Subharmonic Synthesizers:

Comparing the Peavey Kosmos Pro and the Behringer Ultrabass Pro EX1200


The Peavey Kosmos Pro


The Behringer Ultrabass Pro

I have always wanted to add a bowel-loosening low-end to my electric bass rig (an SWR Mo'Bass into twin 15" speakers). The Mo'Bass includes an analog subharmonic synthesizer tweaked to track electric bass, but it's an unfriendly sound -- jagged and harsh, not string-like at all so it remains a separate voice rather than marrying itself with my P-bass's regular tone. I'd heard about rack-mount subharmonic synths that DJ's use to fill out their sound, and wondered what they might do to a live bass. So it's off to the shop -- in this case, a DJ shop in San Francisco called World of Stereo II.

Here's what they had to beef up your bottom end:

These latter two are actually bass boosters rather than synthesizers. I was looking for brand-new low end rather than something that tweaked existing frequencies, so I focused on the Peavey and Behringer units (absent was a DBX 120A, or other DBX unit containing that synth technology, or that would be in the mix as well). These units are both marketed more as total-mix enhancers than as live instrument effects, and they definitely do nice things to a mix run through them. Give them some old motown and it almost sounds like it's been re-mixed for the dancefloor.

Anyway, for my visit I brought in a CD with bass lines isolated on one channel, to hear what happens to an instrument track. Surprise! Nothing at all. With the Ultrabass, we simply couldn't get the device's activity indicator to ever light up. With the Kosmos, it was lighting up, but the output sound was unaltered. No idea what went wrong in the store, but it wasn't exactly the best environment for this comparison: it's pretty hard to evaluate subwoofer output details with salsa blaring through a ceiling-high street-side speaker stack and a porn soundtrack coming from the back room and a homeless guy hitting you up for change while you're wiring up the inputs (I do love this place). Given the hype I'd been reading about these devices I had to believe they could do something. So I rented them both to test at home over the weekend (another fine thing about House of Stereo), and these are my notes.

Part 1: Connections and Controls

This section will mention just enough about the device hookups and controls to support the discussion in later sections. If you want more detail, you can pull down manuals for each of these products (Ultrabass, and Kosmos, and of course the respective websites will have been reorganized and the links broken by the time you are reading this).

Back Panel Connections


The rear panel of the Kosmos Pro


The rear panel of the Ultrabass Pro

Both units have balanced/unbalanced stereo ins and outs and subwoofer out, TRS jacks and XLR for each. Additionally, the Kosmos has a Mono input mode (left input only) and SPDIF I/O. The balanced ins and outs don't do anything for my bass rig, since the unit will be patched in via the amp's (unbalanced) effects send/receive jacks. But I still appreciate that they're there, especially when stealing the unit away and plugging it into my home sound system in the off hours, where I'm running balanced lines out to powered monitors. As it turns out, the two units send a different mix of original and synthesized output to the subwoofer out, more on this in the upcoming discussion.

Front Panel Controls


The front panel of the Kosmos Pro


The front panel of the Ultrabass Pro

Both units' controls are laid out left to right, mostly mirroring the signal's path from input through effects to final subwoofer output control. The Kosmos control path begins with input and main output gain knobs, metering select, and global effect bypass, followed by cryptically named bass synth controls Quake, Dynamics, and Thud. The controls of the Behringer unit are more sensibly named: Frequency (low-frequency input filter cutoff), Bass Mix, Bass Level, Limiter Threshold, Satellite (main) Level. It turns out there's a rough but direct correspondence between controls on the two, though their parameter ranges and sonic effects aren't exactly equivalent. Here's the rough pairing: Kosmos Subterranean Shift button and Ultrabass Frequency knob. Kosmos Quake and Ultrabass Bass Level. Kosmos Dynamics and Ultrabass Dynamic Punch (maybe, more on this later). Kosmos Thud and Ultrabass Bass Mix. Ultrabass Cut Sub from Main vs Kosmos Add Bass to Main.

Is anyone besides me bothered with the control names on the Kosmos? At this point, I should say that everything I learned about the Kosmos, I learned from the Ultrabass. That is, because the Kosmos control names are so cute and the manual is such a joke (go ahead, check it out), I ended up understanding the unit's parameter space better after having read the excellent Ultrabass manual, experimenting with controls there, then tweaking the Kosmos to achieve similar effects. My ultimate goal was to see if I could produce the same sounds with each unit. At which point I'd go buy an Ultrabass, with a street price of $99 vs a Kosmos street price in the $400-$500 range. More about that experiment, and what each of the controls do to your sound, in the next section. Meanwhile, the control panels diverge moving rightward. On the Kosmos appears "XPanse", "Barometrics", and "Stratos." I can't tell you exactly what they do, nor will the manual, but they're concerned with the upper end of your mix. "Stratos" is a high-frequency EQ, though the manual doesn't mention frequencies. This isn't relevant to my setup, as I have a perfectly nice high-frequency post-effects EQ on the Mo'Bass, one octave at 10KHz, it says so right there in the manual. The XPanse and Barometrics knobs have something to do with stereo imaging, again, wish I could say more but then Peavey would have to kill me; and again, not a concern for a mono bass rig (nor did I care for the sound of it in a regular mix). So let's just forget these knobs and get back to subharmonic synthesis.

One control on the Ultrabass not found on the the Kosmos is a crossover switch, a 70Hz cutoff that splits the main and subwoofer outputs. That is at the final output stage -- what ever mucking and mixing of input and synthesized signals you have done earlier in the path, here you make a clean split between main and sub out. You cannot use the Kosmos in this way, as a standalone subwoofer crossover, because it does not let you filter the input's low-frequency content from the main outs and redirect it to the sub. Nor can you remove the input's low-frequency content from the subwoofer out and thus output a pure synthesized signal (accomplished with the Ultrabass by setting the Bass Mix knob hard over to sub-synth). Another interesting Ultrabass-only feature is a select button for generating subharmonics two octaves below the source rather than one octave. Of course, at those frequencies the resulting sounds are toneless puffs and thuds, and not appropriate for the live bass application considered here. Maybe they're for feeding into one of those tactile booty shakers.

One control on the Kosmos that is sorely missing from the Ultrabass is an input level control. As it happens, the Kosmos and Ultrabass both want input levels that are hotter than standard consumer line levels, and you can use this control on the Kosmos to boost the input level. If you're feeding the Ultrabass a signal from, say, a cd player, you'll need some additional pre-amplification to wake it up. Levels within an effects send/receive loop are just fine however. One really annoying thing about the level controls on the Kosmos is that the Global Bypass switch does not bypass the Input Level control. This means if you've boosted the input level for better synth tracking, and then compensated by reducing the output level, hitting the bypass switch suddenly boosts the output level. If you want to be able to use the Global Bypass switch unobtrusively, you need to leave the Output knob at its high noon detent, and let the downstream power amp handle the volume adjustment -- not an option when driving powered monitor speakers, and a real annoyance.

Part 2: Playing a CD

My first listen to these units was from a cd player source, as I just couldn't see hauling my bass rig into the DJ store. But I was also curious to hear what they did to a full mix, as this seems to be their more popular application.

Feed the Beast

The first time you play a CD through one of these bass synths, you'll wonder (as we did in the store) why nothing is happening. As mentioned above, a big part of getting the units to do anything with a consumer level signal is to boost the input level. Each has an indicator light showing when the bass synthesizer is receiving a signal above threshold in the bass synth's input frequency range, which really helps dial this in visually. An input level adjustment should really be viewed as a formal bass synth control, because you get very different tracking behavior depending on how much time your signal spends above versus below threshold, and this is mostly determined by the input signal level (with some minor adjustments available via downstream "dynamics" controls).

In keeping with their "whole mix" target audience, both units sample a frequency window from the input signal for synthesizing the subharmonics (contrast this with the sub generator on the Mo'Bass, which will follow you all the way across the strings and up the neck). The input frequency range is controlled on the Ultrabass by the Frequency knob, a high-frequency cutoff ranging from 80-130 Hz, and the Dynamic Punch knob, a low-frequency cutoff whose range is not clearly indicated (perhaps it is doing more than simply controlling a low cutoff and therefore couldn't be labeled in Hz?). The Kosmos gives a choice of two center frequencies using the Subterranean Shift button. What are the actual values? I guess we are not worthy to know. The Kosmos Dynamics knob controls how quickly the synth cuts in and out, and is described as an envelope control in at least one other review. But I wonder if it might simply be a filter width control like the Ultrabass Dynamic Punch Knob. The tracking behavior of the two units is different, but similar enough with respect to this knob that I can't quite decide.

Punch

Both units give you mixing controls to add "punch" to the synthesized signal (oh, wait, Peavey calls it Thud). In this context, punch means amplifying the original bass content. The idea here is that the sub-synth signal can sound disconnected from the original material, and by boosting the original bass content you can sonically stitch them back together. I've noticed this is very true sometimes, and not so true other times. If you're soloing the sub out, as you're want to do in this kind of evaluation, you'll find that it is thud/punch that defines much of the note attack for you. But whether you want that as an additional signal boost in a full mix is less clear. Certainly if you find that you prefer thud/punch and don't much care for quake/sub, lucky you, you're just wanting some bass EQ. Go check out an Aphex Big Bottom or BBE Sonic Maximizer and save a few bucks.

Limiter

One of the Ultrabass features that I just don't get is the limiter. Not that it's a bad idea, but that it's a really obtrusive limiter. The manual suggests setting input and limiter levels such that the limiter is barely engaged, but when I did this the result was noisy. And at settings where the limiter is not always on, there's something of a thump when it switches in and out. I've seen similar complaints elsewhere, but I'm willing to believe I'm just too dim (or just too afraid of really loud output) to make it work as intended.

Tracking

A big part of how these units differ in sound is in their envelopes -- how synthesized notes sound as they start and stop. The Kosmos tracks the input envelope much more musically, though I don't quite have the words for it. The Dynamics knob controls how long the tracker holds onto a note. I never found the sustain to outlast the actual note in the bass material I tested, and could never convince myself that the attack was affected by this control. but other reviewers note these effects when applied to drum tracks, where the actual input signals are shorter in duration.

At most control settings, the envelope of the Kosmos's notes has a more musical attack/release shape to it, whereas the Ultrabass is more of a big cotton thumpy sound. If you loosen up the Kosmos dynamics control it begins to lose some of its sharpness and sound more like the Ultrabass. Input heat seems to modulate the Ultrabass tracking in a way similar to the Dynamics knob on the Kosmos, but this is with regards to the tail. I've found few settings that make them both the same on the attack. For the longest while I suspected that this was because the Kosmos does not let you send a pure synthesized signal to the sub, and that the original input signal content was what I was hearing as defining the attack; but after adding input signal back into the Ultrabass's sub output, I concluded that there really is something different about the Kosmos Pro's attack envelope shape. For my envelope tracking workout, I used Charlie Haden's solo at the beginning of "Nobody Knows the Trouble I've Seen" on Steal Away (even though we're not supposed to be putting acoustic jazz through these things). My other tracking test was Jamerson's bass line from "I'll be there", a mix of 16th notes and sustained notes, where one is listening for air between the 16ths.

Whole Mix Autopilot

Because you hear these units described as mix enhancers, there's a fantasy that you could just add one to your house sound system and magically have bigger, thumpier beats all the time. I've spent a good deal of time with these units connected to my home stereo, running a wide variety of music through them in pursuit of this fantasy. I'll echo the consensus found elsewhere, that jazz and acoustic music just sound weird through them. But I was also surprised at how often I wanted to tweak the settings with a pure dance/electronica feed like Groove Salad, Beat Blender, or the evening streams from KCRW. The problem is that a good number of these tracks already have sub-harmonic content, and doubling it is a bit much. Still, if you were consistently listening to something like motown or early hip-hop or classic rock, you could likely set it and forget it (and be very happy with the result).

For either unit, if you are going to leave it on all the time in your house sound system, you'll want to hook it in between your pre-amp and power amp, and use the power amp's output level control as your master volume (with a mixing board, you would patch this in as a pre-master-fader effect). That's so you can tailor the unit's input signal level to give you the bass behavior you like, and not have it affected by the overall volume at which you are driving your speakers.

Part 3: Playing Live

Finally, we're down to the acid test: what's it like to play through one of these things? Short answer: a blast. These tests were performed with each unit stitched into my signal path via the Mo'Bass post-effects effects send/receive loop. The test material was me thumping on a P-bass, playing the sustained line from Gin and Juice, various walking bass-lines, and the earlier mentioned Jamerson motown line. But it's really, really satisfying just to hit a single note and feel your insides vibrate.

The Kosmos Pro preserved overall bass tone best while adding sub to it (in both cases, the synth's full spectrum main output was used rather than the subwoofer output). The Utltrabass was a much lower-level output, requiring a big boost from the amp, which magnified a 60Hz hum that might have come from the unit, or might have come from the home-made wiring in the warehouse space where i was practicing. The Kosmos levels were high enough that this background hum was not boosted by the amp, if it was there (a roundabout way of trying not to accuse the Ultrabass of emitting a 60Hz hum, since i didn't hook up a spectrum analyzer and see for myself). With the Ultrabass, the bass's original tone seemed depressed in mid/highs on most settings, and it required careful tweaking to get back something that sounded like the original bass sound plus a generated subharmonic.

On slow lines, quarter notes or longer, I preferred the sound of the Ultrabass -- it has a smoother, rounder sounding subharmonic synth that will agree with the P-bass lovers in the crowd. The Kosmos has more of an edge to it, though not disagreeable in the way that the Mo'Bass sub-synth is disagreeable and harsh. Both sounds complemented and melded with my bass sound to produce a unified tone with broader harmonic spread. The Kosmos tracks faster though -- in 16th note passages you could hear well-defined space between the notes with the Kosmos, much less so with Ultrabass. No amount of tweaking could work around this.

Conclusions: The Perfect Unit

As you've likely guessed, I think the perfect unit would be a hybrid of of the Kosmos Pro and Ultrabass Pro. Each has complementary strengths, and complementary flaws.

Ultrabass Pro Pros:

Ultrabass Pro Cons: Kosmos Pro Pros: Kosmos Pro Cons:

So, what did I buy?

I would've been happy to own both units with their complementary strengths (I enjoyed running them daisy-chained while I had both). But I got a good price on a demo Kosmos Pro unit, all scratched up but working fine. For the very specific use I had in mind (my bass with my bass rig) the Kosmos produced a more musical sound that preserved my original bass tone while augmenting it with sub. That trumped all the other annoyances I found with the unit.

Thinking back on the Ultrabass, it's a tweaker's delight, with much more flexible signal path control than the Kosmos. But there are a narrower range of settings I'd consider musical, so the flexibility can be frustrating at times. If you can set it and forget it, you are golden. The Kosmos had a much narrower range of settings, and it was musical at nearly all of them. Perhaps the Quake and Dynamics knobs are tweaking several things at once that are individually tweakable on the Ultrabass. I did find myself bumping up against boundaries with the Kosmos, especially when I wanted to change the balance of subharmonic output and original signal.