Hifisonix Speaker Protection Board

You can buy a Hifisonix Speaker Protection Board over in the Shop.

This simple, low cost speaker protection board uses the ubiquitous UPC1237 from Unisonic, a Taiwan based semiconductor manufacturer. The circuit will work from 25V through to 75V and uses mosfets to switch the speaker load rather than relays which are problematic and unreliable. A current overload input is also provided, allowing builders to arrange to sense an amplifier output short circuit and rapidly disconnect the amplifier output. The presentation below describes the operation and how to select resistor values for different supply voltages (updated April 2023)

Here is the BOM


Here is a short presentation on the use of mosfets for speaker switching in amplifiers


30 responses to “Hifisonix Speaker Protection Board”

  1. Andrej says:


    When this DC protection will be available to purchase?

    best regards,


  2. Andrej says:


    Any alternative to currently not available opto VOM1271T?



  3. Gregor says:

    Can I use Vishay SUP40012EL-GE3 mosfet.

    • Bonsai says:

      Gregor, this is a very high current mosfet with high input capacitance (c. 10nF). As such, it will switch slowly. I see its only rated at 40V Vds.

      My recommendation is you go for something with lower input capacitance – a good figure to aim for is 2-3nF . The Vds rating must be c. 10% higher than the amplifier supply voltage – so for example for a +-50V powered amplifier, use a 63V part.

  4. Ranko says:

    Is it possible to use different optocoupler, MOC3010, for example, because both of those you suggest are not available in my country and if I order them from abroad, I will have to wait at least one month?

    • Bonsai says:

      Hello Ranko, what are you building? I assume it is for the nx-Amplifier if so you must use a random phase triac on the amplifier module boards so the MOC3010 should work ok but I have not checked the pinout.

  5. David says:

    Would you consider designing a variant incorporating configurable V- I limiters, with a table of component values for the most popular BJT output devices? I assume you would use the emitter resistors of the output devices to measure the current?

    • Bonsai says:

      Hello David,

      This is not something I’ve looked at but may be one for the future (I’m tied up finishing off the new preamp at the moment).


  6. David says:

    Thanks for responding

  7. Tl says:

    Hi, I can’t seem to access the BoM for these boards, but I am interested in purchasing some. Are you able to check if the link still works?

    • Bonsai says:

      Hi – the link is working for me – let me know if you still can’t download it and I’ll email it to you


  8. Andrej says:


    It is possible to use this protection with Hypex SMPS3kA400 power supply? Voltages available are:

    2x65Vdc main PSU

    2x20Vdc boostrap PSU

    2x12Vdc regulated AUX PSU

    Problem is that all three voltages start at sime time. I could use both main or bootstrap voltages, but not sure how to connect AC input in this case, to have proper start up/off delay.

    Best regards,


    • Bonsai says:

      Andrej, are all these voltages from the Hypex PSU, or are any derived from standard transformers supplies? If you have a standard transformer supply somewhere, you can just tap the AC off that.

  9. Andrej says:


    Would this protection board work also with AB class amplifier, powered by Hypex SMPS. Main voltages are +-65 Vdc. I have an option to use also separate AUX +12 Vdc voltages. Idea is to power protection board with +-65Vdc, but not sure what put on AC input terminal. Maybe omit D2 and connect +65Vdc directly? +12Vdc is probably to low voltages…

    Best regards,


    • Bonsai says:


      The problem here is the AC input uses the loss of 50/60 Hz to quickly shut the mosfet relay off to prevent any output thumps from the speaker. If you have any sort of c. 12-20V DC signal that shuts down VERY quickly after the AC power is removed, then this might work. Another option is to wire an opto-coupler from the mains side, but that will entail a dropper resistor (or better a capacitive divider to reduce power dissipation). It’s a pity the Hypex PSU’s don’t have this kind of output.

  10. Bill says:

    Is it possible to set the delay as high as 30 seconds. My Sissy sit 3 takes about that long for the bias to stabilize and I would like to delay any offset that may happen until then.

    • admin says:

      Hello Bill, let me take a look tomorrow and come back to you. The timings are governed by the UPC1237 internal circuits but in principle I don’t see why not.

  11. Andrej says:


    I built DC protection few days ago and I powered it (both AC sense and V+ input) with Hypex SMPS AUX supply (+20V). Test results shows that turn on delay and off state works as it should, providing thump free start up/off. Only thing that bothers me, is that if I put 12 DC offset voltage, protection will turn off speaker outputs in 1-2 sec, not in 100 ms as shown in manual. Is this normal, or should I need to modify values?

    BTW: Main power supply is +-45V.



  12. Andrej says:

    I am using Toshiba TK5R3E088QM, S1X mosfets.



  13. Mike says:

    Hello. I am considering modifying an Adcom GFA-5800 to use this board. The supply rail to the output MOSFETS is +70V. Will this board work as is purchased from your shop? Also I read your write ups and still do not understand how the current overload circuit works, i.e. how I would wire this input up to my amp.


    • Bonsai says:

      Hello Mike,

      1. The board will work with 70V rails – you just have to fit the 57V Zener – details in the presentation.
      2. For the short-circuit protection, I’d haver to see the protection circuit of the GFA5800. It might be that this already has some sort of built in short-circuit protection, in which case the hifisonix speaker protection board will just piggy back off that.

      Are you on diyAudio.com? maybe you can post the diagram up there and we can discuss it on the forum?



      • Mike says:

        Thanks for your reply. I don’t believe the GFA-5800 has any short circuit protection other than multiple fuses. It also has a simple circuit for cutout at thermal overload. I will sign up for diyAudio and post the service manual with schematics. I most likely will not have time to do this until next week. Looking forward to a discussion.


  14. prasi says:

    hello Andrew, whats the size of boards and are they still available?

  15. Mo (HaroldHill on DIYaudio) says:

    Hi, Andrew, A few questions for your: Does one board protect both right and left channels? Are the costs on the site in US dollars? And what country do your boards ship from?

    Thanks! (And season’s best to you?)

    • Bonsai says:

      Hello Hrold,

      the Speaker Protection Board does 2 channels. The boards are shipped from the UK.

      Unfortunately, I have to send the boards tracked otherwise I get problems with Paypal withholding payment to me. You can bundle orders and the $15 price will cover another 3 or 4 boards from the shop.

      Seasons Greetings to you as well!



  16. Tylor says:

    Is this just the blank PCB and no parts?

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