School me on the Klon Centaur gain control.

[chumbox]

Hi All

I've been working my way through the Klon schematic to teach myself a few concepts and understand the inner workings of a 'larger' overdrive pedal.  However one thing has got me stumped.  Why does the gain control wiper connect to +Vb?  What is the purpose of this and how does it work/effect the gain?  I'm probably slow to the game but I don't feel I've seen this type of control/schematic before.



Is it just dumping to ground via this resistor and cap or is there something else going on?



Thanks in advance

[PRR]

It goes to AC/Audio Ground.

However all the stuff it connects to is sitting up on a DC voltage (half supply, "Vb"). If the pot wiper went to DC ground it would screw-up the DC levels on important points that need to be up at Vb.

VB is heavily bypassed (47uFd). It is a good ground for AC/Audio, yet the right DC level for the circuit.

[Groovenut]

Hi All

I've been working my way through the Klon schematic to teach myself a few concepts and understand the inner workings of a 'larger' overdrive pedal.  However one thing has got me stumped.  Why does the gain control wiper connect to +Vb?  What is the purpose of this and how does it work/effect the gain?  I'm probably slow to the game but I don't feel I've seen this type of control/schematic before.



Is it just dumping to ground via this resistor and cap or is there something else going on?



Thanks in advance

The gain control you are refering to has the wiper tied to the bias network to provide bias for U1B. For audio signals the bias voltage is essentially ground. When the gain control is turned all the way down, the C5/R6 combo is directly connected to the bias network (ground) forming a low pass filter for signals flowing through C16/R7. This also sets the gain for U1B to 4.14 for signals above the C7/R11 frequency knee but there is no output on U1B as the input is grounded. When the gain control is maxed, the C5/R6 becomes a high pass filter in conjunction with the 100k value of the gain pot (now connected pin 3 to ground) and sets the gain for U1B to 211 for signals above the C7/R11 frequency knee.

I hope that helps and is a good analysis. If not Im sure someone will chime in 

[chumbox]

Thanks Groovenut and PRR.  These are great explanations.  Once again even though this is not my favourite sounding overdrive, it certainly is a well thought our and somewhat original design to learn from.  The use of the gain control to also control a low/high pass filter arrangement is a great idea along with sending signal to an excellent grounding point.

Appreciate the input guys.

[Keppy]

Good analysis from Paul and Lawrence. Further thoughts:

When the gain control is turned all the way down, the C5/R6 combo is directly connected to the bias network (ground) forming a low pass filter for signals flowing through C16/R7. This also sets the gain for U1B to 4.14 for signals above the C7/R11 frequency knee.

Note that when the control is turned all the way down, the (+) input is connected to Vb, so no signal is present and gain is effectively zero.

The pot creates a voltage divider with R6. As the pot is turned down, the signal present at the opamp (+) input is reduced. This operation would normally be smoothest with a log taper pot.

The pot creates a voltage divider with R12. As the pot is turned down, negative feedback is increased and gain is reduced. This operation would normally be smoothest with an antilog taper pot.

Since the pot has two functions, the "not-smoothness" of the linear taper for one operation is canceled out by the "not-smoothness" of the other operation for a nice smooth overall response. The Bluesbreaker does a similar thing, using a linear pot that affects gain in two ways that would otherwise require log/antilog pots.