Amp freq response: Sonos vs Peachtree

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84 replies

Peter Mc
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.
"The presence of those seeking the truth is infinitely to be preferred to those who think they've found it" - Terry Pratchett
K

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?
Peter Mc
Here are the results from plugging in an unconnected RCA cable to the Sonos Amp sub output:

With no RCA connected, turning the Sub setting on or off in the app makes no difference.

With an RCA connected, it DOES make a difference, and the crossover frequency also makes a difference. The low frequencies are reduced as expected, but as already described by hodgeal in the LS50 thread, 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. This has to be done manually. I'm a bit surprised that the amp can sense the presence of an electrically inert plug, but that this information is not used to turn on the sub setting.

Presumably Sonos can confirm that this is all as designed.

Measurements below - all calibrated (mic, pre-amp, sound card). There are 3 curves underneath the dark blue - look at the very low frequencies. The green and pink do not mask any other curves. See the legend for which is which.



Cheers, Peter.
"The presence of those seeking the truth is infinitely to be preferred to those who think they've found it" - Terry Pratchett
K
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.
Y
... 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 🙂.
K

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.
Peter Mc
... 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.
"The presence of those seeking the truth is infinitely to be preferred to those who think they've found it" - Terry Pratchett
Peter Mc
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.
"The presence of those seeking the truth is infinitely to be preferred to those who think they've found it" - Terry Pratchett
Y
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 🙂
K
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?
L
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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?
Peter Mc
Now consider that real speaker impedance at high frequencies may be far from nominal 4 ohms or 8 ohms, and possibly the high end response could be somewhat variable with different speakers. So I wonder if this could partly explain what some reviewers are hearing.

That's a really interesting thought. Presumably all amplifiers have to be able to handle the frequency-dependent impedance of a whole range of speakers. I seem to remember my Duntech Marquis have impedance ranging from 3-6 ohms (nominally 4 ohms) at different frequencies. If DDFA-based amps are more susceptible to this variable load, I can imagine there could be some issues. However, presumably all DDFA-based amps would be affected in a similar way. I'm not sure if this has come through in reviews of different brands. Of course there could always be design choices unique to Sonos that exacerbate the problem.

Cheers, Peter.
"The presence of those seeking the truth is infinitely to be preferred to those who think they've found it" - Terry Pratchett
L
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Thanks again Peter for your measurements. Something else occurred to me. The DDFA technology used in the Sonos Amp is known to have a high end frequency response that is load dependent. For example, see the curves below from measurements of the NAD M2, which also uses DDFA (from https://www.stereophile.com/content/nad-m2-direct-digital-integrated-amplifier-measurements). Now consider that real speaker impedance at high frequencies may be far from nominal 4 ohms or 8 ohms, and possibly the high end response could be somewhat variable with different speakers. So I wonder if this could partly explain what some reviewers are hearing.


[Fig.2 NAD M2, 4 ohms speaker compensation, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green). (2dB/vertical div.)
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T
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In any event, it's a bit silly to have loudness and sub turned on by default.

Can other AMP owners confirm that the default settings after initial setup is indeed Loudness=ON?

IIRC with the Connect and Connect:AMP the loudness is disabled by default.

If I recall correctly, Loudness was enabled after initial setup both times I have set up an Amp.

FWIW, I don't think the Amp is brighter than other amps. Mine replaced a Grace Design M920 + Hafler power amp. I did have to futz with the sub crossover and level when I switched to the Amp, but once I did, the setup up sounds the same to me.
K
Peter, see the linked review page that has the most detailed comments in elaboration of the brightness finding. Strangely enough, the reviewer did not see this go away by toggling Loudness to off, but by a radical reduction of treble in the EQ control.

Of course the review also talks about things like the amp being more dynamic than another, to the preference of the reviewer. What dynamic means is a mysterious part of the equally mysterious PRAT attributes of amplifiers that some reviewers love to talk about: Pace, Rhythm, Attack, and Timing.

https://www.cnet.com/reviews/sonos-amp-review/2/
K

The volume taper used by Sonos is quite different to most amps.
It is more honest and therefore makes the volume control useable across the entire range of its motion, allowing for finer control at the lower end unlike most amps of today that seek to sound more powerful by very quickly reaching peak distortion free sound levels.
Peter Mc
I see that there are two lines for 75%, but are seen as one because they overlap completely?

Correct.

I am little surprised to see that at 50%, which is moderately loud where my play units are concerned, there is still a significant effect of the loudness toggle and that it takes levels to be as high as 75% for it to vanish.

Agreed. I would rarely listen at this level.

The volume taper used by Sonos is quite different to most amps. My Peachtree reached very high volumes at about 20%, but didn't change as much thereafter. The Sonos Amp is just warming up at 50%, but gets much louder after that. I have run it at 100%, and was surprised at the lack of any distortion. I never ran the Peachtree at 100%, but I probably could have. Just wasn't game!

Cheers, Peter.
"The presence of those seeking the truth is infinitely to be preferred to those who think they've found it" - Terry Pratchett
K
Very nice, thank you!
I see that there are two lines for 75%, but are seen as one because they overlap completely?
This bears out what I hear in other Sonos units - a palpable elevation of bass and even some vocals, and I expect that I don't hear the smaller boost to the treble given my already age affected hearing of those frequencies. And because a lot of my music is low/midrange dominated jazz, with vocals not being the opera/soprano kind.
I am little surprised to see that at 50%, which is moderately loud where my play units are concerned, there is still a significant effect of the loudness toggle and that it takes levels to be as high as 75% for it to vanish.
Peter Mc
Here are graphs of the effect of the loudness control on the Sonos Amp at volumes of 25%, 50% and 75%. The legend indicates which plot is which, but it should be fairly clear. The largest effect is at 25% volume, where the bass is boosted by about 5dB and the treble by about 2.5 dB. At a volume of 50% the effect is roughly halved. At a volume of 75% the loudness control has no effect. The reason there is more boost to the bass than the treble is because our ears lose sensitivity at low volumes more at bass frequencies than at treble frequencies (still true for Equal loudness ISO 226:2003 curves).



Again, note that it is only differences that are meaningful in this plot. Pairs of lines at the same volume have been shifted vertically for clarity, and as discussed earlier, a single line includes the effect of the speaker and microphone (as well as mic pre-amp and sound card response, room effects at low frequencies etc).

Cheers, Peter.
"The presence of those seeking the truth is infinitely to be preferred to those who think they've found it" - Terry Pratchett
K
I see that the other LS50 thread has a graph that shows what the loudness toggle on Sonos Amp does and it shows a noticeably higher boost at the low end compared to that at the treble end.
That's at 50% volume levels; I wonder how it would look at the 20% mark.
K
Does your graph allow you to see what Sonos actually does via loudness?

See this for my understanding:

"The most famous, well-used measurement for plotting our perception of loudness against the frequency of tones is the Fletcher-Munson curve(s) of equal loudness, published in 1933. The graphs were updated in 1956 by D.W. Robinson and R.S. Dadson. They were refined again in 2003 and adopted by the International Standards Organization as ISO 226. The purpose of the graph is to show that for humans to consider two pitches equally loud, the amount of energy necessary to produce the tone at one frequency may be completely different than at another. The multiple lines, spaced in 10 dB increments, also show that the energy/frequency differences are steeper at lower intensity levels and flatten out at extremely high intensities, though not quite as much as Fletcher and Munson first determined.
The "loudness button" on a stereo amplifier is intended to boost bass frequencies at lower volume levels where the curve is the steepest."

Perhaps it is the 2003 revision that drives the boost to just the bass frequencies. In my listening as well, the impression conveyed by the loudness switch is a higher presence of the lower end at low volumes. But it would be interesting to see what a measurement shows, for Sonos.

It is quite possible that my ears don't hear much treble in the first place, for the increase to be noticed!
Peter Mc
Peter,
As to the loudness thing - the usual design for that is to boost the lower mid to low side of things at low sound levels to bring back some depth/richness to the sound that is otherwise missing at those levels. This boost tapers off as sound levels approach 50%. Treble is usually not boosted by the loudness switch.

I disagree. The usual design of a loudness switch is to boost both bass and treble. The reason is that human hearing is less sensitive in both the bass and treble frequencies at low volumes. This has been traditionally demonstrated by the "Fletcher-Munson curves", although I believe they have been superseded by the similar "Equal-loudness contours".

Cheers, Peter.
"The presence of those seeking the truth is infinitely to be preferred to those who think they've found it" - Terry Pratchett
K

Can other AMP owners confirm that the default settings after initial setup is indeed Loudness=ON?

At least one of the reviews I linked confirms this.
K
Peter,
One that talks about the effect of the loudness thing:
https://www.whathifi.com/reviews/sonos-amp

One that says it is bright and claims that this is also how other reviews have found it:

https://www.theverge.com/2019/3/7/18254803/sonos-amp-review-speaker-specs-features-price

Another one that says the same about brightness:

https://www.soundandvision.com/content/sonos-amp-streaming-amplifier-review-page-2

No surprise, none have any measurements attached.

I suspect that posters here that say the same thing have read these and other such reviews and been influenced by these. There is nothing to be said about one poster that found it bright, but found that burning in takes care of that:-).

As to the loudness thing - the usual design for that is to boost the lower mid to low side of things at low sound levels to bring back some depth/richness to the sound that is otherwise missing at those levels. This boost tapers off as sound levels approach 50%. Treble is usually not boosted by the loudness switch. I have no idea how Sonos has built this function for the Sonos Amp.
passopp


In any event, it's a bit silly to have loudness and sub turned on by default.

Can other AMP owners confirm that the default settings after initial setup is indeed Loudness=ON?

IIRC with the Connect and Connect:AMP the loudness is disabled by default.
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