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Old 10th August 2008, 06:36 AM   #1
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Default Replacement woofers for Martin Logan Monolith II's

I was looking for replacement woofers thar are better than the current ML offerings.It seems that they are using the same woofer as they did 20 years ago.I was curious to see if anyone has used any others with good results.Martin Logan is asking $200 a piece for theirs and I wanted to see if there was an upgrade aftermarket woofer.Does anyone know what impedance the stock woofers are and the volume of the Monolith bass section? Thank you for any information.
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Old 11th September 2008, 06:14 PM   #2
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Default Yeah...

...I'm looking too. I may end up building frames (CLS clones) to mount the planars in and just supplement with subs. Of course, then I'd have to store the Monolith frames.
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Old 13th September 2008, 05:40 PM   #3
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Consider first what the requirements are for these "woofers"?

There are two separate and distinct issues at play:
- sucessfully meeting the ESL panels at some frequency & sensitivity.
- producing bass.

On the second one, beating the ML woofers is quite easy.
The first one is non-trivial.

So, the place to start is to find out how low do the ESL cells actually go, and what slope do they fall off at? Once you actually measure this, then you can begin to see the scope of the problem, and what the possible solutions will be.

You'll want a proper rolloff, Q and flat frequency response to meet the ESLs...

The next part comes in working with the fixed volume that the ML cabinet provides - you can always go smaller, but not larger. You can switch to multiple drivers, or reduce down to a single driver too... different benefits from each...

Myself, I might look for a better "mid bass" solution (drivers in the ML cabinet) and add on an external subwoofer/power amp/xover to cover the difference below the F3 of the new midbass drivers...

Please post your findings, and measurements?

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Old 13th September 2008, 08:59 PM   #4
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Given my nonscientific tests using an active EQ to determine by ear... I don't feel comfortable passing the panels below 120Hz 3rd order or 100Hz 4th. If I recall correctly the panels are 89dB sensitivity (1 watt at 1 meter) with 4 ohms impedance. However, impedance drops to ~1 ohm at 20Khz.

I'm looking for reasonably priced transformers to bring the minimum impedance up to at least 4 ohms. I don't think my old Forte model 7's like low impedance loads but I don't want to swap them for anything else. If one stays within their limits, these oldie amps are very sweet indeed.

I'll be using dedicated amps with active crossovers with mine so impedance and sensitivity matching is unnecessary.

The original 12" woofers are dual VC. The total bass output is varied by driving either one or both VC's but I don't know how ML configured the X-over to make this work.
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Old 15th September 2008, 10:07 PM   #5
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Quote:
Originally posted by Mike1234
I don't feel comfortable passing the panels below 120Hz 3rd order or 100Hz 4th
Looking over the III's manual, since II is not on Google, it appears that the 6db/octave dipole baffle effect comes in ~ 125Hz (~27" effective width), and that ML used this Xover frequency to simplify the design to common slope rates.

You may want to test the current woofer T/S parameters and look for a similar clone. OR measure the box volume and modify the port to get a deeper f3 with a more modern 4 ohm woofer, but with the same Xover slopes and/or the old crossover parts. I would do the second... modern woofer with new port for deeper f3 using old Xover.

=====================


Martin Logan III
Crossover frequency 125Hz
12db/octave low pass
18db/octave low pass

89db/2.83 volts/meter

Impedance
High Pass
Nominal 4 ohms below 10Khz
Minimum 1 ohm @20Khz

Low Pass
Nonimal 4 ohms
Minimum 4 ohms
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Old 20th September 2008, 04:32 PM   #6
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Default Sorry about the lengthy delay between posts

I've been dealing with issuse here and have been a bit ill.

I'm going active x-over and amplification for the subs so I won't need to use the old crossovers. I may plug the port and look for subs that will work well in the original enclosures. Or I may build some clones of the CLS II frames and mount the panels in those and go dipole on the subs :-)
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Old 20th September 2008, 11:30 PM   #7
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Quote:
Originally posted by Mike1234
Given my nonscientific tests using an active EQ to determine by ear... I don't feel comfortable passing the panels below 120Hz 3rd order or 100Hz 4th. If I recall correctly the panels are 89dB sensitivity (1 watt at 1 meter) with 4 ohms impedance. However, impedance drops to ~1 ohm at 20Khz.

I'm looking for reasonably priced transformers to bring the minimum impedance up to at least 4 ohms. I don't think my old Forte model 7's like low impedance loads but I don't want to swap them for anything else. If one stays within their limits, these oldie amps are very sweet indeed.

I'll be using dedicated amps with active crossovers with mine so impedance and sensitivity matching is unnecessary.

The original 12" woofers are dual VC. The total bass output is varied by driving either one or both VC's but I don't know how ML configured the X-over to make this work.

You can't use a different transformer to change the load impedance.

The problem is matching.

The ESLs are dropping in impedance WRT frequency.
Given the impedance/reactance that the ESL cells present at lower frequencies - 125Hz being the xover point - the transformer has been set to "match" down there, or for "best match" over the widest range that includes the lower target of 125Hz. In practice you can't "match" to a wide range and get a decent "sensitivity". If you want a higher sensitivity you have to match to a narrower range. If you match so that the input Z looks higher at 20kHz. then you end up with a bad match at lower freqs.

There is one solution: the Jim Strickland/Acoustat method. Check his patent for the details. He uses two transformers matched high and how to maintain a reasonable senstitivity and a wide bandwidth.

Peter Walker explained this stuff in his Electronics World articles back in the 50's. Strickland found a "way around." There have been some other tricks where the same idea is done using a single transformer...

At least this is my understanding of the problems...

_-_-bear
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Old 21st September 2008, 02:17 AM   #8
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Bear,

From what I understand the impedance matching transformors of which I spoke are independant of any downline components... like the output transformers in MacIntosh amps. These just sit outside near the loudspeakers instead of within the amplifier case.
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Old 21st September 2008, 03:35 AM   #9
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Perhaps, as long as you can take the hit on the extra power required.
And, the extra transformer does not wallop the HF response, it might work.

But something seems inherently wrong with this approach, and clangs in the back of my head... the ESL doesn't "know" about the second transformer, it just thinks it is being fed by a composite transformer, one with a lower step-up ratio. While the secondary of the ML transformer still presents the same DCR and VA/turn, the overall transformation is now different.

It's hard for me to see how this isn't the same as merely replacing the ML transformer (or using a different tap on the primary, if it had one), which is the same as matching to a different capacitive reactance of the ESL cells, which gives a different bandwidth or "Q"...

Hmmmm... interesting proposition here!

_-_-bear
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Old 21st September 2008, 08:19 PM   #10
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It's obvious that I'm no expert. However, unscientifically speaking, I owned a Mac 2205 and have no complaints regarding its SQ or flatness of FR with either dynamic drivers, ribbons, or ESL's. How their output transformers accomplish this is unbeknownst to me. I would sell my Forte amps and replace them with Macs but I don't have a personal Ft Knox to finance the transition. :-)
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