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|27th May 2004, 06:22 PM||#1|
Join Date: Apr 2002
I have started a new project: building a Ladegaard. I have read the articles of Poul Ladegaard, Jeremy Epstein and Thomas Dunker which answer most questions.
However some issues remain:
About the smoothing tank: Is it important where it is placed. Near the pump or near the arm?
And about the pump: is there anybody in Holland who has build this thing who has some advice about which pump is best?
hope you can help me,
Keep an open mind. It helps.
|28th May 2004, 01:58 AM||#3|
Look at the hose as a resistor and the tank as a capacitor. We'll
assume that the arm looks like a constant current drain and that
the overall air flow that can be provided by the pump far exceeds
the requirement of the arm.
Thus, in circuit terms you have a Voltage source (pump) driving
a series R (hose) a shunt C (tank), another series R (more hose)
and a shunt load (arm, probably modelable as a relatively high
R in this case).
Thus, for maximum attenuation of high frequencies (pulsing of
the pump) you want the R (hose) between the pump and the
tank to be fairly high.
Conclusion: One should probably put the tank reasonably
close to the arm.
|28th May 2004, 07:51 AM||#5|
Join Date: Nov 2003
Location: Brighton UK
Well electrical analogies are fine if they are appropriate.
I was thinking along the lines once the system is up to pressure
due the net airflow the "resistance" of the tubing would not be
particularly significant, I may be wrong here.
And whilst the analogy uses an RCR filter it neglects to point
out they are usually added to capacitive smoothed rails,
driven by a rectified half sine wave performance is awful.
IMO you should use a large tank near the pump, to reduce
pulsing in the tubes and hence transmission of mechanical
noise to the arm.
If the airflow of the tubing is restrictive the electrical analogy
indicates a smaller subtank somewhere along the tubing is
a possibility, this is equivalent to a CRCR fiter.
Perhaps a car petrol line filter would be ideal ? With its
built fine gauze I can't think of anything better off hand.
The smaller extra tank with restrictive tubing will only work if
you have plently of excess pressure and even with the small
tubing enough airflow (current) is provided for the arm.
IMO the main tank should be near the pump, unless
of course the pump already has its own built in tank.
(Or the main capacitor bank near the transformer,
which is always the case in amplifiers AFAIK)
|28th May 2004, 02:49 PM||#6|
Join Date: Jan 2002
Location: Munich, Bavaria
meanwhile i am on the Ladegaard path, too.
Makes sense what has been said here already.
the air hose itself is a resistor with its length affecting resistance in 1st order and inner diameter in 3rd order. Means: don't use a thicker hose than you need.
For a fixed air flow resistor you can take whatever you use to connect two hoses together. Make a plug of silicone rubber seleing the orifice and after curing of the rubber, pierce the plug with a hypodermic needle. Done.
If you want to change resistance, just take another hypodermic needle with a different diameter or length.
Cutting off the sharp / adjusting the length can be done with a Dremel's cutting disc.
Beauty of this method is that once you've chosen the right air flow resistance, just forget it, will never change its value.
if you manage to use an old fridge's compressor, fine. I didn't, it would refuse to start. On several occasions.
What i choose: a small airbrush compressor. It is even less noisy than a fridge and and not too expensive. And it has the excess pressure to alow a CRCR filter and get the air flow ultra smooth.
And it would not be wrong to give the filter enough capacity to provide air for atleast one records side and to equip the compressor with a switch forcing the compressor to blow its tank up to full pressure.
Having experienced a cheap aquarium pump on a Dennesen ABLT, i would not even think of using such. This is a cheap solution with any negative properties cheap solutions usually have.
|28th May 2004, 06:42 PM||#7|
Join Date: Aug 2002
I've finished with my tank made from 4" plastic sewage tube (use new one or clean thoroughly ) 1.5 meter long, filled with syntetic fibres, using for soft furniture stuffing. Why such long? I'm using two aquarium pumps - they are placed into MDF box, filled again with syntetic fibres - absolutely quiet. The pump produces strong 50 Hz pulsation, but, there is at least partial phase cancellation, due to parallel connection, and the rest is very effectively damped at quater wave peak (50 hz wave length is about 6 meters).
Works very well for me
|28th May 2004, 09:16 PM||#8|
Join Date: Feb 2002
Probably got the wrong aquarium pump because it could not get the arm lifted. Not even the arm bearer alone! So I used an old fridge pump. Works fine with a bit of bleeding. A few 2l soft drink bottles as buffer coming after the radiator of the compressor and its small buffertank. You will need a water separator if you recycle a fridge compressor as they may spit out some oil.
My problem lies in the holes I drilled. Didn't like the perforated tape, so I drilled the 0.3mm holes through the Al profile. This creates a lot more flow resistance.
Dice: you didn't forgot the capacitor when trying the fridge compressor?
|28th May 2004, 09:19 PM||#9|
diyAudio Senior Member
Join Date: Aug 2002
The hose is already smoothing most of the pumps' turbulences caused by the push-pull action of the pumps' membrane.
Having the tank closer to the arm allows for a slow drainage of any air left in it in case of an accidental cut-off of the air supply giving you enough time to lift the arm of the record or correct the problem at the supply side.
Someone stepping on the air hose cutting of the air supply.
A hose connection coming lose.
Most of the time, if the hose is long enough a smoothing tank isn't even needed and in any case it really doesn't have to be big to be effective.
Someone suggested a fuel filter and that's what's often supplied with commercial airbearing arms.
Aquarium shops also carry hose clamps, T-connectors and all kinds of related accessories but I think the important part is to have a system where the pump(s) don't interfere or add any noise so you can enjoy the music to the full.
For the most part a system like this just runs by itself and other than an occasional cleaning of parts it should run fine without any special attention from the user.
|29th May 2004, 04:04 AM||#10|
"Electrical analogies are fine if they are appropriate" ????
Whether it's plumbing or electrical circuits, it all works out the same.
And given the 1:1 correspondence of the two "domains", my
analogy is, in fact, correct. The only thing open to debate is
the exact values of the various components. And if you go back
and more carefully read my posting you'll see that your "what-if's"
are already considered. In fact, the pump's output really is
a good pneumatic analogy to half-wave rectified power. pfft...pfft...
And yes, people have been using the multiple resorvoir approach
for many years RCRCR. I don't recall if it was a DIYer in Audio
Amateur or people tweaking various early air-bearing arms, but
in any case it was tried a good 20 years ago. So go ahead and
put a reservoir at each end.
If I were doing this arm, I'd have the pump in some acoustically
isolated out-of-the-way place with a reservoir relatively close
to it, and then another reservoir a few feet from the arm.
Remember that VERY LITTLE air actually blows out the holes
of the arm.
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