# 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.


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## Sonnie

:unbelievable: 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?


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## 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)


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## Exocer

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


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## soho54

Thank you for posting this. :bigsmile: :T

I love a good Ilkka post with graphs. :daydream:


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## no. 5

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

Is 56% THD even audible at 10Hz?


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## OvalNut

> 56% THD


eeek! ..... :unbelievable:


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
:drive:


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## 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


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## Doctor X

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 ?


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## Doctor X

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,


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## Sonnie

I don't know about the distortion testing, but the response testing I believe has been done by more than one person. And the tech is technically right, the unit only drops by 2-3db at 20Hz, but what he failed to tell you is that it drops more at 19Hz and more at 18Hz... 12-13db at 10Hz. However, as you suggest, that roll-off would not necessarily be a factor with your sub. The issue would be if you ever upgrade (buy or build) to a sub that does have significant response below 20Hz.


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## Doctor X

It's funny because my uneq'ed response seems to be pressurizing my room more on extreme deep bass moments compared to my eq'ed response which has higher levels of low bass in the graphs from 20 hz right through to 80 hz (there was like a 18 dB null inbetween 50-80 hz. After all of this, my previous unaltered response gives me more deep bass for some unknown reason.

I assumed it was the subsonic limiter. Maybe it's something else but according to the graphs, it definitely should be the other way around.


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## SteveCallas

It's very difficult to say anything politically correct about this product. 

Ilkka, I think you should also test to verify that the FR measurement this unit displays on screen matches up to the real FR it is measuring - let's just say I wouldn't be surprised if it leads one to believe their low end is better than it really is, ignoring the extreme smoothening it uses.


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## Ilkka

no. 5 said:


> That is interesting (thanks Ilkka!), but how much of a problem is it?
> 
> Is 56% THD even audible at 10Hz?


It's not just the 56% THD at 10 Hz. Look at the distortion graph, it's also very high at higher frequencies.

And yes, 56% THD at 10 Hz is audible. It means that 56% of the signal is something else than 10 Hz signal.


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## Ilkka

brucek said:


> 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.


Believe me, it clips well below ~1.25 V RMS. Both RCA and XLR inputs.



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


Naturally there is always that chance since I only tested one unit. Hopefully someone will be able to confirm my findings.


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## Ilkka

SteveCallas said:


> It's very difficult to say anything politically correct about this product.
> 
> Ilkka, I think you should also test to verify that the FR measurement this unit displays on screen matches up to the real FR it is measuring - let's just say I wouldn't be surprised if it leads one to believe their low end is better than it really is, ignoring the extreme smoothening it uses.


That has been measured already by others. Other than the extreme smoothing, the measuring display is accurate.


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## brucek

> Believe me, it clips well below ~1.25 V RMS


That would be considered unusual. I suspect Velodyne understands basic electronics, so it makes me think there's something amiss with your test unit.

Perhaps some industrious electronics type can examine their unit with a scope or even better, a distortion analyser.....

brucek


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## jpmst3

brucek said:


> That would be considered unusual. I suspect Velodyne understands basic electronics, so it makes me think there's something amiss with your test unit.
> 
> Perhaps some industrious electronics type can examine their unit with a scope or even better, a distortion analyser.....
> 
> brucek


Illka,

Yes, please test another unit and/or send the unit you tested to someone else to have it tested.


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## brucek

> or send the unit you tested to someone else to have it tested


I suspect Ilkka is quite capable of accurately carrying out a simple THD test....

brucek


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## jpmst3

brucek said:


> I suspect Ilkka is quite capable of accurately carrying out a simple THD test....
> 
> brucek


I don't think anyone would argue with that. Either way that is not the point.

Unless the results can be verified by another test or another unit they cannot be trusted as fact for all units.


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## brucek

Fair enough, but I think it most important to test _another_ unit.........


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## jpmst3

Agreed.


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## Ilkka

"Slartibartfast" over at AV Talk forum has confirmed my measurements. Our absolute levels seems to be slightly off but still the results show that the SMS-1 clips its RCA input rather easily. Also the ~0.5% - 1% THD already at lower levels is not very good performance.

http://www.avtalk.co.uk/forum/index.php?t=msg&goto=243382&rid=5915&SQ=1190058560#msg_243382


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## Doctor X

Ag, no. I can't believe this ! I bought this thing last wednesday, and I'm really starting to regret the purchase. I'll give it my best shot tommorrow with the graphs (I'm off work) but the distortion levels are absolutely insane.

I still can't believe that Velodyne would release a product with such high distortion figures. 

--Regards,


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## brucek

> I still can't believe that Velodyne would release a product with such high distortion figures


Do you feel you are hearing that distortion when listening?

brucek


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## Doctor X

I may or may not. But that isn't the point right now. If the product has high distortion figures then that leads me to believe that there is a engineering fault.

Why would I want to keep something in my system that adds additional distortion at even low levels ? Right now I'm just very frustrated.


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## Ilkka

brucek said:


> Do you feel you are hearing that distortion when listening?
> 
> brucek


While 0.5% THD might not be audible but >5% definitely is, especially when it's 'electronic distortion', meaning it contains a lot of high order components.

Here's the spectrum of the SMS-1 THD at 50 Hz, 0 dBu. 50 Hz spike is the drive signal, all the other is distortion.

*SMS-1*









And here's the FBQ2496 at the same level. Notice the lower level of distortion.

*FBQ2496*


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## Doctor X

Ikka, if I adjusted my sub level in AVR to -10 (it's lowest setting) would I be save from this input stage distortion on the SMS-1 ? Or could I still increase the sub level without fear of having added distortion entering into the mix ?

Thanks.

--Regards,


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## Ilkka

Vaughan100 said:


> Ikka, if I adjusted my sub level in AVR to -10 (it's lowest setting) would I be save from this input stage distortion on the SMS-1 ? Or could I still increase the sub level without fear of having added distortion entering into the mix ?
> 
> Thanks.
> 
> --Regards,


I wouldn't set it higher than -5. You can always raise the SMS-1 output level or the gain setting on your subwoofer (plate) amp.


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## Doctor X

I'm really struggling whether I should keep the SMS-1 in my system or not. I probably haven't utilized it properly in my system yet although my graphs look pretty flat. I haven't messed around with polarity and phase on the SMS-1.

I wonder if I can even return it. I hate being in this situation.


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## Ilkka

Vaughan100 said:


> I'm really struggling whether I should keep the SMS-1 in my system or not. I probably haven't utilized it properly in my system yet although my graphs look pretty flat. I haven't messed around with polarity and phase on the SMS-1.
> 
> I wonder if I can even return it. I hate being in this situation.


Well I don't your situation more specifically but this distortion issue alone isn't an issue as long as the subwoofer out level is kept low enough.


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## Doctor X

I bought this unit thinking that it would make a big difference to my system. So far that hasn't happened. And I've achieved a virtually flat response.

For movies I haven't heard any differences whatsoever except that there is now less percieved bass than before. The increase in bass from 50 to 80 hz seems to go unnoticed. 

With music, on some specific tracks, it sounds a little more full bodied. Considering the levels of boost inbetween 50-80 hz to achieve the flat response, I don't know what I was thinking.

The end result is that while music does sound more full bodied (which is a good thing), the bass still sounds boomy so I'm not sure the parametric EQ actually affected anything besides level.

I thought that deep bass was minimum phase. If I correct a frequency anomaly then the time component should be rectified as well. Or at least in theory. But if I think about it, I could rather spend the same amount of money on acoustic treatment that actually would make a far greater improvement.

I'm deciding what to do. I just hope that the dealer decides to take this thing back. I was so very excited to get this unit but the price does not warrant the small percieved SQ differences given the massive FR change in the graphs.

Decision pending.

--Regards,

Perhaps my room isn't as bad unequalized as the next person although I certainly thought it was.


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## Ilkka

If your unequalized frequency response wasn't that bad, then you shouldn't expect a huge difference. Even though subwoofer is a minimum phase device, it only means that the phase response will be flattened (i.e. group delay will be low), but it doesn't mean that the general room decay time would be much improved. Acoustic treatments are the only solution for that problem.


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## RayJr

Ilkka,
How does the Behringer DSP1124P compair in distortion to the Behringer FBQ2496?

Thanks
RayJr


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## mojomike

Thanks, Ilkka, for helping to expose some of the shortcomings of various "hi-fi" components from presumably respected manufacturers. Hopefully this sort of vigilance will cause these component makers to think twice before passing off their under-performing products as being truly "hi-fi".


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## Doctor X

And Ilkka, could you please confirm for me that the distortion levels on the input stage of the SMS-1 is nonexistant if I choose a sub level in AVR to -7 ? 

Thanks !


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## Doctor X

I spoke to Curt earlier and he promises that the roll off will cease after the subsonic filter has been eliminated. He also said that he'll look into the distortion on the SMS-1 and report back.

--Regards,


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## brucek

> And Ilkka, could you please confirm for me that the distortion levels on the input stage of the SMS-1 is nonexistant if I choose a sub level in AVR to -7 ?


There is simply no way to know what the maximum voltage from your model of receiver is when you have it at -7, without measuring. The trim values that receivers use are dimensionless.....

brucek


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## Doctor X

What does that mean Brucek ? I have the Yamaha RXV-450 receiver. Would modern receivers not have a problem with this ?

--Regards,


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## brucek

> I have the Yamaha RXV-450 receiver


What RMS maximum voltage level is associated with a Yamaha RXV-450 receiver when its subwoofer trim output is set at -7 ?

brucek


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## Doctor X

I have no clue. How would I even calculate that ?

--Regards,


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## Ilkka

RayJr said:


> Ilkka,
> How does the Behringer DSP1124P compair in distortion to the Behringer FBQ2496?
> 
> Thanks
> RayJr


I haven't tested the DSP1124P but if I had to guess, I'd say it will be very close to the FBQ2496.


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## Ilkka

Vaughan100 said:


> And Ilkka, could you please confirm for me that the distortion levels on the input stage of the SMS-1 is nonexistant if I choose a sub level in AVR to -7 ?
> 
> Thanks !


As Brucek has said, we can't tell that unless we know the maximum output voltage of the subwoofer out during the highest level LFE spectacular you listen.

If I had to guess, I'd say you're safe at that level.


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## Doctor X

Ilkka, sorry if you've answered this before, but what should the output voltage be for the subwoofer out in order for distortion to be nonexistant ? 

I'll check in my AVR manual if there are documented output voltages for the subwoofer out. 

Thanks.

--Regards,


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## Ilkka

Vaughan100 said:


> Ilkka, sorry if you've answered this before, but what should the output voltage be for the subwoofer out in order for distortion to be nonexistant ?
> 
> I'll check in my AVR manual if there are documented output voltages for the subwoofer out.
> 
> Thanks.
> 
> --Regards,


The documented maximum output voltage doesn't help us. I know for a fact that most of these modern AVR's can output up to 7 - 8 volts from their subwoofer output when using the maximum subwoofer output level setting, 0 dBFS signal and maximum MV setting. Here's a measurement I did some time ago on my NAD T743 AVR. You can see that its subwoofer output stays clean up to 5.5 volt. That would horrendously clip the SMS-1 if such signal would be inputted (in reality levels stay much lower). The only way to find the maximum voltage in your subwoofer output during normal use is to measure it using an oscilloscope or other suitable device.


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## Doctor X

So in other words then there is no way for us to know for sure if there is any clipping or not unless you verify it yourself with test equipment.

Well, that doesn't sound too good.  But in general, as per your recommendations, setting sub level low (in AVR) enough should negate the distortion characteristics that you measured.

As long as that is the case, I have nothing to worry about. But as with other members I've spoken to, that is simply not good enough. I have no idea what Velodyne is going to do about this IMO.

--Regards,


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## Ilkka

Oh dear! It seems that the Velodyne DD series has the same "flaw".

Check here: http://www.avtalk.co.uk/forum/index.php?t=msg&goto=243457&rid=5915&SQ=1190152037#msg_243457


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## Ilkka

Vaughan100 said:


> So in other words then there is no way for us to know for sure if there is any clipping or not unless you verify it yourself with test equipment.


Measure or listen. 5-10% or higher electronic distortion is clearly audible to anyone. Just keep your subwoofer amplifier gain low enough so that you won't overdrive your subwoofer while testing (it will produce its own distortion of course if pushed too high).


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## Doctor X

The way I generated my graphs, Ilkka, is use gain control on sub at just over 30% and then I'm using sub level in my AVR at -7 (from +10 to -10) and SMS-1 volume at 19.

So the DD subwoofers have this problem too you say ? How on earth could Velodyne not know anything about this ? This is absolutely ridiculous. 

--Regards,


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## Geoff St. Germain

Hmmm... it's interesting that the DD line has the same problem. I know that the DD18 has been in Stereophile's Class A recommended components for a few years now. Has no one else measured the distortion on a DD subwoofer?


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## Sonnie

Hmmm... :scratch: Does this place Stereophile's testing procedures in question?


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## Kal Rubinson

Sonnie said:


> Hmmm... :scratch: Does this place Stereophile's testing procedures in question?


Stereophile never did any lab bench tests on the DD-18 or the SMS-1.

Kal


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## Sonnie

Is that normal? Pardon my ignorance, but it's been a long time since I've read Stereophile (they had just enlarged the magazine size when my subscription ran out). 

I figured to get a class A recommendation, it would at least be tested... or is it based on subjective listening test?

It appears none of the big time reviewers of these products have actually done distortion testing... nor have they noticed (heard) the distortion. I'm thinking several of these reviewers would generally have ears to die for.


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## Geoff St. Germain

IIRC, the only testing done on the DD-18 in Stereophile was using the Digital Drive System Response video output to a TV. They put the sub in a couple of different rooms and had screen shots before and after EQing. Mostly they just said it sounded good, was loud and had lots of features.


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## Kal Rubinson

Sonnie said:


> Is that normal? Pardon my ignorance, but it's been a long time since I've read Stereophile (they had just enlarged the magazine size when my subscription ran out).
> 
> I figured to get a class A recommendation, it would at least be tested... or is it based on subjective listening test?
> 
> It appears none of the big time reviewers of these products have actually done distortion testing... nor have they noticed (heard) the distortion. I'm thinking several of these reviewers would generally have ears to die for.


Certain categories are not bench-tested. These include subwoofers and turntables, along with most products that are reviewed in a regular column.

Kal


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## OvalNut

Interesting finding Ilkka, well done. :clap:

I believe it certainly has gotten Velodyne's attention, particularly with the potential/probable tie-in with the DD series. :mooooh:

Yes, it will be interesting ... 


Tim
:drive:


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## JimP

Not trying to oversimplify, but if the problem is that the input voltage is higher than the circuits were designed to cleanly handle, just turn down the input voltage in the receiver/preamp and make up the gain in the SMS-1 or the subwoofer's volume control.

Wouldn't that take care of the distortion problem? 

Would this cause any undesirable side effects? i.e. Do manufacturers use a higher subwoofer voltage because the circuits need a higher threshold to work properly?


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## brucek

> Not trying to oversimplify, but if the problem is that the input voltage is higher than the circuits were designed to cleanly handle, just turn down the input voltage in the receiver/preamp and make up the gain in the SMS-1 or the subwoofer's volume control.
> 
> Wouldn't that take care of the distortion problem?


Yes, but I think the issue is the clipping level of the SMS is rather low in comparison to other devices, and in relation to (quasi)-standard maximum input levels.

The problem is that every device has a noise floor that can't be changed. If I feed that device with a standard level that can express the loudest and softest signal without being lost in the noise, then everything is fine. If I had to turn down the input level to that device by half (for example), and try and make that loss up in the stage following the device, the loudest signals would pass, but the softest signals would be lost in the noise. Once a signal is in noise, it can't be retrieved. It results in a loss of resolution. In addition, since the device in question is a digitizer, by lowering the input level by half, I wouldn't be defining the signal with as many bits - again, a problem.

I suppose the question would be the level at which the SMS device has to be fed to provide a clean signal without clipping its input. Most consumer receivers etc output a maximum line level of about +2dBV (~1.25vRMS) and a nominal level of about -10dBV (~315mvRMS). 

In Ilkkas graphs he shows the distortion becomes a problem with a level of about 0dBu (~775mvRMS) (note he uses the units of dBu and I use the commercial level units of dBV. It's OK, I'll translate all to voltage). Clipping at 775mvRMS is not too good. It's a concern and just on the edge of manageable. I took some solace when I looked at the AV Talk discussion where the poster did the same test, yet didn't find problem at 0dBu (~775mvRMS), instead the clipping seemed to occur at +3dBu (~1vRMS). That's encouraging and acceptable for most applications, in my opinion. I was concerned when zippy (Ilkka) questioned the levels accuracy of that test and the poster responded with _"I then verified that a test tone of 100Hz and an output level of 0dBu gave close to 0.4 V rms before running the sweep tests."_. Ooops, that's not correct. 0dBu should output a level of 0.775vRMS??? That's off about half, which is the amount his and Ilkka's charts disagree... oh, well.

Anyway, if you set the input level of the SMS to a standard +2dBV (~1.25vRMS), you'll be operating at around a nominal level of ~315mvRMS. The peaks that exceed ~775mvRMS up to 1.25vRMS will contain distortion, (if we accept that all SMS's act like shown in the graphs)......... Any tests over +4dBu don't mean a lot to me because that amount would clip a BFD (for example) and shouldn't be used anyway (+4dBu=~+2dBV).

brucek


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## cjwhitehouse

brucek said:


> _"I then verified that a test tone of 100Hz and an output level of 0dBu gave close to 0.4 V rms before running the sweep tests."_. Ooops, that's not correct. 0dBu should output a level of 0.775vRMS??? That's off about half, which is the amount his and Ilkka's charts disagree... oh, well.


A slip on my part trying to post a reply quickly when I was supposed to be doing something else. It should have said 0.4Vrms at -6dBu. Now fixed, thank you. :T

While you say there is no problem at 0dBu, there is clearly some distortion starting to take off at that level.

Would you care to comment on the reason for the apparent frequency dependence of the distortion? Simple clipping is normally pretty much frequency-independent, at least when dealing with line-level signals.


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## JimP

Can anyone suggest a way to measure either the output voltage or distortion level using REW or other common software or hardware to determine where we should set levels in our preamp/receiver?


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## brucek

> Now fixed


:T



> While you say there is no problem at 0dBu, there is clearly some distortion starting to take off at that level


I suppose, but it appears to behave fairly well down to 10Hz with 0dBu input level. Below that we know the response is down quite a bit so that you won't be hearing any distortion (or sub signals for that matter).

I consider 1% distortion at 0dBu not bad at all for a sub chain. I don't think most users would be exceeding that input level too often. This device is unlike a BFD that is designed to be used at Pro levels. The FBQ has a maximum input level of +20dBu. That's +7.75vRMS before it thinks about clipping. You can't fairly compare this to a device like the SMS that makes it quite clear that it's for home audio equipment. 

Most receivers are lucky to output a maximum level of 1vRMS. That's maximum - full volume. They'll typically output around 300mvRMS. The SMS would be fine for that. Driving that device with +6dBu is not realistic for the average guy with a retail receiver. It's fun to do testing at max levels of different devices, but it often _scares the troops _more than it should unfortunately.. 

I can't imagine why the THD appears to take such a quick jump at a certain frequency. I certainly suggest its not input stage related and would lie in the DSP. We can't even speculate what they're doing inside. :huh:

brucek


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## cjwhitehouse

OK, but for example, my Theta Casablanca III can output 10Vrms via RCA and 20Vrms via XLR so I will still have to be careful. Anyway, I'm glad I now know about this potential issue.


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## brucek

> OK, but for example, my Theta Casablanca III can output 10Vrms via RCA and 20Vrms via XLR so I will still have to be careful


hehehe, fair enough, but yeah, that and my Bryston SP2 are both $8000 processors. They both output 20volts on the XLR's. I consider them both Pro equipment...... you wouldn't use a consumer level device like an SMS - you'd get a Pro level BFD or FBQ.........

brucek


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## cjwhitehouse

All very well, but the sort of people that buy such processors are the very people that are likely to buy Velodyne DD subs. I myself have a pair of DD-18's. It appears the same issue afflicts them so it is a potential issue for those owners. I certainly don't classify Theta gear as pro equipment and as a user I would expect to be able to use it with other high end consumer equipment.


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## brucek

> I myself have a pair of DD-18's. It appears the same issue afflicts them so it is a potential issue for those owners. I certainly don't classify Theta gear as pro equipment and as a user I would expect to be able to use it with other high end consumer equipment.


Yeah, I meant that the processors output a pro level............ 

I sure understand why you'd want to test the DD-18's for distortion. But, I admit to being a bit confused about the test conditions used for the graph when you said:

_The reason this has not shown up on our sub tests up till now is that the 85dB sweep levels correspond with a drive signal of about -33dBu. That means that even when we get to 115dB sweep levels, the highest we have ever sent a DD sub, we are still only at -3dBu._

So, you're saying that -3dBu fed to the DD-18 produces an output (nearfield?) of 115dBSPL? How does that test condition relate as compared to listening levels in your HT? Are you saying that you may never send the DD-18 a level of +6dBu that produces the distortion in the graph?

brucek


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## cjwhitehouse

We are talking about two distinct sorts of tests here. The recent test of my DD-18, to try and confirm if it had the same distortion issue as the SMS-1, was conducted nearfield, in-room. I had turned the gain on the sub right down so as not to clip the microphone. I didn't mention the absolute SPL measured, but for the record, the +6dBu sweep registered an average 106dBspl average across the range 30Hz-80Hz. Not exactly straining the sub or mic.

The other sub testing I was referring to was our tests conducted outdoors at 2m ground-plane. These and others were what inspired Ilkka to start his own such tests. We have pretty much tested the entire Velodyne range now at various times and with various firmware versions.

We start our sweeps at a voltage drive level that delivers 85dBspl at 2m ground-plane averaged across the range 30Hz-80Hz. We then increase sweeps at 5dB increments and measure FR, distortion, compression etc. We refer to these sweeps as at 90dB, 95dB etc. The drive level for a 85dB sweep is normally arranged around -33dBu so that we have sufficient headroom to increase the drive by up to 35dB to get to a "120dB" sweep at around +2dBu. In practice, the only DD sub we sent a "115dB" sweep to was a DD-1812 here. As you can see this pushed it into some pretty hefty compression. Because of the dip in the response (which we won't go into here - we are already enough off-topic!), the "115dB" sweep actually resulted in a maximum output of about 117dBspl at 120Hz. We are hoping to re-test one of these subs soon - Velodyne fixed the dip issue some time ago and it would only be fair to put the record straight. For the DD-18 we never went beyond a "110dB" sweep for which the drive level would have been about -8dBu by my reckoning. Hence why I said we never saw the distortion take off through Ilkka's discovery during our earlier sub tests. Hope that clarifies things. We are starting to wander off the topic rather, but I wanted to make the point why this may be an issue for a certain group of users.


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## Guest

brucek said:


> Do you feel you are hearing that distortion when listening?
> 
> brucek


the times i have heard the sms-1 i think it's done a good job  it might be wrong at my ears though


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## JimP

Vaughan100 said:


> I bought this unit thinking that it would make a big difference to my system. So far that hasn't happened. And I've achieved a virtually flat response.
> 
> For movies I haven't heard any differences whatsoever except that there is now less percieved bass than before. The increase in bass from 50 to 80 hz seems to go unnoticed.
> 
> With music, on some specific tracks, it sounds a little more full bodied. Considering the levels of boost inbetween 50-80 hz to achieve the flat response, I don't know what I was thinking.
> 
> The end result is that while music does sound more full bodied (which is a good thing), the bass still sounds boomy so I'm not sure the parametric EQ actually affected anything besides level.
> 
> I thought that deep bass was minimum phase. If I correct a frequency anomaly then the time component should be rectified as well. Or at least in theory. But if I think about it, I could rather spend the same amount of money on acoustic treatment that actually would make a far greater improvement.
> 
> I'm deciding what to do. I just hope that the dealer decides to take this thing back. I was so very excited to get this unit but the price does not warrant the small percieved SQ differences given the massive FR change in the graphs.
> 
> Decision pending.
> 
> --Regards,
> 
> Perhaps my room isn't as bad unequalized as the next person although I certainly thought it was.


If you're are running your sub flat against the mains, try experimenting by raising the subwoofer level to 3 to 5 dbs.


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## JohnM

JimP said:


> Can anyone suggest a way to measure either the output voltage or distortion level using REW or other common software or hardware to determine where we should set levels in our preamp/receiver?


Use the REW signal generator to play a sine wave through the system (connecting the final output that will feed your sub to your line input) and keep an eye on the SPL meter distortion percentages while you adjust the level, as you get into clipping the 3rd harmonic distortion will increase rapidly. Make sure you are not clipping at the soundcard line input though. You can also look for clipping by taking a measurement sweep with the same connections and looking at the scope plot for clipping on the input.


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## Guest

*Updated firmware is out to some 2.1.3*

Just saw this on Audioholics after seeing a post at AV123. 
The info below is from a post at Audioholics. I didn't do the quote part right. I do not have the firmware but am trying to get it.

"Just coming down the pipe, but curt c from velodyne sent me the firmware update already... claiming the elimination of the drastic rolloff and added distortion... 

Here's the info... quite possibly a must for those with an SMS and deep digging subs...


Quote:

Subwoofer Management System (SMS-1) Release 2.1.3

Release Notes 

This document discusses the latest release of Velodyne’s Subwoofer Management System (SMS) firmware – release 2.1.3. This firmware addresses two issues related to the low frequency management of the SMS. First, it allows the subsonic filter to be set low enough that the only remaining rolloff in the low frequency is restricted only by the hardware, and secondly, corrects some distortion that previous versions of the firmware introduced to the audio spectrum.



Figure 1: SMS-1 Frequency Response 

Please refer to Figure one. This graph shows actual output from the SMS-1 with the subsonic filter set to 5 Hz. The various curves shown reflect the subsonic filter as set to 6, 12, 18, 24, and 30 dB/octave -- the steeper the slope, the higher the subsonic slope setting. As you can see, the actual point at which the curves converge is 5 Hz, and at 5 Hz, the audio frequency is down about 5 dB. The typical 3 dB down point is about 8 Hz. This point is a limitation in the hardware and the low frequency response of the SMS cannot be extended any further without changing components in the hardware. In release 2.1.3 the subsonic filter can be set as low as 1 Hz., but any setting below 5 Hz. has negligible effect on the frequency response. 

Note that the low-pass crossover in this example is set to the default of 80 Hz and 24 dB/octave. 

Some users of the SMS measured the low frequency performance and discovered that the SMS was adding some distortion to the lowest frequencies. Most users did not notice this added distortion due to the low frequencies affected. The distortion was inadvertently introduced as the frequency processing software, which was based on Digital Drive subwoofers, was rewritten for the SMS. The 2.1.3 firmware corrects this problem. 

There was some confusion that this distortion was also present in the DD series. This is not and was never true – the distortion was introduced in the SMS only, and has now been remedied. 

If you have any questions on this information, please contact Velodyne customer service at [email protected]."


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## Warpdrv

I sent it to your email you gave me on Audioholics there RPM2...

Your good to go... remember to rename that main file to .exe


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## TheEAR

Thank you for the testing and data Ilkka. :clap:

Good to see actual measured performance,I have to say I have the Velo SMS and the Behringer.The SMS will take a trip to the nearest closet. 

Behringer surprises me these days,from the low cost and great performing EP2500 and now this.


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