CV controlled resistance//opposite result obtained

jacopo1919

Joined Apr 12, 2020
107
hi everyone, i need an advice!
i'm relpacing this part of the decay section of a 808 bassdrum (pic1).
IF 500k resistance on decay pot= max decay, whereas at min. pot resistance= min. decay:
In order to voltage control that resistance, i'm using a vactrol (pic 2) but here my problem comes:
as more CV as i get, as less resistance i get out of the vactrol (more CV --> less decay, which means the opposite of what i want).
Any help on this much much appreciated!
cheers

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crutschow

Joined Mar 14, 2008
34,060
What is the range of the control voltage?

jacopo1919

Joined Apr 12, 2020
107
Hello there
Does something like this sound good.
Version 1 by MR 808
Whenever Roland Service Notes speak of a “trick”, you know it’s going to be cool. In this case, it’s what gives the 808 & 909 their characteristic attack (that “crisp” bass sound). Another fun feature of this circuit is that it goes slightly flat (just like a real drum). Listen to the the 808 at maximum decay BOOOOooooommm…. hear it go flat?
Check This out & tell me what you think?
http://smd-records.com/tr808/?page_id=91
thanks! good stuff indeed, but basically it explains where are the G points of the decay section.
what i'm trying to achieve is control that decay resistance via voltage source!
What is the range of the control voltage?
will be approx 0 to +5v

crutschow

Joined Mar 14, 2008
34,060
will be approx 0 to +5v
Okay.
You could add another transistor to invert the signal (0V input gives maximum control current to the VACTROL).

LTspice simulation of an example circuit below.
D1 represents the VACTROL input diode.

Veracohr

Joined Jan 3, 2011
772
Edit: crutschow got to it first.

So add a circuit between the CV input and the transistor that inverts and shifts the CV so that 0V CV input gives max voltage at the transistor base, and max CV input gives minimum voltage at the transistor base.

jacopo1919

Joined Apr 12, 2020
107
oh guys those advices are gold. will try out.
at the moment i worked it around this way. Is using an inverting summing ampand it works ok...( would love a comment about this solution)

Your suggestions looks much more elegant though

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Veracohr

Joined Jan 3, 2011
772
The buffer between the summer and the vactrol is unnecessary. I would instead buffer the offset pot from the summing amp. I would also move the CV pot to the input, that way you still have the same 100k input impedance, but now the pot is buffered from the summing amp.

crutschow

Joined Mar 14, 2008
34,060
Does your circuit give the desired proportional control of the decay versus input voltage?
If not, then there are ways to change the relationship between the two.

jacopo1919

Joined Apr 12, 2020
107
Does your circuit give the desired proportional control of the decay versus input voltage?
If not, then there are ways to change the relationship between the two.
at the moment this is what i came up with and is working well! the decay is quite proportional to the CV input.
curious to know what you meant with those ways to change they relationships
in the next days will try out what you suggested!

The buffer between the summer and the vactrol is unnecessary. I would instead buffer the offset pot from the summing amp. I would also move the CV pot to the input, that way you still have the same 100k input impedance, but now the pot is buffered from the summing amp.
indeed that was unnecessary!

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crutschow

Joined Mar 14, 2008
34,060
curious to know what you meant with those ways to change they relationships
Instead of having the vactrol control current proportional to the control voltage, you could add some elements, such as a diode or two, to make the current non-linear with voltage.
But if it's working okay for you as is, then there's no need to go into that.