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 Post subject: Rectifier question
PostPosted: 04 Nov 2020, 21:08 
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OK folks put on your thinking caps. I read about an issue (maybe) with rectifiers that I had not considered before and may not even be an issue. But I wanted some good think on it. Comparing a standard rectifier like a 1N4007 to a faster one like a UF4007, they way they turn off is naturally different. The thought was that the because the faster one turned off faster (duh) that it would be likely to generate a much steeper wave front. That even though it would be only a small voltage it would find its way into the power supply and cause sonic issues.

My thoughts are that possibly this might happen, but a small ceramic or poly cap would easily filter it out. Plus the percentage of crud would vary on the voltage applied. With a high voltage the actual effect would be nil. At low voltages it might be significant. Any thoughts out there.

Good listening
Bruce

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 Post subject: Re: Rectifier question
PostPosted: 04 Nov 2020, 22:31 
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It's really not the time, it's the stored charge. Diodes do not follow the static IV curve under dynamic conditions. When the diode shuts off there is an overshoot due to the stored junction charge. The UF rectifier stores significantly less charge than a typical 1N diode. The shorter Trr of the UF diodes actually generates MORE harmonic spikes. However, the total energy contained in those signals is much less so that the overall effect on the noise floor is lower (the energy is distributed across greater bandwidth).

Since the noise figure of the amplifier is proportional to bandwidth (Fn = K * To * BW). The added noise bandwidth from the UF rectifiers in any particular audio band is smaller. That's why on the spectrum analyzer, band limited to 20kHz, the switching noise from UF rectifiers is lower than the 1N diodes by a handful of dBs. And that can make all the difference.

But if you really want to use solid state rectification, then inserting an inductor in line will kill virtually all the switching hash. In the case of this architecture, the choice between 1N diodes and UF diodes makes much less difference.

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 Post subject: Re: Rectifier question
PostPosted: 05 Nov 2020, 08:57 
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Hi, Thanks. It fits in with my experience, but nice to know why. I don't see or hear any noise in the stuff I build, but was curious about the topic.

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Bruce

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 Post subject: Re: Rectifier question
PostPosted: 14 Nov 2020, 14:37 
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Or use amo beads.

Then you can forget the capacitor suppression.

https://www.digikey.com/catalog/en/part ... ries/60228

NB Amo beads are not ferrite beads they are not interchangeable:
https://www.toshiba-tmat.co.jp/pdf/en/p ... _ab_ds.pdf

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M. Gregg

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 Post subject: Re: Rectifier question
PostPosted: 15 Nov 2020, 05:30 
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This is obvious but worth a mention,

Some people worry about the forward voltage drop across the diode and the effect of it following the sine wave of the AC supply conduction.
The operation point of the diode is not just following the sine wave applied voltage.
And the cut off is a relationship between the applied voltage and stored charge in the PSU.
So you have to remember that the diode only becomes forward biased when the voltage applied to the diode exceeds the voltage stored in the smoothing capacitors.
If the charge time of the capacitor bank is longer than the recovery time needed after a transient before the next transient the supply will SAG.
So its a trade off of smoothing - charge time and transient supply.

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M. Gregg

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 Post subject: Re: Rectifier question
PostPosted: 15 Nov 2020, 10:29 
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Thought I would throw this in just for fun,

If you have a snubbing capacitor across each diode of a full wave bridge.
You do actually have two series (possibly ceramic caps) from the positive of the rectifier to the negative.
This acts like a bypass capacitor on the first electrolytic in the PSU.

So the question would be does using a bypass cap in a PSU effect the "sound" of the equipment.
But lets not go there.
Does the type of rectifier effect the sound, well I think it does but many would disagree. :D

Hexfred is an old example later was schottky is another example.
The sound of the rectifier. :)

Ultra fast and soft recovery but why..

Regarding putting an inductor in the supply to cut the noise is OK but I find it helps to put a HF choke in the other lead of the supply.
You can "tune out" the noise with L/C networks and filters but that's obvious as well, Just worth mentioning.
Trouble is you have to use capacitors in the filters :D

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M. Gregg

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