I have been trying to figure out the best way to cover this without going deep into the weeds and boring
the pants off of everyone. I want to keep this post focused on what the preamp is, does and how to alter it.
I promise......there will be no math!
This preamp design is based on the Baxandall Active EQ circuit. Most particularly, the 3 band EQ type.
A Google search of “Baxandall Active EQ” will give you a wealth of information on the history of the circuit,
inventors, and one crap load of math calculations for finding frequency centers. We’re not going to go down
those roads. It gets a bit hairy. My goal here is to dig right into the preamp and look at what it’s doing
and what you can do about it.
Let’s begin. First off, there are 2 different circuit boards used in the M-Series basses. The one that is used on
the M-2000 and M-2500 is called the BEQ3 (Bass EQ with 3 bands). The one that is used for the MJ-4 and the
MJ-5 is called the MBEQ3 (MJ series EQ with 3 bands). OK, not overly creative on the naming of the PCBs but
marketing was never my strong point. The only difference between these 2 boards is that the MBEQ board has
adjustable gain on the front end before the 3 band EQ circuit. This was done because the split-coil J-Bass type
pickups used on the MJ basses are so much weaker than the MFD pickups.
Here is the schematic for the BEQ3 preamp.
And now, here is the schematic for the MBEQ3 preamp
You can see that the only difference between these two preamps is the addition of 2 resistors (R14 and R15)
and 1 capacitor (C10). This is for adjusting the gain of the input buffer. There are a few component value
differences on the input stage (R5 and R8) but this is to deal with the pickup loading differences between
an L-2000 MFD type pickup and a split-coil J-Bass type pickup.
Here’s how the whole thing wires up in your bass. I drew this out as clear as I could so you can see
where all the wires go.
The preamp I will be using for this analysis is the MBEQ3. This is the USA version with the dark green PCB.
The PCBs from Cort for the Tribute line are white. Here’s a picture of the 2 different boards.
USA version of the MBEQ3 with green PCB.
Cort version of the BEQ3 with white PCB.
OK, let’s get started. Below is a frequency plot of the bass preamp with the volume at max and the
tone controls at center position (flat frequency response).
As you can see, the frequency response is not perfectly flat. This is on purpose. While evaluating the bass
it was decided that the sound was tighter and more preferred if the low frequency was rolled off a bit.
As I talk about components, and their values, I will be referring to the MBEQ3 schematic. OK, the low frequency
roll off of the M-Series preamp is controlled by C3 (.022uf) and C2 (1uf). If I want to modify the preamp to
have a flatter frequency response at the low end I can simply make those 2 capacitors larger in value.
Let’s see what happens when I change C3 from .022uf to .1uf and C2 from 1uf to 4.7uf.
RED TRACE = Preamp with default values for C2 (1uf) and C3 (.022uf)
BLUE TRACE = Preamp with C2 at 4.7uf and C3 at .1uf
I’m going to keep the low frequency mod on this test PCB. I think this will make the upcoming bass frequency
evaluations easier to see. Now, let’s do a plot of the preamp showing the Treble, Middle and Bass controls at max.
Here you will be able to see the frequency and boost level of all 3 bands on this particular board.
RED TRACE = Preamp with all EQ controls at center position (flat)
GREEN TRACE = Treble boost at maximum
MAGENTA TRACE = Mid boost at maximum
BLUE TRACE = Bass boost at maximum.
Here’s what the boost/cut plot looks like for all 3 frequency bands.
RED TRACE = Flat EQ
GREEN TRACES = Bass
BLUE TRACES = Mid
MAGENTA TRACES = Treble
OK, so what if I want to move some of the frequencies around. As you can see from the MBEQ3 schematic,
the frequencies used for Treble, Middle and Bass references are 8 KHz, 600 Hz and 40 Hz. What if you
wanted to move the Bass frequency up or down a small amount. The capacitor controlling the bass frequency
is C1 (.022uf). Let’s see what happens when we adjust that value.
Frequency plot of the Bass control.
RED TRACE = Preamp Flat
GREEN TRACE = C1 value at .047uf
BLUE TRACE = C1 value at .022uf (Default Value)
MAGENTA TRACE = C1 value at .01uf
Now let’s play with the Mid band. We do this by adjusting the values of C4 (.015uf) and C7 (.022uf).
Here is a frequency plot showing the Boost and Cut range of the Mid control with different values
of capacitance for both C4 and C7.
Frequency plot of the Mid control.
RED TRACE = Preamp Flat
BLUE TRACE = C4 value at .015uf and C7 value at .022uf (Default Values)
GREEN TRACE = C4 value at .022uf and C7 value at .033uf.
As you can see, by increasing the values of C4 and C7, we can lower the frequency of the mid band
from 640Hz to right around 360Hz. If I wanted a frequency somewhere in between the default
frequency of 640Hz and the modified frequency of 360Hz, I would simply use a smaller increase of
capacitance from the default values. To have the Mid Band frequency go higher than 640Hz
I would need to make the values of C4 and C7 smaller.
So let’s play with the Treble frequency. We do this by adjusting the value of C18 (.01uf).
Here is a frequency plot showing the Boost and Cut range of the Treble control with different values
of capacitance for C18.
Frequency plot of the Treble control.
RED TRACE = Preamp Flat
GREEN TRACE = C18 value at .01uf (Default Value)
BLUE TRACE = C18 value at .015uf
As you can see, by increasing the value of C18 from the default value of .01uf to a value of .015uf
we have lowered the Treble control frequency from around 10 KHz to approximately 8 KHz.
If you wanted to move the Treble frequency higher you would simply make capacitance value of C18
a bit smaller.
Just a quick note on why I put the default frequencies where I did. The 3 Band Baxandall Active circuit
is very interactive. Adjusting one of the EQ controls has a small impact on the frequency and boost amount
of the tone control next to it. It’s not out of control or anything but it is noticeable. I tried to have as even
of spacing as I could while selecting the 3 frequencies of the controls. I started at 40Hz as my reference (Bass),
went up 4 octaves (80Hz, 160Hz, 320Hz, 640Hz) and used 640Hz as my Mid frequency. Then up another 4 octaves
(1,280Hz, 2,560Hz, 5,120Hz, 10,240Hz) and used 10KHZ as my Treble frequency.
Well, there you have it. I hope this all made at least a little bit of sense. I tried to keep this as light
but meaningful as I could. My goal was to make the M-Series preamp a bit more easy to understand and possibly
get people poking around, digging deeper and starting a conversation.
