Friday, 21 December 2012

Boss FA-1 FET Preamp

Well it's the 21st here so I'm getting this one in early in case the world ends soon so we can get it verified first.  :o)

Another must try for fans of U2 and The Edge and another one of the boost pedals in his arsenal.  Info from Boss about the long discontinued original:

The FA-1 is an incredibly small Field Effect Tramsistor (FET) based pre-amp. The controls are real easy. There's controls for volume, bass, treble and a low cut filter switch. The amplification is as much as 20 times (26dB) and it has an input impedance higher than any other device.






16 comments:

  1. Looks interesting, but 26dB is more like 250 times stronger. I guess that's what they meant. This is why dB is used in the first place to get large amplifications having smaller, more conceivable and tangible numbers by using logarithmic scale. Factor of 20 is more like 13dB.

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    1. The decibel is used because it correlates more to how the human ear perceives sound amplification - almost logarithmic.

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  2. The 20 times amplification may be the AC output voltage over the input which is audibly realized with a plus 26dB change. The decibel scale is logarithmic so every 10dB is an increase by a factor of 10. 20dB is 100 times louder than 1 dB as an example.

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  3. There is also a schematic here which seems to be identical to original schematic. It has two IC, as the original
    http://www.paulinthelab.com/2012/06/boss-fa-1-stripboard-veroboard-layout.html

    What do you think about it ? This pedal will be my next project, as soon as my Naga Viper catalinbread is finished (first pedal, work on progress. I am finished etching with ferric chloride, and then will work on soldering)

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    1. Yes Paul's a good bloke and does some great layouts. As is often the case with these I've opted for a dual opamp to replace two singles, the LM1458 is essentially two LM741's looking at the specs. So go for the one you prefer.

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    2. aye or you could build both mine and Marks! you can never have too many of these preamps.
      I have one of the originals which I bought for £10 in a bargain bin 10 years ago now they are like gold dust.
      it is the kind of thing that everyone should have more than one of and you could maybe experiment with mine. I've got 2 different op-amps designed for space travel in one of them for example

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  4. Thanks for answering.
    If they are equivalent, I will probably goes for your layout, because it is a little more compact.

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  5. Has anyone built this yet? I got parts inbound and just wanna know that it works :)

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  6. I built this yesterday using the 2SK246 transistor. The vero layout works perfectly, apart from if you use the transistor symbol as a visual guide for pin out of the 2SK246. It needs to be turned 90 degrees, so the flat side is on the right. Great booster, probably too much for a solid state amp but great to push a valve hard. A master and volume, or a 500k pot might be more useful for the many.

    Thanks for everyone's continued commitment to this site - it lifts me each day.

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    1. Excellent thanks for verifying, I'll update the layout

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    2. I just checked my 2SK246's and mine are the opposite way, needing the flat side to the left, so there must be differences depending on the manufacturer. So anyone else building this, check whichever JFETs you use and make sure the marked pinouts match.

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  7. If it's any help for anyone using a 2SK246 in a build I used a TOSHIBA TO-92 for the flat side to be on the right

    I also didn't have enough 5n6 caps so used a 4n7 on the treble side and still get a good range of tone. I used the suggested LM1458 IC.

    This has a very sweet hi-fi clarity about it until it starts to push the amp into distortion. The glassiness works well with a classic chorus sound.

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    1. Have tried at both 9v & 18v. For me 18v is much better.

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  8. What can I use instead of 2sk246? I ´m thinking about j201 (by these vero http://forum.musikding.de/cpg/albums/userpics/23524/Boss_FA-1_vero.gif I know there is different pinout) or 2sk117, 2sk30?

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  9. could work also as a preamp for piezo acoustic guitar?

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