Help bi-amping Monolith I's and replacing the bass drivers

[Amayze]

I am interested in bringing a pair of Monolith I's up to the best performance I can get, on a modest budget. I really like the idea of using the original MLs, and I am fond of their looks.

Dave Gilinsky, System 38, apparently accomplished bi-amping of I's with a DriveRack 260. And JonFo and others have used a Peerless replacement driver on later models.

But two things are concerning me. The first is the design of the I versus the III. The back panel only has a pair of binding posts, not two strapped pairs like the later Monoliths, which makes me wonder what I'll find inside. Stereophile's original review said:

"The Monolith is a two-way hybrid system that uses a large electrostatic radiator to span the range from 100Hz upwards, crossing over to a 12" woofer below 100Hz. The Monolith's woofer "isolation" from the power amplifier is minimized through the use of a huge crossover coil—about the size of those two-gallon plastic jugs of distilled water sold in supermarkets."

and the original brochure talks about 20 lbs of copper in each low pass inductor. Seems like it may be an adventure.

The second issue is that the woofer box is ported, not sealed like the III's. Not sure how to proceed - should I try to match the characteristics of the original driver if I can identify it, go with the Peerless drivers others have used, consider sealing the box (is this even possible?), or??

So, am I setting out on a fool's errand (I am not at all a speaker designer), or is there a reasonable path? Thanks for any help!

Greg

 

[Amayze]

 

[Amayze]

Modavid was so kind as to share his gently used Monolith III stat panels with me. The first clue that this might be a less than straightforward process was that the box label showed different SKUs for I and II's versus III's (first photo). The second photo shows the pigtail (harness) of my I's. The III panels have three, apparently smaller gauge wires with stripped ends (no pins). Does anyone recognize the connector? I would like to find them for the newer harness. Also the I's are red, black, and green wires, and the III's are red, black, and blue. The third photo shows that there are four horizontal rails on the back of the frame. These are screwed (and possibly glued?) to the vertical stiles. If you look closely in photo 3 you will see the the panels are glued into the stiles, with something resembling liquid nails. Photo 4 shows the front, where the vertical stiles are two part (maybe glued). They cover about an inch and a half of the panel on each side. Can the panels flex enough to clear 1.5 inches? Either way, am I better off trying to disassemble the frame, because of the glue? The panels would not have to flex at all to come out the back, with the four rails removed.

Any opinions, on either the electrical or structural components? Thanks!

Greg

 

[Amayze]

Well, the first panel is out, and so is the first woofer. It came out the front, as the back rails were glued in. There was a small amount of damage to one of the inner trim pieces of the stile. Turns out, it is held in place by double sided tape. I think I should have removed them before pushing the panel out. The 35+ year old glue (liquid nails?) was dry and provided little resistance. The inner trim pieces on the stile aren't structural, so are they for looks, or is their rounded profile there for sonic reasons (to soften the square edges of the stile)?

The woofer has no identifying marks on it, just a couple of numbers I will try to google. The lead from the woofer is actually a Monster (TM) cable! I will try to get the batting out and have a look at the electronics later today.

 

[Amayze]

OK, here's a peek inside the Monolith I.

On the two sides you can see the gallon-sized inductor coils. To the left of center, starting closest to the camera, are a terminal block, the binding posts, and 120V power (hard to see well). To the right of center is a circuit board and step-up transformer. Here is a (very) gross overview of the wiring.

The circuit board, starting from the furthest away, has some larger caps:

then a small transformer:

then a bunch of Caps, resistors, and inductors:

The step-up transformer hangs off the back of the circuit board bracket, seen in the shadows of the first photo.

 

[Amayze]

I am definitely not an EE or speaker designer. The goal is to externally crossover and bi-amp the speakers. Where would I need to break into the circuits with the leads from the power amps? And what parts of this are the current crossover, that would no longer be necessary with the external one? I hope someone(s) out there knows their stuff!

Also, would it of service to the community to try to document the differences between the I, II, III, IIIp, and IIIx Monoliths? I would be happy to contribute and help organize.

Thanks,

Greg

 

[JonFo]

Hi Greg, sorry I missed the first few posts. I'll definitely have some input for you on this project.

Glad to hear the goal is to go full-active, as that will vastly simplify many aspects of this upgrade. It does add some complexity in terms of setup, but I've been down that road and back again multiple times and can share configs and such for active crossovers.


Good Idea on starting to document the differences between the Monolith models. I've started a thread here, with the variances across the III series. Please add what you learn about the I's to that.

Mods to your units to go active will involve the following:

For the woofer
Add new binding posts and feed to the woofer, use fresh wire. Posts: https://www.parts-express.com/dayton-audio-bpfi-g-fully-insulated-binding-post-pair-gold--091-630

All crossover components for the woofer will be ditched, so you can lighten your speakers by pulling out those copper coils.

Replace the woofer. Next post will discuss options for your vented alignment.

For the panel.
Similarly to the mods I made for my SL3XC center speaker, you will be ditching/bypassing a lot of those caps and resistors in the high-pass crossover. You will be keeping everything related to the step-up transformer and the diaphragm HV supply.
I do recommend keeping at least 1 ohm resistance in the path of the panel hot lead, as many amps do not like to direct-drive an ESL.

 

[JonFo]

Bypassing the high-pass crossover is going to be tricky, as it looks like the entire x-over and step-up is on a single board that uses the etching to link evertyhing up. Other models are more modular, and the HV board is largely independent of the HP crossover.

So we need to dig up a schematic for the Monolith I or you'll need to reverse engineer it from the board traces and components.

 

[JonFo]

...
"The Monolith is a two-way hybrid system that uses a large electrostatic radiator to span the range from 100Hz upwards, crossing over to a 12" woofer below 100Hz. The Monolith's woofer "isolation" from the power amplifier is minimized through the use of a huge crossover coil—about the size of those two-gallon plastic jugs of distilled water sold in supermarkets."

First, that crossover point was way, way to low for the panel. Even the later 125Hz point used in the IIIp was too low. I use somewhere between 315Hz and 360Hz at 24dB/octave on my Monoliths, and I still see the mylar shimmering significantly during loud passages, The orginal crossovers were also only 6dB/octave, meaning the panel is still operating down to 80Hz and below, which IMO is ludicrous.

Anyway, to pick an approariate driver, we need to know the volume of the bass cavity of the Monolith I. Can you please provide the measurements for that. Also, what is the port length and width (to figure total port volume) would be good.

 

[JonFo]

Woofer box options

I note that the Monolith I (and I assume II) are two-part designs, with the panel frame completely independent of the woofer box.

This gives you the option of building your own woofer box that would fully sit underneath the panel, and not have a portion obscuring the panel like it currently does. There's a reason the III moved away from that.

You could go with a sealed design like the III and use the same Peerless driver I do, as long you also use a sub (which you should).

Or you could go with a vented design with a deeper box, but unless you are into woodworking and have plenty of space, probably not the first choice.

Simplest is just find a good driver match for the existing vented box and let the DriveRack deal with the rest.

 

[Amayze]

Jonathan,

Thank you for all your advice.

First the woofer. The box is two stacked rectangular volumes. The top one that obscures a portion of the panel has internal dimensions of 4.5"x23.5"x12.5" or just over 0.7 cu.ft. The bottom portion is 9"x23.5"x22.5" or just under 2.8 cu.ft. for a total of 3.5 cu.ft. minus the woofer, electronics, and two large coils. My first idea would be to truncate most of the upper volume, leaving a couple of inches as a shield to the panel connections, without blocking the panel. I would like to remove the coils, but they are each held in place by a 1"x 2" glued front to back of the box. I don't think cutting them out would present structural (vibration) issues to the box (all 3/4" MDF). I guess I could replace them if necessary. That would leave 2.9 cu.ft. minus the woofer and electronics. The port is a 2" inner diameter PVC pipe two inches long (I thought the ports were typically much longer). Copying the Neolith and you, to some extent, it would seem a sealed woofer and a vented sub would be a good idea. Overall, still keeping the original design cues, but (hopefully) improving the sound. The woofer is 12.25" OD in a 12.375" recess. There are eight mounting holes at 11.75" diameter.

I am working on the tracing of the circuit board (one head-scratcher so far) and will post it when done. It appears that there are only the two large coils and three or four capacitors for the crossover - that's it. Anything you can share about what you have for power circuits in your IIIs would be appreciated. Did you install the 1 ohm resistor on your panel? Are your woofers and subs directly connected to the amps?

There is a product https://www.minidsp.com/products/minidsp-in-a-box/minidsp-4x10-hd called miniDSP 4x10 HD. It seems simpler for a novice than the DriveRack - do you have any familiarity?

Thanks again,

Greg

 

[JonFo]

Thanks for the details Greg, I like the idea of shortening the box so it no longer covers the panel. 2.9 cubic feet fine for that Peerless 830669 woofer. Double-check the mechanical fit, but it should do. Just close off the port to make it a sealed alignment, add some stuffing to the cavity to dampen internal resonances and as long as the crossover to the sub is 60 to 80Hz at 24dB/Octave, you should be fine. The nice thing about DSP-based active crossovers is the actual physical constraints are much broader, as you can easily adjust in the XO if needed based on measurements.

In my setup, the woofers are driven by one channel of a Sunfire Stereo (300wpc) amp, nothing but speaker wire between the amp and the woofer.
Same for the panels, XO feeds a Sanders ESL (600wpc @4ohms), which in turn directly feeds the step-up transformer with no further resistance, as the ESL amp is designed to, well, drive an ESL
My advice to use 1 Ohm is because many other amps could have issues with the highly reactive and capacitive load of the ESL.

The MiniDSP line is a good choice. It is well supported in general, and tools like REW support it natively (for creating EQs and such). The UI is fairly straightforward. I gather that by now, they are also pretty robust and run for extended periods.
As a data-point, my DriveRack 4800 has been on 24/7/365 for over ten years, never needs to be rebooted, and just works. Which is why I like them so much.

I gather you are looking at the 4x10 so you can run bass management for a 2.1 or 2.2 (or heck, a 2.4) configuration along with the two-way active XO for the Monolith, right?