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Old 16th July 2008, 11:55 AM   #1
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Default Cracking noise at clipping

I have 2 prototypes of class D audio amplifier: one is selfoscillating, other is carrier based 250khz. When i overdrive with music, at clipping it produces a different distorsion than class AB amplifier, something like cracking sound, very unpleasant.
Inductor at output is T106-2 double stacked, rail +/-90v, 2ohm-8ohm.
I have "seen" the same cracking noise at comercial Behringer PM5000 classD powered mixer.
My question: it is a normal clipping distorsion for class D amp? Something that i cannot remove without a limiter or compressor at input of module?
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Old 16th July 2008, 12:26 PM   #2
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why dont you use a limiter at input programed to not clip?
I saw in jbl commercial amp a limiter at input and a circuitry around integrator that i think is to reduce the gain at a certain level.In fact the good amplifiers have any sort of circuit to avoid clipping.Google lm13600 northon amplifier as a limiter.There are also some optpcoupler+ldr used in this application.
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Old 16th July 2008, 12:32 PM   #3
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Tryiet already this (clamping diodes, compressor, monostable sensing) - some reducing in level, but at high transient (ie - kick drum), the crackle sound is here
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Old 16th July 2008, 12:32 PM   #4
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the sound of all class d without limiter i have tested is a bit strange,but since we we dont need to listen the clipping sound,it is not very important.Which of yours amp sound better,clock or self?
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Old 16th July 2008, 12:43 PM   #5
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i had a similar problem sometime ago and after some research with scope,i saw an interference at audio input caused by emi only at clipping.the problem was in preamplifier ground picking rf from power stage only at clipping.After grounding the potentiometer of gain and changing the way i did the preamp ground ,cracking at clipping desapeared.
Did you see any strange ringing at integrator at clipping?Another thing that helped was snubber at switching node,maybe because the distance between switching node and filter was very big (20cm).
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Old 16th July 2008, 01:01 PM   #6
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Pro audio = overdrive

Carrier based suffer from noise problem, ucd style is superior to S/N. Both suffer from cracking sound at clipping
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Old 16th July 2008, 01:29 PM   #7
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Have you got a 'scope so you can look at the output waveform during clipping? I'm very used to 'ucd-style' amplifier designs and they should all exhibit clean clipping. The oscillation frequency drops drastically just befor clipping, which is audible but it shouldn't give you the sound that you describe. Are you sure some other instability isn't happening at the onset of cliiping?
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Old 16th July 2008, 03:04 PM   #8
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Quote:
Originally posted by Ouroboros
Have you got a 'scope so you can look at the output waveform during clipping? I'm very used to 'ucd-style' amplifier designs and they should all exhibit clean clipping. The oscillation frequency drops drastically just befor clipping, which is audible but it shouldn't give you the sound that you describe. Are you sure some other instability isn't happening at the onset of cliiping?

Where'd you read that? If you hear Fs drop you're either part bat or there's refinement to be had somewhere. It can clip without Fs dropping much at all. "Clean clipping" is considered soft clipping which does not manifest as an audible crackle.

UCD enters and leaves hard clipping cleanly, big difference, it crackles. I am aware of no class d amplifier with inherent soft clipping, this must be achieved by additional measures.
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Old 16th July 2008, 03:36 PM   #9
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Exactly at the onset of clipping a UCD amps oscillation frequency drops way down, but the clipping is 'hard' and clean, with no 'hangover' effects as it comes out of clipping. I'm talking here about ucd designs based on a modified form of the old Philips ucd app-note, of which I've now tested getting on for a hundred units, and not the more sophisticated Hypex versions.
Due to old age and inherently poor hearing, my upper frequency limit is now around 10kHz so all I hear is a normal clipping effect, but my younger work colleagues all claim to be able to hear the effect of the drop in switching frequency (dropping into the 10 to 20kHz region) just before true clipping starts. I did not say that ucd amps have soft clipping, merely that the transition in and out of clipping is little different to that of a conventional linear amp, apart from the (slight) effect of the switching frequency drop.
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Old 16th July 2008, 04:09 PM   #10
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Seems to me that carrier can allias the overdrive high frequency products into audio range.
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