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Old 2nd April 2012, 11:14 PM   #21991
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If to take Thorsten't input stage and bootstrap JFET followers like Sansui did I would like it even more.
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Old 3rd April 2012, 02:27 AM   #21992
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The MPSA18 is related to the 2N5210, which is complementary to the 2N5087. These are devices that we used more than 40 years ago for input stages. That is WHY they have a WHOLE PAGE devoted to Noise Figure. However, look what happens when you run at 10ma or more. That is why I put it up. These devices normally have a beta between 250 and 500 or so, which lowers the base current. They are fairly high voltage, and their rbb' is slightly on the high side, but typical for this range of devices.
OF COURSE, you can insist on a complementary pair, low Rbb' device with a similar voltage rating and a beta of about 500. I might sell a few to some serious customers. Not cheaply, though. Not made anymore.
It would be a waste of device, since Rs will probably be more than 100 ohms and typically 1K ohm. Then you are in trouble.
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Old 3rd April 2012, 02:36 AM   #21993
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Everyone, please note what type of transistor this is defined as by the manufacturer. There are 'better' devices, but they are not available anymore.
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Old 3rd April 2012, 02:41 AM   #21994
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Everyone, please note the designation of this transistor. There are better transistors, but they are not available, in general. Still, running at 10-25ma would not be optimum. Self heating, you know.
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Old 3rd April 2012, 02:45 AM   #21995
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Hi,

Quote:
Originally Posted by elektroj View Post
You weren't giving credit to originators of the idea, you published schematic and attributed it to Accuphase,
Please upgrade your reading and interpretative skills and stop making silly claims.

If you misunderstood what I wrote, tough...

Ciao T
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Old 3rd April 2012, 04:55 AM   #21996
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In any case, there are usually better ways to make a low noise input than a comp. diff. bipolar transistor array, even though it can be made quiet enough for most uses.
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Old 3rd April 2012, 06:17 AM   #21997
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In late 70'th I found that this transistor works better in mic preamp than specialized low-noise transistors.

Click the image to open in full size.

The guy who designed it in our Tomsk Institute said that he did spiral shape of emitter because it looked better, but I think it was a joke.

It is КТ-626, high frequency medium power transistor.
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Old 3rd April 2012, 06:34 AM   #21998
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Yes, Wavebourn, many medium power transistors work well enough for low noise inputs. I used the Fairchild 8050 and 8550 two amp complementary output transistors in the late 70's in the Sota MC Head Amp, and got 0.4nV/rt Hz.
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Old 3rd April 2012, 11:50 AM   #21999
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Quote:
Originally Posted by john curl View Post
Yes, Wavebourn, many medium power transistors work well enough for low noise inputs. I used the Fairchild 8050 and 8550 two amp complementary output transistors in the late 70's in the Sota MC Head Amp, and got 0.4nV/rt Hz.
Similar devices are still available.
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Old 3rd April 2012, 02:15 PM   #22000
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I might point out that while some 'unusual' devices might work OK for a MC pre-preamp, they would usually make poor candidates for a general purpose complementary differential stage, being discussed here at the moment. This is because of other compromises, such a low beta or low maximum voltage, or both. Practical designers cannot always get in quantity the 'ideal' parts for a product, and we have to find reasonable substitutes.
Perhaps an example of a 'super device' complementary pair is the Toshiba 2SA1316/2SC3329 parts-pair. These parts do just about everything right: Low Rbb', relatively high voltage, very high beta, etc. Unfortunately, they are hard to find.
In future, we will be stuck with GENERIC devices like the MPS-A18,2N5087, etc. it would seem.
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