DIY Simple Active Preamp

Discussion in 'DIY' started by HypnoToad, Oct 30, 2011.

  1. HypnoToad

    HypnoToad Ms Puss Puss Subscriber

    I was looking into making a simple preamp I considered the Pass B1 Buffer but was worried if it would have enough gain.

    I decided to make one out of parts I had and make it as simple as possible.

    I used a pretty standard design which I found on Rod Elliot's site, I simplified it greatly so now it has few parts and only one film cap in it, the opamp I used is a AD823.


    I etched a PCB and assembled it and tested it out, surprisingly it worked first time. It has too much gain, I am going to change out the 100k resistor in the feedback loop to a lower value which should take care of that.

    I hooked it up between my phono stage and the main amp inputs of my Yamaha CR-1020 and then did a comparison with the Yammy using it's preamp. The bass is much more extended for a start the sound being more full bodied and open it should improve once it burns in as I used a 2.2uf Panasonic ECW-F polypropylene which may open more up after some hours.

    Initially it sounds much better than the pre in the Yamaha, the acid test is going to be when I try it in my main setup and put it up against my Yamaha C4 but the way it sounds now it might just be better.:music:
    Last edited: Oct 30, 2011
  2. botrytis

    botrytis Small Speaker Hoarder Subscriber



    You knew that was coming....
  3. HypnoToad

    HypnoToad Ms Puss Puss Subscriber

    Don't laugh I just threw it together to see if it worked.

  4. bsabel

    bsabel Well-Known Member

  5. HypnoToad

    HypnoToad Ms Puss Puss Subscriber

    After some adjustments I still have plenty of gain and listening to Grover Washington Jr - Strawberry Moon and my bedroom system has never sounded so good. With a little burn in time it got a whole lot better.

    More air around the instruments and bass to die for, I am thinking of putting one in a case with a 3 input switch including the phono stage so it can run off the same batteries using decent components, my volume control is a ratshack one. Simple seems to better.

    But the sound is amazing a huge jump from the internal Yamaha preamp, still haven't tried it against my C4 but I am pretty sure of the result already.:music:

    You can build one of these really cheaply.
    Last edited: Oct 30, 2011
  6. roger2

    roger2 . Subscriber

    very cool
  7. sfox52

    sfox52 AK Subscriber Subscriber

    This is great! You have to give this a name! I'm gonna try it, though I'll use point to point wiring cuz I've never etched a board. I have the ESP P05 board that I could use instead of 9V batteries..
  8. HypnoToad

    HypnoToad Ms Puss Puss Subscriber

    It's the Simple Preamp or SP.

    I changed a few things, I put in 51k resistors instead of the 100k in the feedback loop and changed out the 1.5k for some 27k in the input. I would also use a 250k pot instead of the 100k as it builds up volume very quickly. I am going to try one as soon as I get on to a decent one reasonably priced. You could also try a nice axial 2.2uf cap as well, although for the money the Panasonic's are very good.

    I also put some polar 47uf caps from the op amp supply rails to ground with some 0.1uf bypass caps on them for stability and low noise.
    Last edited: Oct 30, 2011
  9. roger2

    roger2 . Subscriber

    I did not realize that a pre-amp could be done so simply. Very interesting. I agree that simple is better. I prefer not to have tone controls, multiple inputs, etc. I have never DIY'd anything but this project has got me thinking :scratch2: In post #1 you mentioned that you simplified the base design, in what ways did you simplify?

    What software do you use to make the schematic?
  10. audiozaz

    audiozaz rtfm!? - tl;dr

    Cool project. Bonus point on the minimalism. I have a class-A power buffer amp project with the same general vision. I should kickstart it this fall/winter; been sitting on shelves for some time despite having most parts.

    All you need is a good enclosure and it will look as good as it sounds!

  11. bobrown14

    bobrown14 AK Subscriber Subscriber

    Hey Toad, wanna do a group board offer???


  12. HypnoToad

    HypnoToad Ms Puss Puss Subscriber

    I use ExpressPCB and ExpressSch. I usually just make the design in ExpressPCB as I can print it out for etching, I wouldn't use it for a commercial board as not many companies accept the files.

    I removed the balance control and took out some other parts in the output, it resembles the output stage of the CNC now. I just wanted something simple to see if it would work, I measured the outputs and there is no DC. It's virtually just a low gain stage with a volume control at the front.

    You can add different inputs if you like.

    I am still not sure about the volume control as it is very sensitive. If anyone has any ideas about implementing it better let me know.
  13. bsabel

    bsabel Well-Known Member

    Maybe use Rod's log faking circuit with a linear pot?
  14. Russellc

    Russellc AK Subscriber Subscriber

    search J-fet BOZ, (Bride of Zen) over on DIY... nice, gain, very simple...

  15. BinaryMike

    BinaryMike Pelagic EE

    Aside from an abundance of caution, there's really no reason to use an output DC-blocking cap in op-amp circuits with such low gain. You could eliminate that item unless you expect to be using op-amps with substantial input offset voltage. Even in that case, you might be better off with an offset trimmer.
  16. HypnoToad

    HypnoToad Ms Puss Puss Subscriber

    I agree and think all those warnings about DC have overcome sensibility, every where you look you see something about blocking DC from entering the next stage. I may short it out and remeasure to see if there is any DC at all.

    A nice ALPS pot might be the ticket here also as my cheap ratshack one varies a lot between gangs.
  17. jwalker

    jwalker Well-Known Member

    I would rework the design to run from a single supply and use a decent rail-to-rail audio op-amp like the OPA2134. You need more caps, but at least you can get rid of one of the batteries or run it from a common 9 or 12v wall-wart. You already have the output coupling cap anyways and a preamp that goes to DC isn't really a huge benefit.

    Which op-amp are you using in this design? If it's a dual like the schematic suggests then we can use the other op-amp to provide a low impedance bias point for a single-supply design.

    I drew up the attached schematic will show you the added complexity. Converting an inverting topology to a single rail is a lot easier than what this schematic shows. You can ignore the op-amp type, I just reused that for this schematic.

    Attached Files:

    Last edited: Nov 1, 2011
  18. HypnoToad

    HypnoToad Ms Puss Puss Subscriber

    I appreciate your efforts but I am trying to simplify things, the op amp is a Rail to Rail AD823 that is ideal for a battery supply. The OPA2134 would be a backwards step IMO. I have 47uf filtering caps on each supply rail and those are bypassed with 0.1ufs as well.
  19. HypnoToad

    HypnoToad Ms Puss Puss Subscriber

    After putting in 33k / 51k feedback loop resistors and returning the 1.5k at the inputs I have the right amount of gain. The volume increase is steep up until 9.00 o'clock where it smooths out.

    I am still using it between my Phonoclone 3 and the power amp section of my Yamaha CR-1020 in my bedroom system and it has made a huge impact on the sound quality, so much so I haven't even bothered to try it in my main rig.

    I think it's a case of it not getting in the way of the sound, the less we mess with the sound from the input to output the better, and it's certainly working out that way. Just smooth, effortless sound with detail and soundstage to die for.

    I am going to build one in the same case as my other Phonoclone 3 with two more inputs and sharing the same battery supply with a decent pot.

    The only caveat is you have to turn the power amp on last and off first to avoid some pops from the preamps. I could add a relay but it would be against my simple is better philosophy, but then again it could just be mental illness on my part.
  20. Pio1980

    Pio1980 AK Member Subscriber

    My biggest active stages bitch is with line stages, they typically exemplify TMS syndrome (waay Too Much Stuff in the signal chain of custody). You can definitely hear the increased clarity of a simpler signal path. I use these things off e-Bay for basic building blocks as they are quite simple, have good parts, and come with an excellent lo-noise PS. The PS section has plenty of reserve capacity to piggy-back a phono stage. FWIW, DC blocking caps are generally unnecessary as there is usually no DC offset to block.
    Search for "OPA2604 servo preamp kit" .
    Last edited: Nov 5, 2011

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