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uPC1237 Amplifier/Speaker Protection Module

uPC1237 is a well known IC used for protecting the speakers form DC as well as amplifiers from over current. Almost any Sony amplifier starting from the lower range and right up to the higher-end ES series are using this chip.

So I thought was a good idea to make a stand alone module which could be used by the DIY community.

To start of, here is the datasheet of the uPC1237:

One important aspect is that this IC could be used to prevent the output devices from over current. This happens when a load too low is introduced at the amplifier’s output or a short circuit is present. Most power amplifiers have a dedicated circuit built in them, to monitor the current flow trough the power transistors. This part of the schematic is forcing the current to stay within the recommended SOA of the devices.

It is a common situation where in case of a short circuit at the output, the output transistors get damaged “to save the fuses” which is absolutely unwanted fact.

Using uPC1237 for over current protection allows the designer to drop the SOA limiting elements in the power amplifier. This will save space on the PCB, lower the cost of the project and increase the reliability.

Due to some patent issues one can not find a recommended schematic to do the current limiting job in the uPC1237’s datasheet. There is no built-in circuit either. However all you need is to monitor the current flow trough one of the emitter resistors at the output of the power amplifier. According to the datasheet pin 1 must be driven above 0.67V. This is the threshold. Rising the voltage above 0.67V will trigger the relay opened. Using a current source will require the pin 1 driven with current above 110uA.

This is what I use in my amp to do the job:


Now the schematic of the module itself. It can be downloaded HERE


A few words about the PCB. Now I wanted to have a rather non standard approach here. I needed a module which could be mounted directly on the rear panel and bolted through the speaker terminals. I’ve seen this type of modules around the eBay. However those were using small current relays not suitable for more powerful amplifiers. For my project I wanted to have a good quality DPST relay with at least 10A rated current. Those can be quite bulky so the PCB is quite large. However, reliability is more important than the cost of the PCB. The devices protected by this module can be quite expensive so no cheap tricks here.

Here is the relay – OMRON – G4W PCB Power Relay

Those are available for purchase from Farnell in Europe:

This is what the PCB looks like:


The relay is located between the mounting pads to keep the paths equal and at the same length. The mounting pads are of 15mm diameter. The mounting holes are at 5.5mm. Those can be drilled to a larger diameter if necessary. My speaker terminals have 5mm threads and fit just fine.

A PDF file of the PCB is available for download here:

PLEASE NOTE – this project is provided to the DIY community for free. It’s free for personal use. Any commercial usage is prohibited.



Universal I2S interface. Line + cable


Tubes: 12B4 Single Stage Preamplifier.


  1. Anonymous

    Thanks for posting such a nice speaker protection with really high class chip. But can you write all resistor values? Because not all of them are written in he schematic?
    Thanks a lot.

  2. Vencislav Simonov

    some of the resistor values depend on the supply voltage. All the values can be calculated using the charts published in the chip’s datasheet.


  3. Andrew

    Hello Vencislav
    greetings please is it possible to give the resistor values for 95 VOLT
    dc rails +/- so i can try the short circuit protector i am just a hobbyist so my electronic knowledge is limited
    hope you can help me please
    warm regards

    • Hi Andrew,
      the IC can not work at this power supply. The maximum allowed is 60VDC. There is a solution around this however, if you can provide a low power auxiliary voltage of about 18VAC.

      Regards, Venci.

  4. Andrew

    Hello Ventsislav
    greetings very kind of you for replying i can provide the low power auxiliary 18 volt ac
    for a long time i have been searching for a short circuit protection schematic little help
    from you and i can make it from schematic i can design the pcb please schematic if you can share
    warm regards

  5. Andrew

    Hello Ventsislav
    greetings please can you share schematic even if low voltage one
    warm regards

  6. Kunalagon

    Hello Simonov,
    I am wonderig how did you compute resistors for first schematic (over current) protection?

  7. Kunalagon

    It seems something with overdrive protection does not work. I have ~0.7V at pin 1 and about ~112uA measured and relay does not goes OFF. I am wondering if this works for you Ventsislav?

  8. Дим

    Здравствуйте! К сожалению ссылка на печатную плату не работает. Можете на почту выслать файл? Спасибо

    • admin

      Ссылки обновлены.

      Спасибо 🙂

  9. Geoff Conway

    Hi Ventsislav,
    I am very interested in this particular design – it is one of the few that has considered mounting and using quality parts for higher powered amplifiers.
    Do you know if this PCB is available for sale anywhere – or has been produced anywhere previously that I would be able to obtain further boards from ?

    Thanks for your help.

  10. isaac

    I need help

  11. Mischa Gielen

    Hi Ventsislav,

    Great article about a very usefull item which no one ever hopes to use. 😉
    Do you happen to have pcb’s available for sale?

    Best Regards,

  12. Carlos Martinez

    The relay is no longer manufractured.

  13. gash

    “It is a common situation where in case of a short circuit at the output, the output transistors get damaged “to save the fuses” which is absolutely unwanted fact.” It really pisses me off to see expensive 5200s and 1943s exploding due to a short in the speaker outputs, leaving the fuses and 0.33 ohm droppers intact, Puh! I hope this “magical” IC can protect both the speakers AND amplifiers.

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