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-   -   Bipolars for low noise input - Alternatives to 2SC2240 (http://www.diyaudio.com/forums/analogue-source/182490-bipolars-low-noise-input-alternatives-2sc2240.html)

hifidaddy 4th February 2011 12:46 PM

Bipolars for low noise input - Alternatives to 2SC2240
 
I wonder about bipolar transistors which are best suitable for moving coil input stages.

I have here 2SC2240/2SA970, 2SC2546/2SA1084 (and many other bipolars) lying around. The 2SC2240 sounds better in my amps so far, but the Rbb is about 40R which is too high for low signal inputs. The 2SC2546 has much lower Rbb, but I do not like it soundwise.

Somewhere in diyaudio.com I read about transistors which sound better than 2SC2240 in input stages, but I cannot find this place anymore.

I want to drive the input stage at 5 to 10mAmps idle current each transistor, to keep voltage noise low down for my 0.1mV output moving coil cartridges (Ortofon 7500, Accuphase AC2, Fidelity Research). I have in mind a simple (non-paralleled, non mirrored) balanced LPT, or alternatively a balanced zero-impedance input, all without feedback.

After many years of playing around, I myself dislike the sound of JFETs and opamps in general, so these devices should be excluded rather.

Thanks in advance for discussion.

Hartmut

Blackjack8635 4th February 2011 04:32 PM

The 2SA1084 and 2SC2546 series that you dislike are pretty much what is commonly available.

The much lamented Rohm 2SB737 is still available at a price.......2 each from Gareth Connor of the Signal Transfer Company near Cleckhuddersfax, can be googled or via the Self site.

Transistors such as the 2SC2546 etc can be safely paralleled WITHOUT the use of emitter balancing resistors to reduce the effective rbb' and hence an improvement in noise figure. Especially if they have been purchased as a single lot. Any difference in rbb' will cause an imbalance in current, but that is not a problem :) And they are a lot less than 2 ;)

rayfutrell 4th February 2011 07:08 PM

Toshiba recommended the 2SA1316 and 2SC3329 for MC head amps. Toshiba no longer manufactures transistors so they may not be available. You could also try some of the DIODES (ZETEX) medium power transistors as shown here. http://www.diodes.com/_files/design_...zetex/dn11.pdf

Ray

Blackjack8635 5th February 2011 07:11 AM

Quote:

Originally Posted by rayfutrell (Post 2457682)
Toshiba recommended the 2SA1316 and 2SC3329 for MC head amps. Toshiba no longer manufactures transistors so they may not be available. You could also try some of the DIODES (ZETEX) medium power transistors as shown here. http://www.diodes.com/_files/design_...zetex/dn11.pdf

Ray

I am afraid you are correct, a quick google will list them with suppliers, but they are always out of stock or made from unobtanium. I certainly have not been able to find any in the UK.

It all comes down to rbb', and I can only go back to 1084 and 2547 and for which the data sheets show comprehensive plots of the noise figure curves. These imply a low rbb' but still a dB or two or three above 737, 1316 and 3329.

The mention of Zetex is good, they used to do a range of switching transistors for things like print heads, small package TO92 for low duty cycle, but high peak current rating.

JensH 5th February 2011 12:39 PM

You should probably be careful with the Zetex Design Note. While looking for transistors with low rbb I came across the following, where there is a quiz around the Design Note. See post #9 and 10.

Ancient ultra low noise narrow band preamplifier - Geotech Forums

Not exactly a positive evaluation!

I am leaning towards the 2N4401 (or rather the SMD variants of this like the MMBT4401), but this is for a 200-250ohm source impedance and since it is for mass production, the cost is also an important parameter. I know the 2N4403 should be better, but I have some other constraints in my design making me prefer the NPN device.
By the way, does anyone here know the rbb of the 2N4401? Reference?

5th element 5th February 2011 01:01 PM

There's always the KSC1845/KSA992 from fairchild. These come in both through hole and SMD packages and are inexpensive too.

jez 5th February 2011 01:23 PM

Quote:

Originally Posted by rayfutrell (Post 2457682)
Toshiba recommended the 2SA1316 and 2SC3329 for MC head amps. Toshiba no longer manufactures transistors so they may not be available. You could also try some of the DIODES (ZETEX) medium power transistors as shown here. http://www.diodes.com/_files/design_...zetex/dn11.pdf

Ray

Oh yes they do!
Product List | Bipolar Transistors | TOSHIBA Semiconductor Company

hifidaddy 5th February 2011 01:56 PM

In the current Linto thread there is the schematic, and I can see there ZTX PNP BJTs at 30mA each device in the input stage.

BTW, I once had borrowed the Linto from a dealer but did not like its sonic performance. The Linto had a lot of bipolar "sand", and bass did not boogie.

Hartmut

rayfutrell 5th February 2011 06:49 PM

Interesting comment on the Zetex design example. Quad used the Zetex transistors in the MC preamp of the Quad 34 and Quad 44. I forget what transistor Quad was using prior to redesigning the MC preamp to use the Zetex transistors. I don't think Quad would have made the switch if the Zetex transistors weren't better. Of course, that was in the 1980s and transistors have improved since then.

JensH 5th February 2011 09:14 PM

I think the Zetex transistors are actually excellent as mikebg writes in his comments, but he thinks the design using them is bad. And I think he does have a point there.


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