Is the UcD modulation scheme less than optimum?

[ALW]

Bruno,

You can't do much more optimum than to minimize the effect near rated impedance.

Is this modulator optimisation complex?

The reason I ask is is it feasible, or beneficial given the often complex nature of the speakers impedance, to optimise a given amplifier installation for a given speaker?

Andy (waiting for his UcD180's to arrive )

[Bruno Putzeys]

Originally posted by ALW
Bruno,



Is this modulator optimisation complex?

The reason I ask is is it feasible, or beneficial given the often complex nature of the speakers impedance, to optimise a given amplifier installation for a given speaker?

Andy (waiting for his UcD180's to arrive )

Feasible:yes. Beneficial:not sure. Complex:takes a lot of simulation time. Wouldn't bother, really...

Besides, the modulator is covered in epoxy resin, so it's not intended to be modified by the user.

[Pjotr]

Hi,

With any load (i.e. the speakers) the load impedance is part of the feedback function. If you want to keep this part constant (that is, independent of frequency) you need to correct the speaker impedance itself with zobel networks. If this is of much audible benefit? Maybe Bruno can say

Cheers

[Bruno Putzeys]

The whole point of UcD is to make the output of the amplifier as independent of the attached load as possible, such that nobody need bother about using zobel networks and all that. Other class D amps produce marked frequency response aberrations when not driving a pure resistive load. The effects of several dB's worth of response error on perceived sound can be very significant.

By contrast, the variations in modulator linearity discussed in this thread are fairly academic, and not very relevant to practical use. Linear amplifiers (be they tube or solid-state based) also produce different distortion behaviour in different loads, and few people care - for good reason. Class D amplifiers without post-filter feedback will also exhibit variations in distortion with variations in load, although the source of the effect is different.

So, the import of this discussion lies in providing a deeper understanding of modulator behaviour, not so much in looking for anything practical to do or try out

To answer your question directly: the effect of adding a zobel network is very small. You're more likely to hear the tonal character of the type of capacitor used than a change produced by the mathematical effect of its presence.

[Pjotr]

Thanks Bruno,

More or less what I expected

[SSassen]

Adding to Bruno's post I'd like to comment that from my simulations and measurements on a number of class-D topology prototypes (including UcD) I often noticed an increase of the THD when a zobel is used at high output power into a pure resistive load. Furthermore although a zobel is often employed in a class-A/AB amplifier to guarantee stable operation (i.e. prevent the amplifier from self-oscillation) this is inherently not desired with a self-oscillating class-D amplifier for obvious reasons.

I've often ascribed the perceived changes in the 'sound' of an ampifier with different loudspeakers to be caused by the specific load of the loudspeaker attached and its interaction with the amplifier. Obviously not all amplifiers cope well with a reactive load at their outputs and suffer from this in a number of ways including increasing the THD, exhibiting a 'wobbly' frequency response or suffering from poor control due to high currents circulating between the amplifier and the reactive load.

I therefore subscribe to Bruno's notion that a zobel might well influence the 'sound' of an amplifier and hence are best left out, which yields one parameter less to worry about. From the number of different class-D topologies I've experimented with UcD exhibits the least amount of ill effects from connecting a reactive load however.

But maybe Bruno has some further comments on this?

Best regards,

Sander Sassen
http://www.hardwareanalysis.com

[Bruno Putzeys]

Originally posted by SSassen
But maybe Bruno has some further comments on this?

Not for the time being :P

[phase_accurate]

Apart from what has been mentioned so far: While there might be better feedback topologies out there than UcD - regarding the feasibility of mass production of decent amps at good prices it is a clear winner.

What advantage would be in an advanced feedback topology when practical amplifiers get too expensive and/or impractical to manufacture ? Would they SOUND better at all BTW ?

Regards

Charles

[SSassen]

Charles,

What advantage would be in an advanced feedback topology when practical amplifiers get too expensive and/or impractical to manufacture ? Would they SOUND better at all BTW ?

Good question let me illustrate this with an example. I've looked at controlling all parameters of a class-AB amplifier quite some years ago. By using an active DC-servo at the output, precise adaptive current controlled bias circuitry and all other gimmicks one can possibly conceive to keep the amplifier 'under control'. The end result was an amp that measured beautifully but sounded dull, uninspiring and was basically a failed experiment.

Obviously one can do without feedback on a class-D amplifier as well, but that puts the emphasis on excellent linearity and impeccable phase response throughout the audio band. Although it is possible to build such an amplifier the complexity and cost would probably yield it impractical. There have been commercial examples of this in the past that have all come and gone without even leaving a ripple in the water.

Best regards,

Sander Sassen
http://www.hardwareanalysis.com

[Pjotr]

Originally posted by SSassen
Adding to Bruno's post I'd like to comment that from my simulations and measurements on a number of class-D topology prototypes (including UcD) I often noticed an increase of the THD when a zobel is used at high output power into a pure resistive load. Furthermore although a zobel is often employed in a class-A/AB amplifier to guarantee stable operation (i.e. prevent the amplifier from self-oscillation) this is inherently not desired with a self-oscillating class-D amplifier for obvious reasons.

Hi Sander,

I meant compensation networks to make the speaker impedance resistive and constant at the side of the speaker, although that is seldom done for woofers anyway. Not the network that sits in most common amplifiers for stability reasons.

Cheers