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Old 13th April 2010, 12:05 AM   #1101
iko is offline iko  Canada
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Funny stuff wrt the reference point (gnd)?
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Old 13th April 2010, 12:35 AM   #1102
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Yes. +/-12.5V. It has been working very well. When I first soldered the 0.1uF between the rails, the negative rail only went up to 9.5VDC and positive rail 11.5VDC. I then replaced the 0.01uF, the negative rail went up to 9VDC and positive rail 11VDC. Put the scope probe on the rails, saw graphs jumping all over the place. Removed the cap, it was measured very quiet. Turn on the music, Wow, very nice.
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Old 13th April 2010, 12:59 AM   #1103
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I can imagine the two regs playing tug-of-war with each other.

But it works OK with 0.1uF from each rail to GND right? Stability for the price of an extra cap. I say it's worth it.

This idea has come up before, I now recall, and it oscillated wildly even in the simulator. I forgot about it.
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Old 13th April 2010, 01:05 AM   #1104
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Yes, the regulator works superbly without rail to rail bypass.

But why the tug-of-war? The rails are supposed to be very quiet DC. Perhaps the active devices don't work as fast as a capacitor?
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Old 13th April 2010, 01:26 AM   #1105
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If I get a good answer I'll get back to you.
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Old 13th April 2010, 07:50 PM   #1106
klewis is offline klewis  United States
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Quote:
Originally Posted by HiFiNutNut View Post
I have a few RF chokes up to 470uH sitting on the shelf that can be used.

But your regulator has already some 160dB plus ripple rejection, why bother?

My LCLC raw supply would have got rid of all high frequency junks.
Bill,

May I ask what values you use for your LCLC? How did you arrive at the values?

Thanks,

Ken
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Old 14th April 2010, 07:57 AM   #1107
cedricb is offline cedricb  Europe
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@ikoflexer: when do you think we are going to see a PCB to buy?
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Old 14th April 2010, 11:31 AM   #1108
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Quote:
Originally Posted by klewis View Post
Bill,

May I ask what values you use for your LCLC? How did you arrive at the values?

Thanks,

Ken
I used two programs: 1) Duncan's PSUII. This program gives you an accurate modelling of the power supply from the Transformer, bridge to the CLR filters. It calculates the RC constants, voltage, current and power dissipation at each stage (hence you work out the power rating of your components) and let you see the rectified and smoothed ripples according to your CLR filters. 2) LTSpice. I modelled the LCLC filter with LTSpice to see how much ripple rejection at various frequencies.

The chokes I used are RF chokes. I consider them to be good. Normal LCLC filters use much larger chokes than the ones I have. But remember the regulators we use here have very high ripple rejections, especially at lower frequencies. Actually, most regulators are very good at rejecting ripples at lower frequencies, only poor at higher frequencies. So my idea is to use capacitor for low frequency smoothing, and RF chokes for high frequency filtering. With smaller chokes, the price is low (I paid no more than $2 each for some very good quality 470uH 3A rated), size is small, easy to fit.

In the schematic below, the resistor is 4.7R. It can be anything from 1R up. Don't use anything less than 0.5R to 1R, because you need the resistance for damping. To high value will drop the voltage too much and produce heat.

As you can see, with this LCLC, I can achieve over 143dB at 20kHz, and 186dB above 1MHz. Of course, in real life, parasitic capacitance will reduce that figure. But it is still VERY VERY good.

By the way, you don't need to use the same values. I happened to have the components with those values. For example, the 3,300uF are the Panasonic FC 35V caps I had on hands.

Click the image to open in full size.

Last edited by HiFiNutNut; 14th April 2010 at 11:34 AM.
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Old 14th April 2010, 12:15 PM   #1109
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Quote:
Originally Posted by cedricb View Post
@ikoflexer: when do you think we are going to see a PCB to buy?
One day not too far from now.
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Old 14th April 2010, 02:47 PM   #1110
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Positive rail pcb layout finished again and done from scratch after the schematic was modified; please have a look and comment. Double layer, 3x2 inches, pads, GND and power traces on both sides, signal traces only on copper side.

You will see the V+out connector is right next to the mosfet, which will interfere with using those tiny wrap-around heat sinks. The idea is to have the distance between M1 drain to the V+out connector as short as possible. A flat heat sink that extends from M2 to M1 will be best for this; another possibility is the case side wall. The mosfets can also be mounted horizontally and attached to the bottom of the case.

There are thick traces where it matters, and a blob for GND. There is no ground in and ground out. The best here is to run a thick wire from the regulator ground to star ground.

The negative layer needs to be done from scratch. Argh

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