[sdiy] Converting rack mount to DC input?

cheater cheater cheater00social at gmail.com
Mon Jul 13 17:09:05 CEST 2026 

Someone off-list made some comments, and here's what came out of it:

1. All my devices use balanced audio
2. The rack items actually tie together their grounds by using rack
screws and the rack rail
3. I don't know if the rack devices have isolated audio grounds, but
they're almost fully analogue, except for stuff like LED bar graphs
and on-off status LEDs for functions

On Mon, Jul 13, 2026 at 4:34 PM cheater cheater
<cheater00social at gmail.com> wrote:
>
> So a person on another forum mentioned that converting the devices to
> DC input could create ground loops.
>
> As a reminder, I would be creating a "DC Bus" that carries +18, -18,
> 0V, and +48V, and I would wire it in a daisy-chain configuration going
> from unit to unit. All of those units are in one rack on top of each
> other.
>
> Here's what they said:
>
> > If you're intending to run them all off a single raw supply, that could
> > introduce ground loops. The safest would be a bunch of small
> > transformers, or one with multiple secondaries. Hammond 229 are
> > small low profile transformers that could fit in a 1U cabinet.
>
> So I've been thinking about this since they mentioned ground loops.
> Here's what I think.
>
> Currently all those rack devices already use a three-prong IEC C14
> receptacle, with the earth wire connected to the chassis.
>
> Instead of the chassis grounding point being connected via the C14
> receptacle, and through a mains cable, to mains earth, it would now be
> connected via my DC bus cable, which would have an earth conductor as
> well.
>
> So in terms of ground loops, not much changes.
>
> However, given that we're talking about LOOPS, the area of the ground
> loop would be important as well.
>
> Currently each rack has a cable going to a power strip.
>
> However, with my DC bus, I would be daisy chaining it, so the bus goes
> to the bottom rack mount device, there's a plug there, then from that
> plug there's a short cable going 1U above it, and so on.
>
> That would create an extremely small area for the devices to create
> ground loops.
>
> I could (and maybe should) additionally put an earthed shield around
> these wires, in order to shield them from EMI.
>
> So in total, either nothing changes regarding ground loops, or it's
> purely improved compared to the current situation.
>
> I would be interested in everyone's thoughts on this matter.
>
> Thanks.
>
>
> On Fri, Jul 10, 2026 at 6:03 PM cheater cheater
> <cheater00social at gmail.com> wrote:
> >
> > there's one that's particularly noisy, but there are others that are a
> > little noisy too, and it adds up. plus the heat just makes the room
> > uncomfortable to be in. I've gone to great lengths to move all heat
> > and noise generating equipment out of my day room, including moving
> > the pcs to another place and connecting everything via fiber optics.
> > so having 80W less heating my head will be an improvement once more.
> >
> > On Fri, Jul 10, 2026 at 5:58 PM el macaco <elmacaco at hotmail.com> wrote:
> > >
> > > Is the noisey transformer just one unit?  Would a torroidial transformer have less physical noise?
> > >
> > > Or if it is vibration related maybe mounting it with rubber washers and making the electrical contact with wires?
> > >
> > > Or move the units away from where the mic can pick up the noise?
> > >
> > > Not my area of expertise, just wondering if there may be a simpler solition.
> > >
> > > Ed
> > >
> > >
> > >
> > > Get Outlook for iOS
> > > ________________________________
> > > From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Ben Stuyts via Synth-diy <synth-diy at synth-diy.org>
> > > Sent: Thursday, 09 July 2026 17:39:07
> > > To: cheater cheater <cheater00social at gmail.com>
> > > Cc: synth-diy <synth-diy at synth-diy.org>
> > > Subject: Re: [sdiy] Converting rack mount to DC input?
> > >
> > > You could use both rails, i.e. connect the relay coil between the + and - supply. Not + and gnd. If you want anything more fancy you should build some sort of power-good circuit. But that applies to your homebrew connector solution too.
> > >
> > > Just look at any general purpose or small power relays from e.g. Panasonic, Omron, TE. They usually have an endurance of millions of cycles. A quick search on digikey shows lots of options, starting at a few euro’s.
> > >
> > > Ben
> > >
> > > > On 9 Jul 2026, at 18:23, cheater cheater <cheater00social at gmail.com> wrote:
> > > >
> > > > Thanks. A few questions:
> > > >
> > > > What if only some of the rails are connected, but not the one used for
> > > > steering the relay (a fault)?
> > > >
> > > > Relays are kind of expensive, how much do you think that could end up costing?
> > > >
> > > > If they don't get cycled a lot at all, wouldn't they end up getting stuck?
> > > >
> > > > On Thu, Jul 9, 2026 at 5:57 PM Ben Stuyts <ben at stuyts.nl> wrote:
> > > >>
> > > >> You could use an internal relay for the switch-over. If there is power on the DC connector -> switch over. Otherwise keep the AC rectifier connected.
> > > >>
> > > >> Ben
> > > >>
> > > >>> On 9 Jul 2026, at 16:06, cheater cheater <cheater00social at gmail.com> wrote:
> > > >>>
> > > >>> In order to provide the bridging functionality it has to have at least 8 pins.
> > > >>>
> > > >>> On Thu, Jul 9, 2026 at 1:41 PM Ben Stuyts <ben at stuyts.nl> wrote:
> > > >>>>
> > > >>>> Perhaps a 4 or 5 pin XLR connector is an option? They are not that expensive, and lots of ready-made cables are available.
> > > >>>>
> > > >>>> Ben
> > > >>>>
> > > >>>>> On 9 Jul 2026, at 10:18, cheater cheater via Synth-diy <synth-diy at synth-diy.org> wrote:
> > > >>>>>
> > > >>>>> One way to have DC input, but retain the capability of using the
> > > >>>>> internal power supply, is to have a 4P2T switch for configuration.
> > > >>>>>
> > > >>>>> It seems that 4P2T switches are super expensive, especially if they're
> > > >>>>> supposed to withstand, say, 1A.
> > > >>>>>
> > > >>>>> So I started looking around. TLDR: classic pcie 8-pin power connectors
> > > >>>>> are probably the best, and also the cheapest.
> > > >>>>>
> > > >>>>> My first thought was to look at PCIE x1 slots. The cheapest advanced
> > > >>>>> connector on earth, for 0.4 Euro at unit price, you get a connector
> > > >>>>> that handles 1.1A per pin as per eg this document, page 6, 4.4
> > > >>>>>
> > > >>>>> https://cdn.amphenol-cs.com/media/wysiwyg/files/documentation/gs-12-233.pdf
> > > >>>>>
> > > >>>>>> 4.4 CONTACT CURRENT RATING
> > > >>>>>> 1.1 amp per contact minimum per EIA-364—70, method 2 and PCI Express Connector High Speed Electrical Test Procedure. The temperature rise shall not exceed 30 degree C. Ambient condition is still air at 25°C.
> > > >>>>>
> > > >>>>> The connector has 36 pins, so while I don't think it would handle 36A,
> > > >>>>> I'm sure it would handle something like 2A, especially if you share
> > > >>>>> pins. You can even leave pins empty to prevent shorts during
> > > >>>>> insertion.
> > > >>>>>
> > > >>>>> You could put the connector out the back, and either insert a plug-in
> > > >>>>> "card" (really just a small edge connector with wires soldered on)
> > > >>>>> that provides DC, or a pass-through "card" that shorts some pins
> > > >>>>> together to carry power from the internal power supply. And now
> > > >>>>> instead of $20-30 per unit, this costs $1 per unit. My main question
> > > >>>>> is how I would fix the connector, but maybe a simple screw hole in the
> > > >>>>> connector that mates with a threaded hole in the case could do the
> > > >>>>> trick.
> > > >>>>>
> > > >>>>> A pcie x1 port is 25mm long, so it can fit upright in the back of a 1U
> > > >>>>> rack unit, which is 45mm, so it doesn't take up much space either.
> > > >>>>>
> > > >>>>> The cheapest one that can be found at Mouser currently and can be
> > > >>>>> bought in low volumes is roughly 0.4 Euro.
> > > >>>>>
> > > >>>>> https://www.mouser.at/ProductDetail/Amphenol-FCI/10018783-10200TLF?qs=V%252BXmToedwojeZUI4fPwmPA%3D%3D
> > > >>>>>
> > > >>>>> Qty.  Unit Price  Ext. Price
> > > >>>>> 1     € 0,439     € 0,44
> > > >>>>> 10    € 0,372     € 3,72
> > > >>>>> 25    € 0,332     € 8,30
> > > >>>>> 100   € 0,316     € 31,60
> > > >>>>>
> > > >>>>> By making the connector require a dummy plug to connect the internal
> > > >>>>> power supply into the circuit, it makes it impossible to connect both
> > > >>>>> DC power and mains AC, so that makes the design intrinsically safe
> > > >>>>> without using switches.
> > > >>>>>
> > > >>>>> Alternatively to a PCIE connector I could use some panel mount plug
> > > >>>>> with 8 pins, have 4 pins for DC input, and have the other 4 pins carry
> > > >>>>> power from the internal power supply, and similarly use a shorting
> > > >>>>> plug to use the internal power supply.
> > > >>>>>
> > > >>>>> Circular DIN connectors start at about 8 Euro per pair of socket and plug.
> > > >>>>> Circular metric aren't better.
> > > >>>>> MIL Spec connectors seem to be cheaper. But the mouser search sucks,
> > > >>>>> so I can't find them by number of pins.
> > > >>>>>
> > > >>>>> PCIE 8-pin power connectors are real, real cheap - 10 cents a piece.
> > > >>>>> They're latching (no need for screws and stuff). There doesn't seem to
> > > >>>>> be a panel mount version, but one could mount a through-hole connector
> > > >>>>> to a pcb, and have the pcb have screw holes for mounting to the rear
> > > >>>>> panel. They handle high power - 150W meaning 3A per pin (half the pins
> > > >>>>> are return pins). Seems like a winner to me. They're also tiny so they
> > > >>>>> will easily fit in the back of a 1U unit even vertically. The biggest
> > > >>>>> pain here might be making the rectangular hole and having it look any
> > > >>>>> good. might be a case for custom die, maybe a small steel job that
> > > >>>>> uses two bolts to screw together the two cutting parts through the
> > > >>>>> sheet metal.
> > > >>>>>
> > > >>>>> "Pin and socket" connectors are the same kind of thing, just not
> > > >>>>> specifically PCIE 8-pin. Still cheap at roughly 20 cents a piece...
> > > >>>>> useful alternative if more than 4 rails are necessary.
> > > >>>>>
> > > >>>>> D-sub connectors carry up to 3A... and are very cheap and can do panel
> > > >>>>> mount of some sort. But I'll be damned if I use one of those cursed
> > > >>>>> things for power.
> > > >>>>>
> > > >>>>> Automotive connectors seem inexpensive too, but I haven't really
> > > >>>>> looked into them much other than a quick parametric search.
> > > >>>>>
> > > >>>>> I wonder what everyone thinks of this.
> > > >>>>>
> > > >>>>> On Tue, Jul 7, 2026 at 6:39 AM cheater cheater
> > > >>>>> <cheater00social at gmail.com> wrote:
> > > >>>>>>
> > > >>>>>> Don't you think this may have had something to do with you
> > > >>>>>> distributing AC power, rather than DC with 0V potential?
> > > >>>>>>
> > > >>>>>> On Sat, Jul 4, 2026 at 11:33 PM Mattias Rickardsson <mr at analogue.org> wrote:
> > > >>>>>>>
> > > >>>>>>> Den lör 4 juli 2026 14:23Florian Anwander via Synth-diy <synth-diy at synth-diy.org> skrev:
> > > >>>>>>>>
> > > >>>>>>>> Maybe, it's a stupid thought, but...: could it be that a device relies on beeing galvanical separated from other devices. I this case the common supply might clash with the devices concept.
> > > >>>>>>>
> > > >>>>>>>
> > > >>>>>>> This reminds me of a somewhat similar issue I had many years ago:
> > > >>>>>>>
> > > >>>>>>> I wanted a Korg MS-20 and a Korg KR-55 to share one external power puck. (Swedish-sold old Korgs had a big external power transformer to step down from 220 V AC to 16 V AC, which is then what the instrument then accepts via a plug with 2 flat prongs that doesn't seem roadworthy with European electrical standards.)
> > > >>>>>>>
> > > >>>>>>> So I made a short splitter Y cable on the 16 V AC side and attached both instruments and turned them on. But then I connected an audio cable between them (or audio cables from both units to the same mixer) and I had a blown fuse! It was apparently not okay to connect those two grounds together when running off the same AC supply.
> > > >>>>>>>
> > > >>>>>>> /mr
> > > >>>>>>>
> > > >>>>>>> ________________________________________________________
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> > > >>>>>
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> > > >>>>
> > > >>>
> > > >>
> > > >
> > >
> > >
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