Behringer FBQ2496 / Velodyne SMS-1 distortion

[Ilkka]

As I mentioned in another thread, a while ago I found something really alarming regarding distortion on these popular subwoofer equalizers. We already know about the quite drastic low frequency roll-off of the SMS-1, but now it seems that it has some other issues too.

Here's the frequency response comparison as a reminder. The SMS-1 had the newest v2.12 firmware and the lowest possible setting was used (5 Hz, 6 dB/oct.).



Velodyne has promised a new firmware which should allow the user to completely bypass the high pass filter.

But even alarming than the roll-off, is the distortion of the unit. Using the same test sweep as I test the subwoofers, I ran a couple sweeps through both units at various input levels. Here are the results.



The green sweep shows the THD of the FBQ2496 at around 0.4 V RMS input level (-6 dBu). Looks really good, THD is around 0.01 % at higher frequencies, and goes to around 0.1 % at the low end. Shouldn't be audible when considering how much distortion the subwoofer itself produces.

The blue sweep is the SMS-1 at the same 0.4 V RMS input level (-6 dBu). THD is considerably higher, around 0.2 % at all frequencies. Not very flattering result when thinking about Velodyne's history and design philosophy.

The red sweep is the SMS-1 at 3 dB higher input level, around 0.8 V RMS (0 dBu). Everything goes fine until at around 75 Hz the signal clips pretty heavily. THD passes the 10 % limit at 30 Hz and ends up at whopping 56 % at 10 Hz.

The black sweep shows the SMS-1 at again 3 dB higher input level, around 1.5 V RMS (+6 dBu). THD is really high at all frequencies, ending up at 120 % at 10 Hz.

I also tried the FBQ2496 at these higher signal levels but the distortion was practically identical to the lower level curve (+6 dBu was slightly clipping the input). I was also using the -10 dBV input sensitivity setting, so switching that to +4 dBu gives one more headroom if needed.

I also tried the XLR input of the SMS-1 but the result was pretty much identical. And it is clear that it is the input stage of the SMS-1 that clips, not the output.

So for those owning the SMS-1, I would definitely set the subwoofer output on the receiver as low as it goes. Typically the subwoofer output can easily give a couple volts, and as you can see, it doesn't need much more than 0.4 volts to clip the input stage of the SMS-1.

I have nothing against Velodyne or their products but this is something that can't go unannounced.

[Sonnie]

That's a pretty remarkable discovery. I wonder how many SMS owners out there have their output level set that high and don't realize what's going on?

[Geoff St. Germain]

That's pretty interesting. I'd been considering an SMS-1 despite the roll-off issues, but now there's just no way. For a third the price the FBQ2496 makes a lot more sense. The SMS-1 has the auto EQ feature, but it's at a pretty big cost apparently (both in money and performance)

[Exocer]

For such a pricey unit one should expect much lower THD...that is ridiculous.

[soho54]

Thank you for posting this.

I love a good Ilkka post with graphs.

[no. 5]

That is interesting (thanks Ilkka!), but how much of a problem is it?

Is 56% THD even audible at 10Hz?

[OvalNut]

56% THD

eeek! .....


In the for what it's worth category, I put in a FBQ2496 a few weeks ago, and used together with REW it really isn't that hard to dial in. Fun too, in a tweaky geeky kind of way. Just some futzing for an hour or so with the concepts going in, but once you get used to it, the actual setup is easy. I'm very happy with the results, particularly the sound quality, and even happier now after reading things like what is posted above. It is remarkable that Velodyne would let something like that go to market.

Tim

[brucek]

THD is really high at all frequencies, ending up at 120 % at 10 Hz.

I don't know what to think about this. Something is amiss - it has to be. The only way to get that type of result is with extreme clipping of a signal.

I don't know the maximum input level spec on the SMS, but if it was somewhat standard, it would be 2dBV (~1.25 voltsRMS). I suppose that's the spot to compare it to other devices. It shouldn't clip if all filters were disabled and that signal fed to it.

Perhaps the unit under test has a fault....

brucek

[Vaughan100]

Hi there,

I just purchased the SMS-1 and I was advised to check out this thread. It's pretty ridiculous how high the distortion levels are. Hopefully that is a faulty model.

But will these distortion levels disappear if you use a low enough subwoofer level in the AVR ?

[Vaughan100]

Also, I've been told by Velodyne representatives that output only drops off by 2-3 dB's at 20 hz. But a lot of people have complained that there is significant roll off at low frequencies. Looking at the graph, it seems to me that output is rolling off above 20 hz and then is nonexistant below it.

I have an SVS PB-10 ISD. Not a subwoofer that will defy the laws of physics by any means but it is capable of output down to 15 hz and is pretty respectable at that. I eq'ed by system to relatively flat levels and I've found that my deep bass response seems extremely low even though according to graphs, my frequencies from 20 hz and up are even higher than my unequalized results.

Something is amiss it seems.

--Regards,