EXPERT two-spring v four-spring soundboxes

[gregbogantz]

I do not own, nor have I ever heard an EMG machine. But I can speculate from an engineering point of view what is going on with some of the reproducer design. First, there is a good reason for increasing the mass (and thereby the weight) of the reproducer and then recommending the use of a counterweight with it. This at first appears to be counterproductive, but the net result is that there is a greater MASS present at the reproducer needle bar pivot. This will lower the reproducer/tonearm resonant frequency which will extend the bass performance to a lower frequency near that resonance.

Most people don't realize that as high as the tracking force and reproducer mass is of most acoustic phonos, the moving stylus assembly with needle bar and diaphragm combination is still pretty stiff and low compliance which results in the tonearm resonance being still fairly well into the audio range, maybe around 100 to 200 Hz. With the entire tonearm wiggling at 100 Hz, the relative motion of the diaphragm relative to the body of the reproducer is reduced, and you don't get much bass transmitted to the horn. Below the resonance frequency, the tonearm is moving rather than the diaphragm within the reproducer, and little to no sound pressure is created within the horn. So in order to improve bass transmission, you must reduce the tonearm resonance frequency. Most designs, particularly with metal diaphragms tend to be pretty low compliance which will produce fairly high tonearm resonances with the attendant poor bass performance. One fix for lowering the tonarm resonance frequency is to increase the mass of the reproducer. This must be offset by a counterweight or spring to keep the tracking force no higher than necessary. An advantage to using a counterweight rather than a spring is that its mass and inertia is added to the moving mass of the entire tonearm which further increases the net mass at the needle tip.

Note that engineers were beginning to understand this situation, particularly with the dawn of electrical playback systems. Electrical recordings had more level overall and especially much more bass energy. By 1929 the playback amplifiers and speakers were capable of producing much more extended bass frequencies than most acoustic phonos could extract from the electrical records. To complement the rest of the playback system, the early horseshoe magnetic pickups were massive and were made even moreso with the RCA "inertia" style tonearm of the 1930s. It's called the inertia arm because of its added mass which increases the motional inertia of the arm. The extra mass that was deliberately added to the front end of the tonearm was offset by a counterweight added at the back, the entire design being intentionally to increase the mass at the needle bar pivot to lower the tonearm resonance and improve bass performance.

Attached is a picture of the RCA "transcription" version of the inertia tonearm which is even more radical than the original inertia design, and dating from about 1935. The original version included masses added underneath the flared scroll design that you see at the top front of the arm. The transcription version adds the two massive "wings" to the original flared shape for even more mass, concentrated very near the front of the reproducer to produce the most effective mass near the needle tip. This frontal mass is counterbalanced by the sizeable counterweight that you can see hanging from the back of the tonearm. This balanced arrangement produces about the same tracking force of around 160 grams that the standard straight tonearm has without the additional masses.

[chunnybh]

Thank you Greg for that enlightening insight into the importance of MASS in reproducers.
I always wondered about those heavy winged inertia tonearms on Electrola radiograms. They always sound marvelous.
It also explains why both EMG and Expert would increase the weight of the 4-spring soundboxes. They could have quite easily made them a lot lighter than the 2-spring. Even the needle bar guards are excessively heavy.
Grahams points on the diaphragms and even effects of annealing the aluminum are all very interesting and need further experimenting and study.
I took one of my 4-springs which did not sound very good and removed the two lateral springs and was well surprised at the improvement. I then replaced the lateral springs and removed the other two and was even more surprised with the improvement. I then loosened the two lateral springs, barely holding the stylus bar so they just buzzed when played. Attached the two others until there was no buzz. Now it sounds the best it ever has. Nowhere as good as the best one but definitely on the right track.

[soundgen]

Most people don't realize that as high as the tracking force and reproducer mass is of most acoustic phonos, the moving stylus assembly with needle bar and diaphragm combination is still pretty stiff and low compliance which results in the tonearm resonance being still fairly well into the audio range, maybe around 100 to 200 Hz. With the entire tonearm wiggling at 100 Hz, the relative motion of the diaphragm relative to the body of the reproducer is reduced, and you don't get much bass transmitted to the horn. Below the resonance frequency, the tonearm is moving rather than the diaphragm within the reproducer, and little to no sound pressure is created within the horn. So in order to improve bass transmission, you must reduce the tonearm resonance frequency. Most designs, particularly with metal diaphragms tend to be pretty low compliance which will produce fairly high tonearm resonances with the attendant poor bass performance. One fix for lowering the tonarm resonance frequency is to increase the mass of the reproducer. This must be offset by a counterweight or spring to keep the tracking force no higher than necessary. An advantage to using a counterweight rather than a spring is that its mass and inertia is added to the moving mass of the entire tonearm which further increases the net mass at the needle tip.

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So can a better sound be got on any gramophone by putting say a 300 gm weight on the soundbox and adding a counterbalance to the back ?

[Steve]

I took one of my 4-springs which did not sound very good and removed the two lateral springs and was well surprised at the improvement. I then replaced the lateral springs and removed the other two and was even more surprised with the improvement. I then loosened the two lateral springs, barely holding the stylus bar so they just buzzed when played. Attached the two others until there was no buzz. Now it sounds the best it ever has. Nowhere as good as the best one but definitely on the right track.

That's very interesting feedback from Chunny. What it appears to suggest is that if you only tighten the lateral springs barely enough to hold the stylus bar in place on their own (without the two end springs), this is the "right" level of adjustment for them when all four springs are in place. The level of tightness is therefore considerably different between the lateral and end springs. My mistake (or one of them) might have been to tighten too loosely all four springs about the same amount. Of course the tension on the springs can be adjusted by the addition of flexible material under the fixing point.

[old country chemist]

The weight of the Expert and EMG soundboxes made of brass, are usually 7.5 to 8 ounce each. I was told many years ago that the whole idea about using a heavy box with a bamboo needle was that the weight compacted the needle point into a hard ball in the groove. Using a weight adjuster perhaps halving the weight, might not then let the soundbox sompact the needle sufficiently....? I have a fitted Ginn weight adjuster. I use it when playing discs with thorns.Excellent item, well made and fits into the quincke filter orifice. I would like to see a picture of an EMG weight adjuster, as it must be attached to the tone arm in a different way to the Expert. I probably saw one years ago, but have forgotten what it looked like. Perhaps one of you gramophiles can supply a photograph. Many thanks.

[epigramophone]

The weight of the Expert and EMG soundboxes made of brass, are usually 7.5 to 8 ounce each. I was told many years ago that the whole idea about using a heavy box with a bamboo needle was that the weight compacted the needle point into a hard ball in the groove. Using a weight adjuster perhaps halving the weight, might not then let the soundbox sompact the needle sufficiently....? I have a fitted Ginn weight adjuster. I use it when playing discs with thorns.Excellent item, well made and fits into the quincke filter orifice. I would like to see a picture of an EMG weight adjuster, as it must be attached to the tone arm in a different way to the Expert. I probably saw one years ago, but have forgotten what it looked like. Perhaps one of you gramophiles can supply a photograph. Many thanks.

Welcome back to the forum Alastair. Whilst we have always kept in regular contact off line, I know that your input here has been greatly missed.

Roger.

[emgcr]

Welcome back to the forum Alastair. Whilst we have always kept in regular contact off line, I know that your input here has been greatly missed.

I would very much agree with that statement---welcome back Alastair.

[gregbogantz]

Yes, Soundgen, better bass performance can be had from any acoustic phono by adding mass to the front of the tonearm and balancing that with a counterweight at the back end so that the tracking force remains as it was originally. The typical tonearm resonance of acoustic phonos is in the 100 to 200 Hz range which means that they can't produce bass well below that frequency. This wasn't much of a problem in the days of acoustic recording because the recording system also did not support extended bass frequencies. However, most acoustic horns are either too small or poorly designed such that they don't produce much output below 100Hz anyway. But if you have one of the premium acoustic phonos such as an HMV 203 or a Victor Credenza, those horns are big enough and properly designed to support bass reproduction down to around 60Hz or so. You will probably hear some improvement in the lowest bass tones while listening to these large-horn machines if you beef up the mass of the tonearms as suggested. As I said earlier, the improvement in bass due to increased tonearm mass was much more noticeable in electronic players because the speakers and amps were capable of producing bass down to 40 or 50Hz on machines such as the Victor RE-45 or RE-75 as early as 1929. Victor started out using a simple straight tonearm in these machines, but later developed the inertia arm to improve the bass performance.

The downside of increasing the tonearm mass is that the tonearm is less capable of tracking a warped or eccentric record, but that isn't a problem with records in good condition. If you are familiar with the earliest electric pickups and arms used in professional and radio station applications, you will notice that they are VERY massive looking. This was the reason for that, because it allowed the highest fidelity to be achieved in those applications with the limited technology of the electric pickups of the day which were still fairly low compliance.

[chunnybh]

Here is an article from The Talking Machine Review.

Not sure where the Expert sketch came from.

[emgcr]

The Expert sketch was drawn by the late and much missed Miles Mallinson. This example would also be suitable for early EMG goose-neck tonearms.

Here are a few examples of various types of counterweights.