Hi Brian, yeah I always understood 25k pots in active systems.
its not that simple when you are designing an active preamp. the pot values will depend on the capacitors used and the center frequency of the frequency band you want to affect. it depends on the type of circuit you are using. will the preamp be op amp based or FET or transistor? how many stages of boost are there. will the bass and treble boost and cut the freq? do you have a circuit in mind? even a simple bass and treble cut tone circuit needs to be designed around the cutoff frequency . will treble cut start at 1KHz? 3KHz? 10KHz? bass roll off at 80Hz? 200Hz? 500Hz?
the blend pot is basically a mixer. the input of each pickup goes to one of the outside legs of the pot and the mixed signal comes out of the center or wiper contact then fed into the preamp. this pot has to be a linear pot not a volume/log pot sometimes also called a audio taper pot. your tone section will likely be in the feed back/gain loop of the preamp. then a master volume pot goes to the output jack. eventually to the guitar amp.
your original post said you were feeding the output of the blend pot to an active preamp. is this a prebuilt module? if it is it probably already has the tone and volume pots in it.
here is a good example. this is stereo but just ignore one channel..lol
the tone controls are inside the feedback loop that controls gain. its been a few years so i am not sure if this has both cut and boost at the desired frequency . here is a link to the page i found this it might help your friend or at least give him ideas..
these type of tone circuits are based around a fixed frequency. if you want to cut and boos but also shift the center frequency then you will need to look into parametric tone control circuits. but that would add an extra 2 pots.lol 1 for cut/boost and one for frequency. double that for bass and treble add in one more for balance of the two pickups and 1 more for volume..wow thats 6 pots on 1 CBG..( gonna need a big box) lol
here is another one , this might be better as it has a 10M input impedance which is good for piezeos. the TL072 is a 1 package dual op amp with very good noise characteristics. http://www.zen22142.zen.co.uk/Circuits/Music/guitar_tc.htm
Hi again Brian, I see you also asked about using piezo disc's with a preamp. My preference when working with full size acoustic guitars is for disc type soundboard transducers, and always with a preamp.
I do not use the cheep flat disc's that I use in a CBG. The ones I use in mine and others guitars are far more expensive and are encapsulated in a plastic "jacket" or covering and have substantial wiring. Three are used under a 6 string bridge.
With piezo transducers the idea is to recreate the acoustic sound of the guitar as faithfully as possible. This is better achieved with a soundboard transducer rather than the under saddle type. One reads mainly string vibrations, due to being so close to the string [but gives the added "quack" sound so disliked by acoustic guitarists]. The other of course reads the top timber vibrations, the main producer of the acoustic sound we hear. However it is also not without certain drawbacks.
Ok Bryan, I'm not big on design and complex details of electronics soooo........
i follow the guidines of the manufacturer and place 3 discs on the bridge plate as instructed, I figure they have done the testing for me so why reinvent the wheel.lf want to blend with anther transducer or magnetic pickup I use a dedicated panpot, as they would have worked out the specs on that too. And if I remember correctly they have centre detent position.
But with a cbg there are many physical differences that would effect a piezos response, so trial and error method may be best for your situation.
If you get feed back there are few options an eq normally has controls for this, you can cover the soundhole/s and the method you suggested.
Not much help, sorry.
So if I'm getting things right, we're talking about a passive EQ to adjust the tone and a mixing stage.
The hard thing will be figuring out the value of the caps to use.