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Old 2nd March 2010, 12:14 AM   #941
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I never saw a built one, but the design was probably mine, as I developed it for Anthony Michealson in 1979 in California.
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Old 2nd March 2010, 01:00 AM   #942
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The correct spelling (I think) is Antony Michaelson, but I could not edit it in time.
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Old 2nd March 2010, 03:04 PM   #943
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A lot of medium power and video transistors have low Rbb.
I am just building an Rbb meter so letīs see what i can find.
We used BC140 / BC160 in MC stages in the 80th. We selected them by listening over headphones.
What a problem can be with transitors not made for audio is that Hfe is sometimes quite low and not very linear with collector current. Hfe can also be very different in NPN and PNP devices, so is 1/F noise corner frequeny.
By the way, i have a working prototype of the BJT INA. To get that going i needed more time then i thought. A lot of circuit parameters had to be changed.
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Old 2nd March 2010, 03:11 PM   #944
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Joachim, the most important thing that you are overlooking is non-linear input capacitance. It can be very high in power devices. Normally, the easiest way to study Rbb' is to use a Quantech Noise Analyzer.
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Old 2nd March 2010, 03:41 PM   #945
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Personally, I like to buy quality INA's based on IC'. Twenty five years ago we first started with Scott Wurcer's AD524. While it was not super quiet, it was very quiet, and it have VERY good common mode rejection. This is most important. Today, you can buy even quieter INA's and use them.
Of course, the INA concept can be used for balanced in microphone preamplifier stages. In this case, all discrete designs are best. I have an 8 channel mixing board, returned to me, after years of use, using Toshiba 2SK146 diff pairs in a discrete op amp board, configured as an INA. I also made a similar configuration for Crystal Clear Records, decades ago.
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Old 2nd March 2010, 03:42 PM   #946
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Quote:
Originally Posted by Joachim Gerhard View Post
A lot of medium power and video transistors have low Rbb.
I am just building an Rbb meter so letīs see what i can find.
We used BC140 / BC160 in MC stages in the 80th. We selected them by listening over headphones.
What a problem can be with transitors not made for audio is that Hfe is sometimes quite low and not very linear with collector current. Hfe can also be very different in NPN and PNP devices, so is 1/F noise corner frequeny.
By the way, i have a working prototype of the BJT INA. To get that going i needed more time then i thought. A lot of circuit parameters had to be changed.
So far, i did made some tests for a MC preamp with some well known
transistors.

BC550/560 are quiete good in respect of the age of these devices.

2SA970/2SC2240 are not better, or only marginaly.

The best results were those of the 2SA992/2SC1845 , on par with
the old 2SA872A/2SC1775A.

Using four in paralel rejected the preamp noise contribution to
a mere 0.32 nV/sqrt hz.....

Personnaly, i use 16 of them, as i did choose a symetrical differential
as topology, so i had to compensate for the increased noise of the
differential pairs compared to single ended by increasing the numbers.

Anyway, great work by there, we just wait for the final cut if
ever there is one, as you seems to be a die hard perfectionnist...

all the best in your tries..
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Old 2nd March 2010, 07:11 PM   #947
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It is important the amateurs and professionals alike to bring to the front what is really happening with transistors and noise. I hope this gets through.
First, it is important to note that there are 5 major sources of noise in a bipolar transistor.
The first two are related to shot noise. This is both input and output current shot noise. One gets manipulated mathematically and turned into En The other is converted to In. This is where we get the En and In figures on the spec sheet. En can be virtually as low as you could ever measure, BUT In will rise proportionally and ultimately dominate the noise. All you have to do is to change the output current in the bipolar transistor to adjust this.
The other three major noise sources are the departures from the ideal bipolar transistor. These are what makes the essential noise difference between transistors measurable.
These are Base Resistivity or Rbb', BETA (the higher the better, all else being equal). and 1/f noise (a number of mysterious causes).
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Old 2nd March 2010, 09:39 PM   #948
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Before i started the MPP thread i build a balanced phonostage with INA163 at the input.
Then comes the 75usec passive and the rest active in an OPA228. At the output i added a fast 100mA transimpedance OP as buffer. The stage is DC coupled with servo. At the output i added a foil safety cap. I used very good parts and even made an elaborated battery supply. I used this stage in the beginning as benchmark.
It does not sound bad at all but the problem is that the internal feedback resistors are 3kOhm. So for a gain of 38dB that is optimal to avoid noise in the next stage ( why this is so i will try to explain in a later post) you need an Rg of 80 Ohm. That brings the total noise to 2.2nV/qHz. Compare that with my 0.6nV/qHz at the same gain ( i am now so far that i can change the gain to virtually any value without added noise) and you see that my design is in a totally different ballpark.
I am not extremely sensitive to noise, because when the music is playing it is usually swapped when it is not totally incompetently designed but many more or less naive listeners crank their systems full and want noise as low as posible.
I know that a stage with extremely low noise is not a recipy for good sound "per se"
( look for example to some good sounding tube units that have a fair amount of noise) but it does not heart provided everything else is the same.
P.S. working on the BJT version i swapped the plus and minus supply today and have blown my only bord. I am quite frustrated. This BJT stgae is a pain the b.
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Old 3rd March 2010, 01:32 AM   #949
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Quote:
Originally Posted by scott wurcer View Post
I would like to see a meeting of the minds on the dynamic headroom for RIAA issues. That is how to best partition gain and time constants and of course a definitive way to test it.
Wondering, why use the phrase "dynamic headroom" for a preamp. This is usually used for power amps with unregulated supplies that are able to provide more short term peak power. Isn't this simply headroom?
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Old 3rd March 2010, 02:51 AM   #950
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Dynamic headroom is really based on the amount of high frequency, well above 20KHz that effects phono cartridges most. This can extend to above 200KHz on a regular basis.
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