BCbox's VCA "Click be Gone" Mod

[Altitude]

Thanks to Brian for this.

The VCA click that is apparent in many x0xen is the result of a DC offset on the 6110 pin 2. Looking at the schems on the BCbox site, there is a 50KB trimmer and a 10m resistor in series to adjust for this. Convieniently enough, there are unused pads for the BA662 right next to the BA6110.



All you need to do is to bend the outside pins of the trimmer to fit the adjacent pads (GND and +12) and the middle pin needs to be bent up and soldered to the 10M resistor lead. The other lead of the resistor simply goes into pin 2 of the BA662 pads. Once in, all you need to do is to adjust the pot till the click goes away! There is a sweet spot so it helps to turn the OSC off (or decrease the cutoff to silence if you have that mod).

[Luap]

Thanks for posting this Altitude. I will try it soon on one of my x0xb0xes that shows a slight click problem.
I fixed it on my other x0xb0x by changing the VCA to a BA662. But in hindsight, I see this was perhaps more by luck than anything else.. (Also a costly fix).
I had tried another BA6110 in there too, but that didn't appear to help much, if any..
Heres hoping this does the job

[Altitude]

it works like a dream. Thank Brian, not me.

[Luap]

Thanks Brian

Right, so I got the parts today and got them fitted. With the oscilattor disabled (I just held the waveform switch in a middle position) the mod certainly allowed me dial out some clicks to the point i'd get total silence. Which is good!
However, on my x0x it appears the clicks I can hear are coming from elsewhere, as they are still there. Which is bad The mod seemed to get rid of noise that must have been barely audible before, yet left the main click unchanged.
This only happens with the filter reduced to low levels, and perhaps res too. It's also slightly random, because it doesn't happen on every note (which I notice seems to be a fairly common report when people describe clicking x0xb0xes).

Here is a quick demo I recorded, with the res and filter knobs at minimum: http://tinyurl.com/x0xclick
Quite audible, and not correct, right?

See the waveform containing 4 notes from the sample above:

(Sorry its a bit fuzzy)

The first note does not click at all. Notes 2, 3 & 4 do click. All I noticed is that the clicking notes all start the first half of the waveform cycle as a negative value first. And that first half of the waveform appears to be sharper/spikier looking than the rest of the waveform, which you don't see on the non clicking note 1. It's like a very brief higher frequency spike, which must be the click im hearing.

Given that I now get clean click free silence with the waveform switch held in a middle position, then im thinking this particular click must be coming from the oscillator (or possibly filter?), and not the VCA.

I know Guest mentioned he was looking into this click gremlin, so hopefully I provided some good clues in here..
Although my example was a particularly bad patch that I cropped out of a longer sequence. Most of the time with the knobs in other positions, it is not noticeable, least of all in a mix with lots of other instruments. Still, if the click could be reduced, it would be nice.. Any ideas?

[dnigrin]

Check out Brian's comments on the various types of clicking... http://alphazone1.com/x0xi0/support/ - the clicking discussion begins in the "excessive clicking" section, and then more at the very bottom....

[antto]

Luap: some notes click more than others: this is very simple - because the oscillator is freeruning and when some notes start - the waveform is somewhere near zero (doesn't matter if it's on the positive or negative side) while others - not (so they click more)
the thing is all notes click, just you can't hear the clicking that much on notes that start at a point in the waveform near zero

why do they click? it's the VCA Gate signal
now, i'm not an electrician, but i know how this Gate signal looks on a TB-303 that doesn't click
it's strange because on the 303 it looks like the Gate signal is filtered by a LP-filter (or is that just the slew of an op-amp - i don't know exactly, but the result looks like it's filtered by a LP)
on the x0xb0x - it's like the opposite, the Gate signal doesn't raise up "first-fast then-slow" but "first-slow then-fast" and stops at once which is very bad

in your audio example - check your very last note to see what is the shape of the Gate-attack.. the curve looks like (x*x) while it should be more like (1-((1-x)^2))

one thing i'd try to do if i was good with electronics (and i'm not) would be to find where this damn Gate signal is and put a LP filter on it
the exact frequency of this filter - i can't say right now.. it has to be as high as possible as long as it doesn't sound clicking

[phono]

The 303 also clicks but no where near the levels on the xox

[Luap]

Hey dnigrin, thanks for that. I had a good read through it, and for the most part it was discussing noise related to the BC mods, which my x0x doesn't have. Still, there were 1 or 2 clues in there.. Like LED's/switching possibly causing some noise. My x0x currently has the wrong resistors in it for the LED's which means they are drawing too much current, so at the moment I am busy swapping all 40 of them out. Maybe it will help, maybe it won't!? We'll soon see.

Antto, thanks for your post too. You say you are not good with electronics, but evidently you know more than I do! I got the gist of what you were saying, but I confess that I didn't understand everything entirely. Still, it was interesting and i'll certainly investigate more.

Thing is, my other x0xb0x doesn't do this. Or at least not nearly as much anyway. Yet they are nigh on identical inside. The main difference I can think of being the clicking b0x has mostly 536 transistors in it (and the wrong led resistors). My non clicky b0x uses the 945 transistors.

[Altitude]

Paul,
you very well be on to something there. My first x0x years ago was the stock kit and it clicked like crazy also.. Since moving to 945s and 6 x0xb0xes later, it is not an issue anymore

Interestingly, the shurti-1 has a similar problem and the solution there was to put a cap between the trigger out to the env to smooth out the rough edge. Wonder is something similar would work here..

[antto]

here's an audio (4.4MB wav) of the same pattern on my x0x (which clicks bad) and a 303*
audio contains "no_reso" first (x0x then 303) and "reso_full" (x0x then 303)
as you can see from the image the "attack" of the VCA gate on the x0x is odd
on the 303 it's quite normal and smooth, yet fast and not clicky
i've drawn with red how the Gate looks like

the second part "reso_full" demonstrates how the x0x is still audiably clicky when cutoff is low but the clickiness is then hidden away from the higher cutoff (when i raise the EnvMod knob) .. then the "303" notes still sound "sharp" but they don't click otherwise

* the "303" is actually my soft-synth but the Gate and VCA behaviour is fairly close approximated to darffader's 303

[phono]

Paul,
you very well be on to something there. My first x0x years ago was the stock kit and it clicked like crazy also.. Since moving to 945s and 6 x0xb0xes later, it is not an issue anymore

Interestingly, the shurti-1 has a similar problem and the solution there was to put a cap between the trigger out to the env to smooth out the rough edge. Wonder is something similar would work here..

a cap and a resistor would technicaly make a slew limiter so in theory it would work

something like this (this is technically a low pass filter, but the principle is the same)

[antto]

Paul,
you very well be on to something there. My first x0x years ago was the stock kit and it clicked like crazy also.. Since moving to 945s and 6 x0xb0xes later, it is not an issue anymore

Interestingly, the shurti-1 has a similar problem and the solution there was to put a cap between the trigger out to the env to smooth out the rough edge. Wonder is something similar would work here..

a cap and a resistor would technicaly make a slew limiter so in theory it would work

something like this

exactly what i called a "LP filter"

[phono]

Paul,
you very well be on to something there. My first x0x years ago was the stock kit and it clicked like crazy also.. Since moving to 945s and 6 x0xb0xes later, it is not an issue anymore

Interestingly, the shurti-1 has a similar problem and the solution there was to put a cap between the trigger out to the env to smooth out the rough edge. Wonder is something similar would work here..

a cap and a resistor would technicaly make a slew limiter so in theory it would work

something like this

exactly what i called a "LP filter"

lol i had just edited my post, but yes thats correct

[Altitude]

So the million dollar questions are what values and where?

[antto]

So the million dollar questions are what values and where?

i can only tell the value for the digital LP filter i used in the synth, in form of Frequency
in theory, the digital 1-pole LP is equivalent to the 1-pole Resistor-Capacitor LP filter, and there is a formula to convert the frequencies and stuff

just checked my code, it looks like i used 132Hz there
now we can emulate this digital filter in the analog domain ;P~