Questions Regarding Horn Design and Dimensions

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TN Allen
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Re: Questions Regarding Horn Design and Dimensions

Post by TN Allen »

This is an interesting possible project, one I probably should not pursue, given it would also be a sinkhole for time, effort and material, nonetheless, I've worked out a CAD file for the full length horn in Modern Gramophone and Electrical Reproducers, and thought through some of the obstacles to producing it. I am fortunate in that I teach machine shop and design courses at a small engineering college with access to a Haas CNC machine, a Dimension 3D printer, and a 4X8 CNC router. This project may become fodder for a few courses. I used an Edison in a design course once, played a disc, then removed the soundbox and had a student play his iPhone through the horn. For the most part the students had never seen a "record". Perhaps a large exponential horn may be a useful educational object.

The Dimension printer can do the red and blue long small diameter pieces, the Hass can mill the intermediate 90 degree bend in several pieces of foam. I am considering a few different possibilities for turning the large bell. Given it is 5' long and 68" in diameter, I might set up off a lathe I rebuilt with an outboard faceplate on the headstock and support the large end of the bell with a large plywood donut rolling on wheels supported by the floor. The donut would be one of several external molds used to glue up staves bent inside the molds. It appears a long arm could be swung off several different pivot points to guide a cuter, or more probably a sanding drum, to cut the horn interior fairly close to the exponential curve that defines the bell.

The Dimension has the advantage of printing sacrificial support filler inside the tapered tubes, the filler is dissolved after printing. The complete tubes would need to be printed in several pieces to fit the printer platten, and then assembled. The best foam for the intermediate section is signmakers foam. It mills beautifully, remains stable, is easily finished, and stiff enough to provide accurate locating reference nubs and pockets. However, it is also very expensive, so perhaps blue 2"X4'X8' builders foam can be used, however, it cuts irregularly. I plan to try a 2" diameter mounted grinding wheel dressed to ~1" radius rather than a cutter. The foam does grind nicely, but what a mess.

In order to provide the best transmission line for the sound, perhaps it will be best to pivot the platter rather than use a separate tonearm. This will require careful attention to making it pivot easily, but that may not be too much trouble, although it will weigh more than a tonearm. Perhaps the pivot arm can be longer than most tonearms and mounted in good bearings to minimize drag, a counter weight might also work.

I have looked for examples of projects based upon the 168" exponential horn. Does anyone know of any?
Also, how critical is the length of the tonearm pivot for 78RPM records? I know there are 33 ⅓ RPM turntables that track in a straight line, it seems likely 78RPM record lathes may have also tracked straight across.
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Full Length Exponential Horn.jpg
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TN Allen
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Re: Questions Regarding Horn Design and Dimensions

Post by TN Allen »

I've been thinking further about this project, and although a curved horn would take less space and be a more interesting project, a straight horn could be done in less time and with less cost. I primarily want to hear the sound, and space is not a concern as the complete assembly could remain in the shop. Nonetheless, the 68" diameter section would be made of 2 "C" sections. It could then be removed if necessary. It would be ~73" long, the straight section 95", and the short curve section at the reproducer would be a uniform ½" diameter section around ~3.8" curve. I read in the MG&ERs that a 6" long uniform diameter section is advisable between the reproducer and exponential horn to improve the bass response.

It seems possible to carefully gimbal the complete horn near its center of balance using appropriate bearings to allow for pivoting laterally for tracking, and vertically to set the reproducer and stylus on the record. Of course mass and inertia are likely to cause problems. Roughly calculating, the weight probably can be kept. under 100 lbs. using Basswood at 20 lbs./cubic foot in a .2" wall thickness. The hypothetical center of balance is approximately at 31" from the 68" diameter along the horn center line(the red vertical line). An adjustable weight near the sound box would adjust the stylus for the correct force on the record. There might also need to be a way to correct for near 0 tracking force by tilting the entire assembly slightly, or adding a pulley, weight and string mechanism. Fortunately there are more conventional alternatives if pivoting the entire horn fails.

I am curious regarding other full length versions of the Modern Gramophone & etc. (pg. 100) horn. Perhaps there were examples similar to the Balmain device? I've known of the Nimbus horn for a few years, but perhaps there were others tried since 1929.
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MG&ERs Type Straight Horn.jpg

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Inigo
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Re: Questions Regarding Horn Design and Dimensions

Post by Inigo »

Our colleague old country chemist is building a new version of the Balmain gramophone, watch this thread :
viewtopic.php?p=277714#p277714
It seems that the difficult part is the method of suspension of the horn...
Inigo

TN Allen
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Re: Questions Regarding Horn Design and Dimensions

Post by TN Allen »

Hello Inigo,

Thank you for the link to the Balmain thread. I skimmed it and find many similar concerns I have for a very large comparatively heavy horn. However, as it can be pivoted in gimbals I think some of these can be solved with careful design, machining, good bearings and adjustment with a precise adjustment mechanism.

The real problem though, as pointed out in the thread would probably be the effect of records in which the spiral groove does not run true to the center hole of the record. Or, to put it perhaps more accurately, where the center hole is not centered accurately in the spiral groove. The unnecessary lateral oscillation of the needle and needle arm, and resulting distortion would be difficult to correct. One solution though would be modifying the platter center pin so that a separate slightly eccentric sleeve can be turned to accurately recenter the record. This would require a good eye to adjust the record to run true. On the other hand, perhaps records that run out noticeably could be avoided.

Your colleagues on this forum are so clever, insightful and resourceful. It a pleasure simply reading through the evolution of their thinking.

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Re: Questions Regarding Horn Design and Dimensions

Post by TN Allen »

Here is further thought regarding correcting undesirable lateral reproducer/needle oscillations. The red section of the horn is 95" long and has quite a small diameter for the most part. It probably would be, or might be made flexible enough to bend sufficiently, it certainly wouldn't require much, to comply with the oscillations maintaining the needle accurately centered in the groove.

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Inigo
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Re: Questions Regarding Horn Design and Dimensions

Post by Inigo »

TNAllen... These people aren't our colleagues, they are your colleagues too, as long as you've joined this forum! :)
And regarding the off-center records... This matter always reminds me of my old Dual turntable which had a phenomenal feature : the spindle was detachable, actually a simple metal pin that you introduced in the center hole. It was detachable because you could remove it to install the automatic turrets for lps or for singles in the same hole. It's the well known dual system of models in the 1200 series. That was a marvel for miscentered 78s, for you removed the spindle and then could place the record as you wanted to eliminate the wow. Actually we could cut the upper part of the spindle of a gramophone and make something detachable instead. What I'm currently doing is to enlarge the record hole as needed to correct the off-center. But it would be better to cut and modify the spindle. Just cutting flush at the turntable, then drilling a hole in the center of the remaining part, and making a new upper part with a corresponding locating pin so you could remove it or insert it with the pin into the center hole. This would help to play miscentered records removing the upper part of the spindle.
EDIT July 19th: this drawing represents what I said. No miscentered spindle, much simpler: just the upper part to the turntable is detachable. Could be threaded or plain. If threaded, the thread must be counterclockwise, or the friction at the record centre will loosen the upper part when it is on its place.
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Last edited by Inigo on Mon Jul 19, 2021 3:16 pm, edited 1 time in total.
Inigo

TN Allen
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Re: Questions Regarding Horn Design and Dimensions

Post by TN Allen »

Tomorrow I'll post a sketch of what I have in mind. Your suggestion would work as well, perhaps with a slight modification, it could work even better. A picture will be an easier explanation.

TN Allen
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Re: Questions Regarding Horn Design and Dimensions

Post by TN Allen »

Following up on Inigo's suggestion, an eccentric spindle to center off center records. The large blue knob might make centering a little easier than turning the 9/32" diameter spindle with fingers. There are 2 versions, one requires drilling the spindle off center, the other turning the spindle eccentric. Both new spindle extensions would be either drilled off center, or turned off center.

An oscilloscope comparison of the sound of a record playing off center and on center might be interesting. A small microphone temporarily located on the soundbox to pick up the sound generated by the diaphragm might provide an adequate signal.
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Eccentric Spindle.jpg

TN Allen
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Re: Questions Regarding Horn Design and Dimensions

Post by TN Allen »

Inigo wrote: Sun Jul 18, 2021 6:10 pm TNAllen... These people aren't our colleagues, they are your colleagues too, as long as you've joined this forum! :)
And regarding the off-center records... This matter always reminds me of my old Dual turntable which had a phenomenal feature : the spindle was detachable, actually a simple metal pin that you introduced in the center hole. It was detachable because you could remove it to install the automatic turrets for lps or for singles in the same hole. It's the well known dual system of models in the 1200 series. That was a marvel for miscentered 78s, for you removed the spindle and then could place the record as you wanted to eliminate the wow. Actually we could cut the upper part of the spindle of a gramophone and make something detachable instead. What I'm currently doing is to enlarge the record hole as needed to correct the off-center. But it would be better to cut and modify the spindle. Just cutting flush at the turntable, then drilling a hole in the center of the remaining part, and making a new upper part with a corresponding locating pin so you could remove it or insert it with the pin into the center hole. This would help to play miscentered records removing the upper part of the spindle.
EDIT July 19th: this drawing represents what I said. No miscentered spindle, much simpler: just the upper part to the turntable is detachable. Could be threaded or plain. If threaded, the thread must be counterclockwise, or the friction at the record centre will loosen the upper part when it is on its place.
I understood. How would the record be centered and remain centered while playing?

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Inigo
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Re: Questions Regarding Horn Design and Dimensions

Post by Inigo »

By the friction of the felt only.
I've also played records in gramophones using thick cork mats on the turntable to gain height until the spindle disappears; the record on top can then be placed as you want, so the miscentering can be counteracted. They play fairly well. It's a similar situation. But I don't like this method because it changes the needle angles on the record, that must be then readjusted.
This is why I think a better idea to remove the spindle tip, the record being seated properly on the platter.
Inigo

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