Improving nonlinear distortion of a full DSP loudspeaker system by adding passive speaker-level filter networks

[Lars Risbo]

That can also be done with a DSP, if you got dedicated DSP channel for each driver. When creating notch filter equivalent with DSP peaking

no, only the linear frequency response can be the same. The effect on the nonlinear current is not the same due to the changed impedance. It’s explained in the blog post.

 

[bmc0]

or use REW which has recently baked FSAF in I think

I'm planning to try FSAF at some point. The residuals with and without the passive filters should be interesting and may give me a better idea of whether or not the improvements I hear are only in my head .
Just in case someone here is looking for it in REW: the current stable release (V5.31.3) doesn't have FSAF, but the beta version does (starting with V5.40 beta 32).

Anyway, if the goal is to reduce HD, shouldn't THD be lower too, otherwise there would be no point in doing it, right?

Not necessarily because nonlinearities differ in audibility/annoyance depending on their order and underlying cause. One good example is the difference between Kms(x) vs Bl(x) nonlinearity, which Purifi covered here. Harmonic distortion for the two examples is very similar, but the intermodulation distortion (and the sound) is very different.

That can also be done with a DSP, if you got dedicated DSP channel for each driver. When creating notch filter equivalent with DSP peaking filter of high Q, you get essentially the same effect.

This was discussed previously. The effect on the linear part can be the same, but the effect on the nonlinear part is not. As @Lars Risbo (the author) says, this is explained in the Purifi application note. Also see posts #4, #5, #7, #8, #10, and #11 in this thread.

During current drive experimentation, some have said that woofers sound quality suffers from series resistance. That can be due to the damping factor being lower and the woofers inertia is less controlled. I fail to sees how this does not apply to mids and highs as well

See the article @tmuikku linked in post #18. Low driving impedance only offers meaningful control near a driver's LF resonance.

 

[Wolf]

Just a note or 2 about the 'sine-cap' filter, also known as capless highpass or RL highpass, that series resistor will get hot being it is the load the amplifier sees. It is not bandwidth limited due to no capacitor, so low frequencies pass and then shunt through the coil. A 50W resistor with heatsink is recommended for this kind of circuit, and externally mounting the resistor to the loudspeaker cabinet is a benefit here to avoid starting a fire with internal damping, etc.

If using a capless highpass is something you want to try, but don't want the heat issues, a passive series xover can get you most of the way there. However, since that kind of xover cannot be biamped, then you can't use DSP or active filters separately for different drivers. This means general EQ is about all you can do there.

 

[Lars Risbo]

for a tweeter it’s best to add a series resistor directly in series with the tweeter. this increases the drive impedance and can be used for padding down the tweeter sensitivity. An L pad is very popular but almost the worst solution. The L-R (cap less ) is burning much energy as pointed out and capacitors are typically close to ideal than inductors.

 

[Wolf]

for a tweeter it’s best to add a series resistor directly in series with the tweeter. this increases the drive impedance and can be used for padding down the tweeter sensitivity. An L pad is very popular but almost the worst solution. The L-R (cap less ) is burning much energy as pointed out and capacitors are typically close to ideal than inductors.

@Lars Risbo...
There are reasons you prefer the aft resistor, as it reduces distortion as cited in your document. However, an aft resistor will also tilt the response down in the treble the larger the value utilized. It also changes the effective damping Q of the xover to where coils get larger and caps get smaller for the same outcome. Copper is expensive, and can increase parts cost over that of a lesser driver net impedance. Having a resistor out front does not tilt the response in this manner.

A resistor across the tweeter is a bad idea in terms of the back EMF operation, but has so many other positive reasons. It damps the Fs of the tweeter, readjusts to the normal Q range of xover components, and attenuates without tilting the response.

Adding series resistance, while decreasing distortion, can also; take the life out of the driver at times, increase sibilance (if not using the right value or type), and if using too long a speaker cable or too high a DCR coil- make a speaker sound overly muted.

I'm sure you already know all of this. I'm thus far not convinced that reducing HD by added resistance or electrical damping is actually more beneficial than using the network with these other positives in play. It shifts the balance of more benefits in the other direction, IMO.

 

[bmc0]

Just a note or 2 about the 'sine-cap' filter [...]

Good points to keep in mind if one intends to use it in a passive system. In this case, the power dissipation falls rapidly below 1kHz because the digital filters on the tweeter channel have a ~12dB/oct high pass response (see the third image in post #1). I use three 30Ω 10W resistors in parallel for the 10Ω series resistance. Massive overkill[1] for this application really, but resistors are fairly cheap.

1: A rough calculation says that if I were to play Bargain by The Who at 95dB(C) avg. RMS (>110dB(C) peaks), the series resistor's average dissipation over the duration of the track would be less than 50mW (~4W instantaneous peak).

 

[Lars Risbo]

there are of course pros and cons to adding series resistance. It decreases the electrical damping (increases Qts) which typically increases distortion down closer to the driver fs. In that sense, the shunt inductor offer the advantage of keeping the damping at low frequencies.

 

[KSTR]

@Wolf, we're talking DSP/active systems here, so altered response is a non-issue. It really depends on the tweeter which drive impedance vs frequency is optimal. Most tweeters improve in sound/distortion with high source impedance overall, from a simple series R (but keeping the target frequency response the same, of course) but not all.

 

[KSTR]

there are of course pros and cons to adding series resistance. It decreases the electrical damping (increases Qts) which typically increases distortion down closer to the driver fs. In that sense, the shunt inductor offer the advantage of keeping the damping at low frequencies.

Yep, I've found some compression drivers really hate high source impedance around resonance, actually they performed best with as much internal feedback as possible (negative drive impedance). My working theory is that the VC (as a motor and as a sensor) is very linear as it is usually underhung but the flat suspension is extremely progressive. Strong internal velocity feedback helps then quite a bit.

 

[bmc0]

My working theory is that the VC (as a motor and as a sensor) is very linear as it is usually underhung but the flat suspension is extremely progressive. Strong internal velocity feedback helps then quite a bit.

That mirrors my thoughts, hence why I designed the tweeter filter the way I did.