[sdiy] Powerline LC filters etc.

Wed Feb 29 17:38:11 CET 2012

Hi Tom

Why are you looking at two DC-DC converters for a bipolar supply and not one with a bipolar output (cost?)

There are potential troubles with two converters, especially if they are the same model.  The switchers may run at'very close to the same frequency, but not exactly. There can be a resulting beat frequency which may be converted to some more offensive frequency by non-linear circuit
elements (essentially, everything in a synth :^).  These new frequency components can be MUCH harder to isolate and filter, and may change
with load etc...  Some switchers include a sync pin so that multiple units can be slaved to the same frequency.

The DC filters are usually better for high frequencies, ie to meets emissions standards.  They need to be chosen with a DC current in mind... if the
current is too big or too small they lose effectivness. This is more a problem for modular where the loads are not fixed.

I'd consider putting the second set of filters on each module, so you can fine tune them individually.

Watch out for the trouble I pointed out earlier... separate ground impedances mean different DC voltage drops, you may create ground loops by
connecting various modules together.

I think your point of 'circuit design' issues is quite good. If you avoid places for the modules to interact by using proper references and
decoupling, you avoid a lot of trouble up front. I use simple zener diode regulators on all my modular front panel pots... to eliminate that
issue. Cheap insurance :^)

H^) harry

----- Original Message -----
From: Tom Bugs <admin at bugbrand.co.uk>
To: 'Synth-DIY DIY' <Synth-diy at dropmix.xs4all.nl>
Sent: Wed, 29 Feb 2012 11:00:23 -0500 (EST)
Subject: Re: [sdiy] Powerline LC filters etc.

Cheers Harry,

Sorry - lack of clarity due to lack of clarity in my mind.
I was referring to DC power systems.

I'm looking at these areas for two ongoing projects -- first power for 
standalone processing boxes and then also for investigations into DC-DC 
converter based power for the modular.
The troubles I've been coming across may be down to power-line noise or 
(perhaps more likely) due to other fundamental flaws in my 
quasi-engineering approaches////

For the standalones - 9v DC operated boxes powered by wall-warts - now 
that many wall-warts are switchers.
- but I have a feeling that audio noise I was experiencing is actually 
down to my circuit approach rather than due to power-line born noise.
(Douglas Self's book 'Small Signal Audio Design' is useful for these 
areas - very good readability too)

For the modular - I've been trying DC-wall-wart to DC-DC converters to 
local LDO V-regs - the recent discussion on SMPSs gave me some pointers 
for lowering switching noise, but I still feel a bit out of my depth. My 
first build was something of a failure, but that may have been down to 
the choice of DC-DC converter - I have changed type and this seems to 
have improved things greatly. Again, problems may be down to my circuit 
approaches - I was finding that modules would pick-up other module 
signals when un-connected - something I'd never had with linear 
supplies. (at least now I'm learning to put local voltage references in 
for critical parts)

Here's a quick diagram of the approach I'm trying - do let me know if it 
is full of foolish holes...
http://bugbrand.co.uk/images/PowerPlan.jpg - first half is a module with 
DC-DC converters which then has output headers to go to distro boards 
which each have LDO Linear VRegs
- external wall-wart supplying 24V DC
- ? input DC filter ? Murata BNX016-01
- two 20W DC-DC converters (Cincon) with trims to take the output 
voltage up to c.15.5V
- output DC filters - note that the negative side one is connected the 
other way around as detailed in the Murata datasheet
- system ground banana connection comes post DC filters
- headers to take the +/- 15.5V to the distros

- distros then have a pair of LDO Linear V-Regs (a positive and a 
negative type) to regulate down to +/-15V

Does this sound like a sound approach? I'm keen to try to make DC-DC 
converters work, but have had my doubts so far.. They would bring many 
benefits (compactness, worldwide voltages, no mains worries, etc) but 
obviously I don't want to be opening a can of worms.

Cheers!
Tom

On 29/02/2012 14:33, Harry Bissell wrote:
> I'm not sure I understood the original question. Are we talking about AC Power Line Filters,
> or common mode filters used on DC power system rails ?
>
> In either case, the usual use for these filters is to reduce the passage of EMI (RF) signals in or out
> of the system. Usually this protection exists for frequencies from the 10MHz and higher range, its rare
> to get down as low as 1MHz with any efficiency.
>
> If I got the original point, the use of an inductor (as part of a common mode choke) in the ground has
> several purposes. In the DC case, the return current to the supply cancels out the current from the supply,
> resulting in a nearly zero flux in the coil (so it will not saturate easily).
>
> This does have the downside that there is impedance in the ground line... OK if the circuit is an "island"
> but maybe not so good if the circuit needs to be connected to other items in use (such as other modules, effects,
> amplifiers etc).
>
> Its a very complex issue... what is good for one unit may totally screw up the rest of a larger system.
>
> Tom... what is the filter supposed to do for you ?  (then we can dive deeper :^)
>
>
> ----- Original Message -----
> From: Jeff Brown<guitaricon at verizon.net>
> To: 'Tom Bugs'<admin at bugbrand.co.uk>, 'Synth-DIY DIY'<Synth-diy at dropmix.xs4all.nl>
> Sent: Tue, 28 Feb 2012 20:15:01 -0500 (EST)
> Subject: Re: [sdiy] Powerline LC filters etc.
>
> Try searching for "power line filter".  One that I found:
>
> http://www.ecoca.ro/papers/09_ARtim991.pdf
>
> shows a basic LC filter on the 3rd page.  You can also just buy these - even
> ones built into a chassis mount female receptacle.
> They're also called "EMI filters".  Most medical electronic equipment that
> is mains powered has them.
>
> -Jeff
>
> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Tom Bugs
> Sent: Tuesday, February 28, 2012 5:40 AM
> To: Synth-DIY DIY
> Subject: [sdiy] Powerline LC filters etc.
>
> I would like no learn more about using LC filters for power-line noise
> suppression. On this front, therefore, a couple of questions::
>
> - can anyone recommend a good source of info - a book or a particular
> site - google has got me a bit confused/overloaded
> - I've been looking at the Murata BNX devices -
> http://www.mouser.com/catalog/specsheets/Murata%20BNX%20Series.pdf --if
> you look at the BNX002-01, you see a schematic diagram (similar is used
> on several other devices).
> I am curious about i) having an inductor in the ground line and ii) how
> you may calculate suitable values for the caps&  inductors (as I say,
> linkage to a book/site with info on these areas would be fantastic too).
> Of course, these devices seem to be designed with some specific areas of
> importance (http://www.rapidonline.com/pdf/26-6010e.pdf mentions
> everything being aligned in the same direction) so perhaps it is best to
> use these parts 'off the shelf' rather than trying to DIY - but, still,
> I would love to understand better.
>
> Cheers for any pointers,
> Tom
>
>

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