Warning: Long nerdy post.
So I have had a bit of time play with the newly released Klark Teknik
(aka Behringer) BBD-320. This is KT's clone of the classic Roland
BBD-320 Dimension D rack unit. Straight out of the box it sounds pretty
good. But I did detect some differences so it was time to have a look
inside. Of course, being surface mount I've not been able to trace out
the whole schematic but I've looked very closely at what I think are the
important bits.
The circuit as expected is very similar to the Roland. The key
difference being that the Roland used MN3007 BBDs running from 0V and
-14V while the KT uses the V3207 running from +8V and 0V. This doesn't
really have a massive impact on the sound of the unit on its own but the
swap from a negative supply to a positive one does have an important
effect on the high frequency VCO circuits that directly control the
delay time of the BBDs.
In both the original and the clone, the low frequency modulation
oscillator has an output that goes from around +3V to -3V. This is piped
through a resistor (or two) to the VCOs that control the delay time. Now
because of the different operating voltages in each unit, Roland's BBD
control circuitry always sees an LFO signal above its own operating
voltage, while the KT sees the LFO signal below its operating voltage.
For Modes 2, 3 and 4, this change in BBD supply voltage should not
affect operation too much if the correct resistor vales are used in the
KT's VCO circuits. However, Mode 1 uses a simple FET transistor and
resistor to change the depth of the LFO modulation. When engaged in Mode
1 this crude circuit not only lowers the modulation depth but also runs
the VCOs at a much lower frequency. Thus in the original unit the delay
in Mode 1 is significantly longer than in Modes 2, 3, and 4. Whether
this was a deliberate design decision by Roland's engineers who can say.