My DIY UcD amp from the junk box

[acid_k2]

Originally Posted by stewin

i was wondering how can i adjust the switching frequency in this schematic?

UCD amps are self-oscillating, and the oscillator frequency is not fixed nor stable.
It depends on:
- output LC low-pass filter (L1 and C31 in the schematic you posted): this filter has a cut-off frequency at about 30-35 kHz for a full-range amp, it can be a lower for subwoofer amps.
- feedback RC network (C4, R11, R12) that causes the 180° phase shift needed to keep the oscillator running
and other less important things:
- inductance/load of the connected speaker
- power at the output
- PCB layout
- MOSfets commutation delay
- etc.

For further informations: UcD

As a rule, the feedback RC network keeps the oscillator frequency at about 10 times the cut-off frequency of the output filter, or more, so you can obtain a strong attenuation of the switching frequency on the speaker. A typical UCD amp's oscillator runs from 200kHz to 700kHz, or more.

Slowering frequency too much is generally non a good idea, you have to keep it as high as you can (as high as your UCD amp is running safe and stable), depending on the devices used in the circuit.

[stewin]

Thanks acid it is hard for me to understand the equations and terminologies but i will try to do my homework . just wondering in your schematic why you decided to use one rail vltg and why not use high vltg like +/-80vlts . and can you make a classd amp without the filter cap?

[Th3 uN1Qu3]

He used single rail because that's what he had lying around. That's why he had to use an output capacitor. Normally you would use a split supply so you can do without that capacitor.

Class D without output LC filter is possible, but NOT UcD which relies on the phase shift of the output filter to keep oscillating. And it is only possible at very low power, <10W, so that the amp doesn't put out significant amounts of radio interference. Speaker lead length must also be kept to a minimum, hence this approach is only used for cellphones, laptops, USB powered speakers and the like.

[elevator]

[QUOTE=Th3 uN1Qu3;2753829]
Class D without output LC filter is possible, but NOT UcD which relies on the phase shift of the output filter to keep oscillating. /QUOTE]
Well that's not entirely true, you can create the phase shift separately. The UcD amp I'm building has the two options of feedback from before or after the output LC filter. The design (schematic attached) is based on the Silabs reference design (which only uses pre-filter feedback). Mine is not working yet, admitted, but it will be interesting to compare the two FB methods.

[acid_k2]

I'm not sure a pre-filter feedback is a good idea. Anyway, elevator, you wrote that your amp is not working, what's wrong? Have you tried the classic post-filter FB?

[elevator]

Originally Posted by acid_k2

...elevator, you wrote that your amp is not working, what's wrong?

I'm still building it, that's what's wrong... Non voglio vendere la pelle dell'orso.

Originally Posted by acid_k2

I'm not sure a pre-filter feedback is a good idea. [...] Have you tried the classic post-filter FB?

Well, if you read my post again, I did not say it was a good idea. I just said it is possible, contrary to what Th3 uN1Qu3 affirms. Also, I said that in my amp I am implementing both pre- and post (selectable), for A-B comparisons and tweakings
Cheers,
E

[Th3 uN1Qu3]

Perhaps i didn't make myself clear enough - pre filter feedback IS possible but then it's not UcD. The UcD patent and original appnote are describing a self-oscillating amplifier with post filter feedback.

[elevator]

Originally Posted by Th3 uN1Qu3

The UcD patent and original appnote are describing a self-oscillating amplifier with post filter feedback.

Ok got it. Now would that mean that pre-filter FB does not infringe on the Hypex IP? Sweet...

[Th3 uN1Qu3]

AFAIK no, it doesn't.

[stewin]

hi acid k2 the continuation of my project


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