Looking at my low noise high gain parts. Kind of the gold standard is the 2SC2240 and 2SA970.
the KSC1845 and KSA992 are definitely good, but not as high gain. How do the noise specs compare to the above pair?
BC550C and BC560C have the higher gain I was looking for, but again, how to the noise specs compare?
Now the pair that I question the most. I know that EW is a huge fan of these. ZTX694B and ZTX795A. Very high gain, I have seen them called low noise, but like the others I don’t know how they compare. Are there others that are fans of these devices? I just don’t see them getting recommended all that often, but maybe it’s because they are expensive? if the noise numbers are low enough, I would say these are a good competition to the 2240/970 pair.
Dan
the KSC1845 and KSA992 are definitely good, but not as high gain. How do the noise specs compare to the above pair?
BC550C and BC560C have the higher gain I was looking for, but again, how to the noise specs compare?
Now the pair that I question the most. I know that EW is a huge fan of these. ZTX694B and ZTX795A. Very high gain, I have seen them called low noise, but like the others I don’t know how they compare. Are there others that are fans of these devices? I just don’t see them getting recommended all that often, but maybe it’s because they are expensive? if the noise numbers are low enough, I would say these are a good competition to the 2240/970 pair.
Dan
My opinion is based upon use of the ZTX851/951 and ZTX450/550 -- they are excellent and very low noise. They are spec'd in several of the Pass designs featured here on DIYAUDIO.
I built a very low noise amplifier with the ZTX951 and it rings in around 200 pV/RtHz
I built a very low noise amplifier with the ZTX951 and it rings in around 200 pV/RtHz
Several Zetex transistors are in this useful list of low-noise devices : http://www.dicks-website.eu/low_noise_amp_part6/part6.html
Are you interested in equivalent input noise voltage, equivalent input noise current or both?
For low noise voltage, you need a low base resistance. BC550C and BC560C are not very good in that sense: base resistances of a few hundred ohm while your gold standard is about 10 times lower, if I remember well.
The noise current is mostly related to the base current, so you need high hFE and low base current 1/f noise. Transistors of which the hFE doesn't drop much at low collector currents usually have low base current 1/f noise. JFETs can easily outperform bipolar transistors when it comes to noise current, as they have no base and hardly any gate current.
For low noise voltage, you need a low base resistance. BC550C and BC560C are not very good in that sense: base resistances of a few hundred ohm while your gold standard is about 10 times lower, if I remember well.
The noise current is mostly related to the base current, so you need high hFE and low base current 1/f noise. Transistors of which the hFE doesn't drop much at low collector currents usually have low base current 1/f noise. JFETs can easily outperform bipolar transistors when it comes to noise current, as they have no base and hardly any gate current.
Thanks, I was just looking for ZTX450 recommendation - I can buy them locally for reasonable price.
Naim used ZTX214 and ZTX384 (I think they are no longer available) in their preamps and they are excellent.
See here
The ZTX694B and ZTX795A are high current (0.5A) and high capacitance for a signal transistor. About 5 times the output capacitance than the oft-used KSC1845/KSA992. So they would not be suitable as a VAS, but possibly a good choice for input pair where voltage noise would be an important specification. Interestingly they have SOA curves in the datasheet, not normally a thing for an analog signal device. The big clue is the "darlington replacement" in the list of applications - these were designed as relay and solenoid drivers. They are not perfectly complementary either.