Small form mixer PSU

[morls]

I've got a little mixer, but no external cable. The power input is marked AC, and it looks like the PSU is set up to take mains voltage (240VAC). Before I try anything there are a couple of things I'd like to check?
The AC input says use supplied cable only, and I'm wondering if this is because there is some kind of AC step-down before the onboard PSU. A couple of reasons for thinking this - the wiring from AC input to ON/OFF switch is very light gauge, and the grounding is also light and goes straight from AC in to PSU board, with no direct chassis connection.

I hoping I can install a proper IEC socket with fuse and wire to the existing switch. My instinct is to ground the IEC socket to chassis as for any other build I've done.

 

[Whoops]

I've got a little mixer, but no external cable.

Not really, it's not the "external cable" you don't have,
what you don't have is the External Power Supply

The power input is marked AC, and it looks like the PSU is set up to take mains voltage (240VAC).

No it doesn't look like the PSU inside the mixer is setup to take 240VAC, it doesn't look anything like that.
The Power input is marked AC but it doesnt say the voltage, specially it doesn't say anywhere that it takes 240VAC.
It just says it takes an AC voltage, and it's pobably 16 times lower than 240VAC

The AC input says use supplied cable only,

No it doesn't say that anywhere,
it says "Use the supplied Power Supply only"

They made it pretty clear that you don't need a cable but the External Power Supply (PSU) that was sold with the unit

I'm wondering if there is some kind of AC step-down before the onboard PSU.

Of course there is,
there's an external PSU missing that probably has a stepdown transformer inside so that it receives 240VAC in the input and outputs around 2x15VAC (or maybe 2x 18VAC) to be connected to the mixer

I hoping I can install a proper IEC socket with fuse and wire to the existing switch.

Your post is frighteningly dangerous.

Think about what can happen when you send 240VAC to the input of those voltage regulators...

Read the datasheets for the LM7815 and LM7915 voltage regulators and read the "maximum input voltage spec"

Study Linear PSU schematics and try to figure out whats the important component that is drawn on the schematic that you don't have in your setup


It would also help us to help you out if you included in your first post the Brand and Model of the mixer

 

[morls]

Well, I wasn't going to go ahead and hook it up to mains, when I say "I'm hoping" I don't mean "I'm going to"

I guess I might as well install a new PSU that can take mains and output the voltages I need.

 

[Bo Deadly]

If there's no transformer inside then it runs on a transformer wall-wart and doing anything other than the same is dangerous because you need the isolation of the transformer. If you have a variac and some a power transformer that is vaguely close or higher (still need the transformer because variacs do not provide isolation) you might use them together to find the right voltage by gradually turning up the variac until the voltages on the supply stabilize.

 

[morls]

Thanks Bo.

 

[morls]

It would also help us to help you out if you included in your first post the Brand and Model of the mixer

Sorry, it's an Australian Monitor ProSeries SM16. Hard to find much information about this, but I suspect it's along the lines of Behringer stuff.

 

[morls]

Actually, I just found a Behringer manual that lists the exact same part number for the power supply.



And the specs of an eBay version of the MXUK2. The output connector of this power supply seems to match the socket on the mixer I have.

 

[Bo Deadly]

Yup. That looks right. 18.5V x 2. Definitely just buy that supply if you can find it.

 

[morls]

Thanks Bo, and thanks Whoops.

 

[Boomerang]

You will not find the MXUK2 or MXEU2. They were discontinued many years ago. However, the current Behringer PSU5 is the same power supply but with a different connector, for use with some of the UB range and some of the current Xenyx range of mixers. Buy one of those. None of the other PSUx power supplies will do. I have stock of them, but I am in the UK. Cut the low voltage plug off the end and fit a standard 3 pin female locking connector that should mate with the male connector that was on your mixer. Those connectors are easy to source, and is what what the old Behringer MX range and also many other small mixers used. The power supply is 16-0-16V, 17.5-0-17.5V or 18-0-18V (depending on how old the one you get is). Both the old MX style connector and the later miniDIN connector are wired with the 0V on the centre pin.

 

[Matt Syson]

The 'specifications' shown for the supply say 2 X 18.5 Volts at 0.5 Amps so that is what you need.
18.5 Volts is uncommon in the 'standard catalogue' voltages but 2 X 18 Volts at 0.5 Amp (or greater) will be fine. 18VA is an unusual 'standard' rating so a 20 or 30 VA version would be suitable and only marginally more expensive.

 

[Whoops]

Sorry, it's an Australian Monitor ProSeries SM16. Hard to find much information about this, but I suspect it's along the lines of Behringer stuff.

is the company still around?
Can you contact them to buy the PSU?

 

[Whoops]

I guess I might as well install a new PSU that can take mains and output the voltages I need.

You don’t need a completely new PSU,
You just need the missing part that seems to be the Transformer only.
A 230VAC primary with 2x 18VAC secondaries Toroid transformer inside an external plastic box will probably do.

but better to buy the original thing

 

[morls]

Thanks Boomerang and Matt!

 

[morls]

You don’t need a completely new PSU,
You just need the missing part that seems to be the Transformer only.
A 230VAC primary with 2x 18VAC secondaries Toroid transformer inside an external plastic box will probably do.

but better to buy the original thing

Thanks Whoops.

 

[morls]

is the company still around?
Can you contact them to buy the PSU?

I’ve tried to track them down but no luck.

 

[Whoops]

Checking the components in the photos you showed us, there's 2x LM7815 with are +15V DC regulators
Theres 1x LM7915 which is a negative - 15V DC regulator.
There's a also a TIP41C which is a transistors.
Theres groups of 4 Diodes and those should be Bridge rectifiers.

I have no idea why does it uses 2x LM7815 regulators and not just one,
but part of that circuit will be the same as this schematic as it's pretty standard

 

[morls]

Thanks Whoops, that explains it really clearly

 

[Matt Syson]

Thanks for drawing that out whoops.
The actual circuit will be more complicated but what you have shown is sufficient. The second 15 Volt positive might be for 'LEDs or bargraph meters or whatever, and there is a TIP42?) which is probably to regulate 48 volts for phantom power after some voltage doubling circuitry which is again pretty common. So, transformer in a box should see you sorted.
Matt S

 

[cyrano]

I’ve tried to track them down but no luck.

Australian Monitor is run by a Dutch marketing outfit. They have no tech staff. Warranty is on an exchange base. No repair. Other brands are American DJ and a few others. They also supply stuff to supermarkets to sell under the supermarket's own brand. They largely failed in the EU market.

Generally, it's even cheaper than Behringer. Not bad, but there's always some weak point.