Balsa and ply plinth?

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jeffco
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Balsa and ply plinth?

Post by jeffco » 06 Mar 2015 07:45

Hi. Thanks to your posts I was inspired to make a new plinth for my Planar 3 which I have used since the 1980s. Reading that the chipboard plinth was one of their worst, I guessed that ply would improve things. I did it last week with a sheet of ply from B&Q using the old one as a template. The only thing I changed was the motor mount, made of 1/16 ply and fully isolated from the plinth with a 1/4" clear silicone bead, see pic.
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It sounds better than the original, so I am happy. But it seems that "light and stiff" is the way to go, and ply is NOT light! Being an aeromodeller, I think balsawood in a 1/16" ply sandwich will work! The link between the arm bearing and centre bearing will be solid ply.
The shape will be interesting: I have a diy isolation base, which was a big SQ upgrade for little cash. It is a bike inner tube, barely inflated, with board on top. Three Screwfix concave door knobs carry 25mm ball bearings, and the plinth sits on them. So the shape will follow the oval inner tube.
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Worth a try? Will it sound good or not? The Rega 8 is based on light, rigid foam. Balsa when laminated is surprisingly stiff

NaturalBritain
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Re: Balsa and ply plinth?

Post by NaturalBritain » 06 Mar 2015 09:24

Looks good, I like it!

It's an interesting idea - I know Rega go light and stiff, but most of the 3rd party upgrades are much heavier. Someone like Inspire in Chesterfield for example - their plinth is birch ply, around 50mm thick, and very highly rated - there's a few on the forum who've had the mods.
I think, if it's not going to cost too much, go for it - you may find it's an absolute success, or you may learn something different!

jives11
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Re: Balsa and ply plinth?

Post by jives11 » 06 Mar 2015 10:11

Sounds a great idea, balsa and ply sandwich, you need to call it the Mosquito
http://en.m.wikipedia.org/wiki/De_Havilland_Mosquito

readargos
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Re: Balsa and ply plinth?

Post by readargos » 10 Mar 2015 01:09

Balsa is something I've considered, but haven't gotten around to any DIY plinths. I think it's pretty easy to get "light", but another key is to have enough stiffness to increase the resonant frequency of the system (as opposed to adding mass to deaden it) and make it functional (so it won't break or fall apart easily).

jeffco
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Re: Balsa and ply plinth?

Post by jeffco » 10 Mar 2015 09:00

I am interested to know why you also think balsa will be good?
I have experience working with balsa and ply, especially "making" ply out of 1/16" or 1/18" balsa. Imagine 7 layers 1/8" balsa cross grain, with 7 stiff glue lines. So I don't think mechanical strength will be an issue. But I know little about the science of materials: what do you mean by "have enough stiffness to increase the resonant frequency of the system"?
I have made a cardboard template, got some balsa, and made a ply piece that runs from arm to centre bearing.
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.
The first attempt was so quick and easy, all accomplished with diy hand tools, I am just going to go for this second one and see for myself. I am off to buy the ply etc today, and will let you know the total project cost.
Looking at the picture, it all looks out of proportion, so I will elongate it. The shape is dictated by the bike inner tube on which it all sits.
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jeffco
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Re: Balsa and ply plinth?

Post by jeffco » 11 Mar 2015 00:17

I have made a start, and it is going well, the first two layers of balsa fitted to the ply. The total cost is £27.
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readargos
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Re: Balsa and ply plinth?

Post by readargos » 11 Mar 2015 20:44

Re: Light & Stiff, you can read my post here:

https://www.vinylengine.com/turntable_f ... 1&start=15

Re: my experience with light Rega designs v. a heavier VPI 'table, I posted some comments in this thread. It's basically how I came to embrace the Rega philosophy from firsthand experience:

https://www.vinylengine.com/turntable_f ... 32&t=55115

Lighter designs tend to "ring" or produce a hollow sound when given the knuckle rap test. Mass designs tend to produce a dull thud.

readargos
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Re: Balsa and ply plinth?

Post by readargos » 11 Mar 2015 22:15

Rega's designs are built to a price point, and have certain compromises, which got me thinking about DIY solutions that wouldn't cut corners. Then I started to run into what I regarded as cost constraints for a DIY project. Thus, like Rega, I was trying to come up with the "correct" compromises, at least to test my designs, before venturing into something more expensive (i.e., with less potential compromise). There is also the possibility that something with a theoretical advantage, but higher cost, would not yield an actual advantage in listening (the application). I was also looking at aesthetically desirable finishes, including real-wood veneers, to go beyond the simple gloss or mat finishes on Regas.

You also have to look at what Rega claims, what you read in the reviews, and what compromises the actual design represents. For example, Rega is generally "light & stiff", but there are parts with high mass (the platter) and parts that are designed to control or dampen resonance (the rubber feet and decoupling the motor from the subplatter with rubber drive belts).

In the past, Rega was using chipboard on its lower-end 'tables, and slightly denser particle board on its better 'tables. Some had mat finishes, and some gloss. I believe they all claimed to be a phenolic resin skin, which increased stiffness. The newer 'tables have the polyolefin foam core with a phenolic skin. But if you step back and look at it, it appears that Rega uses lightweight, self-dampening materials for the plinth core, but rigid materials for the plinth skin and braces, while still keeping the overall plinth design light.

I was attracted to balsa, therefore, for its light weight and open cell structure, but I've not worked with it before, other than those airplane gliders I made as a kid. I was also concerned about the balsa crushing when tightening the nuts for the bearing sleeve and tonearm, but I take it this is not an issue with end-grain balsa?

jeffco
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Re: Balsa and ply plinth?

Post by jeffco » 12 Mar 2015 00:20

Thanks for the links. Your explanations of your theories are really clear, and intrigue me. It was after reading about the light rigid foam structure of the Rega 8 that I considered balsa, and a friend with much more hi fi experience than me, also an aero modeller, confirmed it's worth a try.
As for the compressive strength of the balsa around bearing sleeve and tonearm , it should not be an issue, as I have a dense ply beam to carry the two bearings. That would ideally have been carbon fibre or light alloy! I understood from Rega's recent designs with metal straps that a rigid link between the two is a good thing??
I have not yet decided how to isolate the motor, your advice will be appreciated: on my trial ply plinth, I isolated the motor/ ply plate using a 6mm thick silicone bead. I intend doing the same, but make it 10mm. And the top plate will be 1/16" ply. Would fibreglass cloth, resined direct onto the balsa, give a better ring? Ply sounds dead when struck.
Just glued my 9th balsa layer, one to go! If it sounds good, veneer will follow.
The difficulty is always in assessing the result: its so subjective, isn't it? My mood dictates how it sounds more than the equipment.
I appreciate your interest.

jeffco
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Re: Balsa and ply plinth?

Post by jeffco » 12 Mar 2015 00:32

Just re-read this part: "and parts that are designed to control or dampen resonance (the rubber feet and decoupling the motor from the sub platter with rubber drive belts)." which got me thinking about MY favourite trial and error upgrades: I took away the rubber feet, and use rigid feet: ball bearings in cups. (But an inner tube to dampen). I like this sound a lot. I also prefer my acrylic platter to glass, and I prefer no mat. So I may like the sound of a rigid motor-to-plinth mount, without silicone OR mounting pads. I may try that first.
Which of your changes were the big ones for you?

jeffco
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Re: Balsa and ply plinth?

Post by jeffco » 12 Mar 2015 16:32

Nearly complete, the weight is 1/3 of the original, and its strong!
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readargos
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Re: Balsa and ply plinth?

Post by readargos » 12 Mar 2015 16:40

Re: the rigid link between tonearm and bearing hub, this is the latest evolution in Rega's "closed loop" theory of design (which works in tandem with "light but stiff"). Basically, Rega wants all the parts involved in the actual reading of the vinyl groove rigidly coupled together to avoid movement of one part relative to the other. The goal is to prevent the smearing of musical information. Compare this to some suspended designs, or designs where the tonearm has a separate mounting on its own pillar or arm pod. The parts may not all move in synch together, and you may get some micro-level smearing. But a vinyl groove is on the micro-level, so slight smearing can be a big deal. If this closed loop system is also light and stiff, it is kind of self-grounding. To the extent it vibrates, everything will vibrate sympathetically, and evacuate unwanted vibration quickly.

I've looked at fiberglass, and while it has much in common with carbon fiber and is readily available to the home hobbyist, I'm not sure it's ideal. Although fiberglass is light and strong, it is not very stiff. A fiberglass structure will have some flex to it. But this is where practical considerations come into play. Fiberglass doesn't seem ideally rigid compared to carbon fiber or exotic metals, but in the context of a turntable plinth, it may be "good enough".

Like you, most people who try some type of cone footer on the Rega prefer it to the stock feet. Herbie's Isoballs also have a strong following here.

For an interesting take on motor isolation, and use of two types of footers, take a look at Indydan's excellent post on the SRM-Tech Rega Silent Base:

https://www.vinylengine.com/turntable_f ... 32&t=73056

Michael Lim also sells a motor isolation kit you can look to for inspiration. Watercourse has a thread on the Lim Base somewhere in the Rega forum.

Thanks for the photos and sharing your adventure.

readargos
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Re: Balsa and ply plinth?

Post by readargos » 12 Mar 2015 16:55

>>>> Which of your changes were the big ones for you?

I really haven't done many mods. Most of what I'm sharing here in terms of Rega design theory and practical applications are things I researched when looking into designing my own plinth. Other things have intervened, and I haven't actually done any of this, so it's all academic at this point. But you are actually doing it(!), and it's fun to share ideas and experiences with fellow enthusiasts and DIY hobbyists.

I've done the "Spot Mat" on the glass platter, which was my first mod. I've also played around with different subplatters and platters. I wrote about my observations in another thread. Like you, I am presently inclined toward an acrylic platter with no mat:

https://www.vinylengine.com/turntable_f ... 32&t=75969

jeffco
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Re: Balsa and ply plinth?

Post by jeffco » 13 Mar 2015 22:14

Thanks readargos, being new to the forum, much of this is new to me - thanks for all the information.
I have put the bits back, but not tested it yet. Very pleased with the silicone mounting for the motor, it went well.
I will not complete the sanding and finishing if it does not sound great, instead it will become an expensive but very light cheese board.
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jeffco
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Re: Balsa and ply plinth?

Post by jeffco » 14 Mar 2015 21:44

Well, the new deck has been tested. I first rigid-mounted the motor with screws, and no silicone, and it was poor. So the isolating silicone and foam pads were fitted. It now sounds great – improved dynamics, more speed, better female vocals, but no bass improvement. Still testing, but its going to stay. So I took it apart to stain and varnish.
Here is also a picture of the Teddy Pardo i80a which was an upgrade from my Naim amp, and which began all this revived interest in the last months.
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