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Neve circuit .... Total range is only 24 dB.
My partial simulation shows reasonably good V/dB linearity over 40dB of input. Dot-line is perfect, solid line is simulated from partial schematic, notably omitting the rectifier.
(How does that display?)
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the log conversion is very poor.
Hmmm, we disagree. Wonder what the difference is.
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I assumed the MS2027 diode to be 2.7V 500mW
My sim only has a small 5.6V Zener, I edited the model for 2.7V, and let it run. I am aware that a very low-volt Zener is "different", but I didn't think it would be very important?
Clearly Neve was able to get these things sold and paid-for. The BBC PPM is a broad-zone indicator, not high resolution, so maybe "poor" was good enough? Or maybe he painted the dials to match his poor conversion?
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DIN 45406... is "compressed" at the low end
Yes. A nice scale, but hard to do well. Since we have digital overload indicators, and usually good monitoring (broadcast engineers occasionally work "deaf"), do we need really great PPMs? Or do we just want an analog distraction from dull digital machinery?
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redraw the scale on the Siemens
_I_ love the 1950s graphic. _I_ would leave it original, just make a mental note that it isn't dB-perfect.
What is a meter for? To know when you clip; but that's why you have the OverLoad LED on your other gear. To know when the mike went dead; any meter will give a zero indication. To know the general average loudness: soft for the flower show, loud for the football game; just remember what part of the scale seems to be right loudness for several types of program.
If you -need- a -37dB level to tune a compressor, you have DVMs and maybe a 40-segment LED meter somewhere, which can give you 0.011V on the nose. While the 1960s broadcast engineers may have used these PPMs to check -37dB levels, the limiter factory would have used a gain-set with a good voltmeter and
precision attenuator to get reference tone.
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I don't see myself sourcing 2 transformers and a coil
Most of that U370 plan is German thoroughness. Added input switching, supersonic filter, killer rectifier driver. Then "Platte U370a6" is the same idea as Neve's Zener-string. Neve stacked them up and let the errors average-out, TAB gives six separate break-point trimmers and paid some apprentice to adjust every one. "Platte U370a4" is a very high precision voltage to current converter, where Neve just buffered to a 10K resistor.
One difference: Neve log-converts the rectified audio, then the attack/decay filter; TAB rectifies onto the attack/decay filter and then log-converts. This affects your choice of time-constants, but is probably all the same to the user.