Piezo Bass Preamp

[Peter Wilcox]

As for the low input impedance, you can change R1 to the megohm range, as R1 determines the input impedance. The gain of the circuit is determined by the ratio of R4/R1, so R4 would also need to be increased to above the R1 value depending on the gain you want - currently 470k/150k = 3.13. I don't know how the increased resistance would change the signal characteristics, which is why I'd suggest you bread board the circuit before implementing it, so you can substitute different components and see how it sounds.

If you are ordering stuff from Mouser, they have this one at a reasonable price if you don't already have one.
https://www.mouser.com/ProductDetail/Tw ... 3pWcedQ%3D

Here's one of mine in use (years ago when the bench was clean.)

I don't know any electrical theory besides Ohm's Law, which works for lots of stuff in life besides electronics. I just mickey-mouse stuff together, so if anyone has any additions or corrections to the above, please post.

[Mark Wybierala]

Peter, I kinda thought that the R1 option would work like that. I'm using individual piezo Ghost saddles, one for each string (http://graphtech.com/products/brands/gh ... ups---bass) and by the recommendation of their tech, a 2Meg trimmer resistor as an in-series attenuator on each piezo element. As a purchased set of four, the individual saddles are well matched from Graphtech but I want the option just in case an individual saddle needs to be replaced at some point. I'm at a similar expertise level as you but I have played around with etching copper clad boards (via youtube videos) with success in the etching but not so much with the circuit design. I repaired ECM systems in the Air Force (now retired) and have worked professionally as a guitar tech for almost 20 yrs. Unfortunately in the Air Force, the job was more swaptronics than engineering but I picked up a few chops. I just have a brain block when it comes to solid state circuitry. I have built nearly a dozen tube amps and have had some success in playing around with the original circuit designs.

I'm using a FREE computer design program called ExpressPCB (find it with a google search) which is provided by a company that will manufacture circuit boards for you as one-offs or in bulk. I've never used their fabrication service but their free software is a nice tool for doing the groundwork. There is a schematic drawing program and a circuitboard design program (both download at the same time) that takes a little while to grasp but certainly is helpful. There is an on-line help function providing most of the answers. It can be very detailed if you really take the time to input your custom components with the actual size and lead spacing parameters of the components in your supplier shopping cart via the data sheets for the components but its tedious as hell. I am actually doing this. I have not experienced any virus problems with the download. If this is successful, I'll be more than happy to share the *.sch and the *.pcb files.

[Mark Wybierala]

Correct me if I'm reading this wrong but I can't see the input impedance being less than somewhere around 650K due to R4. I can replace R4 with a 2Meg trim pot. The TL062CP opamp itself has a what the data sheet refers to as "input resistance" in the Tohm range.

[Beate Ritzert]

I do correct You 'cause You are wrong R4 does not at all affect the input impedance, it is *solely* R1.
The ratio of R1 and R4 determines the amplification

[Mark Wybierala]

My understanding is that impedance is the AC resistance of a circuit. I do not see a path that the signal input can travel that encounters any resistance less than that of R4. R1 is in series with only the inverting input of the opamp and the path provided by R4. The input signal does not go anywhere else other than these two components. The input of the opamp is in the Teraohm range so this path will only increase the impedance of what the signal sees and then there is path through R4 which is in series resistance to R1 and that elevates the resistance of that path (150K + 470K) to 620K at minimum and the tone stack is going to add at least 25 ohms.

[Beate Ritzert]

If Your idea was right, the input impedance would in fact be 150k||470k, i.e. even less.
Please read the (readily available) theory on OpAmps before inventing You own...

[Mark Wybierala]

I stand corrected.

[Mark Wybierala]

I did not fully appreciate the operation of the inverting input until you inspired additional knowledge acquisition. I honestly thought you were wrong but held out for the possibility that you were right and you were right. I do not wish to invent anything but rather wish to discover a solution to a problem. I need a preamp that I can build and consistently duplicate which suits having four piezo inputs and provides EQ options. So,... based on the fact that the input impedance of this design will be 150K and no less, I will look for another design. The positive side of this is that I have learned something thanks to your insistence/persistence/patience. Honestly appreciated.

[Peter Wilcox]

Mark, sorry you don't think this will solve your problem. I still think one possible solution might be to simply increase the R1 and R4 resistor values to the megohm range. You can make the first stage a summing amplifier by connecting all 4 piezos to the input, or you could add a high impedance summing amp https://www.electronics-tutorials.ws/opamp/opamp_4.html in front of this circuit as is, by using a TL064 (or 074) quad op amp which has all the op amps needed on a single chip.

Another solution would be to get a piezo guitar/bass preamp (they can be had fo $10 or so like this one https://www.amazon.com/gp/product/B00QW ... U6DA&psc=1 ), take it apart and reverse engineer it, or use the guts connected to your configuration of inputs and pots in your instrument.

Anyway, good luck in finding a circuit that completely suits your needs.

[Halgeir Wold]

Piezos in general want high impedance loads, and most piezo mic's solve this problem by using a FET transistor as an impedance converter. This might be one solution to the load problem. Another way is to use opamps in non-inverting mode, where you will benefit from the JFET or other high-Z input opamps. Gain will still be set by the resistor ratio, except that A=R2/R1+1 in non-inverting mode. So - either a noninverting quad opamp for the piezos, and the you can sum and equalize in later stages, or four JFET converteres and summing i a single opamp.. Beware, though, that not all opamps are unconditionally stable when gain approaches 1.. ( -or 2 in noninverting mode.. )

[Rob Kidd]

I'm looking for a simple piezo preamp with a 3 band EQ that employs current available semiconductors/ICs.

Well I haven't written on MIMF for quite a few years, but I thought I'd share one of my designs for this thread. It is exactly what you asked for.