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Old 28th March 2009, 12:26 AM   #1
Aiace is offline Aiace  Italy
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Default Best BJT for audio applications

What's the best signal (<=1W) BJT for audio applications used as amplifier ?

Thanks
Aiace
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Old 28th March 2009, 01:16 AM   #2
Mr Evil is offline Mr Evil  United Kingdom
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There is no one best of anything. There are lots and lots of good BJTs that are good in various circumstances depending on the combination of voltage, speed, gain, noise etc. you want.

Personally, I like BC550/560 for general purpose use. They have low noise and are cheap. Get ones in the 'C' gain group if you can.
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Old 28th March 2009, 02:02 AM   #3
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I agree, ... no one "best" device for input stages. One can have
a favorite based on the $$$/speed/availability ratio.

For high gain/voltage 2sc1845 (npn) 2sa992 (pnp),
at 1.8pF Cob 120v Vce , they fit the bill for most LTP's.
Also the 2n5401 /2n5501 are great for super high rails
but have higher Cob. (MPSA 92/42 are the same).

The BC's Evil mentioned are very widely available
and work well for lower voltage amps ( or if you
use a cascoded LPT with a HV amp).

Everyone seems to go for faster transistors , but sometimes
this "backfires", creating the need for more care in
compensation and layout to avoid oscillations.
Voltage is the more important issue , as I try to derate
40% or more. ( 70v rail = 120 V device).
OS
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Old 28th March 2009, 02:19 AM   #4
Aiace is offline Aiace  Italy
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What I'm discussing about is the sonic performance. Certanly it depends to applcations and personal taste, but I'm sure that everyone has the own preferred one. What's one?

Aiace


PS: If I had to say the trouth I'm looking for a BJT to use in an i/v converter and amplifier. I think it should have a veri good linearity of hfe over a wide range of current. Is it right?
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Old 28th March 2009, 02:49 AM   #5
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Quote:
What I'm discussing about is the sonic performance
Then the opinions become more subjective. most would say
JFET inputs sound better and the japanese designers
seem to agree (70% of their amps have cascoded jfet's).
I have noticed that the topology and the compensation
have more affect on the final sonic quality than the device
used.
There are truely horrible choices one can make as far as
devices (a low gain device in a circuit that "needs" HG ,
like a non-mirrored simple LTP) but the sonics are the
result of the interaction of all the stages + the feedback
and compensation.
The passive components are as much a factor as the
silicon.
If you must know MY favorite , it is the 2sc1845/2sa992- ksa992/
ksc1845, because it comes in 4 gain groups , has a very wide linear region (.01-9 ma)and is cheap($.08),quiet and fast.
( I use the "F" gain group)
http://www.fairchildsemi.com/ds/KS/KSC1845.pdf
http://www.fairchildsemi.com/ds/KS/KSA992.pdf
original japanese 2sc/2sa's are the same..
OS
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Old 28th March 2009, 06:34 AM   #6
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Quote:
Originally posted by ostripper
I try to derate
40% or more. ( 70v rail = 120 V device).
OS
You cannot use a 120V device on +/- 70 V rails [because the peak to peak V swing will be closer to 140V], for that you need atleast 150V device or 200V device with better safety margin.
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Old 28th March 2009, 07:05 AM   #7
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Quote:
You cannot use a 120V device on +/- 70 V rails [because the peak to peak V swing will be closer to 140V],
I was talking about the typical self LTP topology (from the
Re's of the LTP to the positive rail) NOT
rail to rail ! (attached)

For a VAS or OPS and 70v rails , yes... 250+ is the norm.
I would not give advice to make those little devices "pop" like
firecrackers...
OS
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Old 28th March 2009, 08:33 AM   #8
kaos is offline kaos  United States
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Quote:
PS: If I had to say the trouth I'm looking for a BJT to use in an i/v converter and amplifier. I think it should have a veri good linearity of hfe over a wide range of current.
The Toshiba 2SA1015 and the 2SC1815 are decent devices and have a reasonably flat hfe curve. For example:

http://www.semicon.toshiba.co.jp/doc...eet_071101.pdf
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