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Very Simple 3BJT Headphone Amplifier
Very Simple 3BJT Headphone Amplifier
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Old 24th November 2017, 04:25 PM   #1
HotIce is offline HotIce  United States
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Default Very Simple 3BJT Headphone Amplifier

I am fascinated by simple, so yesterday I had a few hours to experiment with this little fellas:

http://tinyurl.com/y7v99ccn

I used BC550C and and the 2SC3852 because I had them around. Love them both for their role, cheap and performing.
I was also looking for high Hfe for power NPNs there, and the 2SC are plenty.
The amp has an input impedance of about 12K Ohm and a gain of about 3.5.
It is fed with 24V DC as I had a power brick around to spare. Final build will also likely use it.
The BC550C is biased at about 8.6mA, while the output stage varies according to the emitter resistor value of the lower NPN (from 40mA to 80mA).
In order to achieve the best performance, the value of that resistor should be somehow dependent on the headphone impedance.
In/Around 1/7...1/8 times that, to be more precise.
A too high value can lead to the lower NPN not being able to drive enough current in the negative side of the waveform, and a value too low will make the lower NPN to over-conduct by increasing dissipation power of the higher NPN.
I am likely going to use a 2P4T switch there, to select different resistor values.
For standard biasing values, there should be no need of heatsinks for the 2SC3852.
The 1K Ohm resistor is there to provide a discharge path for the output coupling capacitor.

The advantage of this circuit over the usual class A, lower current source, design is that this requires much lower biasing current (1/4 to 1/8), and has a much softer clipping behavior.

Power wise, it can deliver more then 1W on 32 Ohms loads, and about 100mW on 500 Ohms loads.


PS: Sorry for the interactive circuit link ... have not had the time to port it to Kicad and export and image file yet.

Last edited by HotIce; 24th November 2017 at 07:43 PM. Reason: Power Info.
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Old 24th November 2017, 07:14 PM   #2
HotIce is offline HotIce  United States
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Not having my Linux box around, EasyEDA did this for me.
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Old 24th November 2017, 07:19 PM   #3
HotIce is offline HotIce  United States
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Some Gerbers too ... not too bad as Online EDA tool!
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File Type: png 3bjt_top.png (24.7 KB, 379 views)
File Type: png 3bjt_bottom.png (11.8 KB, 363 views)
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File Type: zip 3BJ_Gerbers.zip (12.1 KB, 5 views)
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Old 25th November 2017, 04:13 AM   #4
HotIce is offline HotIce  United States
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Free time on turkey holiday ...
Here's a new version which adds some missing pieces and emitter resistor selection.
EasyEDA routing is not stellar, and I like no vias, so I spoiled myself with a 4 layers.
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File Type: png top.png (72.3 KB, 335 views)
File Type: png bottom.png (42.4 KB, 321 views)
File Type: jpg Very-Simple-3BJT-Headphone-Amplifier-copy.jpg (343.3 KB, 141 views)
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File Type: zip 3BJT_V2.zip (39.0 KB, 8 views)

Last edited by HotIce; 25th November 2017 at 04:17 AM.
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Old 25th November 2017, 06:15 AM   #5
HotIce is offline HotIce  United States
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One more round of refinement switching some components to Digikey ones.
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File Type: png top.png (65.9 KB, 104 views)
File Type: png bottom.png (35.4 KB, 55 views)
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File Type: zip 3BJT_V2.zip (38.1 KB, 10 views)
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Old 25th November 2017, 07:16 AM   #6
AKSA is offline AKSA  Australia
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Lovely work, HI.

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Old 25th November 2017, 05:43 PM   #7
HotIce is offline HotIce  United States
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Thanks AKSA!
One more round. Added choke filter to lower ripple from 24V supply, and included full Digikey bound BOM.
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File Type: jpg Very-Simple-3BJT-Headphone-Amplifier-copy.jpg (349.5 KB, 74 views)
File Type: png top.png (73.8 KB, 77 views)
File Type: png bottom.png (40.2 KB, 35 views)
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File Type: zip VS3BJT_V3.zip (43.1 KB, 2 views)
File Type: pdf BOM.pdf (17.0 KB, 6 views)
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Old 25th November 2017, 11:38 PM   #8
HotIce is offline HotIce  United States
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I think it's close to done.
Added precision trimmer to allow tuning the center voltage, and few other touches.
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File Type: jpg Very-Simple-3BJT-Headphone-Amplifier-copy.jpg (350.3 KB, 64 views)
File Type: png top.png (73.7 KB, 75 views)
File Type: png bottom.png (42.6 KB, 30 views)
Attached Files
File Type: pdf BOM.pdf (16.4 KB, 3 views)
File Type: zip VS3BJT_V5.zip (44.0 KB, 4 views)
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Old 26th November 2017, 02:27 AM   #9
PRR is offline PRR  United States
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No ground symbol where emitter resistor returns.

What does that emitter resistor switch do? It seems to spoil your limited open-loop gain and upset your center voltage.
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Old 26th November 2017, 03:08 AM   #10
HotIce is offline HotIce  United States
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Argh, thanks! I owe you $50!

The emitter resistor regulates the amount of pull on the negative side of the output signal.
Consider the old variations of similar circuitry, WRT the low side of the output stage.

The first one you have a class A bias with a relatively small resistor.
With that setup you have large dissipation on the resistor, a class A bias (heat) and a lame pull on the negative side of the output signal. Essentially, your output impedance on the negative side is the resistor value you sink the class A current through.

The second one is a constant current generator, which requires again a class A bias, which also has a bad clipping behavior once you run out of current.

With this configuration you have an active pull on the negative side of the signal, which is essentially dVin/Re.
But if Re is too small for the HP load, you sink that current through the upper NPN, which means more heat for it.

You can play with the simulation, changing HP load and Re, an notice the upper NPN current/voltage behavior.
Attached Images
File Type: jpg Very-Simple-3BJT-Headphone-Amplifier-copy.jpg (351.4 KB, 74 views)
File Type: png top.png (73.4 KB, 76 views)
File Type: png bottom.png (42.9 KB, 32 views)
Attached Files
File Type: pdf BOM.pdf (16.2 KB, 7 views)
File Type: zip VS3BJT_V6.zip (44.1 KB, 6 views)
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