Amp freq response: Sonos vs Peachtree

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Peter Mc
Audio amplifiers are essentially linear devices. They have to amplify each frequency by the same amount independent of other frequencies. If different frequencies interact, this is called non-linear distortion and is highly undesirable. It would mean, for example, that every C# on the trumpet, or every Bb on the bass would affect other frequencies. This doesn't happen to any noticeable extent. So filtering out the low frequencies should not affect any other frequencies. And it would be extremely unlikely to uniformly boost all other frequencies by exactly the same amount, i.e. 3 dB.

My guess is that there is a bug in the DSP part of the Amp. The fact that the boost is exactly 3 dB suggests a coding error in the filtering software. It can be very easy to leave out a factor of 2. And a 3 dB boost is exactly double the power.

Like kumar and lahey, I don't know what the point would be in deliberately boosting the non-filtered frequencies by 3 dB.

Cheers, Peter.
"The presence of those seeking the truth is infinitely to be preferred to those who think they've found it" - Terry Pratchett
pwt
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Do you mean that removing the low end is likely to result in a significantly lower peak amplitude in the output to the main speakers?
Yes, that's the gist of it.
4 x P:5 | Sub | 4 x P:1 | 3 x Connect | 2 x Boost | Connect:Amp | 2 x One | Amp | Move | Port
L
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Correct, I think it's intentional. .
Do you mean that removing the low end is likely to result in a significantly lower peak amplitude in the output to the main speakers? I suppose that could be why they bump the gain. I'm not sure if that reasoning would hold true or not, I would think it would be dependent on what music is playing.
pwt
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Maybe I misunderstood what you were saying then. Possibly we are both just saying that the crossover adds gain (according to Peter's measurement). You think this might be intentional, whereas I think it is likely to be unintentional.
Correct, I think it's intentional. The Amp has more headroom for gain when not required to produce low frequencies, so the Amp's software takes advantage of this. Amplifiers with simple manual controls typically cannot be run at 100% of dial volume without distortion, so one tends to assess by ear how loud they can be run, and one can increase the maximum volume setting if low frequencies aren't required. The Amp can be run at its 100% volume setting without distortion, so it makes sense to recalibrate via software control when the gain available increases.
4 x P:5 | Sub | 4 x P:1 | 3 x Connect | 2 x Boost | Connect:Amp | 2 x One | Amp | Move | Port
K
Some might want to use the main speakers full range, though, and set the sub filter within the sub controls. The only want to do this, as I understand it, is to connect to the sub with speaker level input.
Yes. And there is someone struggling to achieve just this without a hum! See:
https://en.community.sonos.com/components-228996/sonos-amp-and-high-level-input-from-subwooder-6822917
L
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Maybe I misunderstood what you were saying then. Possibly we are both just saying that the crossover adds gain (according to Peter's measurement). You think this might be intentional, whereas I think it is likely to be unintentional.
pwt
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The Sonos Amp output voltage should just be input signal * gain, until clipping. If you remove low frequencies the amp's output in the pass-band should not change if the crossover filter has unity gain for the pass-band.

You can prove to yourself that even a cheap amp is not affected by the presence of the low frequencies. In Audacity, import a clip with a good range of frequencies. Filter out the low end. Then listen to the filtered and unfiltered clips. The mids and highs will be played at the same level for both clips.
None of this is incorrect, but nor does it refute what I stated. The Amp's gain is under software control, and the software can raise the gain for a given volume setting when there is no requirement to produce low frequencies.
4 x P:5 | Sub | 4 x P:1 | 3 x Connect | 2 x Boost | Connect:Amp | 2 x One | Amp | Move | Port
L
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Whatever the reason, the sound levels for the Sub and the main speakers would still have to be manually matched; and once that was done, this trait would have no impact on the sound from the set up compared to one without the Sub, other than the desired extension in low frequency response. Correct?
Yes I think I agree with this. Some might want to use the main speakers full range, though, and set the sub filter within the sub controls. The only want to do this, as I understand it, is to connect to the sub with speaker level input.
K
Whatever the reason, the sound levels for the Sub and the main speakers would still have to be manually matched; and once that was done, this trait would have no impact on the sound from the set up compared to one without the Sub, other than the desired extension in low frequency response. Correct?
L
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As I said earlier, I don't know enough about amp design to opine. But if the across the board uniform boost to frequencies - as Peter suggests is present when a Sub is in play - that are directed towards the speaker terminals of the amp is a design intent, what is the point of doing that?
I don't know what the point would be. Maybe it is just a result of the signal processing that they do for the crossover, and they did not bother to correct for this.
K
As I said earlier, I don't know enough about amp design to opine. But if the across the board uniform boost to frequencies - as Peter suggests is present when a Sub is in play - that are directed towards the speaker terminals of the amp is a design intent, what is the point of doing that?
L
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You can prove to yourself that even a cheap amp is not affected by the presence of the low frequencies. In Audacity, import a clip with a good range of frequencies. Filter out the low end. Then listen to the filtered and unfiltered clips. The mids and highs will be played at the same level for both clips.
L
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But what Peter found (I think) is that the sub crossover does change the gain. So it is not a unity gain implementation.
L
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No-one said that the Amp 'sags'. It has to operate within a total power envelope with a gain that it can deliver across the required frequency range while meeting its target distortion values. Remove the requirement to amplify signals below the crossover and that total power envelope can be applied to a smaller frequency range, hence the remaining frequencies can benefit from higher gain.
The Sonos Amp output voltage should just be input signal * gain, until clipping. If you remove low frequencies the amp's output in the pass-band should not change if the crossover filter has unity gain for the pass-band.
pwt
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Isn't this just the Amp being able to deliver more power to the frequencies above the crossover, now that it doesn't also have to serve the frequencies below it?
Ideally, an amplifier's output is simply the input signal multiplied by a gain factor. The output should not sag by having to provide the low end. That would mean it is a poor piece of equipment.
No-one said that the Amp 'sags'. It has to operate within a total power envelope with a gain that it can deliver across the required frequency range while meeting its target distortion values. Remove the requirement to amplify signals below the crossover and that total power envelope can be applied to a smaller frequency range, hence the remaining frequencies can benefit from higher gain.
4 x P:5 | Sub | 4 x P:1 | 3 x Connect | 2 x Boost | Connect:Amp | 2 x One | Amp | Move | Port
K

To be clear, I don't think it's an issue, nor does it indicate a brighter sound profile.
Right; over to Peter if he disagrees.
pwt
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Exactly. Manually, since in the case of a third-party sub, the Amp has no idea of its power output.
But then once that is done, the 3dB boost the Peter refers to becomes a non issue. And no reason for a brighter sound.
To be clear, I don't think it's an issue, nor does it indicate a brighter sound profile. I was just explaining why the Amp's volume at frequencies above crossover would be increased in the presence of a sub.
4 x P:5 | Sub | 4 x P:1 | 3 x Connect | 2 x Boost | Connect:Amp | 2 x One | Amp | Move | Port
L
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Isn't this just the Amp being able to deliver more power to the frequencies above the crossover, now that it doesn't also have to serve the frequencies below it?
Ideally, an amplifier's output is simply the input signal multiplied by a gain factor. The output should not sag by having to provide the low end. That would mean it is a poor piece of equipment.
K

Exactly. Manually, since in the case of a third-party sub, the Amp has no idea of its power output.
But then once that is done, the 3dB boost the Peter refers to becomes a non issue. And no reason for a brighter sound.
pwt
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Even so, a lot of the mid range on the play 1 is via the woofer; relieving it of low frequencies does not elevate the mid range dB.
It can't though, because there is no equivalent extra available power to elevate the high end (tweeter) to match. The total P:1 volume has to stay calibrated to within what the tweeter amp can deliver.

How would one match volume levels in the case of Sonos Amp, except manually?
Exactly. Manually, since in the case of a third-party sub, the Amp has no idea of its power output.
4 x P:5 | Sub | 4 x P:1 | 3 x Connect | 2 x Boost | Connect:Amp | 2 x One | Amp | Move | Port
K
The first reply to a question I posed, on hydrogen audio:
"No boost is applied highpassed signal for the speakers.
The sub has its own amp (active sub) and the sub has a volume control, so if configured correctly, the volume of the sub matches that of the speakers."
In the play 1, I am guessing this happens by default. One can then change the relative levels via the Sub output settings. How would one match volume levels in the case of Sonos Amp, except manually?
K
Even so, a lot of the mid range on the play 1 is via the woofer; relieving it of low frequencies does not elevate the mid range dB.
pwt
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Isn't this just the Amp being able to deliver more power to the frequencies above the crossover, now that it doesn't also have to serve the frequencies below it?
Do amps work like that? I don't know enough to say. But this would then apply to the amp in a play 1 unit as well. And what it would take to compensate is dialling down volume levels, not anything in the EQ. I don't notice any change in volumes - or treble levels for that matter - when I toggle the Sub on/off in my play 1 set up for which the same priniciple should apply. The midrange clears up subtly for the better with the Sub on, that's it.
The P:1 has two separate amplifiers, and there's no relief for the tweeter amp when you introduce the Sub, so I wouldn't expect the overall volume to increase.

In the case of the Amp, with a single amplifier configuration, it no longer has to supply energy for the frequencies below the crossover (which is current-heavy due to the large amount of air that has to be moved), so that energy can be redeployed to higher frequencies.
4 x P:5 | Sub | 4 x P:1 | 3 x Connect | 2 x Boost | Connect:Amp | 2 x One | Amp | Move | Port
K

Isn't this just the Amp being able to deliver more power to the frequencies above the crossover, now that it doesn't also have to serve the frequencies below it?
Do amps work like that? I don't know enough to say. But this would then apply to the amp in a play 1 unit as well. And what it would take to compensate is dialling down volume levels, not anything in the EQ. I don't notice any change in volumes - or treble levels for that matter - when I toggle the Sub on/off in my play 1 set up for which the same priniciple should apply. The midrange clears up subtly for the better with the Sub on, that's it.
pwt
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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?
Isn't this just the Amp being able to deliver more power to the frequencies above the crossover, now that it doesn't also have to serve the frequencies below it?
4 x P:5 | Sub | 4 x P:1 | 3 x Connect | 2 x Boost | Connect:Amp | 2 x One | Amp | Move | Port
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