Basic class D amp questions

[loheswaran]

I have looked elswhere in the forum, but I can't seem to get a clear answer to the following questions:

1. Does a class D amp turn an anologue signal into a digital one? (I am a vinyl freak and see no point in losing analogue sound);
2. I see that all class d amps have an operating range that cuts off at 22khz - unlike tube and class a designs. Now I appreciate that I am not a bat, but I am curous as to why this is the case.

I am asking these questions because I am thinking about building a three-way active speaker

[sofaspud]

1. No. Class-D uses binary switching for efficient amplification, as opposed to biasing the transistor output in the linear region. I suppose "digital" might fit since it's an on/off output, but it only serves to cloud the real goings-on IMHO.
2. The output must be low-pass filtered to regain the desired signal. I've not noticed that all are cut off at 22kHz so I can't comment on a specific frequency.
I'm sure others can provide more info. Class-D has made huge inroads the past 5-10 years and worth an audition for your project (again IMHO). You might like it.

[theAnonymous1]

Originally Posted by loheswaran

I see that all class d amps have an operating range that cuts off at 22khz - unlike tube and class a designs. Now I appreciate that I am not a bat, but I am curous as to why this is the case.

Not all Class-D designs are the same, but a lot of them do start to roll-off not much past the ~20kHz mark. I don't see this as a problem IMO.

I think the reason this is the "norm" is a combination of the switching frequencies used (200-300kHz) and the fact a first order filter is used on the outputs. If you push the corner frequency too far up, then there won't be enough attenuation at the amps switching frequency (causing too much radiated noise). Multi pole filters cause other problems.

^That's how this layman understands it anyway; a pro is welcome to correct me.

[nigelwright7557]

Originally Posted by theAnonymous1

Not all Class-D designs are the same, but a lot of them do start to roll-off not much past the ~20kHz mark. I don't see this as a problem IMO.

I think the reason this is the "norm" is a combination of the switching frequencies used (200-300kHz) and the fact a first order filter is used on the outputs. If you push the corner frequency too far up, then there won't be enough attenuation at the amps switching frequency (causing too much radiated noise). Multi pole filters cause other problems.

^That's how this layman understands it anyway; a pro is welcome to correct me.

You can run class d at higher frequencies than 200-300KHz.
You can also shift the filter frequency up a bit as the carrier will be rejected by the speaker anyway. Or use a dual filter. In fact you can now get filterless class d amps.

[theAnonymous1]

Originally Posted by nigelwright7557

You can run class d at higher frequencies than 200-300KHz.

Yes, I realize there are >1MHz Class-D amps. The average for a lot of designs falls in the 200-300kHz range though.

Originally Posted by nigelwright7557

You can also shift the filter frequency up a bit as the carrier will be rejected by the speaker anyway.

As I understand it, it has more to due with RFI being emitted by the speaker cables. I don't think it's recommended to have much less than -40dB attenuation at the switching frequency for that reason.

BTW, I meant to say 2nd order filter in my first reply.

[nigelwright7557]

Originally Posted by theAnonymous1

Yes, I realize there are >1MHz Class-D amps. The average for a lot of designs falls in the 200-300kHz range though.



As I understand it, it has more to due with RFI being emitted by the speaker cables. I don't thing it's recommended to have much less than -40dB attenuation at the switching frequency for that reason.

I use 200KHz on my amps and they sound great.
I tried 400KHz but the class d chip got hot so I went back to 200KHz.

The actual frequency content at the speaker leads isnt as bad as might be first thought. Its is a 200KHz sine wave not a square wave so really doesnt radiate that badly.

[TICUL38]

Unnecessary speaker coil heating with HF signals? would this not be a reason for cutting off frequencies beyond what human ears can use?

Louis

[nigelwright7557]

Originally Posted by TICUL38

Unnecessary speaker coil heating with HF signals? would this not be a reason for cutting off frequencies beyond what human ears can use?

Louis

But the inductance of the speaker coil will reject such high frequencies.

[darkfenriz]

The input filter is just idiot proofness. Many class-D amps (analog of course) react histericcaly to slew overload.
There's basically no problem in designing switching amp of, say, 100kHz small signal bandwidth.

[TICUL38]

Can someone suggest technical reference(s) for justifying
the little importance I sense here, for filtering out the high frequency "hash"...
Thank you,

Louis