A Gainclone with the LM3875 IC

I have built a couple of chipamps/gainclones before, but none like this. Firstly, it is my own PCB layout I am using and secondly it uses the LM3875 IC in a non-inverting configuration – pretty much the simplest GC there is.

For those not already in the know, “Gainclone” is a collective term for amplifiers built based on “power opamps”, mostly National/TIs LM38xx and LM47xx models, but also the TDA729x series and others. Since each chip is basically an opamp, the circuits are very simple. From a single chip you can normally get 30-60W class AB but depending on which chip is used, multiple chips can often be connected in parallel and/or bridge mode to get higher power levels.

The circuit I have used here comes from chipamp/audiosector and it is more or less the same that started the gainclone “revolution” (or “craze” if you want) on diyaudio and elsewhere around 10 years ago. The matching PSU board will take either four 18mm caps or one large 35mm snap-in cap per rail – I’ve used the Nichicon “Fine Gold” caps because I had them in the capacitor box anyway. There is some debate as to whether a Gainclone gets better or worse as the PSU capacitance is increased (I won’t take sides here 😀 ) but this board allows more or less all configurations to be tried, i.e. without the PSU board, with several smaller caps in parallel or with one big supply cap.

I have only managed a brief listen to this but it definitely sounds very promising – and extremely impressive considering the simplicity and low cost.

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13 Responses to A Gainclone with the LM3875 IC

  1. Pingback: Pspice ile OPAMP Devreleri | Book of Engineering - Türkçe

  2. Pingback: Project files: LM3875 Gainclone and PSU | theslowdiyer

  3. Pingback: New gainclones… | theslowdiyer

  4. paul says:

    Hello,
    Great project! Would it be possible to get the pcb layout for etching? i would really love to make my own gainclone…
    best regards

    paul

    • theslowdiyer says:

      Hi Paul,

      The board is double-sided, so I’m not sure if it is suitable for etching. If you want to give it a go then download the gerber package from here, render it at 600 dpi using circuitpeople.com and then you should be able to print the top/bottom layers to scale for UV- or toner transfer. Shoot me an email if you have any problems 🙂

      • paul says:

        thank you for the fast reply. is there a link to the download? when i hover over the tet i dont see anything…
        thanks

        paul

        • theslowdiyer says:

          All the GC-related posts are here: https://theslowdiyer.wordpress.com/tag/gainclone/

          Design files are linked from the posts that have headings that start with “Project files” (there are a couple of different versions).

          • paul says:

            thanks again. i managed to download gcnewfiles_v1-0.zip, but it seems to be compromised. on windows and mac i’m unable to extract the file.

            thanks for everything

            paul

            • theslowdiyer says:

              Not sure what was wrong there, but I’ve replaced the file with my back copy and it seems to work now. If you’re still having problems then send me a message through the “contact me” page and I can email you the file directly.

          • paul says:

            Hello,
            thank you very much!!! now everythings works fine!
            I’d like to order the parts to try and test the different gainclones.
            would you be so kind and reveal the values for the resistors and caps and so on?
            i couldn’t find them in the eagle files.
            thank you very much
            paul

            • theslowdiyer says:

              Hi Paul,

              Component designations and -values are the same as in the manual for the Audiosector kit (http://www.audiosector.com/nigc_kit-users_guide.pdf).

              Rz and Cz should be 2R7/3W and 100nF respectively. The resistor for the LED is connected between the supply rails and need to be a 1W type to handle the power dissipation (and calculated to suit your rail voltage and LED).

              Everything else is hopefully pretty self-explanatory 🙂

            • paul says:

              Thank you. Yeah, i think the rest should be self explanatory… One more question:
              what kind of bridge rectifiers do you use in your designs?

              best regards

              paul

            • theslowdiyer says:

              Hi Paul,

              Most of my bridges are PCB-mounted and then I either use TO-220 discrete rectifiers or integrated GBU-type bridges with 10A+ ratings. If you are not board-mounting the bridges then I’d recommend something in a GBPC chassis mount package.

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