[sdiy] Ferrite beads or 10ohms resistors in series with +/-15vdc ??

[Harry Bissell]

simple question easy answer

The 10 ohm resistor forms an RC filter with whatever filter or decoupling
capacitance there is. It also introduces a loss, as you noticed. The trade of is bigger
resistances work better at low frequencies (nearer to DC), but they lose more
voltage. BIGGEST issue is that the drop is proportional to current draw of the circuit.

The Ferrite bead is almost "zero" ohms at DC, so there are usually no discernible losses
in voltage. At high frequencies (MHz) they are far superior as they are essentially 'lossy'
inductors.

At audio frequencies, the ferrite beads may or may not be effective. If I was trying to stop
audio from entering or leaving a board, the resistors will probably work better.

The right technique is to use the RC supply filters at the chip or subcircuit level. Maybe
a sensitive preamp chip alone could survive the voltage drop from even a 100 ohm resistor...
and the RC will work muck better at the lower frequencies.

Another good technique is to use onboard regulation right where you need the low noise supply.
Zener diode shunt references can be very effective...

H^) harry



----- Original Message -----
From: Jean-Pierre Desrochers <jpdesroc at oricom.ca>
To: synth-diy at dropmix.xs4all.nl
Sent: Fri, 22 Jan 2010 11:48:03 -0500 (EST)
Subject: [sdiy] Ferrite beads or 10ohms resistors in series with +/-15vdc ??

 
Hi list,

Browsing a lot of DIY module schematics all around the net
I noticed some builders use ferrite beads in series with their bipolar supply lines
coming to the PCboard. Some other use 1/4w 10ohms resistors the same way.
That makes me wonder.. 
I'm on the making of a MIDI2DAC 6 voices polyphonic module using +/-15vdc and a separate 5vdc
supplies for digital stuff. I started using 10ohms resistors in series with my 15vdc lines
but rapidly found that these dropped my actual 15vdc voltage too much..
I'm using 12 X 12bits DAC's here, a douzen of opamps  and don't want to disturb my 1v/oct outputs
because of not stable supply voltages..
So I changed the 2 resistors for 2 ferrite beads instead
and now my 15vdc supply lines are precises but do I get
a good noise barrier still compared to 10ohms resistors ?

Your thoughts welcome.
JP



 



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-- 
Harry Bissell & Nora Abdullah 4eva