3-Phase PSU for Class A amp? - diyAudio
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Old 12th February 2001, 09:21 PM   #1
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Hi all,

I've just found this site and I'm considering several class A circuits for possible construction/experimentation.

Power supplies seem to be quite a problem for this configuration so I thought I'd start by building a bench supply.

These may be a stupid questions, but would a 3-phase (or more) supply, with each phase feeding the supply caps via it's own rectifier reduce the ripple a bit or just make it far more audible than the original 50/60Hz? AC motors use hefty caps to simulate a second phase from a single phase supply, would this work with torroidal transfomers? Has anyone tried this without being showered in little bits of foil?

Thank's for reading my first posting here,

Graham
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Old 13th February 2001, 06:38 AM   #2
grataku is offline grataku  United States
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This is what I remember about triphase systems, please check on some ee texbook so what I am saying may very well contain major ******** (I took classes on the subject 18 years ago and never used it since). To get the reduced ripple you need a tri-phase line, transformer and rectifier. When you are done you should get one DC line output. I think the phases on the AC line are well...out of phase by some multiple of 360 deg (either 120 or 60) so the ripple frequency is increased but it's intensity is dramatically reduced by some factor like 3 or sqrt(3) with respect to a mono-phase rectifier. It nearly looks like DC even without filtering caps.
I think in order to work this way the system as to be a pure tri-phase system so trick with capacitors to multiply phases won't do what you want, I don't think.
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Old 13th February 2001, 08:08 AM   #3
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Hi there ;-)
It looks like You haven`t forgotten, grataku.
I`ve learned, just as a rule of thumb, that a rectified 3-phase supply, even w/o capacitors, only gives @10% of ripple, so with some big capacitors, ripple vill be a lot less audible.
But, as grataku wrote, no fooling around with simulation caps.
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Old 13th February 2001, 08:26 AM   #4
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Thank you for your reply Grataku,

I think this is worth further investigation as the higher ripple frequency should be easier to filter out anyway.

Besides rotary converters, there must be some elegant three phase conversion circuits out there, if anyone knows of any please let me know.

I'll try creating three phases from a single transformer secondary first as this would save the expense of extra transformers. Here in the UK 55-0-55v 1000va transformers are reletively cheap though, packed in yellow plastic tubs as isolation transformers for power tools.

I'll let you know how it goes.
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Old 13th February 2001, 08:38 AM   #5
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Sorry Hoffmeyer,

You posted as I was writing my reply to Grataku,

I'd be surprised if simulation caps did work, it seems too easy, the values and power ratings needed could be quite hard to find (especially on a 50hz/240vrms supply) and the distorted waveforms generated could cause no-end of trouble.

I'm still intrigued though.
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Old 13th February 2001, 04:49 PM   #6
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Default You need 3 transformers or a 3ph transformer

You need three transformers. If you really want to do this well, why don't you rectify the mains and convert it into 400Hz and get some surplus aviation transformers (which run at 400Hz)?

3 phase is good, but the only advantage you get is a higher frequency (since your ripple is low) output which results in less ripple.

Typically 3 phase lines are also noisy since industrial equipment runs on it.

Do something radical and make a very good switched mode power supply. That would surely benefit this group as everyone could use it.
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Old 13th February 2001, 07:46 PM   #7
grataku is offline grataku  United States
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Petter
how do you propose to do ac remodulation to 400 hz without introducing a bunch of extra noise?
Personally, unless I had other reasons to have a 3-phase line to my house I wouldn't mess with it. Just use a nice pi filter or a cascade of them in your AMP power supply and rewire your house with a big dedicated line from your braker directly to the stereo outlet to try and keep noises form your household appliances, and resistive voltage losses to a minimum.
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Old 13th February 2001, 08:24 PM   #8
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Yes, the ripple frequency is increased, and this reduces the ripple due to the decrease in time between charging pulses to the capacitor.

The rectifier conducts only briefly at the tranformer's voltage peaks. The ripple is defined by

i = C dV/dt

where i is the peak DC load current, C is the total filter capacitance, dV is the ripple voltage, and dt is the time between the charging peaks. This is a simplification, but represents 90% or so of the ripple. The finite time to charge the capacitor through the dynamic impedance of the rectifier is a secondary effect.

In effect, the value of dt (delta time) is divided by three when using 3 phase, which changes delta V by the same factor.

All that said, transformers are more expensive than capacitors. It would be cheaper to triple the capacitance than to go to 3 transformers.
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Old 13th February 2001, 10:28 PM   #9
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A solution I have seen in a commercial high end power supply uses 3 transformers with double secondaries (central tap). I think they should be rated at one third the current of a normal power supply since the three currents add up.
What you do now.
Each phase goes to each transformer´s primary.
The neutral goes to all the transformers other free primary cable.
Each now transformer has it´s own rectifier bridge.
Center tap to ground and each output to bridges ac input.
So we have so far 3 identicle power supplies like we know them with + and - rails.
We now parallel all the + outputs from the bridges and all the - outputs.
We use smaller caps for each voltage and thats it.
For the RF noise we use snubbing caps around 100 nF parallel to each diode of the bridges (better if directly on the bridge).
Also we use a X type cap wired between live and neutral as close as possible to the inlet. 100nF - 470 nF.
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Old 13th February 2001, 10:55 PM   #10
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mmmmm (scratches head after reading all your posts)

Here's an idea that probably won't work but it's a starting point for this experiment. Please rip it apart, especially if any of you lucky people have circuit simulation software to plug a few values into.

Click the image to open in full size.

Thanks for reading.
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