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Old 17th March 2013, 07:37 PM   #21
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Yes, the curves bottom out on the right side.
At least at lower impedances.
I was talking about ultra low Rbb transistors. They really need some current to show improvements over higher Rbb ones. I found many MC heads where idle was just too low.
It makes of cause so sense to run the input stage on high current when you have a cartridge like the DL103 with 40 Ohm DC impedance.
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Old 17th March 2013, 07:42 PM   #22
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I thought it was common knowledge that low noise amps ran at high bias currents. Even jFET input seem to benefit from high bias currents for lowest noise.
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Old 17th March 2013, 07:49 PM   #23
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The question is : what is high bias current ?
Some think that 1mA is already high. Some examples are the Lindsley-Hood designs and early Self.
In the latest version of his preamp ( Elektor ) he has ramped it up a bit for a total of 4mA though. I think that is still too low when you have one of the very low impedances cartridges that come out now, say from Lyra ( Atlas, 4 Ohm ) or My Sonic Lab ( 2 Ohm and less ). Modern neodym magnets make that possible. The Atlas for example gives out nearly 1mV at 5cm/sec. That is of cause a fantastic combination for high S/N.
It would be a pity when the MC head destroys this.
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Old 17th March 2013, 07:52 PM   #24
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5mA per half of the LTP would be high for me.
That is 10mA of tail current compared to 1/4mA, or 1mA, for a higher noise LTP.
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Old 17th March 2013, 08:00 PM   #25
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Here are Syn08s noise calculations. When you go over it you find enormous bias when Rbb is ultra low. He runs his 0.3nV/qHz bipolar head on 28mA.
I battled
with him for month but in the end i gave in.
Of cause distortion has to be considered too, especially in an open loop design.
Taking this into account i found the optimum at 5 to 10mA per transistor.
That works only when the PNP has a high early voltage.
For a transistor like the BC327 with Rbb of ca.30 Ohm it comes out at 2.5mA.
More then that and the early effect comes into play quite drastic.
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Old 17th March 2013, 08:02 PM   #26
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Here is the text....
Attached Images
File Type: jpg Rbb.jpg (41.9 KB, 226 views)
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Old 18th March 2013, 05:56 PM   #27
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Here's the components of noise contribution for a 70R system -- something like the Denon DL103 (40 ohms) and transistors with rbb = 30R

Click the image to open in full size.
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Old 18th March 2013, 07:07 PM   #28
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I looked in my book, Low Noise Electronic Design, and found the formula for transistor voltage noise.
En=4ktRbb+2qIcRe**2
Re**2 means Re squared. The first term is the thermal noise in the base spreading resistance and the second term is the noise in the Shockley emitter resistance, given below.
Re=kt/(qIc)=1/Gm
You can't change the first term but the second term can be reduced by increasing the collector current, Ic. You can decrease the second term to much less than the first term so base spreading resistance noise dominates.
4ktRbb>>2qIcRe**2
If I did the algebra correctly you obtain this.
Ic>>0.026/(2Rbb) or approximately Ic>>1/(80Rbb)
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Old 18th March 2013, 07:18 PM   #29
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Fits approximately to my 5 to 10mA with low Rbb transistors.
1/ 80 x 2 = 6.25mA
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Old 18th March 2013, 07:56 PM   #30
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The red line is SQRT current noise, the green is SQRT thermal noise and the blue is SQRT kernel noise. The violet line is the net. I used the same formulas. In this example, once the kernel noise is 30% below the thermal noise there would appear to be little benefit to increasing Ic.
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