and allowing impedance minima significantly below 16 ohms, I should expect additional sensitivity gains below 200hz not to exceed 1db by very much. I 've already achieved this, but need to deal with an inband response dip of a few db around 200 hz before the xover is finalized for the Iron Lawbreakers.
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Old 8th February 2009, 05:01 PM   #5
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Now the Iron Lawbreakers are a nominal 16 ohm system, but I chose 8 ohm LF drivers because I realized that my xover circuit approach (such as using 600uF of input polypropylene series capacitance to block DC & maximize the utility of the 2220A's limited Xmax) could easily increase the LF impedance minima to be close to 16 ohm, and in the final version of the speaker, that's exactly what happened. The LF minimum is about 12 ohms at 30 hz (the port tuning), but between 16 and 20 ohms all the way from 40 hz to over 200 hz. IOW, I was able to nearly eliminate the typical BR impedance peaks which surprised me.

This is because 1) I chose the 8 ohm LF driver which cut the impedance peak amplitude by half right off the bat. 2) I added about 10 lb of activated charcoal (in gas tight bags with desiccant) to each enclosure which further cut the impedance maxima to about 25 ohms) and 3) I created an air core transformer/series filter choke for the LF xover that I could tune to both flatten the baffle step/driver LF rolloff and incidentally almost remove the remaining upper BR impedance peak.

The result is really detailed, tight bass and lower midrange in a 3.5cu ft cabinet that still sounds good backed up against a wall (natural for the 'world's largest bookshelf speaker') and with a nominal efficiency of 100db/w/m. The bass and lower midrange sound like what might be expected from an 'aperiodic' loading.

Btw, the activated charcoal was added to increase the acoustic compliance of the box air, which seems to work. Modeling shows that the cabinet behaves as if it were more like 5 cu ft, which means a couple more db of response around the 30 hz tuning, on top of the 3db extra by going to an 8 ohm driver, so there's some useable response down to this range.

Listening to the system last night, I noticed a little upper mid zinginess which sounds great with rock, but not quite so good with classical that I believe is probably related to impedance variations in the HF driver/horn combo. So, maybe I'll open the Iron Lawbreakers up just one more time to tweak in a partial notch filter
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Old 12th February 2009, 05:19 AM   #6
GM is online now GM  United States
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Interesting that the charcoal's porosity would still be so effective in gas tight bags. Anyway, great project, thanks for sharing!

GM
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Old 20th February 2009, 11:31 AM   #7
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Sounds like a wonderful project. Will you post any pictures?

Simon
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Old 20th February 2009, 04:46 PM   #8
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in another thread I asked to see Z of Graham's T-circuit with open baffle and Eminence Sigma 18 and was pretty low - maybe ~half of a simple LC (?)

what is your transformer doing overall? - - - got pix of the scheme and Z plots?

I:ve still got 2220H which should be poked back in 100Hz Edgarhorns plus a variety of 15 to try.

high source Z amps may not work for" boost"

how can one go "cheap" and find decent electrolytic for experimentation that won't pop?

what ar good rules to pick values when simulators aren't available?

Small vented K-coupler - IIRC tuned around 44

no network
Click the image to open in full size.

500uf/20mH - - not tuned well to system and L too low (would do better with 30mH) but illustrates low Z which can be typical. Running this into an autoformer would reflect an even lower impedance - I think ...
Click the image to open in full size.

AJ-Horn simulation of 55 liter sealed box with B&C 15" no eq then 500uF --- maybe some 3rd order sealed boxes can also be iron lawbreakers and keep reasonable input Z - ?
Click the image to open in full size.
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Old 20th February 2009, 05:12 PM   #9
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Hi -

I will see if I can post a few pictures. I think the Iron Lawbreaker project was interesting and instructive...and I'm happy with the sonic results. The constant impedance at bass frequencies was a bit of a surprise but welcome. Even though I'm using a 2220A woofer with a DCR of 6.0 ohms, my xover implementation and the charcoal keep the impedance of the system between 15.75 and 20.25 ohms all the way from 40 hz to 500 hz above a minimum at 35 hz of about 12 ohms. It rises somewhat at higher frequencies, reaching a max of perhaps 30 ohms in the upper midrange before dropping somewhat again, so it is a very easy system to drive with my OTL. This well controlled LF impedance curve is a bit of an unexpected benefit. I need to make a potential confession. I have been calling the Iron Lawbreakers a 100db/w/m system, but they may be more like 98-99db - at the end of the day, I decided to go for the slightly smoother broadband response and more benign impedance characteristics.

The charcoal does seem to work in bags, although I did my best to make sure as much of the surface area of the bags as possible is exposed to the internal cabinet air as possible to help minimize any problems with this. The bags also keep the dust from the charcoal from migrating where it shouldn't. When I have time, I may replace the desiccant I'm using with something called 'microsieve' which is supposed to be more effective at keeping the humidity low in the bags.

I plan to use the charcoal in one or two future projects - a closed box HT subwoofer design, for one. My goal here is to put four long excursion 12" woofers in as compact an enclosure as possible. This may turn out to be an ELF design.

Freddyi -

How's it going? Haven't heard from you for a while. I actually evolved the bass boost circuit to make it more effective since I last posted about it. Basically, it can be configured as an all pass network (two LC networks instead of one - one on each terminal of the bass driver - the second one with the capacitor from the amp output and the inductor to ground), it will still give the peaking, but over a broader frequency range than with one LC network since there is no longer a LF rolloff below the peak. I'll probably retrofit the Basement Blasters with this version when I find time.

However, for the Iron Lawbreaker, I decided to remove the inductor and just kept the series capacitor because when I added the charcoal, the upper impedance peak turned out to be damped considerably and shifted higher in frequency which gave the LC boost network less impedance peaking to work with. So, in a sense, I substituted in part the charcoal which gave more bass extension and a couple more db amplitude at the BR tuning frequency (about 32hz) with perhaps a db boost around 40hz added by the series input capacitance (600uF) which I believe is caused by it interacting with the woofer's reactance at these frequencies.
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Old 20th February 2009, 06:02 PM   #10
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Hi, Freddy -

Looks like the 500uF cap by itself can be pretty effective in your simulation. You can increase the impedance minimum with the LC approach by adjusting values, such as by scaling the L and C values for impedance.
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