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Old 2nd March 2005, 07:24 PM   #1
pixpop is offline pixpop  United States
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Default DIY Tonearm split preamp

Would there be any advantage to locating a gain stage (and possibly RIAA) in the headshell?

The advantage I'm thinking of is possibly reduced noise, since the signal in the tonearm would be very low impedance. Of course, I'd have to feed power through the tonearm as well, but not much.

Since the power supply at the end of the arm would be compromised, I was thinking of a gain stage with good CMRR -- a low noise, single supply dual opamp (1 amp per channel).

Alternatively, if the power at the far end of the arm could be made adequate, think of something like the Pearl, with the input transistors in the headshell, and the RIAA & Output stage elsewhere. Or does this circuit already provide noise so low that my arrangement would yield no benefit?
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Old 2nd March 2005, 08:33 PM   #2
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You are not the first to have this loony idea - i used to have an SPU with a step-up transformer in the headshell. This disaster was partially rectified through bypassing the little horror but the huge effective mass was there to stay. More recently i remember seeing an ADC (not the cart brand ) in the headshell as well...

Why try solving an imaginary problem and create a dozen more in the process? Waste a good arm with a bunch of electronic components dangling at the end? Waste a good active step-up by providing inadequate power and decoupling? Introduce more friction by the power wires in the arm?

What would the benefit of this be? Increase the S/N by 1/4db?

A real audible improvement in most arms can be achieved by rewiring with high quality, single piece of wire. A good place for the step-up would be close to the base of the arm.
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Old 2nd March 2005, 09:22 PM   #3
pixpop is offline pixpop  United States
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Quote:
You are not the first to have this loony idea
Hehe, most of my ideas are much loonier than this one
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i used to have an SPU with a step-up transformer in the headshell
What's an SPU? But a transformer in the headshell? That's just plain nuts!

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Waste a good arm with a bunch of electronic components dangling at the end?
Since this arm doesn't exist yet, there's nothing to ruin. (I'm not planning on modifying an existing arm, but building one from scratch).

I don't think mass is an issue, because firstly the components are very light, and secondly, the rest of the arm would have to compensate. I was not imagining adding any wires, so there shouldn't be any additional friction. In fact, many tonearms have 5 wires, but I think this could be done with four.

But of course you're right, and simpler is better. And if I were to build it my way, I would have nothing to compare it to.. unless I built it both ways, which would really be loony
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Old 2nd March 2005, 09:43 PM   #4
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What's an SPU?

Not having heard the most famous cart of all time doesn't help either The transformer was too small to be any good and too heavy to make most arms miserable.
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Old 2nd March 2005, 10:06 PM   #5
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Murray Zeligman and I did some designs like that back in the late '70s. We used a phantom-powered CM860 FET as a headamp-in-a-headshell. It actually worked, but there's no real advantage to doing this. You'd do better running a fully-balanced input with appropriate wiring.
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Old 2nd March 2005, 11:19 PM   #6
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too heavy to make most arms miserable.
I'm not picking a fight here. I just want to expression the opinion of the other side. (And I mean the other side, not the right or only side.) The problem isn't the SPU, but the modern low-mass tonearms. And most vinyl junkies in Europe and, especially, Japan will swear by the 12-inch Ortofon RF-297 or RMG-309.

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But of course you're right, and simpler is better.
No, simpler isn't better. Better is better.

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You'd do better running a fully-balanced input with appropriate wiring.
Indeed. Balanced is the only way to go if you are to use a low-output MC cart. Otherwise you should stick to MMs, which the modern tonearms are made for anyway.
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Old 3rd March 2005, 12:32 AM   #7
pixpop is offline pixpop  United States
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I wasn't thinking low mass. And I wasn't thinking MC either.

Are you saying that MCs tend to be high compliance, while MMs tend to be lower compliance?
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Old 3rd March 2005, 02:39 AM   #8
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I wasn't thinking low mass. And I wasn't thinking MC either.
Perhaps. But the problem I referred to has to do with modern tonearms being low to medium mass.

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Are you saying that MCs tend to be high compliance, while MMs tend to be lower compliance?
No, I said the opposite. But, of course, most MC carts these days are made for the low to medium mass arms out there.
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Old 3rd March 2005, 06:39 AM   #9
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And most vinyl junkies in Europe and, especially, Japan will swear by the 12-inch Ortofon RF-297 or RMG-309

Indeed. I was amazed how much better my SPU G/T sounded in this horrible chunk of steel and lead which was easily heavier than a Rega platter. Way better than in a 3012.
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Old 3rd March 2005, 11:29 AM   #10
phn is offline phn  Sweden
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Nice! If people want to know what vinyl can be they should listen to the Ortofon combo on a Garrard 301 or Thorens TD-124. I actually settled for an EMT 929 for my Garrard 301. I had an SME laying around that I originally intended to use. It's now sold.
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