Monolith by Monoprice M-12 V2 Plate Amp Teardown?

[tpd]

Bit of a long-shot DIY question here, but has anyone seen a teardown of the plate amp side of the M-10 or M12 THX subwoofers from Monoprice?

Monoprice is blowing out the M-12 UK/EU (240V) model right now. Sorry UK/EU folks: they're only shipping these to the US and Canada where they are not directly compatible with most household power. (Link: https://www.monoprice.com/product?p_id=42849)

I have a strong suspicion that the plate amp itself is a common part worldwide (and I believe shared with the 10-inch model) and runs on a standard DC voltage.

The problem here is the power input itself. Presumably this is the EU/UK-specific part. It's possible the transformer has taps for both 120/240 input. But maybe it's a market-specific transformer and you'd have to find a replacement. Not impossible for someone who wants a nice sub for a little extra work. It's a tempting project at this price if the problem is just getting the right voltage from the wall to the amp.

Erin's testing suggested these are really nice subs that go legitimately low. I have the 10" model in my main audio system and it does not disappoint. https://www.audiosciencereview.com/...-12-and-15-inch-subwoofer-review.17335/page-3

 

[arichman]

 

[four_strings_good]

I don't get this. I'm in Canada, and my clothes dryer and electric range were both made in the USA. Both of these appliances run on 240V, 60 Hz power. Any competent electrician can convert a *dedicated* 120V line to 240V. When the 240V outlet is in place in the listening room, the only other thing that's needed is a line cord with a 15-amp IEC connector on the equipment end and a NEMA 6-15P plug on the wall end. Why would Monoprice state that this version will not work in the United States?

 

[tpd]

@arichman you are the greatest. Thank you

 

[tpd]

I don't get this. I'm in Canada, and my clothes dryer and electric range were both made in the USA. Both of these appliances run on 240V, 60 Hz power. Any competent electrician can convert a *dedicated* 120V line to 240V. When the 240V outlet is in place in the listening room, the only other thing that's needed is a line cord with a 15-amp IEC connector on the equipment end and a NEMA 6-15P plug on the wall end. Why would Monoprice state that this version will not work in the United States?

You're not wrong, but most people don't have 240 readily available in their listening room nor a dedicated line to convert. I don't. And I don't need a sub in the laundry room. But yes this could be a viable solution.

Looks to me like the power input portion of the amp might actually have 120 and 240 wiring options, inferring from the sticker right above the transformer. That could be a super easy swap.

 

[EERecordist]

I wonder what the blue jumper does just to the right of the fuse? A step up transformer of the appropriate Wattage would be about $100. I don't need another subwoofer, especially one this large and heavy, but this is tempting. It is possible with value engineering they put a transformer in there with a lower primary Amperage rating.

 

[arrow34]

Can you use a stepup transformer to make this work in the short term until someone can find a way to 1.rewire 2. replace the plate amp?

 

[MatthewK]

I found this image in a listing on ebay.co.uk, it's the M-10, not the M-12. It seems they are probably the same plate amp, just with different firmware. You can clearly see that it says "FUSE: 5x20mm T3.15L 250VAC 230VAC/50Hz". Would it still work with a step-up transformer (2000W?) that doesn't change the hertz to 50? I'm getting mixed information, but from what I gather it should be fine (60Hz → 50Hz), just not the other way around (50Hz → 60Hz).

 

[four_strings_good]

There is some great and relevant information here. The entire article is worthwhile, but chapters 2.1 and 3 are especially useful in relation to this discussion.

Big hat tip to Rod Elliott!

 

[MatthewK]

I wonder what the blue jumper does just to the right of the fuse? A step up transformer of the appropriate Wattage would be about $100. I don't need another subwoofer, especially one this large and heavy, but this is tempting. It is possible with value engineering they put a transformer in there with a lower primary Amperage rating.

Here's what GPT-5.5 xhigh has to say after feeding it images and letting it do web searches:

About the blue-wire jumper: I’d treat it as a factory-installed primary-side mains link, very likely related to the 120 V / 230 V input configuration or the mains rectifier/front-end wiring.

The reason is its location:

• It is right beside the mains input wiring.
• It is near the EMI filter choke — the green/copper-wound part.
• It is in the area feeding the SMPS primary/bulk capacitors.
• It is not near the low-level audio or DSP/control circuitry.

In many switching power supplies, the same PCB can be built for either:

• 120 V operation, using a voltage-doubler-style rectifier arrangement, or
• 230 V operation, using a normal bridge-rectifier arrangement.

A wire link or jumper in that area can configure which arrangement is used. Your amp’s external label and fuse value strongly suggest the board is factory-configured for 230 V.

Important caveat: the blue wire may also simply be a high-current insulated PCB jumper used to route mains/primary current across the board, not a user-selectable voltage jumper. Either way, it is almost certainly on the dangerous primary side of the supply.

So I would not move, remove, or reposition it unless you have the schematic or can trace the PCB confidently. It is not safe to assume “blue jumper = convert to US voltage.” If misconfigured, a 120/230 V SMPS input can fail violently, especially if a voltage-doubler link is wrong.

If you want to verify its exact role, the key thing to check — with the unit unplugged and capacitors safely discharged — is whether one side of that blue jumper connects to the midpoint between the two high-voltage primary capacitors. If it does, it is very likely part of the 115/230 V selector or voltage-doubler configuration. If it just bridges two traces in the mains input path, it is probably only a factory routing/current jumper.

Bottom line: your plate amp is labeled as a 230 V amp, and that blue jumper should be considered part of the factory mains-side configuration, not a user conversion jumper.

 

[arrow34]

Well for science, i am ordering one and will see if a step up transformer will work until i can find a good plate amp.

 

[tpd]

I don't think you will need a new plate amp. I suspect the power supply in the existing plate amp could be reconfigured for 120V use. Or a step up transformer.

Let us know what you learn. Better images of the plate amp would be a huge help for folks who are interested in this project.

 

[arrow34]

I will take some pics when I get it. I have a 18yr old acoustech h100 that has served me well. I just feel the need for a newer more capable sub.

 

[jimroge]

Well for science, i am ordering one and will see if a step up transformer will work until i can find a good plate amp.

Please let us know if it works and the brand and model of step up transformer. I am in the same boat but ordered by accident when too tired to notice it was for UK/EU.

 

[FrantzM]

I am interested. It is as someone remarked earlier, very easy to bring a 240 Vac outlet. Heck, the split AC is wired 240 in my listening room, but ee use 60 Hz. Would that be an issue? Asking those who know

 

[arichman]

No expert here, but that appears not to be a problem since 50 hz transformers are apparently heavier duty and should run cooler at 60 hz. While the wikipedia article cited above flags 240v as a potential issue given the European standard is 230v 50 hz, the model on sale is listed as for the UK/EU, the UK used to have a 240v standard and the older power generation equipment is still within spec and in use. Apparently these devices are designed anticipating a fair degree of deviation, and 240v is clearly within the range of expectation for this market.

 

[roci_big_ear]

Just ordered 2. I originally thought of reworking them, but thinking about it I'll just bring in 240V through the wall. Don't want to be limited by the 110V 15A room circuit.

 

[defec1]

Isn't 240v in the USA 60hz but in Europe it's 50hz? It lists the input at 50hz on the plate amp. It may matter.

 

[arichman]

Isn't 240v in the USA 60hz but in Europe it's 50hz? It lists the input at 50hz on the plate amp. It may matter.

Since hifi components are fundamentally DC powered, the frequency of the AC current coming out of the wall socket is not important. There is a transformer in the component designed to extract the DC power needed. However, the transformer can only do that efficiently if it is designed for the frequency and voltage of the power coming from the wall socket. For technical reasons, 50 hz transformers are larger and heavier than 60 hz transformers. If a 60 hz transformer is fed 50 hz power of the same voltage, it will tend to overheat. However, the 50 hz transformer fed 60 hz power will not, although it will be somewhat inefficient. For a manufacturer selling their wares in different markets with different AC power requirements (50 hz in EU vs 60 hz in the US), they can make different transformers for each frequency, or just put the 50 hz version in all of them because that is cheaper and will work (per Paul McGowan, PS Audio). So both theory and apparently practice suggest this is not a critical concern in the present case.

 

[roci_big_ear]

Since this is a switched mode power supply, the mains frequency mostly affects the PFC capacitor sizing, not the transformer, which operates at kHz range already. 50 Hz would need a bigger cap to ride through the valleys compared to 60 Hz.