Stroh Guitar - Model Z1.2

When I was a teenager, I reinvented the wheel. More precisely, a model Z1.1 guitar using the sound production principle that Augustus Stroh (1828-1914) had implemented in his Strohviol.

Wikipedia defines this violin as a violin whose wooden sound box is replaced by a round box containing a phonograph membrane connected to a trumpet horn, based on the principle of a gramophone. The membrane amplifies the vibrations and transmits them to the horn, producing a very nasal sound, meant to be heard from a distance, but primarily directional.

My Z1.1 guitar began its life in the 1970s when I repurposed parts from an old phonograph. The Z1.1 ended its life in a scout campfire... God rest its soul...

Phonetically, it was a disaster, but the idea grew in my head, to the point that I recently decided to experiment with a DIY phonograph pickup followed by a horn, all enclosed in a guitar body.

Since the guitar I had for the experiment was a toy, I didn't expect the Z1.2 guitar to be actually playable (I just wanted to experiment with a prototype), but to my surprise, I managed to get a few notes out of it... with a very metallic, cigarbox-like sound! The guitar-making principles are therefore validated, and future versions will probably be more elaborate. In the meantime, I had a lot of fun making all these innovations!

An old guitar near the end of its life. A guitar with a truss rod in the neck if you want to use steel strings.

Coffee can diameter 100mm

Standard DIY supplies. If you have a band saw, even better.

Heat gun is mandatory

Wood glue, wood putty.

For this Stroh guitar, it was necessary to:

1. Modify the body to incorporate a bell and pickup.
2. make an acoustic pickup based on a coffee can and PVC pipes (cigarbox guitar style)
3. make a support for the acoustic pickup allowing it to be mounted and secured in the body
4. make an acoustic horn with exponential dimensions, wrapped around the pickup
5. make an original body decoration
6. manage Assembly and adjustments

The body needed to be widened to accommodate the bell.

So I completely removed the acoustic top (using a heat gun and a spatula) as well as the top of the guitar.

Then I wetted the wood of the sides for two days to thoroughly moisten the wood and make it pliable.

Then I gently pushed it back with shims while moistening it (the process is a bit lengthy) until it took the largest possible shape.

Then I let it dry for several days with the shims.

Then I removed the shims, and the wood didn't budge! Then I plugged the hole by gluing a guitar back that was lying around in my basement (after having previously adjusted it).

The body is then ready to receive the sensor (see § The Acoustic sensor and § Acoustic sensor support) and the horn (see § Exponential horn. Calculation and layout).

I decided to install metal strings. This was risky because the guitar I had was a very fragile child's toy. Because of this, I decided to install "steel and silk" strings, which have a stronger gauge than nylon strings, but a weaker gauge than steel strings. As with steel strings, the center of this pickup must be at a distance equal to the length between the nut and the 12th fret + 2 mm.

The principle of this pickup is to transform the mechanical vibrations of the strings exerted on the pickup membrane into an acoustic vibration (a variation in sound pressure, if you prefer). The pickup must also withstand the vertical component of the force exerted by the strings on the bridge.

The sound pressure passes through a tube and goes to the bell, where the sound is mechanically amplified.

I cut the metal coffee can on the back side. I turn it over, and it now becomes the transducer membrane.

I drilled a hole in the center of the membrane to screw/glue a spacer onto which the bridge and its support are screwed.

Inside, I glued a PVC washer that locks and supports the transducer's closure plate, sealing the transducer tightly.

The closure plate also supports the pipe outlet device: it consists of a block from which a pipe extends (the block is made of several flattened rectangular PVC plates, glued together). The outlet of the pipe (which transmits the acoustic vibrations to the horn), with an external diameter of 8, is a small, curved brass tube, diameter 6.

The bridge support is a complex part. There is a fixed part consisting of a tube placed on the periphery of the membrane on one side.

There is a flexible metal sheet (beer can) that extends from the fixed part to another flat metal part, intended to be screwed onto the membrane on the other side. The bridge itself will be screwed onto this part. metallique souple (canette de biere) qui part de la partie fixe pour rejoindre une autre partie metallique plate, destinée a etre vissée sur la membrane d'autre part. Sur cette piece sera vissé le chevalet a proprement dit.

Ultimately, for the guitar to sound in tune with metal strings, the center of this pickup must be at a distance equal to the length between the nut and the 12th fret (=scale length + 2mm).

So I developed a support that positions and holds the pickup in this position. Everything is made with the rest of the metal box and PVC pipes. The PVC pipes are formed using a heat gun. I won't hide from you that this step requires a lot of precision.

It all starts with the bottom.

You need to make a flat, round plate (flattened PVC) with a diameter equal to the inside diameter of the coffee can. The center of this round plate must be at the length of the scale length + 2mm. Next, glue a PVC sleeve to the outside of the box (cut/heat/shape/glue pieces of PVC pipe) to support the sensor.

Then glue another layer to the outside of this layer to encase the sensor.

On my prototype, I glued an additional sleeve, which didn't prove useful later.

Naturally, the dimensions vary depending on the box used. So I didn't note them on the plan.

Note that shaping the PVC pipes is a delicate operation that must be done outdoors to avoid breathing the fumes from the hot PVC, as they are toxic.

Now I have the dimensions of the sensor mount and its location. I therefore know the space and volume available to install an exponential horn. The term exponential practically means that the small initial sound wave (which starts from a small 8mm diameter tube) will expand, expand, and amplify because the displaced air volume will travel a path that will mechanically amplify without distorting it. This is the principle of the megaphone, or the phonograph horn.

The sensor mount is 115mm in diameter => the exponential acoustic horn must have a minimum diameter of 120mm.

First, two exponentials must be calculated: one for the horizontal component of the horn and another for the vertical component.

I have planned 30 segments. Each segment is equal to the value of the previous one multiplied by a constant K1 for the horizontal component of the snail and a constant K2 for the vertical component.

The first segment is equal to 8mm (= the diameter of the pipe).

For the horizontal component, you need to add 60mm (corresponding to the radius of the pickup holder) to each segment.

The challenge lies in finding K1 and K2 by approximation, so that the resulting horn fits inside the guitar body. In my case, the body was about 85mm thick.

Rest assured, Excel does this very well. I've attached my spreadsheet as an example. To check that everything fit properly inside the guitar body, I drew the spiral on cardboard (you can cheat on the final dimensions: it won't have an audible impact).

Then I glued plywood slats to the lower part to stiffen the body (otherwise it would bend under the tension of the strings).

Next came the creation of the spiral.

After transferring all the dimensions to the bottom of the body, I made polystyrene supports (using a hot wire and a file) that allowed me to hold the 20mm wide wooden slats (my parquet flooring definitely provided me with a good raw material), and I glued the spiral slats.

In the photo, you can see that the centering plate for the pickup support has been glued in position (center = tuning fork distance + 2mm).

You can also see the sound outlet.

Then, for the vertical spiral, I glued aluminum sheets. (soda cans) cut to the height that Excel determined for the vertical exponential progression.

Then I placed a sheet of paper in place of the exponential horn cover to take the dimensions, and I transferred these dimensions to a PVC sheet (from a PVC pipe that I had flattened) and cut it.

After adjusting it, I glued it in place, and smoothed the surface with wood putty.

For the body, you need to

smooth out all the rough spots and apply wood filler.

Cut the acoustic top in half lengthwise.

Glue the top of the acoustic top. To fill the gaps, I reused scraps.

Painting: protect the acoustic top and paint the spiral.

Machining the other half of the acoustic top. Personally, I chose to make holes of different diameters for aesthetics and to improve the sound.

For the tailpiece, I used 0.8mm thick sheet metal.

I had made a tailpiece out of PVC, but it exploded because it couldn't support the string tension.

The space between the holes is 11mm.

The design was drawn on a laser printer, the toner was transferred with an iron, and the plate was engraved with ferric chloride. Unfortunately, the toner didn't last, and the result isn't exactly impressive. I'll try again with another process. In the meantime, I'm keeping this one.

The spiral design was drawn by hand, retouched on the computer, then printed and then glued on with varnish.

To secure the pickup mount, I cut a small PVC sheet that can be screwed (from the outside of the body) into the base of the mount. This secures it securely.

Then you need to mount the pipe onto the pickup, slide the pipe into the spiral, and slide the pickup into its mount.

Then you need to mount the bridge mount, screw it onto the pickup membrane, and then mount the bridge.

Then you need to install the strings. I chose Silk and Steel Medium 011-S048 strings.

Then you need to adjust the nut if necessary (string height at the 1st fret = 0.4mm).

Then you need to adjust the bridge height (string height at the 12th fret = 2.5mm).

Et voila!

https://youtu.be/ck_8G5yPsNc Freight train

https://youtu.be/B1eH9U2zeQU Les copains d'abord

https://youtu.be/Wuu8PjG8ahk Palchelbel

https://youtu.be/7_W2Yyxehyk Raquel

https://youtu.be/COni95XzQIw Petite fugue

The Z1.2 guitar is certainly not the guitar of the century, but this prototype exceeds my expectations, and the rewards of having created a playable, original, and unique instrument are immense!

That said, this project is reserved for experienced tinkerers. It takes time and a lot of discipline. The realization requires overcoming all sorts of technical challenges. ...

But the difficulty allows ideas for improvement to come naturally! A carbon fiber cone, for example! Can't wait for the Z1.3!!!!

Greetings from France!

Bruno G.