Small active monitors

[chaparK]

I've recently finished a pair of active medium-sized closed box speakers and i'm thinking now about small active monitors.

I read these inspiring build reports on the net:
Amethyst Micro Monitor
Whetstones - Taterworks Audio

They're both based on the 4 inch TB W4-1052SD and i liked the 2 outcomes equally:
- On one hand a full-range design in an enclosure of 6 litres,
- On the other miniature 2-way monitors.

I decided i wouldn't choose, just try to build both in a single package, preferably for a low cost (but not derisory).

Here's my present BOM:
bom.jpg

There are 2 boards from Helder to power the drivers. One is the TA2020, that one will power the small 4-ohm woofers with nominally 25w. The small board is the TA2024 to power the 6-ohm tweeters. Not shown on the pic, there's also a 14-volt 4A switched power-supply. I got this power-pack for about 50 bucks delivered to my door (makes me wonder why there are still people making passive crossovers nowadays).

Between the source and the amp, there's going to be a dsp with 2 settings selectable with a push-button: full-range or 2-way. Additionally to doing the crossover and the 'coutour' filter, i'd like it also to equalize the bass and help correcting an issue if such a misfortune happens once the build is over. There could be as well an option for sending a line-out to an external active sub-woofer from the shelf.

I liked very much some speakers that i've seen on this forum and built with ikea wooden bowls. But i don't really enjoy going to ikea cause it's far and crowded and these bowls would cost more than the drivers, and i'd have to explain to my partner - who's been very tolerant so far with apprenticeship carpentry at home - why i'm slicing now brand new wooden bowls etc. Maybe an other time.

Then i thought the following: a lid on the top of the enclosures, with the tweeter secured on the lid itself. You lift the lid and the switch on the enclosure selects the 2-way option. You close the lid and the same switch activates the full-range mode. Wow ! I was feeling happy like a man who thinks he's got great idea! And i could even set the lid such a way that it corrects time-alignment!

But well with a dsp time-alignment is done within a finger snap. And it's certainly a pain to make a lid that doesn't cause diffraction issues. A friend to whom i talked about the lid liked the idea and even asked if one could throw in his keys and coins and close the lid.

So i'm more leaning towards a conventionally shaped monitor right now.

[chaparK]

I've spent some time familiarizing with the woofers. Below is the free-air impedance profile, along with an other measurement in a 1.5-litre closed-box.

imp_TB_A_TS.PNG

I measured the TS data (haven't found them on the net except manufacturer's):

fs: 100.1 Hz / 97.7 Hz
Qts: 0.74 / 0.72
Re: 3.7 ohms
Vas: 1.64 ltr/ 1.92 ltr

Manufacturer announces:
fs: 63 Hz
Qts: 0.48
Re: 3.8 ohms
Vas: 7.58 ltr

Despite looking quite different, WinISD provides simulations that are similar (group delay is not similar, however i'm not sure that i understand the sonic impact of larger group delay at 50 Hz).

TB_sim1.PNG

I also looked quickly at a 6 ltr box with the vent i already have, with the measured parameters. I wouldn't like to make them bigger than 6 litres each.
Now i need to understand what i have to look at in order to optimize bass equalization by choosing the proper alignment given the limits of the woofer (Pe 20w/Xmax 3.4mm).

[pythonic]

How is this project going? I'd be very interested to see some photos as I'm finding it difficult to visualize the concept.