I did this one a while ago and it has been posted on a number of forums but just realised I haven’t shared it here yet.
Update 22nd June 2017:
Some information that was very generously shared by Bjorn about this circuit
Hi Mark,
For your blog:
The original BJF SBEQ used
2N3819 transistors selected to give a source Voltage of approximately
2V’s and consequently then a Drain Voltage at a little over 6V’s. In
production SBEQ has used 2N5458’s, 2N5259’s, J113’s selected the same
way for Source Voltage at about 2V’s and Drain about 6V’s
Note that
with J113 that has higher yfs gain limit resistor of first stage was
altered from 1K to 3K Ohm’s. The Source Voltage and the Drain Source
Voltage set the headroom of circuit.
If say J201 or 2N5457 were
to be used in the circuit then connect a 10M Ohm resistor from B+ to
gate to lift the gate a bit and allow some headroom and for second
transistor then add an input capacitor of 47nF and thus allow to shift
DC on the gate of the second transistor.
When biasing check for about 2V gate range to allow circuit to run fairly clean.
You
can replace the lower resistor in Bass control circuit to adjust mid
point- note that center frequency is about 740Hz and so when you have
Bass and Treble turned down you get a peak filter with 740Hz center but
when you use mid control as described you won’t access the exact
frequency since the lower resistor off sets the whole filter.
Coming
soon is a version of SBEQ with Mid control and that is made with a
gyrator connected at Source of first stage and then gain limiting
resistor is increased to 5K1 and the gyrator only makes boost at center
frequency but it allows both Treble and Bass to be boosted while
retaining mid focus and for anyone who wants to build their own it would
be very simple to add a gyrator centered at 800Hz with a Q of 1,6 and
thus neither Bass or Treble can affect Mid control but it will be
centered at reference frequency.
Decoupling of second stage is
set to shift full bass boost from 50Hz to 100Hz by introducing a 6dB
slope and thus limit the bass excursion to 15dB instead of 20dB by the
roll off that then makes a hump at 100Hz; equally the Drain filters
serve to limit excursion of treble at 7KHz coinciding with the bandlimit
of a standard tape head and also with a standard guitar amplifier
speaker and this limit was set by ear from listening to a 70’s HiFi
speaker defining where loss of’ vital’ treble would occour……
You may post this as my answer on your blog should you so wish
At your service
BJ