A *possible* Blue Box improvement

[Mark Hammer]

With a bunch of Blue Box related postings, online and offline, I started thinking about some of the problems with it. 

One of the problems is that tracking is often a matter of sensitivity and there is no way to adjust sensitivity on the Blue Box.  Another problem is that strings and notes (high vs low) are different in their amplitude, so what is above trigger threshold in one place on the fingerboard might be below trigger threshold at another.

So here is a *possible* fix.  I say "possible" because I haven't tried it out, but in principle/theory it should help a few things.

The default gain on the input stage of the BlueBox is 471 from the subaudio range up to around 1100hz where the 300pf feedback cap starts to roll off response.  The gain is set partly by the path to Vb through C5 and R5.  These two components (1k and 10uf) produce a rolloff around 16hz.

We don't HAVE to have a single path to Vb.  We could have a second one that uses different components to produce a different gain and rolloff.  If you have a second 1k resistor in series with a .22uf (220nf) cap to Vb, that produces a path with a rolloff around 720hz.  Dual paths to ground/Vb are often used.  You can see one in the Rat and Expandora.

We're going to make one of these variable.

Wire up a 10k pot so that the wiper goes to the inverting pit of the input stage (pin 2).  One leg of the pot goes in series with the existing 470k feedback resistor, and the other leg goes to R5, which is now changed to be 680R.  An added 6k8 resistor is placed in parallel with the leg of the pot between the wiper and the 680R resistor.

Now add a 1k resistor and .22uf cap to Vb from pin 2.

What do you have?  The gain from 720hz on up is fixed at the typical/stock x471.  The gain below that point can be adjusted with the pot.  With the pot rotated to minimize the Vb leg's resistance, the gain is set to (470k+10k+680R)/680R = 706, with a low end rolloff around 23hz.  In this position, the extra path to ground has little influence.  As the pot is rotated the other way, the parallel resistance of the 6k8 resistor and pot leg are added to the resistance of R5, and the gain of the full input signal is reduced to a minimum of 4.04k, which gives us a gain of (470k+4.04k+680R)/(4.04k+680R) = about 100.

Ah, but the 1k+.22uf path still exists, so even though the gain may now be only x100 for the low end, for things above 720hz or so, the gain is still 471.  So, this change allows the overall sensitivity to be increased in one direction for the entire spectrum, and in the other direction it allows for the upper notes to have a certain advantage over lower ones.

It is quite possible these specific values will not work for you, but the principle seems sound enough.  It may be necessary to change R5's value, and perhaps the value of R8 or even C3.  I guess you'll see when you get there.  For now, it seems like a simple adjustment that can match the Blue Box to a variety of operating circumstances.

Let me know if it works.

[KMS]

I haven't looked at the Blue Box (don't know what it is) but threshold-trigger-frequency roll off controls would be nice to have on my noise gate.