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Noise Filters & ferrite beads....

I am building a preamp based on DACT's attenuator (DACT2) and DACT's buffer CT101. It will also include a DAC (EVAL board) from Analog Devices based on the AD1895 192 kHZ Stereo Asynchronous Sample Rate Converter. For the buffer power supply I use two OMRON S82K-3W single output 24V 130mA power supplies connected in series to generate +/-24V. The output is filtered by a paralel bank of 6 1,000uF 35V capacitors.

The DAC's power supply is also made by Omron but this time I decided on a dual output +/-15V instead of the series connection...

Recently I came accross a 250v-20A noise filter made by TDK on sale at www.allelectronics.com (ZAC2220-11)

To make a long story short: is it worthwhile to insert this noise filter before the switching power supplies? What will be the effects (positive or negative) of doing this?

Also, I've added ferrite beads to the wires connecting the power supplies to the capacitor bank... Don't know if this is going to do any good either... Any comments?

All responses will be greatly appreciated. What started as a small little project is turning into a mountain...
ppereira
twl
3,251 posts
 
In my opinion, placing ferrite beads on signal carrying wires is a bad thing. The musical signal has a varying magnetic field that is present several millimeters outside the wire. Collapsing this field with a ferrite bead or shield will have adverse effects on the musical signal. If you do not have severe RF interference problems, then leave the RF band-aids off the system.
bob_bundus
1,834 posts
 
Agree with Twl. My personal experiences with ferrite bear out the theory above - it definitely causes degradations.

The only way to determine the effects of adding the noise filter is to install it & then listen some more.
htluc
18 posts
 
I was able to clean out a noticeable level of grunge and noize by installing a pair of Ferrite cores on the AC receptacle providing juice to my audio system as well as the receptacle of my refrigerator (surprisingly more effective than the former). Your mileage may vary but I found this method actually performs better than merely wrapping it on AC cords( which I also have done).
bear
141 posts
 
IF the question is about ferrite beads on the output of a switching supply, the answer, is YES.

But beads are too little - you want LARGER inductors. at least a PI filter, and maybe several sections of PI filter...

Do not use ferrite beads on MOST signal carrying wires, in most cases - there are exceptions. Ferrite beads have nil effect in band for the most part, but I still don't prefer them in circuits.

For your switching supplies, I would suggest another stage of voltage regulation AFTER whatever the supply provides. If you end up with voltages 2 volts lower than spec, but far cleaner, lower in noise, that is far better.

Use low drop out types, low noise types for this application.

Watch your grounding.

It is a good idea to put a filter on the input of the switching supplies, as there can be some reflected noise sent back out the lines...

much on the web about DAC design, low noise design, low noise regulation, grounding, bypassing, power supplies, look around! :- )

_-_-bear
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