These are listed as low noise and high gain, with a 60V rating.
In point of fact, the critical ones are the NPN cascoding on the MC stage
and the current source for the diff pair, where they will see about
30V.
I don't know of a direct replacement, but looking at my parts supply,
I would try the MPSA18 or MPS6571 for these two parts and use
a high quality 60V device elsewhere.
In point of fact, the critical ones are the NPN cascoding on the MC stage
and the current source for the diff pair, where they will see about
30V.
I don't know of a direct replacement, but looking at my parts supply,
I would try the MPSA18 or MPS6571 for these two parts and use
a high quality 60V device elsewhere.
2sc1844 is available
B+D Enterprises - 2SC1844
2sa991
http://www.ebay.com.au/itm/2SA991-Transistor-x-1-pieces-/161024086110
B+D Enterprises - 2SC1844
2sa991
http://www.ebay.com.au/itm/2SA991-Transistor-x-1-pieces-/161024086110
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You can use any bjt with suitable voltage rating in these positions, however, noise performance may suffer.
Looking at the datasheet of the 2SC1845, it will work as it has a 120VCE rating, but it will have a bit higher noise level (look at the noise plots).
Try 2SA872(A)E and 2SC1775(A)E
Thanks, i´ve the focus on the 2SA970 for the 2SA991 and it´s available without problems.
Chris
Note: 2SA... and 2SB... are PNP and 2SC... and 2SD... are NPN
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