Piezo Preamp + Mute True Bypass + Clean Boost + DI Box

[antonis]

I really don't know why that cap is there and what impact it's value has in the circuit.

It contributes to Low Pass Filter formation with R3 & R7 (additional to whatever similar filter exists on power supply) for stabilizing Vref value..
It's corner frequency (-3dB point) is calculated via 0.159/[C8*(R3//R7)] formula..
As it is, corner frequency is at 30Hz..
(high enough for a voltage that has a pretention to be considered "pure" DC..)  

You might take a look here for further interpretation..

[antonis]

Another thing that I noticed is that this Barcus Berry 3000A Baxandall circuit is slightly different from the one in the article I have mentioned:

Mine:


Article:


Not only there are 2 capacitors instead of 1 for the Bass, but they are connected to R3 (in my case R4). Why is that?

There are quite a few versions of original Baxandall tone control..

The one with a single cap on Bass control is called "Americanized"..

P.S.
You can also meet variations on items values ratio for the same configuration..
e.g. a dual cap Bass control with values R1/R2 = C2/C1 = 10 and Logarithmic Bass pot or a resistor in place of C7 and capacitors in place of R8 & R9..

[antonis]

Yes, the idea is that the Boost potentiometer doesn't interfere with the preamp signal, only with the Boosted signal.

That's already obtained via Boost On - Bypass switch..

Anyway, if you like to have Boost permanently ON (although I can't see any obvious reason) delete Boost1 pot and make R18 a 50k pot (wired as variable resistor)..

Both the initial and this configuration should have the same result for undistorted signal..
(I presume you understand the difference between amplification and amplified attenuation..)

To make myself more clear:
With 50k pot turned all the way down, you have unaltered Preamp output going into DI input..
(Booster acts like a unity gain buffer..)
By turning 50k pot up, you add extra amplification on Preamp output..

And you save a Switch pole..

[fryingpan]

The Barcus Berry circuit is a lot better.  It has a high input impedance, which a piezo sensor needs.  It has a Baxandall tone control and an output level control, all of which are useful.

Rod Elliott's circuit should actually work even better, because it is configured as a charge amplifier, and therefore it is basically tracking the very element a piezo actually works with. In his notes, he says that the high impedance "regular" circuit and the charge amplifier both work well, but the latter is basically noise free. You do need a good opamp, though.