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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 25th Mar 2019, 11:12 am   #1
mole42uk
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Default Low-noise BC109?

Is there a more modern equivalent for a BC109 that offers lower noise?
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Old 25th Mar 2019, 11:21 am   #2
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Default Re: Low-noise BC109?

There are lower noise transistors, but they aren't direct equivalents of the BC109. They can usually be substituted though.

This thread on diyaudio discusses the subject: https://www.diyaudio.com/forums/soli...tors-gone.html
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Old 25th Mar 2019, 11:45 am   #3
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Default Re: Low-noise BC109?

Back in the day 2N930 was considered lower noise than BC109.
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Old 25th Mar 2019, 11:56 am   #4
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Default Re: Low-noise BC109?

BC549?

Same chip, but TO92 epoxy package.

I believe the BC107, 108, 109 transistors use all the same chip anyway, but originally they were selected for low noise (BC109) or high voltage capability (BC107). Those that didn't meet either were labelled BC108. But as processes improved, the fall-outs reduced, most chips met all specs, so the manufacturers just labelled them according to which types were in most demand that week.
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Old 25th Mar 2019, 12:51 pm   #5
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Default Re: Low-noise BC109?

Possibly:

2N5089 with max NF of 2dB?

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Old 25th Mar 2019, 1:24 pm   #6
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Default Re: Low-noise BC109?

Was there not a BC109C on sale back in the day that claimed to be a low noise version.
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Old 25th Mar 2019, 1:32 pm   #7
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Default Re: Low-noise BC109?

Hi.

Maybe BC549?

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Old 25th Mar 2019, 1:42 pm   #8
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Default Re: Low-noise BC109?

The C suffix means high gain.
As well as the 107/108/109 selection, there was an A/B/C grading for gain bands!
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Old 25th Mar 2019, 3:55 pm   #9
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Default Re: Low-noise BC109?

While all based on the same die as the BC109, current production BC549 or BC550 might offer lower noise because of improvements to the production process. I haven't studied datasheets so no guarantee, but I would look there first.
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Old 25th Mar 2019, 5:52 pm   #10
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Default Re: Low-noise BC109?

I think they have the same spec, but that's a maximum and they may be quieter in practice.
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Old 25th Mar 2019, 6:13 pm   #11
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Default Re: Low-noise BC109?

Quote:
Originally Posted by Maarten View Post
While all based on the same die as the BC109, current production BC549 or BC550 might offer lower noise because of improvements to the production process. I haven't studied datasheets so no guarantee, but I would look there first.

That's what I would also think is the case due to better manufacturing techniques. An instrument (preferably home built) for checking the noise of semiconductor junctions could be a useful addition to the workshop.

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Old 25th Mar 2019, 7:17 pm   #12
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Default Re: Low-noise BC109?

2n5089
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Old 25th Mar 2019, 7:45 pm   #13
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Default Re: Low-noise BC109?

BC123. BC199. These were low noise alternatives back when the BC109 was popular.
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Old 25th Mar 2019, 8:08 pm   #14
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Default Re: Low-noise BC109?

Low noise at what source impedance?
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Old 26th Mar 2019, 12:04 pm   #15
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Default Re: Low-noise BC109?

I'm using circuits developed by Mr. Linsley-Hood in 1969 for RIAA equalisation on a magnetic cartridge input. He specifies a BC109 for the circuit.

I'm afraid I don't know enough about the operation of transistors at audio frequencies, much of my experience uses valves so this is a bit of a learning exercise for me.
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Old 26th Mar 2019, 12:30 pm   #16
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Default Re: Low-noise BC109?

I would go ahead and build it using BC109s/549s as originally designed. There may be little practical benefit to using ultra low noise transistors, especially with a moving magnet cartridge as the input. You can always experiment later.

Small signal bipolar transistors are usually fairly interchangeable within their ratings and gain groupings. Even the gain is less critical than you might expect.

If you want an RIAA preamp for use rather than as a construction exercise, you would be better off building one around a modern dual op-amp designed for audio use. Even a simple design can give excellent results, including low noise levels.
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Old 26th Mar 2019, 2:41 pm   #17
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Default Re: Low-noise BC109?

I would go with the suggestion in post #16 and use a good socket for the transistor/s so you can try different types.
One application where transistor audio spectrum noise crops up is when the transistor is on the input of a very high gain amplifier, like transistor's in effects pedals for guitars. Years ago I experimented with a large range of silicon and germanium types.

It is interesting the different types of noise you can get, sometimes a clean high pitched hiss other times a random rumble and even if the transistors are the same type there can be quite a lot of variation in the quality or type of the noise produced. It is also affected by the collector currents and the particular circuit. Make sure to use metal film resistors in your circuit too.
Oddly some of the better specimens I found were intended for RF work, like the 2N3643 and 2N3644. Some claimed low noise types were no better. In the end you will likely find some very good specimens.

Another thing, a transistor can be very noisy at audio frequencies, but in an RF application, you cannot hear the noise as its outside the spectrum of IF & RF amplifiers so you don't know it's there. And in many cases there can be more noise generated by the collector resistor (in an audio circuit) especially if a carbon comp type, than the transistor itself. As I recall there are some good noise performance graphs for transistors vs Fets in Horowitz & Hill.
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Old 26th Mar 2019, 3:30 pm   #18
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Default Re: Low-noise BC109?

In the L H preamp, I think the 47k input resistor will dominate transistor noise though vinyl surface noise will dominate overall. The resistor will produce thermal noise of about 32nV in 1Hz BW. I sympathise though as I find noisy electronics very irritating, especially on headphones, even though I know it doesn’t matter when music is playing !
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Old 26th Mar 2019, 4:28 pm   #19
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Default Re: Low-noise BC109?

Quote:
Originally Posted by paulsherwin View Post
If you want an RIAA preamp for use rather than as a construction exercise, you would be better off building one around a modern dual op-amp designed for audio use. Even a simple design can give excellent results, including low noise levels.
Agreed Paul, but I have the JLH power amplifier, so I'd like to build a 'period' pre-amp to go with it.
I am also interested to improve my knowledge of transistor use in audio, it keeps the mind active!
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Old 27th Mar 2019, 12:25 pm   #20
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Default Re: Low-noise BC109?

From the datasheets: the noise figure for the BC109 is max 4dB (Uce 5V, Ic 200uA, Rg 2k, F 1kHz, delta-F 200Hz),
for the 2N3904 max 5dB (Uce 5V, Ic 100uA, Rs 1k, F 10Hz - 17.5kHz).
So you could try even the "lowly" 2N3904.
Though the BC109C would have higher hfe.

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