So I picked up a pair of OSD Nero MB5 bookshelf speakers about a month ago and after giving them a listen and measuring their frequency response, I decided these speakers need an upgrade! In a previous post I updated a pair of Jamo Concert 9 Series speakers where we tackled internal damping, port tuning and the crossover. In this update we’re just going to be tackling the crossover with just one very minor change to the damping. First off I think the Nero MB5’s are great little speakers. I got them on sale over at Crutchfield for only $99. Amazon typically has them for around $159. As of this writing they are still on sale at Crutchfield and for this price, in combination with the updates we’re about to go over, these speakers honestly cannot be beat. These are essentially the passive version of the Nero AB5 powered speakers, ditching the internal amplification (and any DSP) for a simple passive solution with a passive crossover. They even left the LED for the logo on the front baffle, but it isn’t connected to anything.
They have the look of a nice studio monitor, but without the finesse or feel. The box is light, lacks any real bracing and any significant damping, save a single piece of poly fill that covers about half of each side and the top. The soft dome tweeter is framed in a very good-looking waveguide which provides a classic 1st order gain response as the frequency decreases, but any benefit here is not really realized due to the paltry 6 dB/octave filter on the woofer which lets an excessive amount of cone break-up modes to enter the equation. The dual ports provide a tuning frequency of about 60 Hz which is fantastic for a speaker this size. The little 5-1/4″ woofer can produce an impressive amount of bass and depending on how close you set these against a wall, the bass can either be just right or way too much. Don’t expect miracles here, but the bass response is quite punchy. Some port noise (chuffing) can be heard for bass-heavy tracks that don’t have a lot of other music to mask it. But is bearable.
At first listen I really enjoyed these speakers, there was nothing that appeared completely wrong or unmanageable. After a quick measurement with REW and my Behringer ECM8000 mic, we start to see some of the issues this speaker actually has. Measured on axis at about 0.5m directly in front of the speaker midway between the woofer and tweeter, we see a 6-7 dB peak around 5 kHz that screws up any hope of expecting these speakers to sound flat or neutral. Note that this response will look different depending on listening position. Move up or down or left or right and this peak is less predominant. However for purposes of this crossover upgrade, the intent was to make the speaker flat on-axis and ear-level being directly between the woofer and tweeter. As it stands, these speakers will sound a little better either above or below the midpoint of this speaker. But for now let’s open this speaker up and see what makes it tick and see if we can figure out what is causing this uneven frequency response and what it might take to make it better.
Disassembly of the Nero MB5 is pretty straightforward. There are a total of (12) 4mm hex wood screws holding the plastic front baffle and terminal cup/crossover. The only real trick is there are two smaller phillips-head screws hidden underneath the 3M EVA vibe isolation pad in the front left and right corners. You have peel back the foam over about a 1″ x 1″ area to access the screw to remove it, thus releasing a plastic tab attached to the front baffle. This seems holey unnecessary and just makes removal of the front baffle trickier. See pics below for an idea of what this looks like. Once that’s out of the way, the front baffle and as it turns out, the tweeter assembly, come off together. The 5-1/4″ woofer is revealed and can be removed as well. After disconnecting the drivers we can remove the terminal cup and crossover assembly and get our first look at what kind of magic the folks at OSD came up with to tame this beast. Turns out to be just another typical cheap crossover consistent with this type of speaker at this price point. The low-pass filter duties are handled by a basic iron-core inductor creating a simple 1st order, 6dB/octave filter network. Boring, and, ineffective.
I have, in my experience, never found a 1st order filter to be sufficient for a low-pass design to suppress cone break-up modes to a manageable level. The best you might get is with a poly cone as they tend to have slightly better break-up modes, but OSD touts this is a “composite fiberglass woofer” and you aren’t breaking any laws of physics here. Once you get above the fundamental 1st mode of this material, it’s going to resonate, its going to break up, it’s going to like being excited at these frequencies and will respond in kind. If you’re gonna design a speaker and expect to get away with a 1st order filter (which by the way can be turned into a 2nd order filter with a cheap-o 79-cent capacitor), you better be prepared to work out some exotic cone material with specialized composites for better damping to control those break-up modes or you’re going to be fighting a losing battle. 1st order filters just don’t have what it takes to be effective in this regard. So that’s the first thing we will fix. The woofer measured about 3.6 ohms at DC and therefore represents a solid 4 ohm speaker. See the impedance plot of the woofer measured in the box below.
Moving on to the tweeter, we have a 2nd order high-pass filter made up of a 1.2 uF poly cap, a 0.24 mH inductor and an 8 ohm series resistor. This actually sets the crossover frequency to be fairly high, you’d expect a gap going from the woofer to the tweeter, however that’s where the waveguide comes in and does its magic. The waveguide is providing about 6 dB/octave gain all the way down to about 2 kHz. When combined with the filter response, you end up with a decent overall frequency response that actually combines really well with the woofer. And the benefit is that this gain comes acoustically, so the tweeter is able to do less, i.e., less distortion, and still provide a decent output. The only adjustments needed here will be to match the crossover point and level with the woofer once we add the 2nd order filter to the woofer. We’re going to make only a small adjustment here, as I felt like the raw tweeter response with the waveguide was quite good with no serious defects. In which case all we need to do is set the crossover point so it sums well with the woofer and adjust its level so it’s not too bright but not too dull. Special notch or other shaping filters won’t be needed here.