First, to clarify: The change in the frequency response is seen by plugging in an unconnected RCA, regardless of the sub setting in the app?
No. To get a change in the frequency response requires both plugging in the RCA lead and turning on the sub setting in the app. Doing just one of these changes nothing.
I am surprised that there is a boost applied to frequencies above the crossover; this isn't normal amp design for a sub line out.
It does seem unusual, although I've not had much to do with subs in the past.
I suppose it is possible that this is not anything done by the amp, but by the speakers which are putting out higher sound levels by no longer having to deal with frequencies below the crossover?
I can't imagine the speaker response to not having to reproduce low frequencies is to uniformly boost all other frequencies by exactly 3dB. It's too perfect. Besides, what would happen in songs where the bass and kick stop playing? I'm sure it's a design decision by Sonos. The exact same response was noted by hodgeal in the LS50 thread using very different speakers.
Cheers, Peter.
With an RCA connected, it DOES make a difference, and the crossover frequency also makes a difference.
the frequencies above the crossover are boosted by 3 dB.
Plugging in an unconnected RCA does NOT turn on the sub setting in the app.
First, to clarify: The change in the frequency response is seen by plugging in an unconnected RCA, regardless of the sub setting in the app?
I am surprised that there is a boost applied to frequencies above the crossover; this isn't normal amp design for a sub line out. I suppose it is possible that this is not anything done by the amp, but by the speakers which are putting out higher sound levels by no longer having to deal with frequencies below the crossover?
I will, nevertheless, try this when I get a chance. Might be a few days.
As far as what happens when the socket is plugged even by a blank plug, I would expect to see frequencies from the crossover downwards - after taking into account some slope - to disappear at the speaker terminals. That is all that the Amp is supposed to do; it should not, as alleged by some, also then mess around by boosting the higher frequencies. However what will happen is that the speakers, once a muddying effect of having to deliver the lower frequencies is removed, will do a better job of more accurately delivering the remaining frequencies. In the case of some speakers, this more accurate delivery may come across as brighter, or a more elevated treble, but it isn't the amp that is responsible for this by any boosting of the signal strength of the associated frequencies. And it is also likely that once the "burn in" period is over, this kind of more accurate sound delivery will become a preference.
... I wonder whether the measured relative rise at 6kHz and above is partly responsible for the perceived brightness of the amp? I also understand you suggest that this rise is due to the mic frequency response.
I can now confirm the rise is due to the mic. I obtained generic calibration data for the mic, and also calibrated the pre-amp/sound-card combination (which was small). The results are below. The top plots include the calibrations, the bottom ones don't (see the legend). The plots have been separated by 10 dB for clarity.
I have seen a number of people mention that they find class D amps bright. My analysis shows that there is very little difference between the Sonos Amp and the well-respected Peachtree. I haven't seen anyone complain specifically about the Peachtree being bright. But you may find all class D amps sound bright. It would be interesting if you could audition another brand of class D amp.
Cheers, Peter.
Many thanks for this extra test Peter, very interesting indeed. I have not listened to many class d amps but the Bluesound Powernode I briefly tested a while back (with same Dynaudio M10 speakers) was fine, smooth sounding and not bright at all. To my ears It did not sound better or worse than the Sonos amp (after treble adjustment on the latter). No personal experience with the peachtree but it does indeed have a stellar reputation. I have also come across people's opinions regarding class d amps in general. Not sure if the older generations were a bit prone to sound quality problems but I think the later / latest generations of digital amps are trouble free 🙂.
I can now confirm the rise is due to the mic.
LOL. That will put a cat among some of the pigeons...but it does not surprise me that either the mic or the speakers were the cause. Of course your test establishes that if the Sonos Amp is bright, so is the Peachtree, changes between the two being the only variable. I believe that the thing about each interacting differently with the load is a red herring. I also do not think that Sonos will put a bright amp on the market, not having done that with the Connect Amp years ago.
The only mystery is of a reliable poster who claims that he found it bright, power recycled it once, and then it was fine. So there may be something in the DSP that is acting up in some cases. Perhaps.
As to my Sub question, I believe it is a physical sensing of some kind. That must be why Sonos recommends that the jack be pulled out if the Sub is disconnected. A recommendation carried forward from the Connect Amp.
What happens at the speaker terminals of the Sonos Amp if a blank jack is plugged into the Sub out jack?
I cannot imagine the Amp would even know a blank RCA plug was connected. There would be no impedance change for it to sense. Unless there is a physical switch sensing the centre prong. I will, nevertheless, try this when I get a chance. Might be a few days.
Peter many thanks for the measurements and the super informative analysis. Very very interesting indeed 🙂. If (big IF) that frequency response is carried over to other setups then I wonder whether the measured relative rise at 6kHz and above is partly responsible for the perceived brightness of the amp? I also understand you suggest that this rise is due to the mic frequency response. Difficult to reach a conclusion on how this measured response will correlate with perceived sound of course but if we assume this is what reaches the ear then it is worth noting that in your case we can observe a rise that reaches about +3dB at 8 - 9 kHz. To quote my sound engineer friend, this is the 'presence - sibilance' range. In contrast we see a drop of about -5dB in the 1-3 kHz range which is in the vocal range. Comparing the two we can consider a total of 8db relative difference between the vocal range and the sibilance - presence range which may lead to a perceived sound 'flavour' as the reviews describe? Having said that, the peachtree shows a very similar frequency response in your setting so no big difference there. Of course this response is very context dependent (speakers + Room acoustics) so difficult to draw conclusions.
In my case, I immediately felt I had to turn down the treble of the Sonos Amp by 3-4 clicks as soon as I started listening to it. I did turn off the loudness before listening. The 's' sounds felt very pronounced to my ears. This was at my dealer's listening room with a pair of Dynaudio M10 speakers which I also have at home. This does not necessarily mean the amp is bright for everyone though. I know I am very very sensitive to frequencies from 3kHz and above and I usually adjust these down if I can. This is probably the reason why I always the Sonos Trueplay results in a very uncomfortable sound for my ears and can never use it (sadly). Once I adjusted the Sonos Amp treble it was sounding fine 🙂
Peter, if you can answer this after measurements, questions that arose in another thread:
What happens at the speaker terminals of the Sonos Amp if a blank jack is plugged into the Sub out jack? Are the speaker terminals then denied frequencies below the crossover setting of 80Hz even if there is nothing at the other end of the jack? And then does turning the Sub on/off in room settings for the Sonos amp in the Sonos app have any impact on what happens at the speaker terminals? Are low frequencies restored to the speaker terminals with the jack plugged in, but the Sub setting on off?
I seem to remember my Duntech Marquis have impedance ranging from 3-6 ohms (nominally 4 ohms) at different frequencies.
Interesting. This fits what the M2 shows if we assume the Sonos Amp is dialed in for lower impedance. Then the amp might be a little bright with higher impedance speakers. I ordered some power resistors so maybe I can do some simple tests with dummy loads. But first I emailed Sonos to ask if they have any curves that they are able to share. I don't know if Sonos people are monitoring this thread and might chime in?
DDFA uses some feedback taken right at the speaker connections and so I thought it would compensate for different loads. But the M2 tests seem to show that this might only work well over a limited range of loading. Maybe Qualcomm has improved this with newer DDFA technology?
