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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: Re: Ceramic Power Supply Coupling Caps - Replace?? posted by Audiophile-in-Training on November 13, 2005 at 10:52:19:
Any comments about the Rectifier Bridge mod?
Sorry, I forgot it.
On class B or AB amps, this mod is intended to to avoid damped oscillations between the diode's reversed state capacitance and the inductance of the wiring between the rectifier and the transformer (not the transformer's Ms inductance: at the frequency of interest of those induced oscillations, the xformer is quite a short thanks to the secondary inter-turn capacitance)
This oscillating tank is triggered twice per mains period thanks to the dv/dt built up just after the inverse recovery time of the diode has elapsed
So, to tame this phenomenon,one can use diodes with small inverse capacitance,this way, one expects to push the oscillation high enough in frequency so that it damps quite immedialtely, or diodes with fast inverse recovery time on the 50 or 60Hz mains, these diodes build up an inverse voltage smaller than standard diodes, since less time is let to make this voltage build up although it is not so noticeable: simulations give 0.6V instead of 0.61V for diodes with trr in the ns range on 50 Hz 70V unrectified xformer output or dampen the oscillation via a RC damper paralleled with the diode. I do think that it is the best solution, although it is the most tricky, since you need to adjust the R value to the tanks' Ã LC for best damping.
But you have a class D amplifier... To comply with regulations, it has a PFC at its inputs (PFC= Power Factor Correction). This thing switches current on and off a coil about 250,000 times per second in order to mimick a sinusoidal-drawn current like a purely resistive load would suck. This switched current is smoothed by a LC filter to be seen from the mains as a weakly distorted sinusiodal current.
You bet that, with this 250,000 times per second switching instead of 100 in grandpapa's power supplies, fast recovery diodes are yet used, or RC damping is applied, or a non-linear choke is used... Anyway, a solution has been found, or the amp woulkd never have passed the EMI/RFI certification.
So, tweaking in this area would be useless at best.BTW, that's what is great with class D amplifier, or any switching amplifier in this matter: it obliges the manufacturer's designer to work properly and to be up todate with power switching techniques, EMI/RFI techniques, thermal techniques too etc. It is far more complex than apply recipes 80 years old.
The result, which can be similar in terms of sounding to the best ol'good designs, is way superior in terms of reliability and stability of characteristics with time.
It is also a socially responsible and sustainable way to listen music, when a tube powered class-A amplifier with its several hundred watts of heat that you have to get rid of by means of HVAC with 60% efficiency is a selfish waste of non-renewable energy like a redneck mom's urban SUV is.
Down from the soap box, not PC here...
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Follow Ups
- About the rectifier - Jacques 01:02:55 11/14/05 (0)
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