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Old 29th December 2005, 12:03 AM   #1
claudio is offline claudio  Italy
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Default Best rbb transistor, for MM phono preamp

Hello,
I am working on a MM phono preamp, a cascode configuration, which uses a Jfet 2sk389 as the low transistor and a NPN BJT for the upper one. For the noise issue, it's important to use an high Hfe and low rbb BJT, and I came up to these 3 options, which are easily available in my country:
2SC2240 datasheet
MPSA18 datasheet
BC550C datasheet

Now my question: which one has the lowest "base spreading resistance" (rbb)? Only the BC550C datasheet show this value.

Thanks,
Claudio
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Old 29th December 2005, 03:21 AM   #2
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The 2n4401 is an easy to find switching transistor with low rbb and excellent noise performance. The hfe is reasonable.
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Old 29th December 2005, 04:13 AM   #3
AKSA is offline AKSA  Australia
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Hi Claudio,

I have used the 4401 in precisely this role; beta is lowish, around 30 minimum and typically 100, but as David remarks the noise performance is exceptional - AND it's cheap.

The gain of a cascode is given by the product of the transconductance of the lower device and the load on the collector of the upper device; in truth the beta of the upper device does not, IIRC, come into it. So low beta should not matter.

Of most concern is the PSRR of a cascode - it's poor, forcing you to use either super-regulation or a battery. But I would assume this is a multistage amp, so there will be perhaps another stage after the cascode??

Cheers,

Hugh
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Old 29th December 2005, 04:42 AM   #4
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Default Re: Best rbb transistor, for MM phono preamp

Quote:
Originally posted by claudio
Hello,
I am working on a MM phono preamp, a cascode configuration, which uses a Jfet 2sk389 as the low transistor and a NPN BJT for the upper one. For the noise issue, it's important to use an high Hfe and low rbb BJT, and I came up to these 3 options, which are easily available in my country:
2SC2240 datasheet
MPSA18 datasheet
BC550C datasheet

Now my question: which one has the lowest "base spreading resistance" (rbb)? Only the BC550C datasheet show this value.

Thanks,
Claudio
The IP Jfet will determine the noise, the cascode BJT will have
no effect on noise as it is effectively fed by a CCS (the jfet)
and as such has very low noise gain.

Go for a bjt that has low OP cap, and linear HFE versus operating
current.

Cheers,

Terry
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Old 29th December 2005, 05:24 AM   #5
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Claudio, a BC550 will be fine at 100-500 uA. You will get around 1 uV noise which is quite sufficient. You can check my QSXM2 RIAA amp where I use lot's of BC550C/BC560C.
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Old 29th December 2005, 05:37 AM   #6
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Using a jfet front is going to mean fairly high supply current. The 2n4401 has best performance (<1dB NF) with a mm cartridge impedance at 50~100uA Ic, which makes battery operation realistic.
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Old 29th December 2005, 06:30 AM   #7
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Just think P-A,
you could have replaced those x4's with one 2SB737/2SD786 each and quartered the complexity with much the same results. They used to be available in small qty directly from Rohm.

Not anymore. Maybe Erno has some 786's still?
Rbb 4R ; En 0.55nV/sqrt Hz Hfe 250.

Cheers,
Greg
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Old 29th December 2005, 06:44 AM   #8
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... yes, but the design idea was to use easy obtainable parts AND matched pairs. Nothing stop you from using any TO92, DIL08, SOT23, SOT363 part. Just deside for yourself.
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Old 29th December 2005, 10:19 AM   #9
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Default Re: Best rbb transistor, for MM phono preamp

Quote:
Originally posted by claudio
Hello,
I am working on a MM phono preamp, a cascode configuration, which uses a Jfet 2sk389 as the low transistor and a NPN BJT for the upper one. For the noise issue, it's important to use an high Hfe and low rbb BJT, and I came up to these 3 options, which are easily available in my country:
2SC2240 datasheet
MPSA18 datasheet
BC550C datasheet

Now my question: which one has the lowest "base spreading resistance" (rbb)? Only the BC550C datasheet show this value.

Thanks,
Claudio
I like 2SC2240. And its complementary PNP, 2SA970.
Useful transistors with low noise.

2SC2240BL has high gain hFE 350-700.
Max 120 Vce, which is a high value.
Only 300mW power.
So, as long as you need only small powers, it will do nicely.
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Old 29th December 2005, 10:56 AM   #10
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Since noise isn't any issue in a MM stage, I think it's better to use Hfe > 450 parts (BC550C) => less current through the cartridge if is DC connected.
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