# 16 ohm vs Class D



## rawdawg (Apr 27, 2007)

Would there be any problems running a 16 ohm speaker on a Class D amplifier? Specificallly the PDX series from Alpine.


----------



## OldNewb (Sep 30, 2016)

You can't go lower. You can always go higher. I doubt you will get enough power to equal a head unit.


----------



## t3sn4f2 (Jan 3, 2007)

If you plan on playing anything higher than midrange then, I would check with the manufacturer directly. Someone on here would have to know about amp design and have tested that particular amp to tell you for sure if there is going to be any interaction between the lowpass filter on the output and that nominal speaker impedance.

Class D Driving High Impedance Loads - diyAudio

Class-D Amplifiers

"6 - Output Filter Design

The output filter is one of the most important parts of the circuit, as the overall efficiency, reliability and audio performance depends on it. As previously stated, a LC filter is the common approach, as it is (theoretically) lossless and has a -40dB/decade slope, allowing for a reasonable rejection of the carrier if the parameters of the filter and the switching frequency itself are properly designed.

The first thing to do is to design the transfer function for the filter. Usually, a Butterworth or similar frequency response is chosen, with a cutoff frequency slightly above the audio band (30-60KHz). Have in mind that one of the design parameters is the termination load, that is, the speaker impedance. Usually, a typical 4 or 8 ohm resistor is assumed, but that would produce variations in the measured frequency response in presence of different speakers. That must be compensated for by means of proper feedback network design. Some manufacturers simply leave it that way so the response is strongly dependent on the load. Surely a non-desirable thing.

The design can be done mathematically or simply use one of the many software programs available that aid in the design of LC filters. After that, a simulation is always useful. Figure 7 shows a typical LC filter for Class-D amplifiers and its typical frequency response.










This simple filter has a -3dB cutoff frequency of 39KHz (with 4 ohm load), and suppresses the carrier as much as 31dB at 300KHz. For example, if our supply rails are +/-50V (enough for about 275W at 4 ohms), the residual ripple will have an amplitude of about 1Vrms.

This ripple is, obviously inaudible, and 1V RMS will dissipate only around 200mW in a typical tweeter (not likely a problem, especially since the tweeters impedance will be a lot higher than 8 ohms at 300kHz). However, care must be taken as the speaker wires can become an antenna and affect other equipment. In fact, although a couple of volts RMS of ripple can seem low enough to run your speakers safely, EMI can be a concern, so the less carrier level you have, the better. For further rejection, higher order filters are used (with the potential disadvantage of increased phase shift in the audio band), although there are other clever ways to do it, as very selective bandstop or 'notch' filters tuned to the carrier frequency (if it is fixed, and that only happens in synchronous designs as the one described).

Well designed Class-D amplifiers have a higher order filter and/or special carrier suppression sections in order to avoid problems with EMI. As can be seen in Figure 8, the response is dependent on the load, and in fact the load is part of the filter. This is one of the problems to solve in Class-D designs. It doesn't help that a loudspeaker presents a completely different impedance to the amplifier than a test load, and many PWM amps have filters that are not (and never can be) correct for all practical loudspeaker loads. Again, only a handful of good Class-D amplifiers use feedback techniques that include the output filter to compensate for impedance variations and have a nearly load independent frequency response, as well as to reduce distortion produced by non-linearities in the filter. Although passive components are thought to be distortion-free, this does not apply to ferrite or powdered iron cores that are used for the filters. These components most certainly do introduce distortion.................."


----------



## rawdawg (Apr 27, 2007)

When I put the two posts together, it sounds like not such a good idea. I guess I'll hold out for a 8 ohm version of the speaker I was looking at.

The speaker in question was a 2204J ran off a PDX F6 Bridged.


----------



## t3sn4f2 (Jan 3, 2007)

rawdawg said:


> When I put the two posts together, it sounds like not such a good idea. I guess I'll hold out for a 8 ohm version of the speaker I was looking at.
> 
> The speaker in question was a 2204J ran off a PDX F6 Bridged.


Sounds like the way to go. 8 ohms bridged is a sweet spot, no worries way to run a bridged amp. You won't have to worry about over heating, sub nominal impedance + phase swings, etc.


----------



## rawdawg (Apr 27, 2007)

Thanks for all the help!


----------



## Patrick Bateman (Sep 11, 2006)

rawdawg said:


> Would there be any problems running a 16 ohm speaker on a Class D amplifier? Specificallly the PDX series from Alpine.


It will work fine.

If you look at the impedance curve of a loudspeaker, it ranges from 4ohms to 50ohms, sometimes higher. 

Although I generally use 8ohm drivers, I frequently wire resistors in series with my tweeters to simplify the crossover. So I'm basically running tweeters that "look" like they're 18-28ohms to the amplifier. And I've used nothing but digital amps for quite a few years now.











About the only caveat I'd offer is that you're wasting efficiency. IE, for subwoofers, you really want to go with a sub that will maximize the output of your amplifier.


----------



## oabeieo (Feb 22, 2015)

Patrick Bateman said:


> It will work fine.
> 
> If you look at the impedance curve of a loudspeaker, it ranges from 4ohms to 50ohms, sometimes higher.
> 
> ...



You called it digital  

(J.k.)


----------

