Actually, the balanced nature of those cartridges makes most shielding issues moot if the designer decides to take advantage of that. I can put my finger directly on the body and the wires of my Japanese MC, no audible hum. It's really not difficult to get that interface right and get an equally good system S/N with an MC as with an MM.
What's the thermally limited noise of your MM?
Several Hundred ?
(Ouch, that really hurt, Mr Wurcer.
What more is there to say, but good night !)
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I first used Teledyne T-05 relays at Ampex Research on a video project we were developing. I have a plastic drawer full, and we used something similar in the JC-80. Today, we usually use plastic cased telephone style relays that do the same thing, somewhat cheaper, and slightly larger in preamps like the Parasound JC-2, and we achieved an A rating. Personally, I prefer large silver on silver switches, but they are almost impossible to remote control and are even more expensive.
While I wait for my test tech to show up, let us look again at the 2 stage topology, this time putting it in a plastic box with some conductive spray paint coated on the interior.
Once, long ago, I found a very nice plastic box at Radio Shack and I made the first Vendetta Research phono step-up inside it, and we used it one time at CES.
How could I get away with it? Well, by ignoring the magnetic component of any potential airborne interference, and not placing the phono pre-preamp near any power transformers, it worked OK. After all, the input cables are only electrostatically shielded in any case.
Once, long ago, I found a very nice plastic box at Radio Shack and I made the first Vendetta Research phono step-up inside it, and we used it one time at CES.
How could I get away with it? Well, by ignoring the magnetic component of any potential airborne interference, and not placing the phono pre-preamp near any power transformers, it worked OK. After all, the input cables are only electrostatically shielded in any case.
Why did I go away from the plastic box? Because the necessary electronics to make the pre-preamp work best outgrew it. Then, that was when I met my associate, Carl Thompsen, super layout man, and box builder, first class. He got a beautiful aluminum box designed and made, after hours at his place of employment. We ultimately got the box made in limited quantity 50 or so, elsewhere, and it cost us, perhaps $35-$50, finished and silkscreened. It LOOKED a lot better than the plastic box, and we could show it against the competition out there. How well did it shield? Electrostatically it was great, electromagnetically it was almost invisible. The aluminum was just too thin, you see, and you had to remote the power supply transformer to reduce the hum. Even during testing, we would sometimes forget, and wonder where the hum came from? Just moving the power supply box (separate) away fixed the problem.
Hi John,
this is the same principle. The grey box has conductive foam inside.
regards,
Attachments
If aluminum works so well for magnetic shielding, why didn't you just make the box out of thicker material? Surely thicker aluminum would hardly have added much to the cost of the box or the preamp. It also gives prospective customers the illusion they are getting a better quality product.
For detailed pictures of the SCP-1 and its aluminum box, go to: Vendetta Research and on our website look at Japanese pictures linked the bottom of the page. This will give you a feeling of what a REAL circuit layout and box design should look like. We are still happy with it.
I am most interested how a phonostage whould come out that sounds better then the Beatle. First we have to decide what cartridge to use. The discussion what sounds best is ages old and you can not turn a dog ( MM user ) into a cat ( MC user ). I can only say that in High End circles the MC cartridge type is considered as performing better. There are some great MMs too from Clearaudio, Benz and Grado but their top models are also over 500,-€ here. The most affordable cart. that is considered "good" is the DL103 and that is not expensive and abundant i think. What i do not like with this cart is the rather high impedance of 40 Ohm and the very basic diamond. One notch up is the DL103R that has 16 Ohm impedance and an eliptic needle. That combination gives lower Johnson noise and better extention in the treble. I could life with that cart. maybe. The really interesting things happen over 1000,- $ but that maybe too much for the purpose here.
It is also perfectly posible to buy an NOS MC from the 70th and 80th. A Technics 305 whould set you back under 100,- $, never used. Concerning record wear, i keep them clean with a carbon brush and wash them regulary with a Loricraft machine.
Ask Jan Didden. I visited him some time ago and we listened to maybe 10 records i had with me and he was really surprised in what great shape they where. Some of them where over 20 years old, abused with Ortofons, Dynavectors, Emts, Van den Huls, Lyras, you name it and played a dozent times also on shows worldwide. Yes i carry them around quite a bit. Conclusion is, that when you treat records like pieces of art they are, they show no heavy signs of wear over time, even when a ( well setup ) MC tracks it. I play my Titam i with 1.75 pond by the way.
Concernig the input Opamp ( have we decided to use Opamps ? ) and we do not want to use the old stablemates the likes of AD797, LT1028, LT1115 we have the choice over the following models from the premier companies Analog Devices, Burr-Brown and National Semiconductor. Here is the list : ADA4896, OPA211, OPA1611, LME49990.
It is also perfectly posible to buy an NOS MC from the 70th and 80th. A Technics 305 whould set you back under 100,- $, never used. Concerning record wear, i keep them clean with a carbon brush and wash them regulary with a Loricraft machine.
Ask Jan Didden. I visited him some time ago and we listened to maybe 10 records i had with me and he was really surprised in what great shape they where. Some of them where over 20 years old, abused with Ortofons, Dynavectors, Emts, Van den Huls, Lyras, you name it and played a dozent times also on shows worldwide. Yes i carry them around quite a bit. Conclusion is, that when you treat records like pieces of art they are, they show no heavy signs of wear over time, even when a ( well setup ) MC tracks it. I play my Titam i with 1.75 pond by the way.
Concernig the input Opamp ( have we decided to use Opamps ? ) and we do not want to use the old stablemates the likes of AD797, LT1028, LT1115 we have the choice over the following models from the premier companies Analog Devices, Burr-Brown and National Semiconductor. Here is the list : ADA4896, OPA211, OPA1611, LME49990.
Soundminded, aluminum and other conducting materials do not shield against STATIC magnetic field (= DC). That's usually not an issue in amplifiers. But it does shield against 50/60Hz alternating magnetic field and shield effect can be easily calculated from well known formula, containing conductivity, thickness and frequency, with permeability = 1. Is it so difficult to understand?
Well, we figured that only a fool would put the pre-preamp on top of a power transformer in a real set-up, and we couldn't justify the added cost, at the time. Today, our Vendetta Research V3 has a THICK ALUMINUM CASE and costs us about $1500 for a set of 2, to have made at a machine shop. The price of the VR-3 is estimated to between $6000 and $8000, if we can ever afford to put it into production. The finished prototype is under my bench at the moment, waiting to take on all competition. ;-) And look who our competition is? And he is right here with us with the initials: NP Why don't you look up his phono design and compare?
Sounds similar to this one designed at the height of the TIM debate (Borbely later published a similar topoloy in Audio Amateur).
Not terribly impressive if you ask me. Looks rather amateurish. The layout could be a fair bit tighter. Wanna keep those loop areas small.
se
Duh, has merely a little over a dozen relays, plus a pair or two in the PS case.
And not an RF seal in sight. (goodlooking symmetrical input stage on the XP25 though, nice job by Mr Colburn)
Just use more DPDT's.
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Yeah, double ground plain shielded, gold plated, no solder mask, circuit board drawn on a million dollar computer, in 1984. Now, Steve can do it in his backyard with his Dell. ;-) The REAL overall dimensions of each board is only 2.8" X 3.5". Could have been a little tighter, but it works really, really, well, anyway.
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