Pork: the pre-Hog

Preface

[2005-02-11] I've played the electric guitar quite seriously for half a year, and so far haven't owned a traditional amplifier. Instead I've used available gear at the school where I practice. At home I can't play with proper volumes, but I occasionally hack around with computer effects.

I'm not interested in following a strict tradition of tube amps and the like — if you want traditions, why use electricity at all? Anyway, for some time I've been thinking of building a preamp for a number of reasons:

More reliable and portable than computer
Good starting point for future electronic hacks
I already have a power amp.

There's actually a possibility that I might appear on stage later this year, with a requirement of the sound going through the PA, which is why I now started to realize these ideas.

Plan

I'm using the schematics of this ESP amp, namely its preamplifier section, with slight modifications. It's built on a veroboard for flexibility and simplicity.

Power supply

I've adapted yet another ESP project to my needs. The only change is using the 8-volt 7808 and 7908, since I happened to have a 9 VAC power brick from a dead modem. 9-volt regulators would have worked too, but the local electronics store didn't have them at the moment :)

Prototype

Construction went on surprisingly quickly. I bought the components and a breadboard yesterday, and now I have a fully soldered (on veroboard) piece ready.

Experiences and tweaks

The first breadboard prototype didn't give enough maximum distortion to my tastes :) So I left out half of the clipping diodes. You can still get a nice clean, round sound with a low gain setting. My guitar is a Charvel Jackson PS-2, FYI.

With my power amp, the amplifier line is almost entirely made of op-amps. There's only one discrete transistor as an emitter follower at the output of the preamp, so it's not doing much to the sound. Incidentally, op-amp systems are nothing like transistor amps when it comes to sound; they are closer to tubes in some ways.

After a little testing, the distortion sounds quite similar to the valve saturation effect from Steve's LADSPA plugin collection (which I use via Ecamegapedal). It reminds me of the old 'rautalanka' sound and the Beatles' guitars. Which is a nice thing to get from my heavy-metal axe :)

[2005-04-06] I've added a switch to disable one of the clipping diodes. The resulting asymmetric clip is warmer, but a distinct edge remains.

Case

[2005-04-06] The neat mesh-like case used to house a cognac bottle. It's a little thin and weak for heavy stage use, but I was in a slight hurry to enclose it for some light stage work. On the other hand, I bet it looks cooler than a standard chip-amp case :)

Future ideas

Op-amp effects, for example integrator, differentiator, logarithm (for compression) and exponent operations. For the real geek rock sound, you know. I tried integration/differentiation with software (2005-02-19), and neither sounded particularly cool, but they'd probably work better in hardware. They are very simple circuits after all, so nothing to lose there.
Modification of the limiter, for example with a diode network. Dave Krooshof's distortion page also has interesting pointers.
Modularization of effects. I have robot voice effect which sounds very crunchy when used after the preamp, but it needs additional volume control afterwards. Thus it and similar effects should be pluggable in the middle. That way they could also share the power supply with the preamp.

Risto A. Paju