Barber Gain Changer schematic link, and few questions

[nocentelli]

Here is a link to Barber's Gain Changer pedal page:

http://www.barberelectronics.com/gainchanger.html

David Barber has provided a schematic (wonderful man!) but requested it not be directly linked without permission, so i have redrawn a portion to illustrate my questions:



I *think* I understand how the gain switch works in principle: When the switch is down, there is a constant boost to the upper (or upper and mid?) frequencies produced by the 10n/4k7 and 22n/13k3 in parallel connected to the 4.5v vref.

When the switch is up, the vref is becomes 5v, and an additional pair of cap+resistors is connected to the new vref, which, having a lower cut-off frequency, boost perhaps the full range signal, hence the gain boost.

Now to the questions: Does using a pair of cap+resistors in parallel give a more complex EQ curve than a single cap + single resistor? How would one calculate or approximate the cut-off frequency for such a parallel pair?

(I've searched the net extensively, but can't seem to find the appropriate search terms - "RC filters in parallel" yields lots of pages of theory about parallel RC filters, i.e. with resistor and cap in parallel).       

[thelonious]

Basically, it provides a "stepped" boost where the frequencies passed by 68n are boosted slightly more (1M/2k2=454 max gain) than the frequencies passed by 100n (1M/2k4=416 max gain). But I guess in practice it would be higher gain than that because at the frequencies where they overlap the resistors would appear in parallel?

[ashcat_lt]

They're all separate filters.  Figure the cutoff separately for each with the gain pot as R and the cap as C.

Edit - these are of course all shelving filters, and the whole thing is "upside down" because it's negative feedback.  What I did when I made the digital version of the Rat was for each of the RC pairs made an HPF with cutoff set by the gain pot and the C, then mixed in the original broadband signal attenuated by the voltage divider created by the gain pot and the R, and the basically divided the whole thing by that same voltage divider, so that the whole curve is gained up to where the flat part is unity.  Then average those curves together - basically add them all up and then divide by the number of filters so that the flat part is still at unity.

That will give you an idea of what the opamp will try to do, and will look a lot like the curves we're used to seeing for the TS.  It leaves out the gainbandwidth properties of the opamp, which acts like an LPF across the whole thing.

[thelonious]

You could probably sim it and get a pretty graph in https://www.circuitlab.com/ or a SPICE thingy of your choice.

[ch1]

I have a question about the EQ switch around the tone control. Is this schema as Barber posted correct? Why switch the two resistor in parallell with the 100k? Isn't this the same as if you just switch between one 8k2 or 13k7 resistor instead?


BTW: Are there any ready pcb for this one?

[nocentelli]

I suspect it's just an easy way to roughly double or half the effective resistance with a single pole, no clicking, and maybe fits easily on the layout. There are several incidences in Barber circuits where I am puzzled at the choice of uncommon resistor values.

[ch1]

Well I just read the manual for the GC and maybe the schematic isn't correct since the manual says it's a three position switch. Or maybe one position is with none of the resistors connected. Or should they be in serie in stead?

" the flat sweet-EQ of the LTD SR , the vintage snarling-fat-EQ of our classic LTD v2 or a new voice that combines flat EQ with a little added fatness"

[nocentelli]

It will be a centre off switch, i'm willing to bet Barber got his own schematic correct.

[ch1]

Yes off course he's right. I just didn't think about that alternative with on-off-on switch.