[sdiy] Powerline LC filters etc.

Tom Bugs admin at bugbrand.co.uk
Wed Feb 29 21:04:04 CET 2012

Thanks for the details, Harry.

I have tried bipolar ones, but they don't tend to have trimmable outputs 
- if I'm going with the scheme of having linear regulation on the 
distros then I need to be able to trim the DC-DC converters up a bit (of 
course, it could be that there's really no need to have the linear regs! 
what do you think?)

I did recently read about two converters locking together - hmm - that's 
one to watch, for sure. (in fact, that *could* have been a problem I 
experienced while trying other converters)

Given the possibility that the BNX blocks are simply ferrites, I am 
thinking to have - i) filtering directly after the converters (maybe LC) 
and ii) on the input of each distro / regulator for a 2nd rolloff. I 
could still implement ferrites on the ground line, but have inductors on 
the V+ and V- lines.
Each module already has ferrite beads, but I'm wondering whether to 
change over to using 1210 50mA polyfuses.
While I'm doing this I'm also considering whether to have a major system 
upgrade and implement clean and dirty ground lines - certainly a wise 
move and hopefully I can do it while keeping backwards compatability.

Emissions tests -- hmm, one of those things I have never really looked 
in to  -- below the radar?

Many thanks for the pointers so far, Tom

On 29/02/2012 16:38, Harry Bissell wrote:
> 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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