So once upon a time I got this whim to glue together a bass body block out of mahogany, purple heart and birch. Dont ask me why, as I said it was just a whim and it seemed like a good idea at the time. Basically because I had some scrap pieces lying around in the shed that I had no idea to do with otherwise. The body blank was glued together with a piece of mahogany as the center piece, the "wings" with mahogany core and birch on the backside and purple heart on the top. The blank was then put aside to dry properly, and two years later I thought it was high time to use it.
As always, I started building two builds at the same time, this "Red Red Wine" and "The Classic". I had never before made a neck out of wenge, so I glued together a neck blank with wenge and two ash stringers.
These two builds was planned to be the same body shape as "The Fenix" and "Blue Lagoon", the body shape which I call "Jayzee", however the rounding of the edges is slightly larger, if I remember correctly I used a edge rounding router bit with a 20mm radius.
The bass builds I've done over the years had always been 34 or 32inch scale lengths, so I had been wondering (for a while) what a 33" bass would feel, sound and play like?
|Construction: 5-string (bolt-on)
Scale Length: 840mm (33")
Strings: DR Lo-rider 45-125
Number of frets: 24
Tuners: Boston (Gold)
Bridge: Gotoh style (Gold)
Frets: Dunlop 6310
Neck wood: Wenge, ash (5-piece)
Body wood: Mahogany, purple heart, birch.
Fretboard wood: Merbau
Neck pickup: EMG HZ40
Bridge pickup: EMG HZ40
Preamp: Home made 2-band
|One thing that has caused some problems in the past is the front strap button that I have mounted facing backwards. This is because I want the strap to go from the front strap button over the shoulder without the strap having to make a 90 degree twist. This gives a more comfortable strap and it makes it feel like you have a better physical connection with the instrument.|
However, this causes a problem when you put the bass in a hard case. The bass will rest on the strap button, not the whole back of the instrument. A simple solution to this is to "resess" the strap button. Looks a bit "unusual" from the back, but is barely visible from the front.
For years I've been looking for a good sounding DIY kind of onboard preamp for bass, found a few ones but never really give it that much thought. I mean, why pay 100-170€ for a factory made preamp when you could make one yourself for like 5-10€? I mean, its not THAT difficult if you have a basic degree in electronics. It's basically just discrete components and some soldering.
In fact, now that I think about it, I've allready have made a preamp for one of my basses, the "Black Pearl". But that was a copy of a StingRay preamp, which may or may not be suitable for the majority of my basses.
So I made some research on the net and found a few interesting preamps, read a lot of stuff about op amps, jfet's, tone stacks and basic preamp designs. One very simple preamp solution was one that I had used and built before, the Tillman jfet preamp.
|The whole circuit consists of just 7 parts. It's designed around the J201 jfet. Why a jfet? Well, jfet's tend to be the next best thing for audio amplification with very similar sound and characteristics as valves (tubes).|
Jfet's are popular in audio electronics because they have a very high input impedance, low noise, high gain (if needed) and low power consumption.
So since the pickups I installed on this bass are passive, this little preamp could be used to make the HZ40's practically active. By connecting both pickups to its own dedicated preamp circuit, you isolate the neck and bridge pickup from each other, which will eliminate the problem of the two pickups interacting with each other when the balance (blend) pot is set to middle (both pickups full on). Another advantage is you can get a little gain boost, you dont have to, but in this case each preamp circuit is boosting the signal about +9dB.
One very neat thing I found was a free software, the Tone Stack Calculator by Duncan's Amp Pages. This is a simple software for experimenting with different types of tone stacks. One stack design in particular caught my eye. It's called the James tone stack, basically a passive baxandall filter type tone stack that has a fairly flat frequency responce when the bass and treble pots are set to 12 o'clock. In addition the bass and treble controls have a wide adjustment range and minimal interaction between treble and bass. Exactly what I wanted. Happy times :)
Below a demonstration of the bass and treble controls max/min range. Approx. +/-18dB on both bass and treble.
As with all passive tone stacks they decrease the overall level of the signal. This particular tone stack has a attenuation of -22dB, as you can see in the picture above. So the signal has to be amplified up to the correct level again, by placing a singel amplification circuit both before and after the tone stack. This is where the Tillman jfet circuit comes in. Below a schematic that I drew together and tested on a breadboard.
|Click on picture for a bigger version (opens in new tab)|
And lo and behold, it works! Made a few tweaks on my own, added a 220pF capasitor from the input to ground to filter out high frequency interference (radio, etc.), used the tone stack calculator to test if the bass pot could be the same value as the treble pot (500k), seemed to work just fine. Lowered the value of the 180kohm resistor to 100kohms, it seemed to raise the level of the mids a few dB for a bit more flat frequency response.
When I compared the level of this circuit and bypassed I realized I had to take down the gain somewhat. The circuit sounded much louder than the bypassed signal. So I increased the values of the drain and source resistors until I got approx. the same signal level both with the circuit engaged and bypassed. Also the 4,7uF and 47uF capasitors seemed a bit big in value so I tested out 1uF resp. 10uF capasitors instead. No audible difference, at least that I could hear in my headphones.
|Click on picture for a bigger version (opens in new tab)|
The interesting thing is, that when I compared the engaged and bypassed sound, the engaged sound sounded more full, deeper, meatier, more "round" and warmer, even with the tone pots at 12 o'clock. It just might be something to it, that jfet's do sound like valve's...
So, this is the version I soldered together on a piece of perfboard and installed in the bass. Below a block diagram of the electronics installed.
Here's a sound sample. The bass plugged directly into my Focusrite soundcard. Both pickups (blend pot in the middle) bass and treble pots at 12 o'clock.