Amplifying a double bass(Letto 235 volte)

Disclaimer: All statements in this article are personal opinions based on my experience and should be treated as such. That said...

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Once upon a time, there were double bass players, the kind who—it's said—were known for their horrible hands, deformed by the calluses they'd gotten from playing the instrument with extremely high gut strings to be heard even by an 18-piece orchestra, or who used the slap technique like Milt Hinton. At best, you'd place a microphone in front of the instrument, and that's pretty much how it worked until the '70s and '80s. Then came pickups and steel strings, and the sound of the double bass went to hell...

The Pick-Ups

The piezoelectric effect of quartz crystals was actually discovered around 1880 by Pierre and Paul-Jacques Curie, and its first applications in the audio field were in the creation of transducers, or instruments capable of transforming an electrical signal into sound (loudspeaker) and vice versa (microphone). The first guitar with a piezoelectric pickup was a 1968 Gibson, and the first double bass pickups appeared in the 1970s. The sound produced was quite rich in mid-highs, not at all similar to that of a double bass, due to both the construction characteristics and the impedance matching, which I'll discuss later. Today, there are dozens of brands of pickups of all types, as well as condenser microphones that can be used with the instrument.

How can you navigate the vast array of products? There are many different types, with different installation options, though the operating principle remains the same: plates to be placed under the bridge feet, tabs to be installed under the bridge wings, single (various brands), double (Underwood or similar), electromagnetic transducers (Schertler), etc. Then there are the microphones: AKG, DPA, Shure, T-Bone, to name a few.

Obviously, if we can afford a DPA for around €500, it's fine, especially if we're playing on an outdoor stage or in a theater. But if we're using it in a club and the drummer is a bit too loud, we risk everything coming out of our microphone except the double bass. A contact pickup remains the most practical solution in these cases, but... 

…my opinion is that most of these solutions suck. No matter how much you spend on equipment, all the commercial pickups I've tried don't deliver good results. 

Barcus-Berry, Underwood, and Polytone were the most popular pickups in the '70s and '80s. The resulting sound was a mix between a children's metallophone and a ukulele in the case of the Barcus-Berry and similar devices, or a kind of meow in the case of the Underwood. All sounds far removed from the natural timbre of the instrument and my taste. I found that the Schertler offered a good compromise between powerful sound and timbre, which still remained very artificial.

I started experimenting with piezo transducers and after throwing away hundreds of wafers and other components, I found the solution that works best for me: a pickup similar to the Schertler but made of balsa wood, but the essential thing is the preamp.

The preamplifier

This is the point that almost everyone overlooks: piezo pickups have a very high impedance (even more than 5 Mohm) and the input of the amplifiers  Solid-state pickups typically don't exceed 25 kOhms. This results in the pickup being short-circuited, and frequencies aren't reproduced across the full range, so an impedance matcher is needed. In the early days of pickups, almost no one used them, and the models on the market weren't that great. To be clear, the preamp in this case isn't used to boost the pickup's signal or equalize the sound, but to match the impedance. I've been using a Fishman Bass Blender preamp with a Crown GLM200 condenser microphone and an underwood pickup for a long time. The rubbery sound of the underwood is somewhat strengthened by the attack and deep bass of the condenser microphone, producing a good compromise, but as I said before, in a small venue the microphone can cause some problems, so I started looking for a design that would work for me. I found an excellent FET transistor preamplifier schematic, with high input impedance, and I built a number of them, for myself and for colleagues and students who requested them. The best solution, however, came from a radio amateur friend of mine (Franco Bachetti, whom I thank infinitely), who gave me a beautiful box of mints containing a tube preamplifier, powered by low voltage (12V). Beautiful, rounded, smooth bass—finally the sound I was craving!

The amplifier

Amplifying a double bass with a 4 x 12" kick drum is crazy. Everything starts to vibrate: the stage, the drum heads, let alone the kick drum. The result: guaranteed feedback, a horrible sound, and always on the verge of triggering even at low volume.

Better a small 1 x 12” or even 1 x 10” amplifier, but with good power. After using the legendary 100W Polytone Mini Brute with a 15" speaker for a lifetime (which I still own and works perfectly), I had a 400W, 22Kg Mark Bass Combo 121 for many years. I'm not at all satisfied with the sound of the Mark Basses; I find it quite fake, even the new, lighter models with neodymium speakers. I must also say that I've had other problems, such as the solder joints on the circuit board that wouldn't hold up because they were too thin for the vibrations they had to withstand (I had to redo everything because the potentiometers had practically detached from the board) and the burnt-out power amps, which I replaced myself. Furthermore, matching the pickup always requires an impedance adapter. I also bought a Phil Jones Bass cube; the tone is beautiful and the output is excellent despite its ridiculously small size, but it's more suited to electric bass. The 5" speakers are too small to have a good output with the low frequencies of the double bass. In the end I solved the problem with a homemade cabinet with a 12” 300W Celestion neodymium speaker and a GK MB200 head. 

Another equally important thing: the amplifier should be placed ON THE GROUND and the volume adjusted accordingly. It's best to place it in a corner, where the low frequencies are more diffused. Experiment, as each room has its own optimal placement. In any case, absolutely NOT off the ground.