There was an article in Nuts and Volts (10 2013) about a headphones amp with op-amp topology and class A output. Claims pretty good performance. The transistors used (except for the power stage) are 2N5551 / 2N5401. Those are high voltage transistors. Would be a good choice for high voltage power amp design, but this headphones amp is powered by +/- 12V.
Is there something else about these transistors making them particularly good here? Or general purpose transistors like 2n4401 / 2N4403 will work the same?
Is there something else about these transistors making them particularly good here? Or general purpose transistors like 2n4401 / 2N4403 will work the same?
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Great question - I'll be watching this thread for the answer !
I've used this 2N5401/5550/5551 pair quite a bit, but that's because I noticed Hugh (AKSA) used them in his earlier amplifiers, which have a great reputation for their sound.
For low voltage I believe the BC546/556 are more popular.
I've used this 2N5401/5550/5551 pair quite a bit, but that's because I noticed Hugh (AKSA) used them in his earlier amplifiers, which have a great reputation for their sound.
For low voltage I believe the BC546/556 are more popular.
Simply the combination of low cost, good Vceo, reasonable Hfe and collector current makes them an unbeatable, flexible component for DIY use where a small inventory saves a lot of time and money. Randy Slone adopted them exclusively for his amplifiers and many other DIY designs. He gives brief reasons like these for choosing them, in his books.
Having said that, they aren't the best component either, when it comes to technically best performance, or lowest noise in the specific roles of say the LTP, current mirror or VAS transistor in many designs. Japanese manufacturers always used the most recently developed, specific types designed for those applications and DIYs still continue with their best selections more often than not. Just check the designs of the engineers and others aiming for ultimate peformance in their designs here. Where test and selection is viable, as it usually is for DIY, I think they make very useful general purpose transistors for medium power amplifiers but it doesn't end there - you can build the protection circuits and drive relays with them too. All for just cents and available almost everywhere.
Having said that, they aren't the best component either, when it comes to technically best performance, or lowest noise in the specific roles of say the LTP, current mirror or VAS transistor in many designs. Japanese manufacturers always used the most recently developed, specific types designed for those applications and DIYs still continue with their best selections more often than not. Just check the designs of the engineers and others aiming for ultimate peformance in their designs here. Where test and selection is viable, as it usually is for DIY, I think they make very useful general purpose transistors for medium power amplifiers but it doesn't end there - you can build the protection circuits and drive relays with them too. All for just cents and available almost everywhere.
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This kind of confirms my suspicion that these particular transistors were used just because they were there. In fact the amp in question follows the Slone's designs quite closely.
Professionally I design mostly digital stuff and usually "drive relays" with 2N2222 because of the high current rating and wide availability. So it's my "any transistor" part.
Professionally I design mostly digital stuff and usually "drive relays" with 2N2222 because of the high current rating and wide availability. So it's my "any transistor" part.
What is your application i.e. VAS, driver or general purpose type?
It's always necessary to know what your actual need is before simply guessing that something with similar general specifications should work anywhere. One look at the case size (TO92L) of 2SC2705/A1145 should tell you they are rated for higher continuous dissipation than an average TO92 transistor. You can assume this really was needed.
If the pair failed in a commercial amplifier product, it could even mean the problem is elsewhere, such as the output stage or a speaker load problem overtaxing the VAS, for instance.
It's always necessary to know what your actual need is before simply guessing that something with similar general specifications should work anywhere. One look at the case size (TO92L) of 2SC2705/A1145 should tell you they are rated for higher continuous dissipation than an average TO92 transistor. You can assume this really was needed.
If the pair failed in a commercial amplifier product, it could even mean the problem is elsewhere, such as the output stage or a speaker load problem overtaxing the VAS, for instance.
Ian Finch and jerluwoo, thank you for the reply.
Indeed I don't feel just to replace them without being sure if the 2N5401/2N5551 (TO-92) would do a good job. The original A1145/C2705 pair of right channel failed (photo) in the preamp section of a Denon integrated after the "double" diode KB-265 failed, too in the right channel. At this point, I'll take the other KB-265 before the same can happen to the left channel.
They both here and you can see the A1145 visible "fried" with both 47 ohms resistors.
The problem is that the originals are difficult to source. They seem to be very specific devices (inclunding fT and Hfe parameters).
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Indeed I don't feel just to replace them without being sure if the 2N5401/2N5551 (TO-92) would do a good job. The original A1145/C2705 pair of right channel failed (photo) in the preamp section of a Denon integrated after the "double" diode KB-265 failed, too in the right channel. At this point, I'll take the other KB-265 before the same can happen to the left channel.
They both here and you can see the A1145 visible "fried" with both 47 ohms resistors.
An externally hosted image should be here but it was not working when we last tested it.
The problem is that the originals are difficult to source. They seem to be very specific devices (inclunding fT and Hfe parameters).
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What's magic about 2N5401/2N5551 is simply this: they are rated to withstand 150 volts from collector to emitter. So they are able to tolerate the power supply voltages found in a 200 watts / 8 ohms solid state power amplifier, with a large margin-of-safety.
HOWEVER, like many higher voltage transistors, the 2N5401/2N5551 start to exhibit anomalous behavior at low VCE, specifically quasi-saturation. In my testing the NPN showed noticeable QS when Ic > 4mA, and the PNP when Ic > 10mA. So they're fine for power amp stage #1 ("LTP") but perhaps not such a great choice for stage #2 ("VAS").
However, don't take my word for it. Put your very own 2N5401 / 2N5551 transistors, built by your favorite manufacturer and sourced from your favorite distributor, on a curve tracer. The Peak Atlas DCA Pro (Link) is a low cost curve tracer that is sold on Amazon and other places.
Have a look at the low-VCE behavior of the 2N5401 / 2N5551. This is where your power amp transistors operate when you play your music LOUD, both before and during clipping. Do you like what you see on the curve tracer? If so, rejoice. If not, look further.
FYI, the MPSA42 / MPSA92 have much worse quasi-saturation, at least in my testing. On the other hand they are rated for 300 volts instead of 150 volts. I'm sure the device designer thinks you are crazy to buy a 300V transistor and then operate it at VCE = 2 volts, but for loud music and/or clipping, that's exactly where we ask the transistors to operate.
HOWEVER, like many higher voltage transistors, the 2N5401/2N5551 start to exhibit anomalous behavior at low VCE, specifically quasi-saturation. In my testing the NPN showed noticeable QS when Ic > 4mA, and the PNP when Ic > 10mA. So they're fine for power amp stage #1 ("LTP") but perhaps not such a great choice for stage #2 ("VAS").
However, don't take my word for it. Put your very own 2N5401 / 2N5551 transistors, built by your favorite manufacturer and sourced from your favorite distributor, on a curve tracer. The Peak Atlas DCA Pro (Link) is a low cost curve tracer that is sold on Amazon and other places.
Have a look at the low-VCE behavior of the 2N5401 / 2N5551. This is where your power amp transistors operate when you play your music LOUD, both before and during clipping. Do you like what you see on the curve tracer? If so, rejoice. If not, look further.
FYI, the MPSA42 / MPSA92 have much worse quasi-saturation, at least in my testing. On the other hand they are rated for 300 volts instead of 150 volts. I'm sure the device designer thinks you are crazy to buy a 300V transistor and then operate it at VCE = 2 volts, but for loud music and/or clipping, that's exactly where we ask the transistors to operate.
Hi AndrewT,
I took the other double diode KB-256 to prevent the other channel possible failure until I find the originals or other secure substitutes...
wq_ski, Yes, I was considering what Ketje just told above, using MJE340 (NPN) and MJE350 (PNP), both 20W capable (!!!), since they are available here...
Mark Johnson, thank you for all the info!
The A1145/C2705 seem to be very special for many amps produced in early 80s...
20 watts with an infinite heat sink. About a watt and a half in free air. But if you stick your finger on it and it makes you nervous, you can bolt a heat sink on it and presto! it runs cooler. Not so easy with a TO-92.
Nah... probably just a 'universal' cheap (at the time) medium power medium voltage workhorse. I had my favorite Jap units back in those days too - but they were TO-126's (for reason cited above).
Hi Guys
The 2N5401-5551 pair is fairly linear - the 5551 moreso than the 5401 - has a high Vceo and decent gain and bandwidth for most positions in a power amp for audio. Motorola's MPS-series had related types, like the MPSL01-51 that were identical to the 5551-5401 except for a 120V rating instead of 150V. It would not be the first choice for an input stage (a bit noisy for there despite having a modest NF), but other positions it works very well. Of course, much depends on the circuit.
If you really want linearity higher voltages get you there. You don't have to worry about base capacitance and saturation if the amp does not push to the rails. Even if it does - and you know it will - there are circuit options that keep things relatively nonproblematic.
The best amps in the world used 2N5401-5551 and still do today, although there is a trend towards surface-mount alternatives that embody most of the same capabilities with even lower noise.
Have fun
The 2N5401-5551 pair is fairly linear - the 5551 moreso than the 5401 - has a high Vceo and decent gain and bandwidth for most positions in a power amp for audio. Motorola's MPS-series had related types, like the MPSL01-51 that were identical to the 5551-5401 except for a 120V rating instead of 150V. It would not be the first choice for an input stage (a bit noisy for there despite having a modest NF), but other positions it works very well. Of course, much depends on the circuit.
If you really want linearity higher voltages get you there. You don't have to worry about base capacitance and saturation if the amp does not push to the rails. Even if it does - and you know it will - there are circuit options that keep things relatively nonproblematic.
The best amps in the world used 2N5401-5551 and still do today, although there is a trend towards surface-mount alternatives that embody most of the same capabilities with even lower noise.
Have fun
Hi Sir,
What's your recommendation for LTP transistors for an amplifier working above +/- 35V dual supply.
I have access to 2N5401, 2N5551 , MPSA42, MPSA92.
Thanks and regards,
Sumesh
They work fine for input stages. You can get parts that are ”better”, but having them on hand is better than waiting 6 months or a year to get the “best” ones.
A42/92 are sometimes used too. They aren’t the best choice for input stage because of the lower gain, but can be used in places where 5551/5401 can’t. When you see someone using A42 input stages, one of three things is happening. 1) The voltage is too high for anything else, 2) The builder only wants to carry one universal TO-92 pair that will work anywhere and sometimes they need 200+ volts, or 3) They don’t know any better.
A42/92 are sometimes used too. They aren’t the best choice for input stage because of the lower gain, but can be used in places where 5551/5401 can’t. When you see someone using A42 input stages, one of three things is happening. 1) The voltage is too high for anything else, 2) The builder only wants to carry one universal TO-92 pair that will work anywhere and sometimes they need 200+ volts, or 3) They don’t know any better.