Armboard Materials

[makinson1]

I have used a home made tonearm and an Alphason on My Thorens TD125. I never had the original armboard so I do not know what it sounds like. I have just had a Grado Platinum for about 2 months. I would like to get a discussion going on the virtues of diferent armboard materials. To get the ball rolling I would say that 7075-T6 Aluminium sounds very solid and yet lively in the upper midrange with the Grado cartridges. Where as the solid oak has a more subdued and laidback sound. Much like the switch from the Grado prestige Gold up to the Platinum Reference. Both boards are 1/2" thick. Please refer me back to previous threads on this subject.

 

[fdegrove]

Hi,

Sure enough different armboards will make for a different sound.

In general I'd stick as close as possible to a material with the same or similar (mechanical impedance-wise) properties as the arm itself.
The armboard can then serve as an energy to heat convector and keep the energy from reflecting back into the cartridge.
That's the basic line of thought anyway...

Unfortunately suspended turntables don't leave you much room for experimenting as the suspension mechanism is easily bottomed out or worse still the platter bearing suffers under the added load.

Cheers,

 

[makinson1]

Somebody had a post regarding laminating a board out of materials with differing acoustical impedances. Wish i could remember if they went up or down from top to bottom. Remember that the tonearm is almost exclusively aluminum except for the brass counterweight, which is considerably heavier and denser than the wand itself.

 

[fdegrove]

Hi,

Wish i could remember if they went up or down from top to bottom.

Probably with increasing damping properties towards the bottom.
Going that far isn't really necessary but the idea behind it remains the same: you want the vibrational energy away from the arm into the board and make it as hard as possible for the energy to creep back in.

Same principle can be applied to the platter, BTW...

Personnaly I choose the material I need for the job and stick with that as much as possible so overall rigidity isn't compromised in the process.
Different TTs may require different measures however.

Cheers,

 

[clivetjm]

I don't know if this helps but it may illustrate how sensitive turnables can be to different armboards

I have a Gyro SE. Originally I used a Rega arm then I switched to an OL Encounter. The arm/arm board combination on the Gyro is supposed to weigh 1Kg. The OL arm is about 400gms heavier than a Rega. In theory the armboard should be lightened for the OL arm. Michell made me a new armboard using an SME sized puck drilled for a Rega/OL fitting. This combo is about 1.1Kg so it's much closer to being correct.

The result: I much prefer the standard (heavy) Rega armboard. The lightweight one has less bass (still ok) but a very cold upper-mid and unnatural sibilance.

 

[makinson1]

I read an interesting review of the SME turntable/arm combination that said they opted for a harder armboard and arm materials so that the resonances would be moved up in frequency, moveing them away from lower ones that would reflect back into the cartridge body and into a higher band that would give the turntable a livelier sound. A subjective decision.

 

[fdegrove]

Hi,

moveing them away from lower ones that would reflect back into the cartridge body and into a higher band that would give the turntable a

First and foremost let's agree that resonances should be drained away, not reflected back.
Once properly drained they have no chance of contributing to the overall character of the TT. That, at least is the goal a designer should set out for himself if he'd wanted to design a neutral sounding tonearm or TT.

Secondly, once the board is properly fixed to the TT and carries the tonearm it shouldn't contribute to the resonant signature of the TT as a whole, it really shouldn't.

Thirdly, yes there are several ways of compensating for colourations downstream but this is just a bad way of designing IMO as you have no way of predicting how this is going to interact with other manufacturers gear.

Fourthly, if you can't avoid resonance in a design, make sure its frequency pattern doesn't bundle with other major resonant peaks as you'll only reinforce them.

Cheers,

 

[makinson1]

Your answer implies that the armboard should be isolated from the plate that carries the main bearing and platter. On the old set up the armboard was isolated from the chassis by a layer of 1/8" sorbathane. Do you think this is better than bolting it hard to the main plate? Another question: The Thorens has the platter mounted to the main plate (a zinc casting) and the motor is mounted to the plinth. Does this lead to the phenomenem known as the "porch glider" effect?

 

[fdegrove]

Hi,

Your answer implies that the armboard should be isolated from the plate that carries the main bearing and platter.

In the case of a Thorens or constructionally similar TT, yes.

On the old set up the armboard was isolated from the chassis by a layer of 1/8" sorbathane. Do you think this is better than bolting it hard to the main plate?

The eternal problem with high damping material is how do you preserve structural integrety?
Using some kind of sandwich structure can help here, the sorbothane sheet playing beef....Washers can help too.

Does this lead to the phenomenem known as the "porch glider" effect?

It could, which is why I like to stiffen the suspension on those TTs as much as I can and put the TT on a wallmounted shelf to prevent the stylus from jumping out of the groove due to footfall disturbances.
You'll immediately notice a huge gain in dynamic range as well provided it's set up properly...

Cheers,

 

[makinson1]

Thanks for the reply. I think my next step will be to get a strain gauge, mount it to various parts of the TT, and observe on an oscilloscope exactly what vibration modes and amplitudes are going on. I wonder if any TT manufacturers do this. It is common practice among people who make barrels for target rifles. I cut my teeth designing the "Stiff Stick" control stick in the F-18 Hornet that uses strain gauges to send signals to the flight control computers that work the control surfaces of the aircraft (wings, trim tabs, canards, etc). It is a 1/2" stainless steel bar that is machines flat on four sides. This is pretty far off topic!