I have a few questions about piezo pickups with an active (9V battery) preamp. In a large body cbg, I’m thinking of installing a system with 2 piezo discs (or perhaps one under saddle piezo rod and one disc) wired in parallel to a blend pot (either mn250K or mn500k) and then to a 9V battery-powered active preamp with either two (1v 1t) or three (1v 1bass 1treble) volume and tone pot knobs. Here are my questions: (1) What value volume and tone pots do you use with a piezo/active system? 25K? 50K? 100k? (2) Have you ever installed an active preamp with piezo discs, or only with piezo rods? On a related note, ave you ever mixed rods and discs through an active preamp?

I realize I could skip the blend pot and use two volume pots, but that’s not my question, as I want to try with a blend pot. My main question is about pot values in a 9V active preamp piezo system.

For clarity, I’m going to build something similar to this item on eBay, except for a cbg instead of for a bass guitar: https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com%...

Thanks for sharing your experience!

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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.

Great reply, Timothy. Yes, you’ve given me several things to think about, and I understand why they matter. So, just to answer a few of your questions: I intend for the bass and treble controls to both boost and cut the frequency. But of course you are exactly right that I have to design it around the frequency band each is to control, so there’s not going to be just one answer to my question.

Yes, the blend pot output will feed into the preamp, but it is not a prebuilt module. A guy in the electronics shop of my maker space (MakeNashville) is going to show me how to build a preamp (I believe he said FET), so it will not come with pots installed. I will need to supply the pots, based on the sound I want to achieve. He knows electronics, but he doesn’t know guitar making or how to shape tone, which is why I floated the question here. You’re right, though. I can’t separate one aspect from the other. That seems obvious now.

Image result for active tone control preamp guitar circuit diagram

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.


Taff -

I love the idea of using multiple discs under the bridge. I have a couple questions:

In my original plan, I wanted to mix the under saddle rod piezo with a soundboard disc piezo, because I wanted to get some of the soundboard vibration sound, in addition to the vibration of the strings. I thought a blend of the two might be nice. If I go with multiple discs immediately under the bridge, does that capture the soundboard vibration enough to make the other soundboard disc unnecessary?

Another reason I wanted to blend an under saddle rod and a soundboard disc is that if I am playing somewhere that feedback or handling noise is a problem, I could shift the blend to less soundboard disc and more under saddle rod to accommodate it. When you use multiple discs immediately under the bridge, are they as prone to feedback and handling noise as discs elsewhere on the soundboard, or is this less of a concern in that location?

Man, great info so far. Thank you so much. I hope you’ll keep it coming.

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.

No, it it is an active onboard preamp powered by a 9V battery. The blend pot will be between the piezos and the preamp (and thus probably 500K, due to the high impedance of the piezos), but the volume, bass, and treble control pots will not be before the preamp (and thus will not need to be such high value, since they will not be in a high impedance system).

In any event, I think I’ve figured out the answer. I think I’ll start with 25K pots for the volume, bass, and treble controls, and if I think they need to be lightened up a bit, I’ll replace them with 50Ks.


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