blog for whoever

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.

Continue reading →

So this is going to be a Part II to my original post from 14 years ago where I first discussed the topic of proper subsonic filter design in conjunction with subwoofer enclosure modeling. It’s been a while, but I recently got around to actually doing what I said I was going to do all those years ago. While my specific example incorporates this super-old (and no longer available) EV XEQ-2 crossover, the methods employed herein are practical for anyone looking into subsonic filter design, how it works, and the best way to incorporate it into you next sub project without sucking the life out of your bass. And more recently since the availability of online, web-based simulation tools, such as National Instruments MultismLive, taking the guesswork out of active filter design is a thing of the past. So let’s get into it!

I’ll do a quick re-cap of my previous post on this topic, but the main idea behind this post is to show how to design subsonic filters and then how to model them into a subwoofer design before you even buy or build anything. The two main tools for achieving this are NI’s MultismLive and Unibox 4.08. Unibox is a bit dated for speaker enclosure design and runs in Excel, which not everyone uses, but is my favorite box simulation software of choice. I took a stab at using WinISD to do this, but it doesn’t provide the option to import .frd files to create custom filters. You can create simple filters using the built-in filter tool, but is limited to certain prescribed filter types. Plus it’s not obvious how to go from the simulation to a real-world active filter, unless you’re building a system with DSP. Unibox 4.08 on the other hand will let you import .frd files which can have any shape, slope, Q, response, etc. you can come up with.

So with that let’s start out with this simple example, which is based on a sub I built several years ago, so we can see how it all plays out. First we’ll start with a simple simulation of the driver and enclosure. In this case I’ve got a Dayton Audio RSS315HF-4 in a 95-liter enclosure tuned to 22 Hz. This looks pretty good in the simulation, has a decent f3, a nice and flat in-band response, without any peaking or drooping, but if you look at the excursion below 19 Hz, you can see that Xmax is widely exceeded, as is the assumed Xmech. Regardless of the driver and enclosure, this is a pretty common behavior that will occur with any vented design. Passive radiator designs are bit better and sealed designs are even better yet, but they too can benefit from a properly designed subsonic filter to accompany them. So even though this sub is already tuned pretty low, if you didn’t want to implement a subsonic filter, you could argue three points: 1. there is little source material that is actually going to push the sub to those levels below 20 Hz and 2. most if not all amplifiers are not even flat 20 Hz, meaning below 20 Hz the amp is starting to roll-off anyway, thus limiting the actual excursion at the driver and lastly 3: eh, it’s just modeled data, who really cares? All valid, but flawed points on their own. A subsonic filter should be implemented to keep the driver under control below tuning to prevent possible driver damage, lower distortion, and keep the amplifier happy (i.e., possibly going into protect mode during playback and interrupting your movie/music).

The biggest problem with subsonic filters is by design, they limit excursion, and in doing so if not set to the correct frequency, will limit the output at and above tuning, which we do not want. Steeper slopes can help, but also make the filter more complex. In this case I am using a fairly simple 12 dB/octave active filter that requires only one op-amp to implement. So how do we determine where to set the corner frequency of this subsonic filter? Well, that’s where MultismLive comes into play. I built a model of the exact crossover that makes up the EV XEQ-2 and ran an AC simulation which lets you see the exact response of the filter. I show that circuit here, which is made up of an input buffer stage, which can be set just about any gain with no EQ, there’s an 18 dB/octave low-pass filter which can be set to any frequency desired, and then there’s the 12 dB/octave high-pass filter (the subsonic filter) which also can be set to any frequency and any Q desired. The input stage is unity gain, and provides a good buffer stage to the source, the LPF stage is set to about 90 Hz, but can be adjusted as shown with three resistors and last stage is the subsonic stage which is set by only two resistors. Depending on the values selected, you can actually create a bass boost circuit, which can be used to model bass boost circuits of common plate amps to see how they will sum with your enclosure, which is what I did with my Sonorous 10S subwoofer. But for this vented sub box, we want zero boost, in fact the ideal roll-off is where the Q=0.7 which can be achieved by changing R9 and R11 at a ratio of 1:2.

So I played around with a bunch of resistor values for gain, LPF and HPF and ran an AC simulation for each one to get an idea of different frequency responses that can be achieved. You can click on this link here and play around with the simulation and model some filter cutoff frequencies of your own. But we’re not done yet. MultismLive lets you export the data into a .csv file which this is where the whole design gets interesting. With that exported file, we can now import that into the Frequency Response Active Filter section of Unibox and actually see how the HPF affects the box response and how it affects the driver excursion. I created about 10 different filters, and imported each of them into Unibox and then compared them all see which one looked the best. And by looked the best, basically what I was shooting for is best control over excursion below tuning, but with minimal impact to frequency response above tuning. Go too low, and excursion is still too high, but FR is minimally impacted, go too high and excursion looks great, but FR suffers. The balance is basically somewhere in the middle. By plotting all of the options, we can better choose where we want this subsonic filter to sit. But even Unibox has limitations, one being you can’t compare more than 6 responses, and you can only compare FR plots and nothing else. So I exported the raw data from Unibox into my own worksheet in Excel where I plotted several configurations and overlay excursion and FR plots on the same graph. I’ve shown that here for this example design. Notice that a filter corner (-3 dB) of 20 Hz does the best at keeping excursion in check, but also has the most impact on FR. The 12 Hz corner is better than no filter at all, but still allows over-excursion. The sweet spot is probably right around 15 Hz or 17 Hz. So that’s the optimum subsonic filter corner for this subwoofer. And that’s the design. Short and sweet.

Continue reading →

I have to admit, I’ve got the bug once again. After building and listening to my little Sonorous 10S Subwoofer over the last couple of weeks, I’ve suddenly got this need for more, more bass, more low-end bump, more rattling, more shaking, more chest-thumping boom. So I’ve started hatching a plan to build a bigger, better, more HT-centric subwoofer, where the size of the enclosure has no limits, no bounds. But before I get into that part of this project (saving that for another post), we’re going to check out the new/old amplifier I just picked up which will power this monster subwoofer. So per my usual browsing on the web, I tend to check out my local Craigslist (aka CL) at least a couple of times a week to see what kind of audio gear people trying to unload. I connected with a guy who was selling a bunch of pro-style amps from Carver, Peavey and Crest Audio that he had collected over the years. None of them really seemed all that interesting for a subwoofer build, though his pricing was reasonable.

But after digging a little deeper, I ended up looking up the datasheets on each of his amps and was surprised to find the Crest Audio Vs900 amp he was selling had some decent specs: class A/B architecture, 2-ohm stable per channel, bridgeable, 1100W max rated power into 4-ohms bridged (at 1 kHz, 1% THD), claimed frequency response down to 10 Hz (none of this 30 Hz high-pass filter nonsense some pro amps have) plus just a nice clean front panel with nothing but a couple of status LEDs. So with a subwoofer from Parts Express already in mind, I thought this amp would make a great pairing (can you guess what sub I’ve got in mind?). He said he’d let it go for $100 and I said sure, let’s do it. If this thing can do its rated power, we’re talking about 9 cents per watt here. So let’s see if this 20-year old, 46-pound boat anchor can actually deliver.

So I fired up REW and pulled out my 400-watt load box, which is just a bank of four 100 watt, 4-ohm load resistors wired in a parallel/series configuration. I picked them up for Amazon for about $10. I’ve got a 45:1 resistive divider network that effectively will take 40 Vrms down to 0.9 Vrms which is just about right for driving the input to my Presonus Firestudio Mobile with some headroom (which can take up to +10 dBu or 2.4 Vrms). Obviously this setup cannot measure the rated power of this amp in bridged mode or into to 2 ohms, so all of my measurements will be limited to one channel driven into 4 ohms. The measurements I was able to collect are as follows:

Maximum power (RMS) into 4 ohms (one channel driven) at 1 kHz
Maximum power (RMS) into 4 ohms (one channel driven) at 40 Hz
THD (total harmonic distortion) from 1W up to rated power
Frequency Response from 5 Hz to 20 kHz (overall flatness from 20-20 kHz)
Channel Imbalance

First things first, anything you buy off CL may not work at all, let alone meet any kind of performance parameters it once had 20 years ago. The guy said he tested it and it worked fine, so I was really hoping it didn’t go straight into protect mode the second I turned it on. At power-up, lights came up, fan spun up and the protect circuit kicked out after a couple of seconds as normal. This was a good start! I played some music through it to a small speaker, just to see that it made sound and that the volume/gain knobs worked okay. Then it was off to the test bench to really check it out. I tested CH B first and ran a quick frequency response plot at about 1 watt. Holy cow this thing was flat! I’d tested some smaller Class D amps recently and was expecting the typical low-cut/high-cut sort of thing, but was impressed to find the response down only -0.3 dB at 20 Hz and -0.4 dB at 20 kHz. I couldn’t even see the -3 dB point since my sweep started at 5 Hz and at 5 Hz the response was down only -2.3 dB. CH A was a touch better at -2.0 dB at 5 Hz. CH B also had a little wobble in the response that was not present in CH A. I suspect it’s due to some possible aging of a 100uF electrolytic cap in the pre-amp stage that makes up the high-pass circuit for this amp. I could replace it, but the effect is small enough that I probably am not going to worry about it. I ran some additional frequency response plots at higher power levels up to 50 watts. The plots shown here are at 50 watts.

With the frequency response plots you can also grab distortion over frequency using REW. Though it’s not as accurate as using the RTA at a single frequency, if you increase the measurement length from the standard to either 512k or 1M, you get some better fidelity on the distortion side of things, but still not as good as RTA. But this does provide a general idea of the overall distortion over frequency and can identify problems or other areas the amp may have at frequencies that you may not notice otherwise when just using RTA. In this case Ch A is slightly better below 20 Hz than Ch B but nothing too crazy. Again, points to possible aging of the inline electrolytic cap (this would be C202 in the schematic) in this channel. I did not run full sweeps up to full power since the protect circuit did kick in when running the sweep starts at 5 Hz. I could have upped the start frequency, but went ahead with RTA measurements instead.

So I ran tests from 1W up to 450W at 40 Hz and 1 kHz on both channels (separately) at about 50W increments and grabbed screenshots from REW RTA with the noise floor set at 130 dBc. This is about the limit of my setup with 128k FFT Length and 4 averages. This kept each measurement to about 6 seconds which helped prevent build-up of heat in the amp or my load resistors during testing. At least once I got above 100W or so. Tests were run about 1 minute apart, give or take the amount of time it took me to grab the screenshot and adjust the gain for the next measurement. I’ll probably only upload a few of them, since basically between 5W and 350W there isn’t much going on. Things only start to get interesting around the 400W mark. At 1 kHz into 4 ohms, one channel driven, I measured exactly 450 watts RMS at 0.097% THD on CH B. This was excellent! And an order of magnitude lower distortion than the spec! Of coursse, this was one channel driven not both as the spec stated, but still, this was impressive. I pushed the amp another 10W to 0.2% THD and the red clip light came on indicating it was time to turn it down. So I’d say max power for CH B was easily 450 watts with the clip light coming on at just around 460 watts.

Continue reading →