--- In DTXpress@yahoogroups.com, "epvk_jahoe" <yahoo@...> wrote:
> 1) there is only a slight difference between the large topright and
bottomleft circuits (the
> small bit just above IC2A). I would guess that I only need 1 of the
two - but which one? Or
> am I completely wrong?
They are the same. The difference is that they both share IC2. It is
a dual comparator but you only need one per channel. .
> 2) Is your comment about the reverse diode protection similar to the
simple DTXPander
> circuit, i.e. just after the input-jack add a diode?
No. That was a "note to self". I was just concerned about someone
being able to plug another power supply into the unit. While it could
take 40V of input, it wouldn't be too happy about someone plugging in
a power supply the wrong way round. If I made more I would put a
diode in series with the 24V input.
> 3) What is the small extra circuit just above that comment?
That is an LM78L12 - a small 12V regulator. It generates half the
supply voltage (i.e. +12V from the 24V power supply) which you will
see scattered around in the circuit.
Perhaps I should explain it's operation so you can see which bits you
really need. Bear in mind it is not just an amplifier, but generates
the rim switch signal as well and tries to be far too clever.
Looking at the bottom half (they are the same) IC1A and IC1B just
buffer the input with IC1A being the main pad input, IC1B is the "rim"
pad. IC1C amplifies the signal to the output.
IC1D is a peak detect for the rim signal. Comparator IC2A triggers
when the peak signal is above a threshold set by VR1, R15, R13. The
peak detect has gain and so is quite sensitive.
IC2A has an open collector output so can drive the Yamaha rim switch
input directly. However, in practice the pull-up resistor on the
DTXpress module is a bit weak (around 100k I think) so it can be a bit
sluggish, so adding R64 & R63 helps to pull it up to 5V quicker. Bear
in mind that the DTXpress only wants 5V on the rim switch so DON'T
play around with those values unless you know what you are doing. It
may be safer to leave off R63 & R64 unless you need them.
IC1B output goes through a lumped approximation of a transmission line
to delay the signal until AFTER the rim switch has been closed by
IC2A. This is the clever bit, but I am not sure how useful it really
is. Certainly if you close the rim switch too long after the piezo
signal it will ignore the switch, but I am not quite sure of the limits.
The signal from the delay line is summed in IC1A so the final piezo
output is the sum of the two inputs, with the rim one having more gain
than the main one (you will notice R11 is smaller than R6). This is
because the DTXpress will ignore small hits on the rim - so this makes
them bigger.
The whole circuit runs on 24V DC because the signals from the pads can
be up to 15V - hence the protection diodes on the input.
There - I bet that's now as clear as mud to everyone.
Keith.
