class-d output filter indispensable?

[motomoto]

At the output of all tripath's chips there is a 2nd or 3rd order filter to cut all the frequencies above 20KHz.
They say you need it to rebuild the audio signal from a PWM signal.

I would like to know if is really indispensable.


Inside a speaker, woofers and midrange already have a low pass filter (with a cutoff frequency significantly lower than 20KHz), so, for this frequencies, the

signal is correctly reconstructed from the speaker itself.

What about the tweeter?

The tweeter is an inductance of low value in series with 8ohm resistance. This means a first order low pass filter. I don't know the value of this
inductance, but, an hipotetic cut off frequency could be 50kHz (even lower, I think).

The frequencies of the output signal are around 650KHz, more than 3 octaves higher, so I think they could be eliminated even from the tweeter inductance.

Three years ago I builded a ta2024-based amplifier. I used a fenice board, and, after some listening tests, I completely eliminated the output filter.

Here some photos.

(the contruction is horrible, but, it was my first project, and it was very low budget).

Please tell me if I am wrong.

I studied electronic only recently and with online courses, so my Knowledge is still little.

I appreciate every suggestion.

Thank you.

[Corben]

I'm by no means an expert, but I believe the purpose of the output filter is to eliminate the switching frequency and in particular it's harmonics, which may have significant power at radio frequencies, if not filtered out. While the speaker no doubt acts as a filter, the speaker wires will carry the switching waveform and cause EMI. A shielded speaker cable might prevent this problem. I am also interested in using D-class without an output filter, since the output filter needs to be designed for a specific load. Without a filter there tends to be less distortion, at least in amplifiers that use no feedback. A shielded cable should have a shielded connector, however I've yet to find anything I'd really like using. XLR might be ok, but I'd prefer a connector that can't be confused with an interconnect.

[Eva]

In a fast switching class D amplifier there may be things happening up to 100Mhz at the switching node. Feding this signal to wires and a voice coil directly is very likely to cause trouble, not only due to strong radiation on each switching edge but because the output stage requires a controlled load at RF to prevent resonances that can hurt reliability and overall circuit performance.

Some form of output filter is always required for anything except driving tiny speakers with low supply voltages (like in a cell pone or a laptop).

However, sometimes a full output filter may be replaced by an EMI filter made with ferrite beads (the ones that appear more and more resistive above 10Mhz and as a low inductance value below 10Mhz) and 1nF capacitors. This may be a good solution for not very long wirings and not too big voice coils.

BTW: There are some control schemes with post filter feedback that achieve load-independent frequency response (no peaking at filter resonance).

[event horizon]

Originally Posted by motomoto

At the output of all tripath's chips there is a 2nd or 3rd order filter to cut all the frequencies above 20KHz.
They say you need it to rebuild the audio signal from a PWM signal.

I would like to know if is really indispensable.

What about the tweeter?

Unless you want to turn into the latest radio staion in your area i'd suggest they are The tweeter still has an inductive reactance & will act as a low pass filter, however all the cables feeding it won't! The idea is simply to make the amplifier comply with IEC regulations on RFI so it can be used & may i say be bought legally..With no output filter you'll be broadcasting a hell of a lot of RF unless you happen to be using screened cable all the way from your amplifier & in certain respects after the crossover in relation to the HF output..I might add that a wooden box containing the amplifier certainly won't help in this matter either

In this case it's not about what you want, more about what other people don't want messing up their lives The filter is there to prevent undue RF from inflicting your music on the local population via the radio. Or disrupting other peoples activities by interfering with them.

[motomoto]

thank you Corben, and thank you Eva.

I'll try ferrite bead + 1nF cap.

Is economic and fast, so is easy to try.

Corben, actually my speakers are all shielded, and I also use shielded and twisted speaker cables (I leave the extremities of the shield disconnected).
I've also shielded everything in my ampli.
Maybe this is the reason why my ampli doesn't suffer the absence of the output filter.

[motomoto]

Thank you event horizon,

thanks to your suggestions now I understand what is the problem.

May be I am you neighbour...