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Headphone Buffer Amplifier
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Old 17th May 2018, 06:07 PM   #11
needtubes is offline needtubes  United States
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Looks like there is, indeed, a wiring error. Lineup's original circuit has what you call R1 connected directly to Q1's gate. Seems to me the input cap is unnecessary, unless one is worried about DC offset from the source.
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Old 18th May 2018, 09:19 AM   #12
xrk971 is offline xrk971  United States
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Headphone Buffer Amplifier
Changing that error did not fix problem in that the circuit clipped badly on negative side. The only way to fix was to add a voltage divider in order to bias the gate of Q1 properly. This sets the DC offset as well as sets the bias current in Q1. With this, the performance for 1vpp gives THD of 0.04% which is no better than my circuits which use a plain resistor vs CCS's.

New schematic:
Headphone Buffer Amplifier-lineup-follower-hpa-sch-v2-png

FFT:
Headphone Buffer Amplifier-lineup-follower-hpa-fft-1vpp-32ohms-v2-png

Distortion driving 32ohm load 1vpp:
Quote:
Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 4.863e-01 1.000e+00 0.13 0.00
2 2.000e+03 2.011e-04 4.135e-04 -89.86 -89.98
3 3.000e+03 1.390e-05 2.858e-05 0.45 0.32
4 4.000e+03 9.866e-07 2.029e-06 104.66 104.53
5 5.000e+03 2.621e-07 5.390e-07 -179.82 -179.95
6 6.000e+03 1.605e-07 3.300e-07 -178.12 -178.25
7 7.000e+03 1.370e-07 2.817e-07 -180.00 -180.12
8 8.000e+03 1.201e-07 2.469e-07 179.99 179.86
9 9.000e+03 1.066e-07 2.192e-07 -179.99 -180.12
Total Harmonic Distortion: 0.041454%(0.041434%)
But, driving 270ohm load 1vpp give very low THD of 0.0008%:
Quote:
Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 4.955e-01 1.000e+00 0.09 0.00
2 2.000e+03 4.216e-06 8.509e-06 89.14 89.06
3 3.000e+03 9.456e-09 1.908e-08 169.62 169.54
4 4.000e+03 9.982e-08 2.015e-07 179.83 179.74
5 5.000e+03 7.973e-08 1.609e-07 -179.98 -180.07
6 6.000e+03 6.628e-08 1.338e-07 -179.99 -180.07
7 7.000e+03 5.666e-08 1.144e-07 -179.99 -180.08
8 8.000e+03 4.942e-08 9.974e-08 -179.99 -180.08
9 9.000e+03 4.377e-08 8.834e-08 -180.00 -180.08
Total Harmonic Distortion: 0.000852%(0.000000%)
This would indeed sound exceptionally clean with higher impedance phones - but they need more voltage gain, so a preamp is needed.

Note, I moved the R1 100k to the left of the cap C1 and it did not make any difference (THD) other than reset the pot R9 to 590R to zero DC offset.

With 6vpp into 270ohms, I was still getting THD of 0.005%, an excellent follower.
Attached Images
File Type: png Lineup-follower-HPA-Sch-v2.png (62.9 KB, 119 views)
File Type: png Lineup-follower-HPA-FFT-1vpp-32ohms-v2.png (23.7 KB, 116 views)
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Last edited by xrk971; 18th May 2018 at 09:34 AM.
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Old 18th May 2018, 10:07 AM   #13
lineup is offline lineup  Sweden
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I am puzzled by that offset you get.
According to my thinking it should not be any offset.
Whatever offset is corrected by the the trimmer.

Otherwise the simulation you get looks very good. Low THD.
This is as expected.
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Old 18th May 2018, 10:08 AM   #14
lineup is offline lineup  Sweden
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Quote:
Originally Posted by needtubes View Post
Looks like there is, indeed, a wiring error. Lineup's original circuit has what you call R1 connected directly to Q1's gate. Seems to me the input cap is unnecessary, unless one is worried about DC offset from the source.
That is correct.
I put this only to protect from sources with offset.
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Old 27th May 2018, 05:23 PM   #15
jhofland is offline jhofland  United States
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R1 should be connected to the gate of Q1. Right now it's floating.
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