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Critic's Corner Discuss a review. Provide constructive feedback. Talk to the industry. |
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In Reply to: Yet another rave review for cables that fit Trump's budget made me wonder........ posted by Rick W on February 17, 2010 at 11:56:03:
"How did all that good stuff get on the recordings made in the 50's, 60's, 70's (or earlier)".
I've wondered about this myself. Two possible factors have already been mentioned, tubes and less EMI, and I'm inclined to agree with that thinking.
EMI-wise, besides there being less of it in the typical environment in those days, normally designed equipment also tended to have better rejection of it than it does now. Several factors contributed to that and they were mostly driven by the tubes themselves. Take RFI for instance, radio frequency interference which is virtually always intercepted by cables and presented to the gear as a common mode signal. An amplifier has three cables: in, out, power and all make potential antennas. So what's the difference between now and then? I'll trot them out but first I want to say clearly that it's all a matter of degree, all the same problems that affect transistors based designs can affect tube based ones also, but not as easily. Here's why...
Power: The typical power supply used to be full-wave rectifier (5Y3/5U4) feeding a choke input (L/C) filter. Two factors here are important, the tube and the choke. Tubes are slower than silicon and so less inclined to rectify RF and have a substantially higher on-resistance, the choke provides a deliberate series impedance to lower frequencies and has a lot of parasitic capacitance to ground to shunt the HF stuff. And, they draw their power over a considerable amount of the cycle reducing hash and flat-topping on the power grid. Those supplies provide a lot more isolation from the line also and produce a lot less noise than silicon diodes dumped into a capacitor.
Input: Tubes are slow. Well, relatively slow and the variety used for audio run out of steam by about 50 MHz largely due to transit time and parasitics. Solid state is fast, even if the circuit is bandwidth limited to essentially the audio region, the devices themselves might happily amplify and rectify signals in the GHz region. The smaller the device typically the faster it is, that's why you use a point contact diode in your crystal set. You do have one, don't you? And nowadays where die-size is money, the active area of a chip may not be much larger than the "point" contact area of a cat whisker.
Output: This is similar to the power system. The output tubes are slow, the plate-VC transformer provides significant isolation and the whole maryann operates at high voltages and is largely unilateral, meaning the output doesn't affect the input to speak of. The high voltages (high impedances) help because they drive a high turns-ratio on the transformer making the stray capacitance an effective filter for incoming RF.
I've prattled on almost enough, I want to say again that it's all relative, a strong RF signal of a few MHz (but <100MHz) will cause grief in tube front ends especially since manufacturers in those days were just as scrimpy as they are now and didn't bother with the cost of an RC filter at the first grid. I had to correct that deficiency for a number of neighbors and relatives to keep them from hanging me with my open-ladder line. The trouble with ham radio is that the antennas are so big everyone knows who the guilty party is.
Now, is this stuff the difference? Who knows? But I bet it's at least part of it.
Rick
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- RE: Yet another rave review for cables that fit Trump's budget made me wonder........ - rick_m 02/18/1005:11:42 02/18/10 (0)
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