Beautiful. Great DAC + streamer straight to Mod-686, and then those speakers! I really like your system. Even the "small" version of the Mod-86 looks quite big next to just the streamer. Of course those streamers aren't really big devices. Haven't heard Philharmonic Audio mentioned but I like the design choices they made in your floorstanders. Mundorf tweeters, midranges I don't know, and dual Purifi 6.5".. Do you find it tough to stop a listening session?
Thanks jwdevos! Listening right now and my system has never sounded better. When you have 3 components that all have objectively stellar measurements, getting good sound is pretty easy. Link below to the speakers, so you can check out the driver compliment.
Philharmonic Audio BMR HT Tower
Philharmonic Audio BMR HT Tower
By coincidence you put me on to Matrix Audio too. My friend needs a DAC with HDMI ARC to control volume with the TV remote (no ARC: WAF == zero). There are only a few options, turns out licensing for the HDMI standard is expensive and annoying to implement. Matrix Audio seems to have their approach down with their own OS. They actually have a viable option below 1000 Euro (mini-i 4). All other options I found so far either measure bad or cost 5000 Euro.
The impedance graph of those Philharmonics looks almost exactly the same as the graphs for the Revel floorstanders. Not surprising as the driver layout is similar too. Mod-686 power should be a good match, I'm getting really interested in trying a similar approach as your system.
A while back I became convinced that active crossovers are the holy grail, reading about JBL M2 etc. I was looking into building a JBL M2 clone and that's probably still the cheapest way to real serious results. But you need to add two great amps and a great 4-channel DAC, and then do the DSP implementation without messing up.
Then I found out what kind of results people are getting with integrating their Revel floorstanders into their room, even without DSP. I guess that using passive crossovers and a single amp has a lot to offer too.
Revel distribution in Europe seems to be a mess. I found one dealer in Slovenia that has stock, good service, and a few options in my price range. I don't know, maybe I should start saving and planning a road trip from the Netherlands to Slovenia. They'll probably ship but it's a cool memory to create and pick them up yourself. I wonder if a shop would be open to connecting a DIY amp when trying out 5k or more speakers.
The impedance graph of those Philharmonics looks almost exactly the same as the graphs for the Revel floorstanders. Not surprising as the driver layout is similar too. Mod-686 power should be a good match, I'm getting really interested in trying a similar approach as your system.
A while back I became convinced that active crossovers are the holy grail, reading about JBL M2 etc. I was looking into building a JBL M2 clone and that's probably still the cheapest way to real serious results. But you need to add two great amps and a great 4-channel DAC, and then do the DSP implementation without messing up.
Then I found out what kind of results people are getting with integrating their Revel floorstanders into their room, even without DSP. I guess that using passive crossovers and a single amp has a lot to offer too.
Revel distribution in Europe seems to be a mess. I found one dealer in Slovenia that has stock, good service, and a few options in my price range. I don't know, maybe I should start saving and planning a road trip from the Netherlands to Slovenia. They'll probably ship but it's a cool memory to create and pick them up yourself. I wonder if a shop would be open to connecting a DIY amp when trying out 5k or more speakers.
I actually have the Matrix Audio I-4Pro and use it in our TV room. Great unit, but it’s not certified yet by Roon - will be nice when that’s finally done. It feeds a Neurochrome Modulus-286 (also built by Tom, like my 686) and a pair of PSB B600 speakers. Pretty awesome TV/music system in its own right!
I‘ve considered Revel speakers in the past, but for some reason never pulled the trigger. Really happy with the BMR HT Towers and don’t see changing anything for a long while.
I‘ve considered Revel speakers in the past, but for some reason never pulled the trigger. Really happy with the BMR HT Towers and don’t see changing anything for a long while.
Hi tom. i just recently discovered your "Neurochrome Modulus 686" and i must say i am impressed by the specifications. i have had dreams of building the ultimate sound system for years. after looking at a lot of my choices of integrated av receiver out there that seem to lack on the sound i can get from a high end dedicated amp. which is how i found myself here looking to build my own custom 5 channel amp. i stumbled on the Neurochrome Modulus 686. i am looking for some advice to my build. For starters i plan to supply my build with a liner toroidal power supply i would need a larger Chassis for sure. To save a bit of space im thinking 2-686 modules and 3- 286 modules. both the 686 to drive my front left and right and the 3-286 to drive my center and rear surround. do you recommend i keep all the modules the same or going smaller drivers on the other? thanks in advance for anyone's input
@steve1555
If you are trying to do a conventional 5ch home theater setup then typically you want the front 3 channels (L, center, R) to be of the same wattage. That being said, the 286 is sonically similar to the 686 so you won’t run into issues from a timbre standpoint. Depending on the load, the 686 can play much louder in comparison (more dynamic prowess if you will). In my own comparisons I have found the 686 have more macrodynamic ease compared to the 286 and on certain tougher 4 ohm loads, it can be quite audible. The ability to hear small details in the aural scene is a hallmark of Neurochrome Modulus series of amplifiers and the 686 is no slouch in that regard. Member groovybassist recently upgraded from the 286 to the 686 and he stated his thoughts on post 1677. I’ve listened to his system…It’s quite a treat.
Best,
Anand.
If you are trying to do a conventional 5ch home theater setup then typically you want the front 3 channels (L, center, R) to be of the same wattage. That being said, the 286 is sonically similar to the 686 so you won’t run into issues from a timbre standpoint. Depending on the load, the 686 can play much louder in comparison (more dynamic prowess if you will). In my own comparisons I have found the 686 have more macrodynamic ease compared to the 286 and on certain tougher 4 ohm loads, it can be quite audible. The ability to hear small details in the aural scene is a hallmark of Neurochrome Modulus series of amplifiers and the 686 is no slouch in that regard. Member groovybassist recently upgraded from the 286 to the 686 and he stated his thoughts on post 1677. I’ve listened to his system…It’s quite a treat.
Best,
Anand.
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@poseidonsvoice
thanks for the input, i was thinking the same thing about matching the center with L R. So your saying the 686 is not only more power but a performance upgrade as well from the 286? maybe ill just bight the bullet and do all 5 686. i plan to just buy the components as my budget allows over time. this will take a while but worth the time and feeling to building something you can sit back and enjoy later. would love anyone opinion on switching power over liner. i know liner has way less noise. but is there any advantage to sticking with a switching supply?
thanks for the input, i was thinking the same thing about matching the center with L R. So your saying the 686 is not only more power but a performance upgrade as well from the 286? maybe ill just bight the bullet and do all 5 686. i plan to just buy the components as my budget allows over time. this will take a while but worth the time and feeling to building something you can sit back and enjoy later. would love anyone opinion on switching power over liner. i know liner has way less noise. but is there any advantage to sticking with a switching supply?
Given the very high PSRR of both the 686 and 286, there doesn’t seem to be large differences other than weight between a linear supply and switching supply. Many have built with switcher supplies and they are pleased (including the unit groovybassist received from Tom recently).
The lower noise of linear supplies as it pertains to this design is rather immaterial to be honest. Save your money. And your back. But if you want to know a fella who went nuts and built it with a linear supply, that would be me. Many others have as well (several links are in the 1st post of this thread). I’ve listened to the 686 (with switcher supplies) in other systems and honestly I would be hard pressed to tell a difference. There certainly isn’t a difference in the measurements. That much is clear. So why would you want to use a linear supply for the 686? Not sure, perhaps you like heavier amps? Or just because? This is the world of diy and a choice has to be made!
Best,
Anand.
The lower noise of linear supplies as it pertains to this design is rather immaterial to be honest. Save your money. And your back. But if you want to know a fella who went nuts and built it with a linear supply, that would be me. Many others have as well (several links are in the 1st post of this thread). I’ve listened to the 686 (with switcher supplies) in other systems and honestly I would be hard pressed to tell a difference. There certainly isn’t a difference in the measurements. That much is clear. So why would you want to use a linear supply for the 686? Not sure, perhaps you like heavier amps? Or just because? This is the world of diy and a choice has to be made!
Best,
Anand.
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I haven't been able to measure any difference in performance for the Modulus-686 when it's powered by a high-end linear laboratory power supply and a (much lower cost) switching power supply like the Connex SMPS800RE.
For a 5-channel setup I would consider building a stereo amp and a 3-channel amp just to keep the size of each amp manageable.
You could consider something like the Modulus-686 Safe-n-Sane for the surround channels. The Safe-n-Sane is powered by two Mean Well RPS-400-27 and fits in a 3U tall, 330x300 mm chassis. I happen to have all the parts in stock for such a build and can deliver it as a kit if you're so inclined.
Tom
For a 5-channel setup I would consider building a stereo amp and a 3-channel amp just to keep the size of each amp manageable.
You could consider something like the Modulus-686 Safe-n-Sane for the surround channels. The Safe-n-Sane is powered by two Mean Well RPS-400-27 and fits in a 3U tall, 330x300 mm chassis. I happen to have all the parts in stock for such a build and can deliver it as a kit if you're so inclined.
Tom
It'll be a challenge to find a fan-less SMPS that can power five channels of Modulus-686. That said, such supplies tend to be pretty small and in a 3U tall chassis you'd be able to stack multiple power supplies on top of each other by using threaded standoffs.
I go back/forth on how much I'm willing to endorse the Connex power supplies. They're pretty decent for a DIY supply, but products by Micro-Audio, Hypex, Mean Well, XP Power, etc. are of much higher quality. I've had a couple of SMPS300RE and SMPS300REh that needed a bit of touch-up soldering on the output terminals because they arrived with dry solder joints. If you're willing to deal with that they're pretty good bang for the buck.
The supplies by Hypex look attractive at first, until you realize that at high mains they'll blow the LM3886es in the Modulus amps.
I currently have an OEM-only supply by Micro-Audio on my workbench that I need to test in more detail. Early data indicate that it's a pretty capable supply. It may not be powerful enough for a stereo Modulus-686, but some of M-A's other offerings might be. One thing I really appreciate about Micro-Audio is that they test the supplies to various EMC and safety standards. I doubt Connex does that; at least it's never been mentioned if they do.
Sadly the Mean Well RPS-400-36 has issues with mechanical whine at light load, so that's not the best candidate for an audio amp. I don't know if the RPS-500-36 has the same issue. All builders I know of who used the RPS-400-36 and had issues with whine were able to solve the issue by turning the voltage up to 40 V, so it's not necessarily the end of the world if you end up with a whiny supply.
The Mean Well LOP-600-36 could be an interesting candidate as well. I'd think a pair of those would power a 5-channel amp intended for surround sound service, though I'd ask Mean Well first to see how well they handle a light load. Figure you'll have each supply loaded by about 350 mA per channel when the amps sit idle. Note that this 600 W supply is intended for fan cooling. It'll provide 400 W with convection cooling. In an audio application the supply will hardly ever see the full output power for more than a split second, so going fan-less makes sense. $90/each is hard to argue with...
I think I might get a pair of these to play with. If they perform well it could be time to bring back the Mean Well Controller so we can use these in audio amps.
... oh and for inspiration: Here's an example of a 6-channel amp that uses two Modulus-686 and four Modulus-286: https://ncforo.com/index.php?threads/power-amp-picture-gallery.7/post-42
Tom
I go back/forth on how much I'm willing to endorse the Connex power supplies. They're pretty decent for a DIY supply, but products by Micro-Audio, Hypex, Mean Well, XP Power, etc. are of much higher quality. I've had a couple of SMPS300RE and SMPS300REh that needed a bit of touch-up soldering on the output terminals because they arrived with dry solder joints. If you're willing to deal with that they're pretty good bang for the buck.
The supplies by Hypex look attractive at first, until you realize that at high mains they'll blow the LM3886es in the Modulus amps.
I currently have an OEM-only supply by Micro-Audio on my workbench that I need to test in more detail. Early data indicate that it's a pretty capable supply. It may not be powerful enough for a stereo Modulus-686, but some of M-A's other offerings might be. One thing I really appreciate about Micro-Audio is that they test the supplies to various EMC and safety standards. I doubt Connex does that; at least it's never been mentioned if they do.
Sadly the Mean Well RPS-400-36 has issues with mechanical whine at light load, so that's not the best candidate for an audio amp. I don't know if the RPS-500-36 has the same issue. All builders I know of who used the RPS-400-36 and had issues with whine were able to solve the issue by turning the voltage up to 40 V, so it's not necessarily the end of the world if you end up with a whiny supply.
The Mean Well LOP-600-36 could be an interesting candidate as well. I'd think a pair of those would power a 5-channel amp intended for surround sound service, though I'd ask Mean Well first to see how well they handle a light load. Figure you'll have each supply loaded by about 350 mA per channel when the amps sit idle. Note that this 600 W supply is intended for fan cooling. It'll provide 400 W with convection cooling. In an audio application the supply will hardly ever see the full output power for more than a split second, so going fan-less makes sense. $90/each is hard to argue with...
I think I might get a pair of these to play with. If they perform well it could be time to bring back the Mean Well Controller so we can use these in audio amps.
... oh and for inspiration: Here's an example of a 6-channel amp that uses two Modulus-686 and four Modulus-286: https://ncforo.com/index.php?threads/power-amp-picture-gallery.7/post-42
Tom
Thanks Tom
This give me lots to consider. I've been playing around with this aside in my head for weeks now part of me is almost considering an external power supply but then harnessing that into the chassis with multiple legs would be unslightly and its own challenge. But I'm sure a lot of us on this form could all agree that we do this because we like these kind of challenges. This is what makes this a hobby 😁. I've been an electrician 26 years now and deal with my own design challenges day to day for work . But with audio I'm finding out it's a different beast. I guess there's part of me that still set on the linear power supply just because of the simplicity of it. But there's definitely pros and cons in both directions between linear and switching. If I'm powering five sources I may be winding my own transformer that alone is a huge project. But after all this is a hobby for some of us. I appreciate all the great feedback from everyone on this form. Last night I watched one of your videos building games and I really like the idea of reversing the phase on one of the other amps to eliminate the demand of power at the same time.
Steve
This give me lots to consider. I've been playing around with this aside in my head for weeks now part of me is almost considering an external power supply but then harnessing that into the chassis with multiple legs would be unslightly and its own challenge. But I'm sure a lot of us on this form could all agree that we do this because we like these kind of challenges. This is what makes this a hobby 😁. I've been an electrician 26 years now and deal with my own design challenges day to day for work . But with audio I'm finding out it's a different beast. I guess there's part of me that still set on the linear power supply just because of the simplicity of it. But there's definitely pros and cons in both directions between linear and switching. If I'm powering five sources I may be winding my own transformer that alone is a huge project. But after all this is a hobby for some of us. I appreciate all the great feedback from everyone on this form. Last night I watched one of your videos building games and I really like the idea of reversing the phase on one of the other amps to eliminate the demand of power at the same time.
Steve
Nooooooooo!!!!!!!
Ok, so seriously: An external power supply for an audio amp will have to deliver significant current. On top of that it delivers current in pulses (assuming a Class AB amp). So it's about the worst case for radiated EMI. In addition, most build the power supply into a chassis that matches the amp, which makes many stack the two on top of each other. So now you have a buzz box sitting above/below an otherwise quiet amp and that just invites strong coupling between the spiky currents in the power supply and sensitive audio circuits.
Then add that finding connectors that will handle the current of five channels of Modulus-686 (15 A, peak, each with a 4 Ω load) is not exactly easy. And you better make it a polarized connector as reverse polarity on the Modulus-686 fries the module instantly. Frying five of them would make for a very expensive and sad day.
If you're considering having a custom transformer wound, I suggest contacting Toroidy in Poland: www.toroidy.pl. They do great work. You'll notice that Anand uses the Audio Supreme series. I 'just' go with the audio series. I can make a few technical arguments in favour of the pretty polished steel can, but mostly it's about looks. And it does look good.
Tom
Ok no external power supply lol. Just thinking out loud. It seems your original recommendation makes more sense doing two separate chassis one stereo chassis and the other my 3 channel. My other dilemma is trying to find the right preamp 5.1 processor. I was looking at some of anthems stuff it's getting really expensive going this avenue. But it seems to be cheaper going integrated amps even with the auxiliary outputs. Not sure what the advantage of going with a standalone preamplifier I'm assuming higher quality for the money but is it really noticeable. Seems like a waste of resources buying an integrated amplifier to not use the onboard amplification circuits and just using the pre-outs. But as soon as you buy the same product without the amplifiers built in it seems the price hike is 400% more in some cases which I want to build and use my own amplifier I think there's a quality advantage there
I've tried various Mean Well supplies, and noticed that same whine with some of them. In all cases the supply's potential output power exceeded what I needed for the amp module.
Later I wondered, given the headroom of the supply, whether I could just put a resistor in parallel to get rid of the whine.
@tomchr Any downside to running ±40 V? It seems pretty extreme, just ±2 V below the absolute max according to the manual.
@steve1555 Cool project, I'm definitely following along.
I don't know much about surround sound, but I've looked into active crossovers a bit. Active crossovers need one channel of amplification for each driver in each speaker. Then you use a software solution to programe active crossovers and direct a multichannel DAC to drive each amp channel.
I'm guessing you could do something similar not to achieve active crossovers but a surround setup. Maybe you could hook up something like this to a computer, then use software to program the surround: https://www.oktoresearch.com/dac8pro.htm
I don't know if this is feasible but I imagine you can use a digital input on the DAC, route that input signal through the software on the computer, then route it back to the multiple DAC channels with the Mod-686 channels behind them. You gain the possibility to apply room correction in this way too, which can certainly help a surround sound setup.
The downside is a bunch of added complexity and possibly lots of extra things you need to learn. For the whole digital stuff there is a nice company that offers remote assistance to alleviate some if this, if needed.
The upside is total control, and getting to choose a multichannel DAC that matches Modulus-686 noise and distortion performance, with remote volume control support built in too.
My main Mod-686 parts arrived last week, thanks Tom! Such a nice package to receive, it's like delayed Christmas.
This will be a stereo build for my living room. Tom set the Mod-686 boards to 20 dB gain for me. I'm drawing a design based on a Modushop Dissipante 5U 400mm for the chassis. Quite a beast, but still within reasonable cost and dimensions, looking for the best thermal performance I can buy off the shelf.
Interesting PSU discussion. I haven't decided yet, but I'm considering the Micro Audio COBRA-S2. It offers 1200 watt of "music power" but I don't know with which crest factor. It also has a good bunch of capacitance at the output built in, just like you'd do with a linear supply. All in all looks like a nice product and it's dual rail so a single one could do the trick.
I also need to build a dummy load rig. When I tested a Mod-86 at 50 watt on my 8 ohm 100 watt resistor I almost melted my desk. I'm gonna get 8x 4 ohm 200 watt resistors, and put them in a Makerbeam rig with some PC fans on it. I'll add a whole bunch of banana sockets so I can easily make different combinations with the resistors using banana cables.
All this will take a while, but I'm looking to test a stereo Mod-686 with a signal generator. Two channels at once to see what the PSU does, and the Mod-686 of course.
I'm looking at applying some area of slatted wall behind my speakers. My room is not bad but slatted wall looks nice and might offer some acoustic benefit too. I've also ordered a custom rack from oak and steel. The current rack is low and wide, the new one is smaller but taller. This means more free air around the speakers.
When my Mod-686 is done, I'm hunting for a pair of used Revel Studio2 speakers. The power requirements of those are not really clear, but I found an impedance graph (from Stereophile):
Nominal should be around 6 ohm, but parts of the graph are near or even below 4 ohm, including low frequencies. These speakers are known to be real power hungry if you want to drive them properly. It's hard to find accurate information though, and most of the discussion is about the big brother (Salon2) which is 4-way so would have extra crossover loss.
Most discussion I've found talks about people using even more power than the Mod-686 can deliver at 4 ohm. I'm hoping the Mod-686 will be enough. My room is not small, but not super large either, so I probably have some room gain. Calculations for listening at 80-90 dB say I can easily drive the speakers (sensitivity 87.7 dB SPL with 2.83 V @ 1m). But I'm afraid I'll lose dynamic range if I'm "short" on power. Hard to imagine but all the discussion about insane amps for Revel Ultima speakers has me worrying a bit. Official word on the speakers is "500 watt".
Worst case I could bi-amp with another stereo Mod-686.
@steve1555 Cool project, I'm definitely following along.
I don't know much about surround sound, but I've looked into active crossovers a bit. Active crossovers need one channel of amplification for each driver in each speaker. Then you use a software solution to programe active crossovers and direct a multichannel DAC to drive each amp channel.
I'm guessing you could do something similar not to achieve active crossovers but a surround setup. Maybe you could hook up something like this to a computer, then use software to program the surround: https://www.oktoresearch.com/dac8pro.htm
I don't know if this is feasible but I imagine you can use a digital input on the DAC, route that input signal through the software on the computer, then route it back to the multiple DAC channels with the Mod-686 channels behind them. You gain the possibility to apply room correction in this way too, which can certainly help a surround sound setup.
The downside is a bunch of added complexity and possibly lots of extra things you need to learn. For the whole digital stuff there is a nice company that offers remote assistance to alleviate some if this, if needed.
The upside is total control, and getting to choose a multichannel DAC that matches Modulus-686 noise and distortion performance, with remote volume control support built in too.
My main Mod-686 parts arrived last week, thanks Tom! Such a nice package to receive, it's like delayed Christmas.
This will be a stereo build for my living room. Tom set the Mod-686 boards to 20 dB gain for me. I'm drawing a design based on a Modushop Dissipante 5U 400mm for the chassis. Quite a beast, but still within reasonable cost and dimensions, looking for the best thermal performance I can buy off the shelf.
Interesting PSU discussion. I haven't decided yet, but I'm considering the Micro Audio COBRA-S2. It offers 1200 watt of "music power" but I don't know with which crest factor. It also has a good bunch of capacitance at the output built in, just like you'd do with a linear supply. All in all looks like a nice product and it's dual rail so a single one could do the trick.
I also need to build a dummy load rig. When I tested a Mod-86 at 50 watt on my 8 ohm 100 watt resistor I almost melted my desk. I'm gonna get 8x 4 ohm 200 watt resistors, and put them in a Makerbeam rig with some PC fans on it. I'll add a whole bunch of banana sockets so I can easily make different combinations with the resistors using banana cables.
All this will take a while, but I'm looking to test a stereo Mod-686 with a signal generator. Two channels at once to see what the PSU does, and the Mod-686 of course.
I'm looking at applying some area of slatted wall behind my speakers. My room is not bad but slatted wall looks nice and might offer some acoustic benefit too. I've also ordered a custom rack from oak and steel. The current rack is low and wide, the new one is smaller but taller. This means more free air around the speakers.
When my Mod-686 is done, I'm hunting for a pair of used Revel Studio2 speakers. The power requirements of those are not really clear, but I found an impedance graph (from Stereophile):
Nominal should be around 6 ohm, but parts of the graph are near or even below 4 ohm, including low frequencies. These speakers are known to be real power hungry if you want to drive them properly. It's hard to find accurate information though, and most of the discussion is about the big brother (Salon2) which is 4-way so would have extra crossover loss.
Most discussion I've found talks about people using even more power than the Mod-686 can deliver at 4 ohm. I'm hoping the Mod-686 will be enough. My room is not small, but not super large either, so I probably have some room gain. Calculations for listening at 80-90 dB say I can easily drive the speakers (sensitivity 87.7 dB SPL with 2.83 V @ 1m). But I'm afraid I'll lose dynamic range if I'm "short" on power. Hard to imagine but all the discussion about insane amps for Revel Ultima speakers has me worrying a bit. Official word on the speakers is "500 watt".
Worst case I could bi-amp with another stereo Mod-686.
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It sounds like you've found this page: https://sound-au.com/project124.htm. I've considering rebuilding my dummy load following that pattern. I've been wanting to change a few things about it anyway, including adding support for remote control so I can perform automated amp testing with the APx555 similar to what I do with the HP-LOAD.
Tom
I would definitely not recommend running at ±40 V if you're using an unregulated power supply. With a regulated supply at least you know that you'll get ±40 V even on days where the mains voltage is running a bit high. The drawback is that you get higher heat dissipation and you could potentially engage the protection circuits in the LM3886es a bit earlier than desired. Turning the supply voltage up was preferable to accepting the lost cost of the SMPSes.
Tom
As you know, I built mine with a 4U/400, which seems plenty. Also know that the largest Modushop makes is 5U/500, so yeah, you can really go bonkers!
Consider GIK Acoustics. I believe they have a European distributor and good knowledge based on the existing acoustics of your room.
I see that R19 is plucked! That’s the way to do it!
Don’t just concentrate on the phase and impedance curve. There is data to show that the EPDR (Equivalent Peak Dissipation Resistance) is even more relevant particularly for Class B(or AB). You can read up on it from articles from Dr. Jack Oclee-Brown (KEF). In essence you have to look at the combination of impedance and phase and simply put, if you have obtuse phase angles along with low impedance, the overall EPDR is lower than the impedance at that frequency. In example for your Revel Studio 2, at 300Hz you are at 3.9 ohms but with a ~(-)15 degree phase angle. That will drop the impedance by a certain magnitude (I don’t know how much). When the phase is closer to 0 degrees there is little to no affect on the impedance. Stereophile, Erin’s Audio Corner and even REW are now calculating EPDR using the phase and impedance curve measurements to give the user a real world look at what a Class AB amplifier has to face. If you have a colleague who has a pair of the Revel model you desire, I suggest you have them measure it using REW. It should be eye opening (particularly when realizing that the speaker’s sensitivity is in the high 80’s).
Best,
Anand.
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