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| Building a sound mixer within my guitar amp with line IN & mic input |
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| dazz:
--- Quote from: Zero999 on June 04, 2019, 09:56:56 pm ---You need to increase the value R9, so it's the same as R8, to keep the CMRR high. You will probably need to add another stage of amplification afterwards. --- End quote --- Understood, I'll do just that :-+ |
| dazz:
OK, I've been playing with the simulation of this preamp for a bit and there seems to be a problem. The op amp is biased very close to the negative rail and it's distorting a lot. Here's the output of the fourier analysis for a sweep in the gain pot --- Code: ---Fourier components of V(micout) DC component:1.90883e-006 Harmonic Frequency Fourier Normalized Phase Normalized Number [Hz] Component Component [degree] Phase [deg] 1 1.000e+03 1.806e-04 1.000e+00 -0.69° 0.00° 2 2.000e+03 3.648e-07 2.020e-03 -91.38° -90.69° 3 3.000e+03 8.163e-10 4.519e-06 177.92° 178.61° 4 4.000e+03 2.217e-12 1.228e-08 83.24° 83.93° 5 5.000e+03 1.890e-13 1.046e-09 -14.35° -13.66° 6 6.000e+03 2.190e-13 1.212e-09 -5.75° -5.06° 7 7.000e+03 2.814e-13 1.558e-09 -10.74° -10.05° 8 8.000e+03 2.764e-13 1.530e-09 -9.19° -8.50° 9 9.000e+03 3.352e-13 1.856e-09 0.70° 1.39° Total Harmonic Distortion: 0.201955%(0.201950%) .step g=10 N-Period=1 Fourier components of V(micout) DC component:2.10847e-006 Harmonic Frequency Fourier Normalized Phase Normalized Number [Hz] Component Component [degree] Phase [deg] 1 1.000e+03 2.247e-04 1.000e+00 -0.68° 0.00° 2 2.000e+03 5.644e-07 2.512e-03 -91.37° -90.69° 3 3.000e+03 1.571e-09 6.993e-06 177.93° 178.62° 4 4.000e+03 5.268e-12 2.345e-08 84.87° 85.56° 5 5.000e+03 2.031e-13 9.038e-10 -19.10° -18.42° 6 6.000e+03 2.822e-13 1.256e-09 -16.49° -15.81° 7 7.000e+03 3.063e-13 1.363e-09 -9.38° -8.70° 8 8.000e+03 3.310e-13 1.473e-09 -1.81° -1.13° 9 9.000e+03 3.624e-13 1.613e-09 -7.81° -7.13° Total Harmonic Distortion: 0.251209%(0.251205%) .step g=20 N-Period=1 Fourier components of V(micout) DC component:2.48944e-006 Harmonic Frequency Fourier Normalized Phase Normalized Number [Hz] Component Component [degree] Phase [deg] 1 1.000e+03 2.908e-04 1.000e+00 -0.68° 0.00° 2 2.000e+03 9.453e-07 3.251e-03 -91.36° -90.68° 3 3.000e+03 3.406e-09 1.171e-05 177.95° 178.63° 4 4.000e+03 1.474e-11 5.069e-08 86.21° 86.88° 5 5.000e+03 3.803e-13 1.308e-09 -21.54° -20.86° 6 6.000e+03 3.849e-13 1.324e-09 -19.06° -18.38° 7 7.000e+03 4.657e-13 1.602e-09 -8.78° -8.10° 8 8.000e+03 4.754e-13 1.635e-09 -6.95° -6.27° 9 9.000e+03 5.518e-13 1.898e-09 -8.37° -7.69° Total Harmonic Distortion: 0.325115%(0.325112%) .step g=30 N-Period=1 Fourier components of V(micout) DC component:3.28645e-006 Harmonic Frequency Fourier Normalized Phase Normalized Number [Hz] Component Component [degree] Phase [deg] 1 1.000e+03 3.947e-04 1.000e+00 -0.67° 0.00° 2 2.000e+03 1.742e-06 4.414e-03 -91.35° -90.68° 3 3.000e+03 8.522e-09 2.159e-05 177.97° 178.64° 4 4.000e+03 5.040e-11 1.277e-07 86.94° 87.61° 5 5.000e+03 7.158e-13 1.813e-09 -11.48° -10.81° 6 6.000e+03 4.867e-13 1.233e-09 -22.51° -21.84° 7 7.000e+03 6.092e-13 1.543e-09 -16.19° -15.52° 8 8.000e+03 6.413e-13 1.625e-09 -11.22° -10.55° 9 9.000e+03 7.496e-13 1.899e-09 -12.83° -12.16° Total Harmonic Distortion: 0.441366%(0.441364%) .step g=40 N-Period=1 Fourier components of V(micout) DC component:5.15258e-006 Harmonic Frequency Fourier Normalized Phase Normalized Number [Hz] Component Component [degree] Phase [deg] 1 1.000e+03 5.681e-04 1.000e+00 -0.66° 0.00° 2 2.000e+03 3.608e-06 6.352e-03 -91.32° -90.66° 3 3.000e+03 2.540e-08 4.471e-05 178.00° 178.66° 4 4.000e+03 2.164e-10 3.810e-07 87.19° 87.85° 5 5.000e+03 2.640e-12 4.648e-09 -9.51° -8.85° 6 6.000e+03 8.161e-13 1.437e-09 -24.29° -23.63° 7 7.000e+03 9.191e-13 1.618e-09 -19.11° -18.45° 8 8.000e+03 9.834e-13 1.731e-09 -14.61° -13.95° 9 9.000e+03 1.089e-12 1.918e-09 -11.45° -10.79° Total Harmonic Distortion: 0.635190%(0.635188%) .step g=50 N-Period=1 Fourier components of V(micout) DC component:1.01764e-005 Harmonic Frequency Fourier Normalized Phase Normalized Number [Hz] Component Component [degree] Phase [deg] 1 1.000e+03 8.787e-04 1.000e+00 -0.64° 0.00° 2 2.000e+03 8.633e-06 9.825e-03 -91.28° -90.64° 3 3.000e+03 9.399e-08 1.070e-04 178.07° 178.71° 4 4.000e+03 1.240e-09 1.411e-06 87.37° 88.00° 5 5.000e+03 1.825e-11 2.077e-08 -4.91° -4.27° 6 6.000e+03 1.496e-12 1.702e-09 -35.16° -34.52° 7 7.000e+03 1.426e-12 1.623e-09 -21.79° -21.15° 8 8.000e+03 1.627e-12 1.852e-09 -20.43° -19.79° 9 9.000e+03 1.763e-12 2.006e-09 -18.38° -17.74° Total Harmonic Distortion: 0.982516%(0.982515%) .step g=60 N-Period=1 Fourier components of V(micout) DC component:2.62225e-005 Harmonic Frequency Fourier Normalized Phase Normalized Number [Hz] Component Component [degree] Phase [deg] 1 1.000e+03 1.486e-03 1.000e+00 -0.60° 0.00° 2 2.000e+03 2.469e-05 1.661e-02 -91.20° -90.60° 3 3.000e+03 4.546e-07 3.059e-04 178.19° 178.79° 4 4.000e+03 1.014e-08 6.826e-06 87.56° 88.16° 5 5.000e+03 2.384e-10 1.604e-07 -3.40° -2.81° 6 6.000e+03 7.550e-12 5.081e-09 -74.17° -73.58° 7 7.000e+03 2.928e-12 1.970e-09 -30.58° -29.99° 8 8.000e+03 3.242e-12 2.182e-09 -26.96° -26.36° 9 9.000e+03 3.450e-12 2.322e-09 -23.85° -23.25° Total Harmonic Distortion: 1.661638%(1.661637%) .step g=70 N-Period=1 Fourier components of V(micout) DC component:8.82526e-005 Harmonic Frequency Fourier Normalized Phase Normalized Number [Hz] Component Component [degree] Phase [deg] 1 1.000e+03 2.788e-03 1.000e+00 -0.51° 0.00° 2 2.000e+03 8.684e-05 3.115e-02 -91.02° -90.51° 3 3.000e+03 3.000e-06 1.076e-03 178.46° 178.96° 4 4.000e+03 1.256e-07 4.504e-05 87.93° 88.43° 5 5.000e+03 5.484e-09 1.967e-06 -2.66° -2.15° 6 6.000e+03 2.547e-10 9.136e-08 -92.05° -91.54° 7 7.000e+03 7.848e-12 2.815e-09 -147.69° -147.18° 8 8.000e+03 6.917e-12 2.481e-09 -37.77° -37.26° 9 9.000e+03 7.515e-12 2.696e-09 -37.99° -37.48° Total Harmonic Distortion: 3.117192%(3.117192%) .step g=80 N-Period=1 Fourier components of V(micout) DC component:0.000356279 Harmonic Frequency Fourier Normalized Phase Normalized Number [Hz] Component Component [degree] Phase [deg] 1 1.000e+03 5.657e-03 1.000e+00 -0.32° 0.00° 2 2.000e+03 3.570e-04 6.310e-02 -90.64° -90.32° 3 3.000e+03 2.503e-05 4.425e-03 179.02° 179.34° 4 4.000e+03 2.124e-06 3.754e-04 88.69° 89.01° 5 5.000e+03 1.880e-07 3.323e-05 -1.66° -1.35° 6 6.000e+03 1.739e-08 3.075e-06 -91.98° -91.67° 7 7.000e+03 1.635e-09 2.890e-07 178.29° 178.61° 8 8.000e+03 1.407e-10 2.487e-08 83.03° 83.35° 9 9.000e+03 3.223e-11 5.697e-09 -35.81° -35.49° Total Harmonic Distortion: 6.325785%(6.325785%) .step g=90 N-Period=1 Fourier components of V(micout) DC component:0.00113987 Harmonic Frequency Fourier Normalized Phase Normalized Number [Hz] Component Component [degree] Phase [deg] 1 1.000e+03 1.023e-02 1.000e+00 -0.03° 0.00° 2 2.000e+03 1.161e-03 1.135e-01 -90.07° -90.04° 3 3.000e+03 1.474e-04 1.440e-02 179.88° 179.91° 4 4.000e+03 2.253e-05 2.202e-03 89.83° 89.86° 5 5.000e+03 3.600e-06 3.519e-04 -0.23° -0.20° 6 6.000e+03 6.001e-07 5.866e-05 -90.30° -90.27° 7 7.000e+03 1.024e-07 1.001e-05 179.66° 179.69° 8 8.000e+03 1.774e-08 1.734e-06 89.49° 89.52° 9 9.000e+03 3.153e-09 3.082e-07 -1.54° -1.51° Total Harmonic Distortion: 11.441146%(11.441146%) .step g=100 N-Period=1 Fourier components of V(micout) DC component:0.00101971 Harmonic Frequency Fourier Normalized Phase Normalized Number [Hz] Component Component [degree] Phase [deg] 1 1.000e+03 1.253e-02 1.000e+00 0.07° 0.00° 2 2.000e+03 1.716e-03 1.369e-01 -89.86° -89.93° 3 3.000e+03 2.671e-04 2.131e-02 -179.80° -179.87° 4 4.000e+03 4.959e-05 3.957e-03 90.25° 90.17° 5 5.000e+03 9.654e-06 7.703e-04 0.21° 0.14° 6 6.000e+03 1.970e-06 1.572e-04 -89.65° -89.72° 7 7.000e+03 4.055e-07 3.235e-05 -177.85° -177.93° 8 8.000e+03 7.317e-08 5.838e-06 89.69° 89.62° 9 9.000e+03 2.300e-08 1.835e-06 -33.04° -33.11° Total Harmonic Distortion: 13.864246%(13.864246%) --- End code --- The THD seems a bit out of whack, doesn't it? Here's a table with the THD for each gain level: --- Code: ---Pot Gain THD 0 -14dB 0.201955% 1 -13dB 0.251209% 2 -11dB 0.325115% 3 -8dB 0.441366% 4 -5dB 0.635190% 5 -1dB 0.982516% 6 3dB 1.661638% 7 9dB 3.117192% 8 15dB 6.325785% 9 20dB 11.441146% 10 22dB 13.864246% --- End code --- It gets a lot better if I reference the opamp to Vref (Vdd/2) instead of GND through R8 (R14 in my simulation below), but can I do that without ruining my CMRR again? Do I also need to add coupling caps to the opamp inputs if I do that? |
| Zero999:
What's the power supply voltage? Try reducing Vref to 8V. That should increase the bias current through the transistors, thus the voltages across the collector resistors. By the way, does anyone know what the diodes are for? They're continuously reverse biased and appear to do nothing. |
| dazz:
--- Quote from: Zero999 on June 05, 2019, 09:55:39 am ---What's the power supply voltage? Try reducing Vref to 8V. That should increase the bias current through the transistors, thus the voltages across the collector resistors. --- End quote --- How the hell do I manage to miss the simplest solution every frigging time? |O Supply voltage is 20V in the simulation. Ideally I'd like it to work down to 12V, or 9V if possible. |
| Zero999:
I think you'll struggle to get that circuit to run down to 9V, because you won't be able to get the voltage across the collector resistors to fall within the TL72's common mode range, at low voltages. You could replace the emitter resistors with constant current sources: try 1mA. See link below. Note that the diodes and resistor provide a reference voltage, which can be used for both current sources. http://www.ecircuitcenter.com/Circuits_Audio_Amp/BJT%20Current_Source/BJT_Current_Source.htm I hope you can see why I suggested an IC, for simplicity's sake, but you won't learn so much and it'll still work out more expensive. |
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