Building an Electro-harmonix LPB-1 from scratch

Thus far my effects building experience has all been limited to assembling pre-made boards and soldering the components. Along the way I’ve tried to study the schematics of the original effect, such as the Tubescreamer, but none of it was really sinking in. To remedy this I have decided to go back to basics and learn more about the principles of audio electronics from the bottom up. I have defined ‘the bottom’ as a guitar tone control.

Research

My starting point then is some kind of boost circuit. Many great tones from the 60’s were achieved by driving the front end of a tube amp quite hard using something like the Rangemaster. Certainly Clapton was rumoured to have used one to brighten up his Les Paul in the Beano sessions.

So I looked for boost circuit schematics. One of the first ones I found was a really good single page explanation on how a boost works:

Beavis Audio – How It Workz

This was perfect as it explained it in dumb-musicians terms. For more detail (and for later consumption) I am reading this:

The Transistor Amplifier

Things of note:

  • Notice how similar a basic common-emitter transistor amp is to a common-cathode tube amplifier?
  • All boost circuits are basically the same. They might bias the transistor differently, or they might have extra components (for protection or good behaviour) but fundamentally a clean boost is a common-emitter amp.
  • I can probably build one of these quite quickly.

Which one to build first?

The LPB-1 schematic I started with

The LPB-1 schematic I started with

My choices were between the ZVEX Super hard-on (the SHO) or the Electro-harmonix LPB-1¬†as they have plenty of reputation in the industry and they’re incredibly simple devices. I decided on the LPB-1 for no apparent reason other than it featured in an article on Beavis Audio’s site which helped me greatly in knowing how to approach this. Here’s that article, go and read it if you’re interested in doing this.

Layout

As encouraged by the beavis article I downloaded DIY Layout Creator, and literally created my own veroboard layout using the LPB- 1 schematic. This was loads of fun and surprisingly simple. It forced me to know the schematic in great detail. I tried to keep in mind the how-it-workz article so I had a feel for why each component was there. Here’s the resulting layout:

Simon Allaway's First LPB-1 veroboard layout

Simon Allaway's First LPB-1 veroboard layout

As you can see, it is pretty simple and even though I haven’t done this in years I was able to assemble all the on-board components in about half an hour. I bet I can do it in half that time now as I kept screwing up.

I made a part substitution as I failed to order some 2N5088 transistors. I was under the impression I already had some, but didn’t. Instead I used a 2N4401.

Testing

So, once I’d wired up power, switching and signal jacks I plugged it into my amp to try it out, but all I got was hiss and distortion. So I unplugged and checked all the solder joints, reflowed a few and tried again. No different. I began to suspect the transistor as that was the only deviation from the schematic. I tried a 2N3904 this time, with much better results. I do get a boost (not as much as expected) and also lots more bass, which isn’t necessarily a good thing. At this point I was discouraged.

Time to re-evaluate. After some thought, I recalled the Beavis document, and some other discussions surrounding the SHO boost; check your transistor pin-outs. I had stupidly placed the transistors pins into the overboard without honestly checking that the collector, emitter, base pins were actually in the right place. I couldn’t tell what brand I was using, so I used a technique I found online here to derive the pins using the diode test function of my multimeter. And sure enough, I had it wired incorrectly. ¬†Literally 180 degrees wrong.

Alright, so put the transistor in the right way around and it now works. There’s definitely lots of gain in this device, and lots of bottom end boost too. And its more than happy driving the hell out of a tube-amp.

The following pictures is before I trimmed the board down to its final size. You can also see I went for an on-board trimmer (as I didn’t have a 100k pot to hand). I may well leave it like this for when it gets integrated into my OD2.

My LPB-1 clone

My LPB-1 clone

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5 thoughts on “Building an Electro-harmonix LPB-1 from scratch

  1. Ash says:

    I’m currently building this circuit for a bass boost. It works well with pickups in passive mode but distorts with the actives on.

    I’ve tried various values in R1 but if I lower this too much then I dont get enough boost. I’ve also swapped c1 and c2 but still not happy with the sound.

    Any advice or tips?

    • simonallaway says:

      It sounds like your active pickups are already boosting the signal a fair amount. If the LPB-1 circuit is in a high gain state then you’re creating a two stage preamp. Which might sound good :)

      These kinds of booster circuits were designed to boost a weak signal. This is why the Dallas Rangemaster was so popular in the sixties; you could get a Strat to be as loud as a Les Paul, and therefore drive an amp harder.

      So the corollary is, these circuits weren’t designed to be driven themselves.

      It also depends on what your intentions are. i.e. do you want ultra clean boost? The success of that approach will depend on what comes after this booster in your rig? Do you regularly switch between active and passive?

      I gave a SHO clone to a bassist family member, and I recommended he use it at the very end of his pedal board. This way it’s being used as a line driver for the cable run to the amp. It gave him the clarity he was missing. And he has the choice of driving the front of the amp too.

      Simon

      • Ash says:

        Thanks for your reply Simon.

        I’m really just looking for a clean boost so my occasional bass solo comes through the mix better.

        I also need to make one for a friend who has passive bass pick ups so I guess I just need an adjustable input gain which I can adjust depending on whether its used on an active or passive bass.

        I’m not sure which components to tweak or maybe swap to a small preset or normal size pot???

      • simonallaway says:

        Well, rather than messing with the LPB-1 you might consider the SHO. I’ve tried that with both passive and active electronics (guitar and bass) with very good success. I.e. Sparkly clean boost. The circuit is just as simple.

        I will try the LPB with actives though as I never did try it with my Steinberger. Perhaps there is some tweaking to be done.

      • Ash says:

        Thanks Simon I’ll give that a go!

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