Piezo Bass Preamp

[Mark Wybierala]

I'm looking for a proven and reliable schematic and parts list to build preamps.

I've been experimenting with short scale basses using strings intended for the UBass and the Taylor GS Mini Bass. This has resulted in a couple of interesting instruments with both positive and negative results. Generally, the goal is a scale length of around 25". In playing around with this idea, I've converted inexpensive manufactured solid body guitars which just happen to fall into my possession for little or no money. The most successful result is from an Epiphone SG that had a broken neck joint. I repaired the neck joint, tapered and plugged the six tuner holes, applied a veneer faceplate to the headstock, and drilled for 4 bass tuners. I made a new pickguard and installed a flat-top acoustic guitar style rosewood bridge with a piezo element under the saddle. I used DAddario strings designed for the Taylor GS Mini Bass (DAddario part# EXPPBB190GS). I'm using an economy piezo preamp unit purchased on ebay with a built-in tuner. The result is a wonderful little bass guitar and if I adjust the EQ on the preamp to cut the highs and mids, I have absolutely no complaints about the tonal capabilities other than the dynamic response is a bit harder to control and that is mainly the result of my playing ability. The compression control on my Hartke bass amp makes this okay.

The strings for the short scale basses do not have a ferrous component in their construction so a magnetic pickup will not function. I've built short scale basses using a wide variety of available conventional electric bass guitar strings and nothing comes close to the intonation accuracy of these non-ferrous strings. There is no flabby tone or intonation quirks that routinely occur with scale lengths under 30 inches. The DAddario Taylor GS Mini Bass strings are essentially very fat classical guitar strings with a synthetic core.

I want to build a few of these basses. I'm not finding any premanufactured preamps that suit hand crafted instruments. Ebay is awash with inexpensive preamps designed to be installed into the side of an acoustic guitar. I've used quite a few of these and for under $30 they work great as replacements for broken preamps and upgrades to intermediate quality acoustic guitars. Unfortunately, a 2" X 2" preamp module is not what I want on an instrument that I design from the beginning to be a solid body bass. I want to locate my volume and EQ controls using more conventional style pots without having to compromise my design to incorporate a premanufactured preamp.

I have built a number of preamps from schematics and designs that I've found here on this forum and other places. Some of these have worked but none have been a great success. Often I find a design that claims to be good only to find later posts with modifications or corrections to the point that I loose faith. I'm looking for a design that works, is proven, and recommended by people who have built it. I'm looking for a simple piezo preamp with a 3 band EQ that employs current available semiconductors/ICs.

After saying that I'm looking for a basic dependable piezo preamp, is it reasonable to ask for guidance if I want to incorporate individual trim pots if I decide to employ individual piezo saddles such as the piezo Ghost saddles manufactured by Graphtech??? In my experience, they are fairly well balanced to begin with but I'd like the option -- I have experienced a problem with this before.

[David King]

Mark,

pardon my blunt response here, it sounds like you are hoping to reinvent the wheel but somehow end up with something better and cheaper than all who have tried this before. You are also hoping to achieve these results with very limited investment. I don't see what the problem is with the current crop of available electronics.
I suspect that one limitation to your design is that your strings won't have much tension unless you can miraculously increase the mass of the strings. (You could try D'Addario's Tungsten wrapped cello strings I suppose but the cost of those is going to stop you pretty quick.)
Have you considered other types of transducers? Schertler and Moses Graphite have both made acoustic saddle designs that use a electret condenser mic or two that wouldn't require the high input impedance of a traditional piezo transducer. They may not work any better than piezo for the low tension but they are extremely simple devices.
Know that the preamp and tone controls are independent so once you get your piezo input impedance down to a reasonable range any bass guitar tone circuit will work for you provided that the frequency centers are where you need them to be. Moving frequency centers is quite often just a matter of changing out a resister or capacitor.
Adapting an existing tone control with fixed pot locations by simply desoldering the pots and adding leads to the pots is still going to be a whole lot simpler than building your own circuit from scratch.

The whole point of the RMC and Ghost individual saddles is that they are sold as matched sets from the factory so that the need for trim pots is eliminated. If you find that they aren't providing even volume then the problem is not yours.

If you really insist on going through the years of R and D and testing and modifications and custom PC boards and sourcing ever rarer components that are actually big enough for you to hand solder then start with the opamp IC and try the manufacturer's recommended circuits that come free in the data sheet.
Generally you want low noise, low current, low distortion audio preamp with extremely high input impedance. ST, Texas instruments, Burr Brown and others have been working on these for decades. There are dozens of fora devoted to discussing this stuff ad infinitum so just dive in the deep end and start learning.

Meanwhile someone over at MEF just posted a very simple looking circuit on the pickup maker's board that uses only one fet. https://music-electronics-forum.com/sho ... hp?t=48575

I'll say once again that the ghost systems are very affordable to OEM clients so maybe give them a call. Bartolini and EMG both sell modules that will do everything you want and do it very well. They are also happy to serve OEMs.

Lastly consider collecting variety of used bass preamps off Reverb or ebay for evaluation.

[Mark Wybierala]

David, thanks for the response. I think you might have missed my intent although you did get some of it. Here's the SG I mentioned. I'm totally pleased with the result and the main thing I want to do is be able to have four normal looking knobs instead of one of these premanufactured preamps. I'm looking for a circuit design that I can build myself. I'm also an authorized Graphtech guy. I like the Ghost system. I've installed a few of them but for a number of reasons I don't want to use it. I don't want unnecessary plugs and jacks throughout the wiring preferring to have solid, dependable and repairable solder joints. As things are, the current piezo transducer which is the piezo coax style purchased by the inch works great with the DAddario nylon core bass strings. If I close my eyes and don't look the bass, I like it a lot.

There are under $20 worth of parts in most preamps. I just want to make my own on a circuit board and have the latitude to locate the control pots where I want for aesthetics.

[Mark Wybierala]

If I'm going to build instruments like this now that I'm satisfied with the proof of concept, I want the freedom to locate the controls as I desire without the constraints of the length of wires provided by the preamp manufacturer -- sometimes too short and sometimes too long. I don't want to construct the instrument around the preamp. My hand crafted instruments, my handcrafted preamp...

The Ghost saddles seem like a good idea. As far as the trim pots, I have encountered mismatch issues. I'd like the option just in case a saddle goes dead and a single replacement is needed. For what ever reason... I think I can simply use a passive trim pot on the input to the preamp but am not sure.

[Peter Wilcox]

I know you're looking for a proven circuit - I don't know the history of this one.
http://www.redcircuits.com/Page69.htm
You could probably substitute an 072 op amp if more available.

If you'd be content with the usual passive volume and tone controls, I'd think you could just use an input buffer followed by the usual control hookup.
http://www.muzique.com/lab/buffers.htm

[Mark Wybierala]

Peter, This is a complete schematic with a parts list. Thank you sir. I talked with Graphtech and they said that individual trimmer resistors should work fine installed as in-series variable resistors. I'll order parts and get back with a review.

[Peter Wilcox]

Hi Mark - I hope it works out - let us know.

You could forego the gain switch (SW1) and R2, R3 in the onboard preamp. Maybe try those other resistor values in a breadboarded circuit with your piezo setup to see which has the best volume for you.

[Beate Ritzert]

Here a really simple solution, which works really well. With a reduced input resitance i am using with magnetic pickups as well.
http://www.till.com/articles/GuitarPreamp/index.html

And here something for the Jazz Bass:
http://ozvalveamps.org/repairs/circuits ... reuzer.jpg

I could also contribute my own version with a PI tone control and switchabel characteristics. Two JFET stages.

[Mark Wybierala]

Posting here on this forum, its always a number of things I want to accomplish. Obviously in this case I am looking for information and resources but I'm also sharing what I believe is a really cool thingy. These DAddario strings designed for Taylor really work to make a short scale bass that overcomes the normal problems of a short scale (23 to 25.5"). I've been on this quest for a while now. There is a downside that the tension is significantly lower and the inability to use magnetic pickups. However, in perspective, the tension is far greater than the rubbery strings used on the UBass and that odd thing that Fender built called the Ashbory. These instruments are super cool and capable of wonderful things but to play them properly requires the development of a whole new discipline as they react inappropriately when dropped into the hands of an accomplished bassist at first. The very short scale lengths of 20" and less are too short to be comfortable for me and the rubbery strings require a lot of discipline and a light touch. On the other hand, a UBass will almost fit into the glove compartment, doesn't weight anything, and blows people away with true powerful deep bass noises coming out of a slightly swollen ukulele. I dig these things all to hell but I'm not wanting to invest my practice time to modify my playing style as much as I'd need to in order to perform with one and not everyone is going to have the difficulties that I do. The DAddario strings and the semi-short guitar-like scale of 23-25.5" is a nice middle compromise. I just want to put this out there cuz I'm diggin it. Another observation with these strings is that I've observed that playing partial or even full chords, the sweetness(?) or harmonic interaction is significantly more pleasant to the ear than any other bass I've ever heard. I'm sure that there is a scientific reason for this.

I've downloaded ExpressPCB and I'm transcribing the schematic (http://www.redcircuits.com/Page69.htm) into it to design the layout. I've used this before and I'm hoping that the free download is virus free. I've found all of the parts at Mouser and have a saved parts list. It'll be a few days and if anyone sees a problem in the schematic let me know.

[Barry Daniels]

Mark, did you ever check out the different versions of the MIMF pre-amp that Vlad designed for us 10 years or so ago?

[Mark Wybierala]

Barry, I got confused with the revisions. Looked for a post with a certainty of success or praise and got lost -- lost faith. Unless someone finds fault with the schematic I'm working with, I'm going to continue with it and hopefully arrive with a winner. I'm going to keep all of the documentation and part#s and hopefully be able to provide the forum a complete package of a successful preamp with parts source(s) and costs.

[Peter Wilcox]

Mark, you probably know how to do this, but you could also eliminate SW2 by using the guitar's output jack as a switch to turn the circuit on and off when the cable is inserted or removed..

https://www.stewmac.com/How-To/Online_R ... 000_8.html

[Bob Francis]

Mark, you probably know how to do this, but you could also eliminate SW2 by using the guitar's output jack as a switch to turn the circuit on and off when the cable is inserted or removed..

https://www.stewmac.com/How-To/Online_R ... 000_8.html

that would be what I would do too.

[David King]

Most 8 pin opamps have the same pin out so make sure you use an IC socket and then you can trade out DIPs (dual inline pins). I'd order an 062, 072, and 082 for starters. The 062 and 072 were introduced in 1978, the 082 in 1977... the specs have undoubtedly improved over the years but we are talking ancient history.

The thing to know about piezos is that they can generate big voltages and can easily overdrive a preamp at the start of a note. Rick Turner did some measurements on solid crystal piezos with a good oscilloscope and hit 100V with a hard pluck. Granted under-saddle sticks are usually ground piezo crystals in a gel substrate and the output is much lower but the point is that a single JFet transistor will do a much more musical job of handling peaks compared to a typical opamp. Bass strings can hit pretty hard and you might not be able to identify the clipping happening in the initial impulse of a note but but you know think it sounds bad and not know why.

[David King]

Is there a reason that the board-mounted pots in your prototype can't be removed and replaced with conventional pots? Also many of the bass preamp makers should be able to customize the lead lengths for tone controls if you're ordering in any kind of quantity. That said I don't think anyone will judge you if the leads are longer than absolutely necessary in a cavity.

[David King]

There is no indication in the redcircuits paper what the input impedance is. Also no indication of whether the bass and treble controls are shelving. Assuming a 6db/octave slope, a bass frequency centered at 30Hz while great for an open low B string isn't going to be ideal for an E string and above. Worth asking the author about these details.

[Beate Ritzert]

There is no indication in the redcircuits paper what the input impedance is.

But You can read it directly from the schematics: 150 kOhm. FAR to small for a piezo preamp.
Piezos need more that 1 MOhm. Even the 3.3 Meg of the Till JFET buffer i linked is on the low side.

As for the Opamps: the TL072 and TL 082 consume quite a lot of energy, and the TL062 is the noisiest (but should be sufficiently low noise).

[Beate Ritzert]

Addition (in German), quoted from a German forum - to link it does not correspond to our rules here. Please paste the text into an online translator if You can't read German. Background is an amplifier for LoZ pickups. The recommended LT1112 is pin compatible with the TL72, less noisy (the noise of an eventual large resistor defining the input impedance is larger than the noise of the chip itself), and it uses only very little electric energy. In a HiFi setting its GBP will allow for a usable amplification of around 30 dB. Which we will not need here.

We do need a *high impedance* buffer - amplification 1 or at maximum slightly more, maybe 1.5, a *passive* tone stack followed by a gain stage which compensates for the loss of the passive tone stack. And then a volume pot of about 25k or 50 k. I would actually suggest starting with a simple AMZ tone stack - one knob, yet really (!!) versatile.

Ein 10kOhm-Widerstand erzeugt bei 10kHz Bandbreite ca. 1,27µV Rauschspannung.
1kOhm entspricht 0,402µV,
100Ohm entspricht 0,127µV.
Ein schlechter OP-Amp (TL062) rauscht mit 4,2µV, ein guter (LT1112) mit 1,4µV und ein seht guter (LT1024) mit 0,3µV
(alles jeweils mit 10kHz Bandbreite und bei Raumteperatur 20°C = 293K).
Rauschquellen werden unkorreliert addiert, d.h. die Wurzel aus der Summe der Quadrate U=SQR(U1²+U2²+...+Un²). D.h. wenn eine der Rauschquellen deutlich größer ist als die anderen, dominiert deren Spannung alle anderen.

BTW: nicht mit jedem OP-Amp ist jede beliebiege Verstärkung machbar, Stichwort "Verstärkungsbandbreite" oder "Gain-Bandwidth-Product" (GBP). Beträgt GBP z.B. 750kHz (LT1112), so können bei 100-facher Verstärkung (+40dB) noch max. 7,5kHz übertragen werden.

Ich habe bisher 500, 750 und 1000 Windungen mit jeweils 0,15mm Draht gewickelt, ergibt ca. Rdc=130-300Ohm. Damit wird eine Standard Tonabnehmer-Spule (Gibson-Humbucker, 7ender-Jass-Bass) halb voll bis fast voll. Verstärkung 5-20-fach mit LT1112 (350µA pro Ämp) und alles wird gut.

[Mark Wybierala]

Using a DIP socket is a given when I build something like this. ESD, solder/resolder for future repairs and the low cost of a socket just makes it a good idea. I will be eliminating both switches and using four 2M trim pots as variable attenuators on the input.

[Mark Wybierala]

Replacing the SW1 setup with a 500K multiturn trim pot for adjustable gain.