Wednesday, 9 January 2013

Runoffgroove Grace Overdrive

From ROGA few months ago, Aron Nelson released the Smash Drive. The circuit ingeniously used an LM386 audio power amp chip to produce great distortion sounds. Aron based his work on that of Tim Escobedo. Tim's Stupidity Box was another great sounding unit and beautifully simplistic. The Smokey Amp by Bruce Zinky was another design that used a 386 for some great sounds.

Depending on your personal tastes, there may be a potential down side to the 386 designs. The input impedance is listed at 50k, according to the datasheet. This is quite low and causes some loading of the guitar signal. There is some degree of high end signal loss. This may be a favorable thing, again depending on personal preference. I chose to try a high impedance buffer before the 386 to alleviate the loading effect.

A simple Jfet buffer was chosen, due to high input Z and minimal parts count. A high value of 4M7 was chosen as the gate resistor, which helps set the input Z. The J201 fet was used, but any N Channel Jfet could be used with similar results.

The result of the input buffer is easily noticeable. There is increased sparkle in the signal. The signal hits the 386 a little harder, due to more signal reaching the amplification stages of the 386. The buffer adds no gain, but there is a perceived increase. The gain control of the circuit is something else unseen this far with the 386 designs. It is simplistic, but functional. A simple 1k pot was chosen and wired as a variable resistor between pins 1 and 8 of the 386. Looking at the datasheet for the IC, you should notice an internal 1.35k resistor. By connecting the 1k pot between pins 1 and 8, you can effectively vary the gain from near the normal condition of 20X gain to the pins 1/8 connected setting of 200X gain with the 1k pot at minimum resistance.



12 comments:

  1. I'm super-excited about seeing more ROG layouts on here. I'm a n00b electronics guy wanting to find a slightly easier way to figure out the circuits other than screwing them up on a perfboard 20 times.
    yea, a breadboard would be a good idea too.
    I'm hoping either of you will get bored enough to make a run at the Dr. Tweed and the Azabache.
    Thanks for doin what you do-do-do!

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    1. Thanks for your words. I'll be looking at those both at some point.
      +m

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  2. I'm loving the low parts count layouts! Would it be possible to tag them so that in between part orders...i can see if I have enough spares around to build one?

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  3. you can tag this one. verified. another quick and easy build. it sounds pretty good as well. i tried it with a few different J201's and they all sounded pretty good, even some questionable ones i got from a seller in china.i am also loving the low parts count builds. you can get a few done in no time at all. also, i know you're not really taking requests, but a low parts layout i wouldn't mind seeing is the lovepedal woodrow.

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    1. Thanks mate, you're steadily becoming Miro2 for the verifications! :)
      And Miro, the Woodrow is mine, you should all see how many he's still got to post! Haha

      Delete
    2. Great to see someone else lost it for builds too :) Definitely one of the best hobbies i've ever had.
      +m

      Delete
  4. Mark- i don't think there could be a miro2! i remember not to long ago when he was building them as fast as you were posting them! didnt he build every big muff variant in a single day? i think he built the green russian in the time it took my iron to get hot! i've just had some free time lately, and you guys have posted some good ones that are quick and easy.
    Miro- i agree 100% . i've never had a hobby that my wife hated more!!

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  5. I built this a couple months ago (on perf) for a friend, it sounded great. Just chiming in to say that I used an NTE823 op-amp and a 2N5951 JFET and both sounded really good, just some more options ;)

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  6. I really like the sound of this - what does the 6.8N cap do? I used a 4.7 that I had on hand. Should I have gone to a higher value as opposed to lower? And I assumed the D1 was a protection diode so I used a 1n4001, is that correct?

    Thanks

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    1. Yup. Both ok.

      Diode is in fact a polarity protection (i'll add a label..).

      That 6n8 is the input cap from buffer to IC. 4n7 will work just fine, but you should use larger value, if you want bigger bass responce.
      +m

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  7. I built this as my first foray into veroboard, after building a few kits with conventional circuit boards. By following the build guide I was able to put it together from scratch in about an hour. Would you consider including a bill-of-materials with your layouts?

    It's quite a bright sounding overdrive with a relatively low amount of drive available. I'm currently using a no-suffix LM386. Has anyone tried different chips and what did they find?

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Should all layouts be verified before publishing? This would mean less layouts but more peace of mind?