Component procurement: Audio transistors and substitutes

Abstract

In the past, there was a huge variety of great transistors on the market which are perfectly suitable for analog audio. The situation has changed dramatically and in year 2021 there only a few transistors suitable for linear audio left. Furthermore, all kinds of electronic components have become more difficult and expensive to procure.

I have a small stock of old transistors for my projects so I'm currently not affected too badly by obsolete and out of stock components. However, I need to manage my small stock wisely and think of suitable substitutes early to save the good parts for applications where they perform best and investigate where lesser grade part are applicable.

Transistor pin-out overview

I try to maintain an overview here about transistors that are still on the market and some obsolete ones to find substitutes more easily. This overview may be helpful to quickly identify a transistor to fit a given pin-out.

Transistor details and substitutes

2N5551 / 2N5401

2N5551 in THT is obsolete - available in SMT
2N5401 is almost obsolete

Complimentary small signal transistors with high VCE. Not directly pin compatible to any other transistors I have. I bought some of them long time ago when they were cheap.

BC546 BC547 BC548 BC549 BC550
BC556 BC557 BC558 BC559 BC560

Mostly still active

This is a very old transistor design that is partially still in production. They differ from each other in maximum VCE and the series features some low noise types. The whole series has high fT of 150MHz as expected from a low power transistor.

Low noise types BC549 / BC559 and BC550 / BC560 availability has become more sparse, especially in high hFE. I was lucky to grab some of the last BC559C as complement to the old BC550C I had in stock. Not a perfect match, but close enough.

In year 2021 I bough large bags of BC547C and BC557C for roughly five cents per transistor as general purpose transistors for applications where low noise is not important. The C grades have high hFE at least. I guess this will be my preferred small signal transistor for future designs.

BC639 / BC640

active

Fast high voltage small signal transistor. In the past, they have been very affordable. Nowadays they cost 19€ per hundred, which I find pretty expensive for a small signal transistor.

BD139 / BD140

active

The specification of those transistors in the manufacturers datasheets is omitting many important aspects. Expect substantial variation between manufacturers. The old BD139 / BD140 from Philips were just BC639 / BC640 put into TO-126 package. The different package slightly alters the electrical parameters. The newer BD139 / BD140 from different manufacturers are not made this way, but have a bigger die with different characteristic.

I could imagine that this pair of transistors may be a good choice for low VCE applications where high VCE may not perform well.

In 2021, they sell for roughly 29€ (incl. VAT) per hundred.

SMD equivalent would be the BCP53 / BCP56 in SOT-223 package. ON Semiconductor specifies 50MHz fT for theirs.

IRF610 / IRF9610

active

Old vertical MOSFET design. While designed for switching applications, this pair is suitable for linear applications as well. Good as buffers or drivers.

IRFP240 / IRFP9240

active

Old vertical MOSFET design. While designed for switching applications, this pair is suitable for linear applications as well. Ideal choice for powerful output stages.

MJE243 / MJE253

active

Lower VCE and very high current transistors with good SOA. Yet I haven't found application for those.

MJE340 / MJE350

active

High VCE transistors with impressive SOA. Specification in the datasheets is omitting many important parameters like fT. Those transistors may have substantial deviations between manufacturers. Pierre Lacombe measured fT of roughly 20MHz for MJE340 and 10MHz for MJE350. Good as powerful general purpose transistors, but not directly in the signal path.

MJE15032 / MJE15033

active

Good driver transistors, but not as good as the old ones. Lower fT, lower hFE, better SOA and pin compatible to many now obsolete ones like SC4793 / SA1837 or SC4159 / SA1600.

MJL3281 / MJL1302

active

Excellent power transistor for power output stages.

NJL3281 / NJL1302

active

ThermalTrak variant of MJL3281 / MJL1302 with built in diode for very fast bias adjustment. This is by far the best option to keep bias constant in class&bnsp;B output stages.

SC1845 / SA992

SC1845 almost obsolete
SA992 mostly active

This pair of low noise transistors offers higher VCE.

The best way to deal with high voltage across a small signal transistor is to cascode the small signal transistor with a more powerful one. This also reduces the Early effect.

SC3503 / SA1381

SC3503 is active
SA1381 seems obsolete

The SC3503 and SA1381 are complimentary to each other and were excellent transistors for linear audio applications. Low Cob, high FT and high VCE with good SOA make them a very good choice in the VAS and as low power drivers.

A lower grade and pin compatible replacement would be the TTC004B and TTA004B. They have higher Cob, lower fT, lower VCE and lower SOA.

SC5200 / SA1943

active

Excellent power transistor for power output stages. Very similar and pin compatible to MJL3281 / MJL1302.

TTC004B / TTA004B

active

As of today, this seems to pretty much the only fast, high voltage, low power complimentary transistor pair in TO-126 package that is still available. Still a good pair of transistors, but not as good as the SC3503 and SA1381.

Transistor comparison table

ECB pin compatible

Trannsistor Manufacturer VCE [V] IC [mA] fT [MHz] hFE min hFE max
BC639 / BC640 ON Semiconductor 80 1000 100 25 250
BD139 / BD140 ST 80 1500 ~50 25
-10: 63
-16: 100
250
-10: 160
-16: 250
MJE243 / MJE253 ON Semiconductor 100 4000 40 15 180
MJE340 / MJE350 ON Semiconductor 300 500 20 / 10 30 240
SC1845  ON Semiconductor 120 50 50-110 P: 200
F: 300
E: 400
U: 600
P: 400
F: 600
E: 800
U: 1200
SA992 ON Semiconductor 120 50 50-100 P: 200
F: 300
FA: 300
FB: 430
E: 400
P: 400
F: 600
FA: 470
FB: 600
E: 800
SC2240 / SA970 Toshiba 120 100 100 GR: 200
BL: 350
GR: 350
BL: 700
SC3503 / SA1381 Fairchild 300 100 150 C: 40
D: 60
E: 100
F: 160
C: 80
D: 120
E: 200
F: 320
TTC004B / TTA004B Toshiba 160 1500 100 80 280

Further resources

I find my small transistor overview pretty helpful. There are more such tables on the internet. Greg Erskine has made some tables, which seem no longer maintained and contain mostly obsolete transistors. Those tables may be most helpful finding substitues for old transistors.

Mark Johnson has presented measured data of quasi saturation for some small signal transistors on DIY Audio:

Outlook

The situation is getting worse quickly. Quality linear analog audio has started to die long time ago and is dying a slow death since nobody cares about quality anymore. Decreased demand together with dramatic semiconductor shortage in year 2021 results in obsolescence earlier than many had expected.