Project files: Universal Mini-preamp

A few weeks ago a reader was commenting on simple buffers/preamps and also asked about ebay-kits to use since I haven’t posted anything with a volume control yet. That got me searching to see what was actually out there and very quickly came the realisation – “I can do this better” 😀 Not sure if I did, but I at least tried 🙂

What is it?
A very simple opamp-based buffer/pre with an onboard volume control that can be used as a “buffered volume control” with a power amplifier module, a real preamp with or without gain or even a “CMoy”-style headphone amp. The board has space for a DIP-8 dual opamp, polypropylene input caps and a full-size Alps volume control and still manages to be very compact. I’m showing the board now as I already have a couple of applications for it in the pipeline myself which you will see later 🙂

How big are the boards?
2″x2″ (app. 51×51 mm) – a theslowdiyer standard size (TM) 😉

What is the status of the boards?
The board file is v1.0. I’ve built a prototype and everything seems to be fine.

Does it use any special/expensive/hard-to-find parts?
None, really. You can get what you need from Mouser/Reichelt and similar places and most of the component values aren’t that critical anyway.

Anything else I need to know?

  • The opamp should be a dual-type with standard pinout. My recommendations would be either the LME49720 (sadly discontinued in DIP) or the OPA2107 (still available but fairly expensive), but there are loads of other options. The board layout should be suitable for using adapters as well (for DIP/SO-8 singles or SO-8 duals) and if you want to go all-out there’s even a discrete option from Burson that should fit as well.
  • The only surface mount components are the optional (but recommended) 1206 bandwidth-limiting caps on the bottom – otherwise it’s through-hole all the way.
  • The PCB should be happy with just about any (regulated) dual power source – linear PSU, switching PSU or even a pair of 9V batteries.

Downloads:
Download design files here

Edit 26th Aug. 2020: Download file for v1.5 version intended for SMD opamps is here. Apart from the change to the SMD opamp footprint, the only thing that has really changed is that the output connector can be 3 or 4 pin (instead of just 3-pin). The original BoM should still be good as well.

Related information:
Even though this is a basic opamp circuit and I can just about draw the schematic and recite the parts values from memory, I went back to look at it once more to try and read up on the theory behind. If you aren’t very familiar with the basic schematic already I can absolutely recommend the old but still excellent articles from Headwize/Head-fi member Tangent here and here. Tangent’s pages also have a ton of other useful information and although the site isn’t updated any more (and it’s quite old) there’s still plenty of good stuff even for inexperienced diy’ers.

If you are more technically inclined then probably the best resource is the “Opamp Applications Handbook” from Analog Devices and edited by Walt Jung.

Note: Always read the “intro post” for additional important information about my designs.

19 Responses to Project files: Universal Mini-preamp

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  4. aldo says:

    Hello,
    I agree with you that it is a very simple opamp-based preamp. The design is perfect for many of us that don’t want a tone control. Would you please tell us the resistors and caps values that you use in the unit depited? It would very useful.
    Thanks

  5. Hello, may I ask if pin spacing for Alps RK27 is the same as the common potentiometer? I have never used Alps pot. Thank you.

    • theslowdiyer says:

      Depends a little on what you mean by “common” potentiometer because there are different types. The spacing between the pins in each row is the same (0.2″/5mm) but the spacing between the rows can differ. On the Alps pot it is 10mm/0.4″ and on some other types of pots it is less. Apart from this point there should be no other differences. If you have a normal pot in hand, checking dimensions against the mechanical drawing of the RK27 in the datasheet should be easy.

  6. David F. Eisan says:

    Hello,

    Do you sell the PCB for the MiniPre/Cmoy?

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  8. aldo says:

    Hello,

    I built the unit using these values pot=25K , R1=1k and R2=250k. Viewing the schematic I noticed that the impedance is affected by the potentiometer adjutment. In order to avoid a very low impedance value, Is it convient to use a higher pot value?
    Thanks.

    • theslowdiyer says:

      The source sees the 25k pot which is fine (and fixed). The output impedance of the pot wiper is variable, but still lower that the impedance it sees which is the 250k R2 resistor, which should then also be fine. This is actually the reason for the “x10” recommendation between pot value and R2. If anything, If the source on the input can handle a lower impedance, a lower pot value actually allows a lower R2-value which all contribute to lower noise. In reality it’s not too critical, but in general lower values are preferred, not higher values 🙂

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