[sdiy] 4017 sequencer

[Theo]

Sounds like you have tied the wipers of the pots together.
You need to add a mixer stage after the pots.
For this you need a extra negative power supply line and a dual opamp IC
(e.g. TL072).
Adding both -12V and +12V for the opamp is even better.

Afraid for a real simple sequencer a 4516 + 4051 combination is the easier
way to go.
This system can work on a single supply without any mixing/buffer opamps.
Also HEF or CD 4xxxx series IC are probably a better pick cause you can run
these on 12 or 15V covering the whole CV range.

For your 4017 approach you need to "mix" the CV outputs of the pots.
First connect a resistor to the wiper of each pot, 100k for 1k or 10k pots
should do.
The resistors come together at the inverting (-) input of a opamp.
Between the (-) input and the output of the opamp you put a feedback
resistor of 100k and a 33pF cap in parallel.
The (+) input of this opamp you connect to ground.
This circuit mixes the outputs of the pots together and amplifies the
signal -1x.
So now your output is 0V to -5V.

To fix this you need a other inverting opamp stage:
10k from the output of the first opamp to the (-) input of the second opamp.
10k feedback resistor over the second opamp and again put a 33pF cap next to
it.
Connect the (+) input of the opamp to ground, and add a 1k output resistor.
The 33 pF caps and the 1k output resistor are there to prevent oscillations
of the opamp,
for this reason you also need to place 100nF caps to ground close to the
opamp power pins.
Now your output will be 0V to +5V, or about 5 octaves range.

You can change the pitch range by changing the value of the feedback
resistor(s) around the opamps.
Best place might be the second opamp.
Change the 10k feedback resistor to 5k (4k7) to narrow the range to about
2,5 octaves.
Change to 20k (22k) to get about 10 octaves range.

Another trick you can play is to add a detuning pot to the circuit.
Connect a pot between positive supply and ground, mix the wiper though a
resistor with the other pots.
Now you can detune your sequences with a turn of the knob.


HTH.
Theo


----- Original Message -----
From: daryl groetsch <groetsch555 at netscape.net>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Thursday, March 13, 2003 6:01 AM
Subject: [sdiy] 4017 sequencer


> I'm working on a 4017 sequencer (actually a 74HCT4017N, all i could find
> locally). It's very basic, clocked by a 555 and each number output 0-9
> has a 1K pot on it, then to a CV jack. I'm running it on a +5V computer
> power supply. My problem is when I change the pitch of a step, the pitch
> of others are affected. All pots and jack are grounded, and I've tried
> various VCOs.
>
> Also, even with bigger pots the pitch range is about 1 octave. How  can
> this be increased?
>
> Thanks much in advance!
> Daryl
>
>
> http://www.synthnoise.com
>
>