Audio DIY Active Monitors

cranky

Skilled
Here are a few pictures of my new active monitors. These are using a 6" Kevlar midbass unit coupled to a low resonance soft dome tweeter. Both drive units are sourced from Peerless India and diyaudiocart.com. The project was started because I want to start recording again, having no job and little else to do. Since I sold my Dynaudios and the accompanying amplifier a year ago, I did not have a nearfield reference speaker, which IMO is absolutely critical for recording and mixing. Since the money I had was not enough to buy me a good pair of monitors, I decided to go homebuilt.

Amplification is 70W on the tweeter and 140W on the woofer, though sustained power levels are gain-limited to 100W due to the very low crossover point of 1.6KHz (4th order, Linkwitz Riley textbook slopes). Speaker drivers are sensitive enough that I've never had to exceed -15dB level (about 25W total). Tweeter will explode if played at full power for sustained duration, but the additional headroom helps to avoid clipping at normal power levels. Sinks are warm to the touch - about 50 degrees - as the power supply voltages are a wee bit higher than I am comfortable with. However many, many parts including the very old transformers were from my spares bin, and little higher voltage is fine because it was free :)

The box is fully closed, resulting in final frequency response 65Hz-22Khz, and there is a matching subwoofer in the oven right now to get this down to about 38Hz. I chose sealed alignment for a flatter response, low distortion and improved power handling. Also, since the monitors will typically be heard from very close distances in untreated rooms, ported boxes would mean poor integration and I would have had to plug the ports anyway. The Peerless woofers are not very flexible, they need quite a large box to extend down to the low 40s, though their parameters mean they can work very well in small sealed boxes with only the slightest of midbass humps. I suspect these are less than optimal for home speakers, but I can't judge that right now.

The speakers sound flat, clean and extended when used in 'monitoring' trim, allowing me to hear deep into the mix. I still have some tweaking and cable management to be done after I get used to the sound and can work out the next set of changes, specially for the 'hifi' trim. The BSC and tweeter levels are tweakable with two three-position switches on the rear plate, resulting in a variety of different options to optimise the response for different rooms and listening axes/preferences.

The finish is hand-sprayed matt black out of a 'rattle can' - or six of them - and the attempt was to disguise the wood ancestry of the speakers. The box is constructed in 1/2" exterior-grade MDF, with a double-thickness front baffle. The wood thickness was kept low to give me additional internal box volume, and compensated with a vertical window brace across the depth dimension as well as mass-loading the walls with bitumen sheets. Also, exterior grade MDF absorbs less moisture and is easier to paint and stays together for longer.

I'm glad I built them. I learnt many new things along the way, specially things like painting which do not suit my ten left thumbs. In the end I can gladly say that these are better than the Dynaudios I sold in many ways, specially the driver to driver integration and the nicer, if not as extended, bass presentation. I hope to put them to duty by early next year once the recording rig is built. With no income, one goes about these things very slowly and carefully.

00Plans.JPG
01Right.JPG
02Left.JPG
03Front Close.JPG
04Rear Close.JPG
05Plate.JPG
06PSU.JPG
07Amps.JPG
 
Yes, that is a truncated frame that allows you to bring drivers closer to each other. The Indian version combines that idea with surface mounting the woofer, which means the woofer edge is bevelled flat to the mounting surface.

The trouble with this adaptation of that design is that the truncated portion presents a very diffractive edge to the tweeter hence we have to orient the driver so that the tweeter wave interacts with the bevelled slope. This increases driver center to center distance, leading to the obviously low crossover point, which prevents lobing. If I had the crossover at the more conventional 2khz mark I would need to be at least four feet away to get the drivers to integrate with that layout.
 
What kind of man hours have gone into this?

Guess only those with better knowledge of sound recording and acoustics can appreciate your efforts here but to rank outsiders like me, the entire setup looks amazing!
 
I haven't kept count, but I'm guessing about a hundred hours of my time including the drawing and design phase, plus another twenty of the people who supplied me with various bits and pieces.

This is actually my bedroom setup which normally houses a pair of small usher bookshelves powered by that amp you see below the computer. There's no recording to be done here, that's in a different room and with a different PC, which is so way away from completion.

Thanks for the kind words :)
 
Excellent work Sangram! The finish of those boxes are terrific! Allen screws look wicked. Doesn't look like a DIY job to be honest. Few questions.

How did the carpenter manage those curves on the edges of the box? Or were the boxes sourced off the shelf?

Why have you mounted those boxes atop marble bases?

I see that the boxes aren't ported? Just curious because I've noticed most bookshelves and studio monitors to be ported.

PS: Regarding the conversation we'd the other day, I spoke to my friend and he confirmed the speakers were submerged under few inches of water! Plugged the power cable and we could only hear a small thump from the speakers. No indications on the 7 seg. display. Will PM you later with the details. Clicked some snaps.
 
Thanks gannu.

The boxes were custom built from scratch. The rounded corners are pretty easy for the carpenter, he creates progressive bevels using a planer and then sands down to the round over. Requires a lot of skill and sweat, and I have an excellent team of woodworkers who can achieve the impossible. For the record, the recess for the tweeter was the most difficult part of the woodwork.

The marble slabs are required because the unit below has no mass at all and resonates like mad when you put any speaker on it. The setup, as I said, is actually meant for a pair of Usher S520 which are being rested as these are settling in. There is a fantastic DAW table in the making that is their permanent home. Should be ready by end of the year.

Regarding the sealed versus ported I alluded to it in quite some detail in the body text. Ported speakers allow more extension and is the default alignment. These, OTOTH were designed to be partnered with a sub from the get go. Some of it was due to the specific parameters of the woofer, and some of it was due to space issues.
 
Awesome looking DIY ! What kind of amplifier are you using in this active design. I have a pair of mini towers (passive) lying around and thinking of making them active. Also how much did the drivers cost you ?
 
Thanks.

The amps are LM3886 chipamps, two in bridged for the woofer and a single for the tweeter. Rated dynamic power is 200W per channel, though long term max is limited to about 100W.

The drivers cost me about 5500 for the whole set, give or take.
 
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