TOPIC:

I thought I'd share some of the progress I've made in my attempt to keep the audio in the digital domain as long as possible in my quest for a 7.1 Dirac Live! corrected theatre system.

My idea was to take the "audio only" HDMI output from an Oppo 103D Bluray player (which is 8ch LPCM and has no HDCP), and feed it through a nanoAVR for bass management and then a nanoAVR DL for Dirac Live! processing and then an HDMI to SDI converter and finally an SDI AES/EBU audio extractor to feed professional DACs and amps. An alternative was to go HDMI to SDI to AES/EBU deembedders and then through a DDRC-88D. Well, a nanoAVR showed up on ebay so that set the course for me.

1. The nanoAVR DL wants to negotiate HDCP downstream of it even if the upstream source does not use. it. I thought my project was dead in the water. Fortunately, HDTV Supply makes an HDMI to SDI converter that strips DHCP. It's intended to be used to provide compatibility so that video over HDMI can be displayed on SDI monitors. It works in this application. WHY minidsp negotiates DHCP downstream of the nanoAVR DL when there is no DHCP upstream of it I don't know, but it took a long time futzing with EDID editing boxes, and using logging firmware from minidsp to find this out. It was very frustrating, but ultimately overcome.

2. I had tried using Blackmagic HDMI to SDI 3G converters but ran into the DHCP issue (since they don't implement it). The HDTV Supply unit saved the day. I did use a Blackmagic SDI to AES/EBU deembedder with success to produce fours pairs of 2 channel 48 k Hz sampled AES/EBU audio! It also provides a word clock over a BNC connector to synchronize downstream DACs. This converter can output either four pairs of 2ch AES/EBU audio, or 4 analog audio output signals on TRS connectors. In analog mode, there is a digital volume control, and two units can be daisy chained via SDI over coax via BNC connectors, so as to extract all eight channels (16 can actually be carried on an SDI 3G video signal). However, the unwillingness of Blackmagic to provide DAC THD and SNR specs led me to uses just one unit to output 4 pairs of 2ch AES/EBU audio and look to AJA for two ADA4s to convert to analog.

3. The AJA converters can convert digital audio to analog or visc versa, or both in pairs. They sport a DB25 connector, and a pigtail fans out in to either TRS or XLR connectors -- 4 male and 4 female. They accept a word clock input via a BNC connector, and daisy chain it along. They are " broadcast quality". Broadcast quality turns out to be 85 dB SNR. Blech. I learned this the hard way. Of course, for $395 new, $195 used, I suppose one gets what one pays for. The wall ward power supply does not help, and damn do they pick up A/C power line noise.

4. I WAS going to have one AJA ADA4 in the front of the theater, run two XLR cables and a word clock cable to the read, and have another in the back of the theatre , with two stereo amps for the rear and side surrounds. But that 85 dB SNR. At first, it was problematic. But, by playing with the gain structuring, I can live with it for now that way, and have an alternative in the works. More on that later.

5. When I first started running Dirac Live! calibration, the default output level is -48 dBFS. I had to crank my Crown amps way up to get a decent signal for the mic to pick up (with the per-channel levels pretty well maxed) and that's how the noise made itself heard. I had figured the AJA ADA4s would be "good", based on their reputation, but email from AJA responding to my query for specs quickly verified the 85 dB SNR figure. I guess that's excellent for "broadcast quality" and I was a fool for bringing in A/V studio audio equipment into a home theatre. I have found a used 8ch Lucid 8824 D/A A/D ADAT converter on ebay for $350 that has much better specs (105 dB SNR both ways). It's only 48 kHz sampling, but I have satisfied myself that that is good enough. (The nanoAVR DL only processes at that rate anyway and it is standard for audio over SDI).

6. If I cranked up the Dirac Live! output calibration level to -20 dBFS, I could pad down the amps enough that the DAC noise was no longer objectionable, at least not for theatre use. Critical audio listening might be a different issue, but there are other, compatible ways to skin that cat. The attenuation settings on the amps were at about "2". Much better than "8". Measurement level was probably 75 dB, so I'd have to crank the amps up a bit if I want to reach THX levels of 105 dB at 0 dBFS (more like -5 dBFS), maybe to a 3 or 4? Livable, at least until the 8824 arrives in a week or so. Don 't use "broadcast quality" analog audio equipment in a home theatre. Use sound recording studio equipment.

7. I actually ran a calibration today! Corrected response is from about 30 Hz to 19 kHz within 5 dB or so. That's with one Klipsch 12" ported sub, not both (dual mono), and there is a hot spot at one of the "sofa" measurement sides. They are O.K. But not great. I am eyeing a pair of Rythmic Audio subs eventually. Subs are at 1/3 and 2/3 along the front wall.

8. While the Crown XLS 1502 and 1002 amps have per-channel input attenuators, the Outlaw M-2200 driving the centre channel does not. While it has a balanced input , it is likely designed for +10 dbV consumer level full scale instead of +24 dBu, some 11.79 dB lower. It's actually quite sensitive: 1.7V XLR input sensitivity, so 4.6 dbV, typical of consumer gear. Given the professional level outputs of the DACs (+24 dBu at 0 dBFS -- adjustable, but only globally across all 4 channels per DAC), and my desire for the highest level possible on the wire (for the best SNR), I thought it prudent to precede the M2200 with a 12 dB pad. I still had to pad the measurement output by some 39 dB to match the levels of the other speakers. I think I need a bigger inline pad. Remember: you want to output the strongest signal you can on the line, and pad down at the receiving end to minimize the effect of line noise. This also helps with the SNR of each output stage, Basic gain structure.

9. The Crowns aren't audiophile grade amps. They don't cost like audiophile grade amps either. If I want to go good class D (for efficiency), Hypex N400 amps are the bomb. SNR on the Crown XLS 1502 is about 103 dB, and the Crown XLS 1002 is about 97 dB. Good enough for me. For now. Also, the XLS 1002 drive surround speakers. The M2200 center channel amp is 112 dB, typical of a budget A/AB/G amp. The XLS1502 drives a pair of Bohlender Graebner Radia 520i main speakers, the M2200 a Bohlender Graebner Radia 220i center channel speaker, and the surrounds driven by the pair of XLS 1002 amps are in wall Bohlender Graebner PD-6LCRi. I am considering swapping the M2200 center for an Emotiva PA-1 -- class D, but 112 dB SNR! That way I could mount the 52 lb 41"x10.5"x12" 220i monster on an Omnimount on-wall shelf under the TV with the PA-1 in the cubby underneath and not exceed the weight limits of the shelf. I am considering upgrading the front left and right speaker amplification to a pair of Hypex NCore 400 monoblocks: 125 dB SNR (weighted, 101 dB unweighted).

10. Signal flow. Nvidia shield into HDMI input of Oppo 103D. Video HDMI output of Oppo 103D to TV, audio HDMI output of Oppo 103D to nanoAVR (for bass management), to nanoAVR DL (for room correction), to HDTV Supply HDMI to SDI converter, to Blackmagic SDI to AES/EBU de-embedder. From here two sets of two pairs of AES/EBU audio, for channels (and daisy chained word clock) are sent to two ADA ADA4 DACs. The second one, along with the two XLS 1002 amps are located at the back of the theatre, and feed wall mounted speaker terminals for the side and rear surround channels, so there is a 20 foot run of two AES/EBU cables, an RG-6 word clock coax cable, and four conductor cable for remote turn on of the XLS 1002 amps. The DACs are always on. The first one drives the XLS 1502 amps and thence the Radia 520i left and right speakers, the M2200 (after 12 dB pad) and center channel, and lastly is split, converted from balanced to unbalanced, and drives two Klipsch Sub12 HG subwoders.

11. Upgrades. Currently signals between front and back of theatre are run back straight down the middle of the room, in a cable protector. It is kind of cheesy. When I install the Lucid A/D D/A converter, I have to decide where it goes. If it goes in the front, under the TV, that's one MORE piece of 2U b y 19" equipment to see, and I have to run four analog balanced cables to the back instead of two AES/EBU cables. I could bring the rear amps to the front of the room and run 20' of speaker cable (they're only surrounds), but that's even MORE equipment up front: another pair of 2U by 19" boxes. With all this rack mount stuff, I'm beginning I should get a stage rack.

12. Equipment location. I am considering rack mounting all the audio gear in the rear of the room, out of sight, with only the source components, nanoAVRs, front channel amps and speakers, and HDMI to SDI converter in the front. That way, I just have to run a single SDI coax cable TO the back, with a control cable for remote turn on of the amps, and an XLR snake of balanced audio back to the front. Yeah, 20' or so, but that's what balanced signals and cables were designed for: combatting line noise on long runs. And, frankly, in that world, 20' is nothing. I'm not keen on putting source equipment in the back and running an HDMI cable to the front. Yes, the Crown amps have fans, but I honestly do not expect to drive them so hard as to have them turn on: they are already over-spec'd for the max speaker ratings by a factor of two, and judiciously padded down for that reason. A stage rack in the back, with the LUCID 8824 and two Crown XLS 1002s might actually look kind of cool. Do I want to put the XLS 1502 and M2200 amps in the back and run speaker cables forward? I don't think so. I like amps close to speakers. And if I upgrade to a pair of Hypex NCore NC400s as monoblocks for the front, I can wall mount them on glass shelves to show them off: they weigh what 5 lb.,? I am really tempted to swap the M2200 for an Emotiva PA-1 to stick into the centre channel shelf cubby: It's rated for 5 lb. and the amp weighs 3! Class D, folks. When done well (and often it is barely adequate), it is amazing. Heck, might get two more for the left and right mains and save a bit of coin over the Hypex ncore N400, and have a cleaner "look".

13. Upgrades II. If two subs are good, four are better, right? I can fill the room with one sub now, but it is uneven. A pair is far better, but the Klipsch's are, well, so so. If I upgrade them to sealed Rythmic Audio F12s, then I can go a bit deeper. What with the two Klipcsh's then? I have this crazy idea of sticking them in the back, at 1/3 and 2/3 wall distance, and reconfigure the subs thus: front left: left low frequency + 1/2 centre low frequency + 1/4 LFE; front right right low frequency + 1/2 centre low frequency + 1/4 LFE; rear left: left surround low frequency + left rear low frequency + 1/4 LFE; rear right: right surround low frequency + right rear low frequency + 1/4 LFE. I know bass under 80 Hz is non-directional. I do the typical bass management now, and run ALL channel low frequencies under 80 Hz along with LFE to be split between the two front subs. But here's the thing: the sub crossover is at 80 Hz, and most main speakers already struggle there. Let's say I crossed over at 160 Hz, or close to the limit of the subs. I could allow some localization, and use "more" of each sub's capacity. Just an idea. A miniDSP 4x10 and 10x10 would get the job done. In the rear rack, of course. I'd have to run 6 channels back to the front. Time to consider an 8ch XLR snake. And a 12U rear rack.

14. Thirteen is an unlucky number to end on.

Hmm. I thought there would be more interest than this. Oh well. Now that the electrical and sonic parts have been taken care of, it is time to finalize the layout.

The seating position is 126" from the front wall and I'll use that from the distance to the screen, even though it will be out a bit from the wall, once it is mounted on it. Right now it is about 18" out from the wall as it is on a combination TV stand/equipment table. Ear height when seated varies from 37" (for me at 5'7") to 40-1/2" (for my son at 6'0"), so I'd average that to 38-3/4". If I give priority to a centre channel speaker at ear height mounted below the TV, then the centre of the speaker should be at 38-3/4" above the floor.

My centre channel speaker is a Bohlender-Graebner Radia 220i. It measures 39" wide, 10-1/2" high, and 12" deep. It weighs 52 lbs., and will sit on an Omnimount OWS60 shelf, which has a capacity of 60 lbs. total when mounted to two wall studs, with up to 5 lbs. in it's "cubby" under the shelf. The bottom of that cubby is 3.8" from the top of the shelf, so at 29.7" above the floor. The top of the speaker is 44" above the floor.

I had been planning to put a stage rack in the rear corner or rear centre, with source equipment, front speaker amplification, and miniDSP processors in the front of the room, but the question arose of what to put that equipment on: the existing table (with the TV stand removed), or a small rack (say Odyssey CRS-08), and run an XLR snake, SDI coax, and control line around the room to the rear rack; or put everything in a 12U stage rack at the front of the room and run speaker cables to the rear surround channels. I already got the Odyssey CRS-12 stage rack with the anticipation of putting it in the rear, but started thinking that (a) the table in the front would look "big" (It's a bit wider and deeper than the Radia 220i centre channel speaker, and until recently, holds it), and purchasing a second rack, with shelves for non-rackmount equipment for the front (again, say an Odyssey CRS-08). This is starting to get expensive.

The idea for the 12U rack for the rear was to accommodate some 4U to 5U of future equipment, and if I go that route, I can certainly buy a small rack for the front. But right now, I think I can fit all the gear in a 12U rack in the front for a slimmer look than the table, and run speaker cable to the rear, so that is the current plan. The Odyssey CRS-12U stands 25" tall and so the top would be 4.7" under the centre speaker shelf cubby. An Odyssey CRS-08 stage rack stands 19" tall and would be 10.7" under the centre speaker cubby.

If I mount the centre of the TV at 64" above the floor, with a seated ear height of 37" to 40-1/2" at a distance of 126", the neck crane angle is 12.1 to 10.6 degrees, respectively. This is well within the recommended neck crane angle of 10 to 15 degrees, when seated at a sofa where one's head can recline a bit against the head support. At 52" TV with a 1-1/2" bezel (it is old) would have it's bottom 49-3/4" above the floor, so 5-3/4" above the top of the speaker. A (future) 75" TV with a 1" bezel (probably less, but I am being conservative) would have 44.6" above the floor, so just above the top of the speaker. Any size in between would fit. The 52" TV yields a horizontal viewing angle of 20.4 degrees, and a 75" TV would yield a horizontal viewing angle of 29 degrees, closer to the ideal 30 degrees. For a 30 degree viewing angle, one would need a 77-1/2" TV. A projector and screen might be cheaper.

As far as loading a 12U rack in the front, I am considering, bottom to top, the back surround Crown XLS 1002 amp (2U), side surround Crown XLS 1002 (2U), front Crown XLS 1502 (2U), Lucid 8824 DAC (2U), Furman M-8X2 Merit Series 8 Outlet Power Conditioner and Surge Protector (1U), a 2U vented shelf, for the Oppo BDP 103D (or replacement), and a 1U vented shelf for the Outlaw M-2200 centre channel mono amp. (It is 2" high, but 1-1/2" without the feet, and so would fit in in a 1U spot, given enough ventilation. 1/4" above and as the Oppo sits 3.1" high in a 3.5" 2U space, 0.4" below. That leaves the Nvidia Shield, which could sit on top of the rack, or behind it as it is controlled by bluetooth. Yes, it is ironic that my best amp drives a centre channel speaker.

The nanoAVR and nanoAVR DL, and HDTV supply HDMI to SDI converter, a power bar, and wall wart power adapters for same all fit in the cubby. Volume control is managed by the nanoAVR DL.

With that layout, two HDMI cables (one carrying video for the TV, and one audio for the nanoAVRs), two ethernet cables, (for the nanoAVRs) and a speaker cable (for the centre speaker) run up to the cubby, along with two power cords (one for the power bar, and one for the TV; and an SDI coax cable runs down from the cubby (to a hidden SDI to AES/EBU audio deembedder and then the Lucid 8824 DAC). Tight, but doable. The Blackmagic SDI to AWS/EBU audio demebedder can be mounted upside down under the vented 2U rack shelf for the Oppo BDP-103D with a couple of tie-wraps as the Furman power conditioner under it is only 7-1/2" out of 13" deep and the SDI to AES/EBU audio deembededer is only 3.63" deep, 0.9" high, and 4.52" wide. Four 30' runs of speaker cable connect to the rear surround speakers. 60' of 12ga is only 0.095 ohms or 0.2 dB loss (4 ohm speakers) -- acceptable for surround channels.

If I'd seek to upgrade the front channel amps, I might add two more Outlaw M-2200 monoblocks. I'd gain 2U removing the Crown XLS-1502, but would have to purchase two more vented 1U shelves and remove the M-2200 feet. That would leave 1/2" between monoblocks -- the same as stacking them with feet. An alternative, would be three Emotiva PA-1s, as these can be mounted in 3U of rack space with one wide and two wide 1.5 RU face plates from Emotiva.

I could also swap the Outlaw M-2200 for an Emotiva PA-1 for the centre channel speaker. Not quite as good an amp, but not shabby: 105 dB SNR. It weighs 3 lbs. and would fit in the cubby with a convenient path to the centre channel speaker. The two nanoAVR and HDMI to SDI converters would take the place of the M-2200 on it's shelf, and the power bar could be hidden in the back of the rack: The Crown XLS amps and Lucid 8824 are only 7.7" and 8.5" deep, so there is room on the floor of the rack.

Well, the Furman power conditioner lights point down (the front of the rack), so the conditioner has to be at the top. It's actually neat: the lights are incandescent, but on a pair of what look like knobs. Except, one pulls on them, triggering a switch that turns on the lights, which are dimmable as a pair.

I racked up the power conditioner, Lucid DAC, five 1U blank plates (for now), and two Crown XLS 1002 rear channel amps, assuming the rack will be in the rear. My son and I agree that it looks "cool" enough that it should be in the front, with the Crown XLS 1502 amp, and other equipment, with a speaker wire run to the back instead of an XLR snake. I ordered some Monoprice 4 conductor 12ga OFC cable (under a dollar a foot, how could I go wrong?), and some 4 pole speakons and banana plugs, and will be making 30' and 6' cables this weekend for either location. By the way: if you're going to run speaker cable around the perimeter of the room without a raceway (at least for now), a white jacket on the cable blends in better than a black one. And it looks cooler.


Said "coolness" was also impacted not so much by the appeal of the equipment (to us nerds), but rather the width of the rack. It looks much nicer than the existing AV table, and certainly will once the TV and centre speaker are wall mounted instead of AV table mounted.

The Oppo BP-103D is spec'd at 3.1" and the Outlaw M-2200 at 2", both with feet, so together they come in at 5.1", 5.17" with shelf thickness. I was thinking of mounting them in a 3U shelf, as I have 3U of space left in the rack, but (a) that will be tight (my measurements showed them bigger but measuring tapes are designed to be pulled taught and not pushed against a surface, and have some slack at the end, about 1/8") and (b) I realized the Odyssey CRS-12 rack is slanted down at about an 11 degree angle. The Oppo transport might not like that.

So, the current plan is to put the Oppo on top of the rack, freeing up almost some 2U of breathing space for the amps. I hope that will not be too ugly. Conversely, I can use a 2U shelf for the M-2200 amp, and a 1U shelf for the two nanoAVRs, taking them out of the cubby. Hmm, If I do that, then I could put a 4k bluray player IN the cubby, like the Sony UBP-X700 and avoid all the up and down of HDMI to nanoAVRs to SDI adapters (and power bar, and power adapters) in the cubby. That starts to look to be worth $100 to $200 to me. As for the Oppo, maybe it is time to sell it (at a premium! since Oppo players aren't made any more and the price for the 203 and 205 is stratopheric as a result). Yup, time to get a networked 4k player, in plans for a 4k TV. When that happens I can move the old LG 52" 1080P, Oppo BDP-103D and Nvidia shield (used to network and local playback, which the Sony does), to my bedroom, along with the existing stand. Hmm, I get to reuse a lot that way. Time to get the Sony 4k player.

Pics, or it didn't happen.

1660W into 8 ohms, 2750W into 4 ohms. You'd blow the breaker driving 4 ohm loads at full power. I run 4 ohm speakers, but they limit out at half that power rating just after reachingTHX peak levels of 105 dB, 115 dB for the LFE channel.

Conservatively: a kilowatt and a half "Home Theater In A Box".

In 4 ohm:

525W each left and right, 103dB SNR
350W each surrounds, 97 dB SNR
300W center, 112 db SNR
Balanced sub output.

SNR is A-weighted.

Digital bass management and Dirac Live room correction.

Lucid 8824 A/D D/A 105dB SNR

24 bit 48 khz processing.

Speakers are:

Left and right: BG Radia 520i
Center: BG Radia 220i
Surrounds: BG Radia PD6-LCRI x4
Subs: 2x Klipsch Sub12HG

KICKS ASS!

Not sure how many are following this thread, but I will continue. Please speak up if you are! I plan to turn this into a diary of sorts of this project with a report at the end.

Two small snags, neither insurmountable:

1) I need to change the 3.5mm mono plug on the end of the cable to trigger the Outlaw M-2200 amp. It is shorted inside. For amp control, I wanted to use the remote and trigger connections. On the Crown amps, these are three pin Phoenix connectors with two pins shorted to turn the amp OFF when the power switch is in, and a third pin to indicate whether the amp is on or off (so, a control line on pin 1 to short to a common line on pin 2, and a status line on pin 3, referenced to that common line, to return amp status). On the Outlaw M-2200 it is a standard consumer mono 3.5mm plug that carries 3V to 30V at a few milliamps to turn on the amp when it is set to trigger mode.

The way I have this wired up is with a 12V 4PDT relay: Three of the poles connect to three pairs of lines, one to each of the Crown amps, making a normally closed contact. When the relay is energized, the contacts open, and the amps turn on. The fourth pole connects the power to switch the relay through a protective diode (to block back emf from the relay coil when it de-energizes -- there is also a reverse polarity diode across the relay coil to dissipate this), to the hot pin of the Outlaw M-220o trigger input, The ground pin connects to the negative side of the power switching the relay. A small 12VDC adapter is plugged into a controlled AC output of a Staples power sensing power bar with the master control outlet sensing when the TV (or blueray player, I haven't decided which), is turned on.

The Crown amps have an annoying feature of FLASHING the power indicators when in standby mode, but thankfully they have a menu option to turn these indicators off.

2) The only way to set the DAC output levels on the Lucid 8824 is via remote control, and these "stick" when control is set to local. Remote control is via MIDI (I do not have the RS-232 version of this unit). So, I have to get a USB to MIDI adapter and software for my Ubuntu laptop to set this. Apparently, the M Audio Uno USB to MIDI adapter is supported, with some effort.

A word about power.

You will note that the system can consume more power than a 15A, 120V circuit can provide (1800W) when driving 4 ohm loads to it's maximum capacity at peak levels. While all my speakers are 4 ohm loads, they are rated to about half the output peak capacity of the amps driving them, so there is no need to drive the amps anywhere close to clipping, This is normal: one sizes the amp to about double the rating of the speaker and never drives it to full power. Of course, half power is only 3dB down. Nevertheless, if someone had less efficient speakers, they might need closer to the full capacity of the system. Again, this is only an issue with driving 4 ohm loads. (IT is also the reason why class D amps were chosen, at least for the surround speakers. The front left and rights could be upgraded to more traditional Class AB amps while still being reasonably efficient. If you want to go full Class A for the fronts, you would be talking outboard amps and separate power sources PER AMP. However, Hypex is starting to may some very, very, good audiophile grade Class D amps, so we will see.)

To recap:

Front Left/Right: Crown XLS 1502, 300W x 2 into 8 ohm, 525W x 2 into 4 ohm
Surround Side Left/Right: Crown XLS 1002, 215W x 2 into 8 ohm, 350W x 2 into 4 ohm
Surround Rear Left/Right: Crown XLS 1002, 215W x 2 into 8 ohm, 350W x 2 into 4 ohm
Center: Outlaw M2200, 200W x 1 into 8 ohm, 300W x 1 into 4 ohm

Total: 1660W into 8 ohm, 2750W into 4 ohm

Assuming 90% efficiency, that's 1850W into 8 ohm and 3060W into 4 ohm,

And we haven't even included the subwoofer(s) and electronics.

Now, my speakers are not rated for these power levels, and represent a more typical power budget:


Front Left/Right: BG Radia 5201, 250W max x 2
Surround Side Left/Right: BG Radia PD6-LCRi, 150W max x 2
Surround Rear Left/Right: BG Radia PD6-LCRi, 150W max x 2
Center: BG Radia 220i, 250W max x 1

Total: 1350W max, or 1500W at 90% amp efficiency.

So, that should leave plenty of room for electronics (excluding subs and TV), on a single 15A 120V circuit. Remember these are PEAK levels, and typically accommodated by amplifier reserve power. How "peak"? THX reference levels of 105 dB peak, and then a bit, representing a NORMAL listening level of 85 dB. That's way louder than any reasonable person would want to listen. Nice to be able to have the bragging rights to reach them, but let's be reasonable.

But, it still begs the question of what if we are overloading a single 15A, 120V circuit? There are THREE power strips in this build, and any of them could be plugged into an independent circuit. Typically, one or two circuits would be used. One handled by the Furman power strip, another (mostly for wall wart adapters for the miniDSP nanoAVRs, network switch, HDMI to SDI converter, and SDI to AES/EBU deembeddder), having several free outlets, and a last one with a sensing master outlet, 4 outlets controlled by that master (one for the trigger and remote controls for the amps), and two uncontrolled ones. So, one has a choice of which of the three power distribution strips, the four amps plug in to, with all four accommodated by two of the three, and the third accommodating up to two. It is expected that the subs will use two of the master controlled outlets.

Peak output levels. You know you want to know.

We have to consider the efficiency of each speaker, how much power it can take, how far from the listener it is, and what type of driver it uses. Ribbon drivers, used here, for the midrange and highs, drop off at 3 dB per doubling of distance, not 6 dB, because their wavefront is cylindrical and not spherical. The ear is most sensitive in this range as well.

Taking all that into account, at their maximum ratings, at the listening position, the speakers will reach:

Front Left and Right: 107.5 dB (250W, 88 dB/W/m, 2.8m)
Center: 107.4 dB (250W, 88 dB/W/m, 2.9m)
Side Surrounds: 109.5 dB (150W, 90 dB/W/m, 1.8m)
Rear Surrounds: 111.2 dB (150W, 90 dB/w/m, 1.2m)

Remember, fall off for ribbon drivers is 3 dB per doubling of distance, not six, at least in the midrange. For the bass, using conventional drivers, it is 6.

I have excluded the subs, though THX reference levels require a peak of 115 dB at 20 Hz! I don't think I can quite reach this with the subs I have as they start to roll off at 24 Hz, and measuring them much below that is hard.

But for the mains and surrounds, we can certainly reach THX peak levels of 105 dB.

That implies an average level of 85 dB, which, again, is far louder than any reasonable person would watch a movie.

Problems.

1) The Crown amps do flash their power indicaters when in standby regardless of the lighting setting.

2) The Lucid 8824 produces transformer hum.

The latter can be dealt with by isolating it from the chassis with rubber washers as some have reported. Others have used simple cardboard between the transformer and chassis to silence the resonance of the 60 Hz vibration. It does not affect the signal passing through it.

The former is more problematic. I can live with it and switch the rack off when not in the room. This allows a power on sequence of powering up the rack, waiting for the minidsp nanoAVR to boot (and other non-amp equipment), and monitoring the TV or source player power on to switch the relay that turns on the amps. As this is a dedicated theatre room, that might not be unreasonable.

I have applied first power, without source signal but with moderate amplification and the system is dead quiet (except for the 8824 transformer hum) with no significant pops on turn on or off. If source equipment is well behaved this should remain the case.

Now this is tempting: replace the Lucid 8824 and it's transformer hum with a Yamaha DA824 with an AES3 card. $125 on ebay. Only DA, BUT it also accepts an SDI input card (currently prohibitively expensive) in place of the AES3 card. Almost worth getting as a backup.

$90 after haggling. It is on it's way as a backup.

Details, details.

The front wall has a stud ALMOST in the centre of the wall. Wall mount brackets are designed to accommodate a bit of off-centredness by allowing where the retaining screws go (via an elongated screw hole cut-out in the bracket). However, while the TV mount can span three studs (32" centre to centre, with one in the middle), the speaker shelf bracket can only span two. It can be mounted into a single stud, with drywall retainers on the ends, but this reduces it's load capacity from 60 lbs. to 25 lbs. Mounting the centre channel speaker shelf off-centre is NOT an option. The TV mount can mount into two studs only, but neither shelf centring adjustment accommodates a whole half-stud space (8") and then BOTH the TV and centre channel speaker would be significantly off-centre on the wall. The room is small enough that this would be noticeable.

After some thought, I realized I could bolt the centre channel shelf to a metal bracket that DOES span three studs: a 2" by 36" by 1/8" piece of metal works fine, and that can be mounted to all three studs (and still somewhat obscured from view under the shelf itself). Steel bar is ugly so I settled on aluminium but I do not expect significant bimetallic corrosion in an indoor, dry, environment.

I can has be structural engineer?

Mounting the BG Radia Omnimount OWS 60 proved to be a challenge. I had to size the bolts holding the OWS 60 frame to the 2" x 36" bar so they would not fail. Four 1/4"-20 bolts work fine. I know know that grade two steel has a proof strength of 55 ksi and the bolt has a cross sectional area of 0.0319 in.^2. So I had to calculate the pulling force on the bolts. About 210 lb. on the top two bolts against a limit of 1749 for a safety factor of about 8.

The other issue was torsion on the bar as it was being pulled in the middle of a 16 inch stud span. If the bolts held, would the bar snap? Calculations predict a 12 degree deflection and that was about what I saw. I have more respect for structural engineers.

Finally, the BG Radia weighs 52 lbs. and sits on four 1/2" feet in the corners. It did not break the class shelf but it bowed disconcertingly. I replaced the four feet with ten, distributed as follows: four where the original ones were, two in line with them at the center of the speaker and four in line where the speaker rests on the bracket. No more glass bowing.

It has held since Saturday.

On Edit: I have since found out that the aluminium bar is 6063-T5 aluminium, and has a shear strength of 17000 psi. Calculated max shear stress (if I did it right: the online calculator was for a bar held at one end and mine is held at two ends, so I cut the torque in half -- the deflection angle is close enough so I think it's right) is 12000 psi. Close, but strikes me as safe since the bar is restrained on it's lower half through it's length by drywall. Still, I like safety factors of 4 or better and so I am wary. Maybe I should have used a 1/4" thick instead of a 1/8" thick bar.

Well, the aluminium bar holds (I expected it to). But, the stress is visible: there is some bend to it, and, according to my calculations, the maximum torsion stress of the 6063-T5 aluminium bar is 12 ksi against a failure limit of 17 ksi (not counting whatever restraint the drywall provides, which I am generally ignoring). That strikes me as too close for comfort. I am thinking of replacing it with stainless steel.

Turns out there is a metal supplier near me that is willing to deal in small quantities, and will even drill or punch a hole pattern for me (albeit at $7.50 a hole, but I only have six holes, and only need one piece, so up to $100 to get it made is not a deterrent.) It would certainly look a bit better and be three times stronger.