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Very Simple 3BJT Headphone Amplifier
Very Simple 3BJT Headphone Amplifier
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Old 27th November 2017, 12:31 AM   #11
HotIce is online now HotIce  United States
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OK, should be ready to send Gerbers to manufacturing.
It was missing an LED, and it is no amplifier if it does not have an LED (strictly blue!)

For this I was not thinking of any enclosed case, but more sandwiched among two 1/4" clear acrylic panels, held with brass PCB standoffs.

New simulink here with updated Hfe:

http://tinyurl.com/ycyxpnqg

The models used by that simulation are much less sophisticated than spice (I use ngspice) ones, but I usually have not found much of a difference once compared them, especially for such static analysis.

Overall, as experiment of a project done 100% with an online EDA tool, it turned out that for simple stuff like this, I can work with it.
Though some times I felt missing KiCad
Attached Images
File Type: jpg Very-Simple-3BJT-Headphone-Amplifier.jpg (367.0 KB, 142 views)
File Type: png top.png (77.6 KB, 134 views)
File Type: png bottom.png (43.2 KB, 122 views)
Attached Files
File Type: pdf BOM.pdf (16.6 KB, 3 views)
File Type: zip VS3BJT_V7.zip (45.8 KB, 2 views)
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Old 27th November 2017, 01:15 AM   #12
xrk971 is offline xrk971  United States
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Very Simple 3BJT Headphone Amplifier
Nice design. I love SE Class A headphone amps. Could a BD139 work here instead of 2SC3852? For circa 100mA bias it probably is not necessary to use a 3amp transistor. Rosina 2SC4795 could work well too?
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Old 27th November 2017, 01:21 AM   #13
Bigun is offline Bigun  Canada
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Very Simple 3BJT Headphone Amplifier
Nice !

another idea - maybe add a small cap from base of input transistor to ground, a small npo ceramic, which in conjunction with the input resistor makes an RC filter for pesky r.f. leaking in from the local radio station.

why did you choose to have the wiper of the volume pot to the source ? when volume is dialled to zero it will place a near-short on the source ? I usually put the wiper of the pot to the base of the input transistor ?
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Old 27th November 2017, 01:38 AM   #14
HotIce is online now HotIce  United States
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Thanks!
I think many BJTs will work.
I just chose those because I had them around. They both have very nice performance for the price (they 2SC are $1.5 IIRC at Digikey).
They tend to dissipate (depending on emitter resistor choice) from 600mW to 900mW, so they should be fine w/out heatsink (they will be pretty hot to touch).

I would not call this class A. More class W (for Weird ).
If you open the simulation, you notice the audio output has about +/- 7.5V swing (using a 2V input), and delivering +/- 150mA on a 50 Ohms load.
A class A will have to be biased to 150mA at least, to not have the signal chopped.
Now right click the signal source, Edit, and set voltage to 0 (DC simulation).
You will notice a 50mA bias current, about 1/3 of a class A.
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Old 27th November 2017, 01:40 AM   #15
HotIce is online now HotIce  United States
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Quote:
Originally Posted by Bigun View Post
Nice !

another idea - maybe add a small cap from base of input transistor to ground, a small npo ceramic, which in conjunction with the input resistor makes an RC filter for pesky r.f. leaking in from the local radio station.

why did you choose to have the wiper of the volume pot to the source ? when volume is dialled to zero it will place a near-short on the source ? I usually put the wiper of the pot to the base of the input transistor ?
Duh!
Another $50 saved (I was bout to submit the PCB order ).
Thanks!
Yeah, does not make ANY sense ... totally missed that when I change from the ALPS to the Bourns POT.
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Old 27th November 2017, 02:33 AM   #16
HotIce is online now HotIce  United States
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Alright, one more round
Attached Images
File Type: jpg Very-Simple-3BJT-Headphone-Amplifier.jpg (363.8 KB, 136 views)
File Type: png top.png (76.6 KB, 133 views)
File Type: png bottom.png (44.2 KB, 42 views)
Attached Files
File Type: pdf BOM.pdf (17.0 KB, 10 views)
File Type: zip VS3BJT_V8.zip (45.6 KB, 10 views)
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Old 3rd December 2017, 05:41 PM   #17
HotIce is online now HotIce  United States
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PCB and components should be here next week
Here are the FFTs of emitting 16V and 4V peak-to-peak on a 30 Ohms load.
Not too bad considering in the first case we are dumping about 1W RMS, and considering the pretty low NFB value.
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File Type: png fft2.png (27.8 KB, 31 views)
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Old 3rd December 2017, 09:25 PM   #18
xrk971 is offline xrk971  United States
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Very Simple 3BJT Headphone Amplifier
Something’s not right with your sim. It should look much cleaner. Here is a sim for a headphone amp with two transistors. SE Class A should have mostly 2nd and 3rd order distortion when driving reasonable output levels.

Click the image to open in full size.
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Old 3rd December 2017, 11:20 PM   #19
HotIce is online now HotIce  United States
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Yeah, thanks for pointing that out. Windowing function effects.
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Old 3rd December 2017, 11:30 PM   #20
HotIce is online now HotIce  United States
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And this is one with an output level which is more similar to your (still on a 30 Ohms load).
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