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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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15th Mar 2018, 8:39 am | #21 |
Heptode
Join Date: Oct 2005
Location: Olympia, Washington, USA.
Posts: 663
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Re: Pre-amp for Golden ears
In the "Good Old Days" here we used to put lead pipe sections around the tubes to damp out microphonics, (And to shield them). They even made heavy metal chrome plated "Top hats" for the early tubes like the 01A's.
This practice continued well into the 60's on some brands of TV sets, where they used the lead pipe to shield and eliminate microphonics in TV tuner tubes. |
15th Mar 2018, 9:20 am | #22 | ||
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
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Re: Pre-amp for Golden ears
Quote:
The only reason to look at it this way, is when vibration isolating something, you only need to look at how much the spring is stretched and that is enough to give you the resonant frequency without knowing anything at all about the mass and the spring. Just as another for instance, the bed plate of an old SME arm used to have grommets, so the arm was isolated in some way from the plinth or arm board. The mounting screw is required not to compresses the grommet, so the grommet compression is only by the arm mass itself. Suppose that is by 0.1mm (which seems plausible). That means that the vibration isolation cuts in at 50Hz. Of course there are more complex rocking modes too, and for that reason the grommets were often removed and the bedplate screwed hard to the plinth. Current SME arms are bolted hard down by design. The only (classical) way of working out the local gravity, for example on the moon, is to use a pendulum. Craig |
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15th Mar 2018, 12:04 pm | #23 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
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Re: Pre-amp for Golden ears
Sorry - dropped a power of ten. The SME grommets cuts off over 500Hz, NOT 50Hz!
Oops |
15th Mar 2018, 1:01 pm | #24 | |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
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Re: Pre-amp for Golden ears
Quote:
Cheers, GJ
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15th Mar 2018, 2:17 pm | #25 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
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Re: Pre-amp for Golden ears
If you want the true nature of non-relativistic gravity, you need a gravity gradiometer
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16th Mar 2018, 12:50 am | #26 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
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Re: Pre-amp for Golden ears
If I have read it correctly, for low noise the article recommends a step-up transformer to raise the signal level as high as possible and use cascoded triodes to reduce the input capacitance to avoid roll-off from the increase in source impedance?
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16th Mar 2018, 10:10 pm | #27 | |||
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,060
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Re: Pre-amp for Golden ears
Quote:
I'm now off to crawl into a corner where I can calculate the resonant frequencies of a few spring-isolated motors and sub-chassis! |
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17th Mar 2018, 1:13 am | #28 | |
Banned
Join Date: Nov 2014
Location: Derry, Northern Ireland, UK.
Posts: 167
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Re: Pre-amp for Golden ears
Quote:
Goodbye |
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17th Mar 2018, 1:40 am | #29 |
Banned
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
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Re: Pre-amp for Golden ears
My apologies tritone, twas nowt more than my feeble attempt at humour, to share some fun over something I have tried and found no difference in the microphonics. Other members have quoted lead pipe, which I have seen, but its something almost impossible to obtain in Australia, the only place I have seen it was old lead covered, paper insulated telephone cables that were being ripped out and replaced with PVC when I was a kid, probably 60 years ago.
Again, my apologies Joe |
17th Mar 2018, 7:06 am | #30 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,800
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Re: Pre-amp for Golden ears
Air springs can give an escape from the deflection relationship. The force they exert is proportional to piston area and gas pressure, but the spring rate involves the volume behind the piston. Add in active control and the world is your mollusc of choice.
David
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17th Mar 2018, 8:41 am | #31 | |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
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Re: Pre-amp for Golden ears
Quote:
I guess your comment comes from the design of optical honeycomb tables, which are fitted with a number of vibration isolation options for the legs, including active control. Last one I specced was an 8' x 4' with super-invar skins, and earthquake bump stops on the legs. That was for an astronomical spectrometer on Gemini South. Unfortunately big telescopes are sited on mountain tops, which are generally in earthquake zones - so immunity is built into all aspects of design. I was system engineer for that, and Maggie Aderin (now the Sky at Night host) was project manager. Craig |
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17th Mar 2018, 9:00 am | #32 |
Moderator
Join Date: Mar 2012
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Re: Pre-amp for Golden ears
There are all sorts of methods of trying to approach the ideal force-source of infinite compliance much as there of creating a current source of infinite impedance.
Air spring arrangements rarely have cylinders long enough for their piston to move to their true unloaded position which would have been the reference position from which deflection would have been measured to use that equation, so the presence of an end stop means that the active range is only a segment of a much larger curve, and the spring constant can be made low considering the mass being supported. Once active control is introduced, the rules can be changed. Supporting a mass and having some freedom to choose the natural frequency is a classical engineering problem. In this case, I guess a positioning servo would have to be implemented with valves David (running!)
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17th Mar 2018, 10:45 am | #33 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
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Re: Pre-amp for Golden ears
If it does not move to its unloaded position, you get the same result by using the ratio of cylinder pressure to atmospheric pressure. So if the cylinder pressure is 3 bar, the relevant ratio is B = 3 and hence the resonant frequency is about 0.3Hz. That will be quite high Q and hence legs have quite careful viscous damping using oil and orifices, which means that the legs start to operate (ie to attenuate) at about 1Hz.
It is more complicated than that, because they have to deal with horizontal excitation too, so the legs also have pendulum mechanisms too (also damped) to handle that. |
17th Mar 2018, 11:02 am | #34 | |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
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Re: Pre-amp for Golden ears
Quote:
Cheers, GJ
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17th Mar 2018, 11:17 am | #35 | |
Moderator
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Re: Pre-amp for Golden ears
Quote:
If we have some sort of air suspension system supporting a mass and the piston has some area and there is some pressure in the cylinder so the pressure times area balances the gravitational force of the mass. If I move the pass up and down a little, the pressure in the cylinder changes due to the change in volume of the space the gas has to occupy. This creates a restoring force which acts to put the piston and mass back to its equilibrium resting position once I've stopped poking it. The proportional to deflection restoring force (for small deflections) and the mass create the classical mass-spring resonator and the resonant frequency is now set by the mass and the spring coefficient. If I now connect or add a large air reservoir to the pressure side of the cylinder, and ump it all up to the same pressure as before. I have the same pressure, I have the same mass, I have the same piston area. What has changed is that when I poke the mass and deflect it a little, I change the volume of the pressure side of the piston by the same amount, but due to the much greater connected volume the change in pressure is now far less, and that means the restoring force is far less and therefore the springiness is far more compliant. This drops the resonant frequency of the the system. If I took the mass off of the piston and the cylinder was long enough, the piston would move far higher to its new equilibrium position than it would have before I added the air reservoir, because there is more air that needs to expand before the pressure comes down to the rest position. The resonance is controlled not by the spring force, but by the slope of the force/deflection curve at the operating point. I think we're just visualising different things in different terms. It would be one heck of a preamp with air sprung valve sockets with say a 1Hz natural frequency and little dampers. Don't tell the audiophiles - they'd only argue over which gas sounds best. David
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17th Mar 2018, 11:34 am | #36 | |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
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Re: Pre-amp for Golden ears
Quote:
I love the last comment though; fill it with helium because of the speed! Craig |
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17th Mar 2018, 6:57 pm | #37 | |
Banned
Join Date: Nov 2014
Location: Derry, Northern Ireland, UK.
Posts: 167
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Re: Pre-amp for Golden ears
Quote:
I have an idea please don't shoot me down as it may sound a bit crazy . I don't know if something like this has ever been done before. Ok, here I go, (phew),, Would it be possible to construct some kind of valve base that could magnetically 'float' and have zero physical contact with the chassis! Probably need some mechanical ingenuity, I rekon but with perserverance, why not? I see those neodynium magnets for sale in all shapes and sizes and they are very powerful for their size, my only concern is, would they (magnets) interfere with with the electron stream within in the valve. Just an idea Joe! |
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17th Mar 2018, 7:46 pm | #38 |
Hexode
Join Date: Aug 2010
Location: Malvern, Worcestershire, UK.
Posts: 340
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Re: Pre-amp for Golden ears
Here goes nothing..... or maybe not? What about submini valves like the American 6112 and its Eastern Bloc clones ie 6n17B.
Designed with rigid low mass structures in order to resist microphonics,much easier to construct an anti-vibration mounting than for a valve with higher mass/insertion force AND poor contact (possibly)via pins eliminated. Maybe a different angle to consider Last edited by VT FUSE; 17th Mar 2018 at 8:04 pm. |
17th Mar 2018, 8:35 pm | #39 |
Banned
Join Date: Nov 2014
Location: Derry, Northern Ireland, UK.
Posts: 167
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Re: Pre-amp for Golden ears
+1 ^^
Happy St Patricks Day forum! |
17th Mar 2018, 11:06 pm | #40 |
Banned
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
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Re: Pre-amp for Golden ears
I do have minature valves that are easy to mount (a simple grommet) and they are designed for very high G forces. I don't know however if they are microphonic or not. As yet I haven't made anything using them so no experience whatsover. They don't need bases at all as they are wire ended, just solder them into the circuit. I suppose one could "roll up" the leads to make a sort of isolation spring as well. I will have to look at them again I think.
Joe |