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Headphone Buffer Amplifier
Headphone Buffer Amplifier
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Old 9th May 2018, 08:44 AM   #1
lineup is offline lineup  Sweden
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Default Headphone Buffer Amplifier

Personally I have 32 Ohms headphones. When impedance is 32 Ohm and lower, there is really no need of voltage gain. This is why I suggest a buffer, a voltage follower.

My circuit is such a follower.

There is no feedback loop. It is only current gain. P1 is used to set the output at 0.0 volt. For no DC-offset.

The output is one current source. There is the possibility to increase the current in output stage. By adjusting the R4 resistor. For my 32 Ohms headphones it is not necessary. 50mA is enough. But we can use for example MJE15030-31 in the output.

BF245B was chosen for the availability. And low price. Can be used for 30V. So, maximal power supply should be 2x15VDC. Regulated is recommended. For example use LM317T/LM337T.
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File Type: png HP-buffer1.png (31.2 KB, 819 views)
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Old 9th May 2018, 10:01 AM   #2
adason is offline adason  United States
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Nice...have you built it? Are you going to?
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Old 13th May 2018, 07:14 PM   #3
lineup is offline lineup  Sweden
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I am going to.
But first I am building a low voltage tube amp. With ECC86.

Headphone amps are nice to test.
You only need one music source, one potentiometer and so you attach headphones in other end.
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Old 15th May 2018, 11:44 PM   #4
xrk971 is offline xrk971  United States
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Headphone Buffer Amplifier
Nice circuit, Lineup. This could probably be done with BF862’s?
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Old 16th May 2018, 05:01 PM   #5
DontHertzMe is offline DontHertzMe  South Africa
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Headphone Buffer Amplifier
Or possibly the BF545? Ostensibly a direct SMT replacement for the BF245 by NXP.
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Old 16th May 2018, 10:54 PM   #6
xrk971 is offline xrk971  United States
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Headphone Buffer Amplifier
Hi Lineup,
I sim’d this circuit in LTSpice and it has large 12v offset at output. Although done with BF862, I don’t think that should change DC setpoints that much. Thd AC performance looks nice though as expected for a SE Class A design. Have you run your own sim yet?
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Last edited by xrk971; 16th May 2018 at 10:56 PM.
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Old 17th May 2018, 07:05 AM   #7
lineup is offline lineup  Sweden
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Quote:
Originally Posted by xrk971 View Post
Hi Lineup,
I sim’d this circuit in LTSpice and it has large 12v offset at output. Although done with BF862, I don’t think that should change DC setpoints that much. Thd AC performance looks nice though as expected for a SE Class A design. Have you run your own sim yet?
You should use a dual supply.
+/-12 or +/-15 Volt.

I can not do sim.
For being a voltage follower, we can expect rather low THD.
As there is no voltage gain.
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Old 17th May 2018, 09:08 AM   #8
xrk971 is offline xrk971  United States
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Indeed I did use dual +/-15v supply in sim. I think I may have had a wiring error. Will retry it.
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Last edited by xrk971; 17th May 2018 at 09:10 AM.
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Old 17th May 2018, 11:34 AM   #9
lineup is offline lineup  Sweden
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I might change the input JFETs to be two 2SK246BL.
I have a supplier who sells 2SK246BL and 2SJ103BL.
Will be very fun to test this amplifier.

I will also buy a labb supply which gives out 0-30Volt.
This will be good for different testing.
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Old 17th May 2018, 05:02 PM   #10
xrk971 is offline xrk971  United States
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Headphone Buffer Amplifier
Default LTSpice Schematic

Hi Lineup,
Here is the 12v DC offset at the output that I was talking about. Changing the pot doesn't do anything. I think you need to apply a voltage divider from the +ve rail (filtered with say 47uF) and apply correct bias to Gate of Q1 in order to get the correct bias on the input and output stage.

I don't think change in type of JFET changes the fundamental DC setpoint behavior.

Headphone Buffer Amplifier-lineup-follower-hpa-1-schematic-run-png

Even with DC, offset, the THD is indeed low and the profile is nice H2 and descending higher orders:
Quote:
Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 2.431e-01 1.000e+00 -0.01° 0.00°
2 2.000e+03 1.458e-05 5.995e-05 93.38° 93.39°
3 3.000e+03 6.961e-08 2.863e-07 9.07° 9.08°
4 4.000e+03 1.921e-10 7.901e-10 -168.65° -168.65°
5 5.000e+03 1.575e-10 6.479e-10 -178.44° -178.44°
6 6.000e+03 5.456e-11 2.244e-10 -177.89° -177.88°
7 7.000e+03 2.971e-11 1.222e-10 -2.57° -2.57°
8 8.000e+03 1.031e-10 4.239e-10 -0.33° -0.33°
9 9.000e+03 1.693e-10 6.962e-10 0.09° 0.09°
Total Harmonic Distortion: 0.005995%(0.005854%)
FFT with 500mVpp into 32ohms:
Headphone Buffer Amplifier-lineup-follower-hpa-1-fft-png
Attached Images
File Type: png Lineup-follower-HPA-1-schematic-run.png (59.8 KB, 597 views)
File Type: png Lineup-follower-HPA-1-FFT.png (31.8 KB, 589 views)
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Last edited by xrk971; 17th May 2018 at 05:08 PM.
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