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sharpi31 27th January 2007 07:34 PM

ultrasonic noise problem side-stepped?
The one major issue with non-oversampling dacs (as I understand it) is the large amount of ultrasonic noise produced.

But if a nos-dac is used with class-d amplifiers and full range drive units, this shouldn't be a problem.

Firstly, all class-d amps have a capacitance across the output to remove the switching frequencies. This is often about 0.1uF, so surely this capacitance will remove both the amplifiers switching frequency and also the ultrasonic content coming from the dac.

Similarly, the rising impedance with frequancy of full range drivers (i have fe207e) due to voice coil inductance will prevent any ultrasonic noise getting through the speaker.

Please let me know if I'm missing something here. It seems to me that the major short-coming of nos-dacs is removed if these dacs are used with class-d amps and full range drivers.

Just an idea.... Let me know what you think :-)

georgehifi 27th January 2007 08:03 PM

The filtering your talking about in your fullrange driver and the .1uf on your class D amp are 1st order (-6db per octave) the kind of attenuation you need for the noise in cd and da convertors is more like 4th order -24db or more that quickly cut above 20k and not to effect the signal at 20k too much. The only way a first order filter would have an effect on this is to start to cut at 10k then you'll be left with no highs from your source.

Cheers George

sharpi31 27th January 2007 08:18 PM

That makes sense.

If both class-d and full range drivers follow the nos-dac, and both roll-off at 6db above 20KHz (for the sake of argument) does this become 12db per octave filtering?

Agreed this isn't as effective as 24db, but is at least a partial help at binning those unwanted ultrasonics.

georgehifi 27th January 2007 08:27 PM

Yes two 6's will make a 12 if they are at the same frequency which is very unlikely, but you will find that it still won't have an effect just above 20k, were it all should happen very steeply.
Cheers George

sharpi31 27th January 2007 08:37 PM

Thanks George :-)

Duo 27th January 2007 08:42 PM

You simply need better DAC output filters if this is actually a problem in the first place.

martin clark 27th January 2007 10:02 PM

No, it makes NO sense.

The problem is not with risking 'hearing' the HF noise from a NOS dac, but the way in which it can upset all the amplification stages that follow. This may be exhibited in intermod distortions, odd DC offests, full-blown oscillation or just...the sense on audition that all is not well.

Duo is right. By feeding a NOS Dac into a Class D amp you are still expecting the Class D amps' front-end to cope OK with the HF noise these dacs produce. That is not a given... all the class D amps I've seen so far still us a linear buffer stage before the modulator, and you just as at the mercy of the power amps' designer as with any other amp chain... given many of these things are sold on being cheap as any actual audio merit (and so use slow opamps here) it's a lottery. Sorry.

georgehifi 27th January 2007 10:23 PM

Yes this is why it must be done heavily and as steep as possible at 20k at the source, to avoid sending any amp/preamp into oscillation. It cannot be done at the end or the middle of the hifi chain, just asking for trouble with anything at is unstable especialy switching/D/T class amps or wide bandwidth linear a class for that matter.

Cheers George

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