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I'm trying to drive a TPA6120 headphone amplifier with TL072. I use single 24VDC supply with a TLE2426 virtual ground.

The TL072 works well and puts out around 21Vp-p sine wave. But as soon as I connect the TPA6120 amplifier to its output the sine gets clipped to around 17-18Vp-p. That is 6V less than supply voltage. Any idea wahat could cause this? Looking at TL072 datasheet Figure 6-12 and 6-13, output swing with 1k load (12mA@+-10V) should still be atleast 22V.

  • The sine looks ok from first TL072 output.(Yellow line on scope screen)
  • The sine comes out clipped from second stage(Blue line on scope screen)
  • The signal then enters TPA6120 which is loaded with 16 ohm resistor at its output. enter image description here

enter image description here

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  • \$\begingroup\$ Your amp is very strong. Decrease amplitude of your signal or select other power supply. Try change R23. \$\endgroup\$
    – nick_n_a
    Dec 14, 2020 at 19:53
  • \$\begingroup\$ I will be driving a very high impedance headphones(400 ohm) so the power will not be as high actually. By select other power suply you mean higher voltage? \$\endgroup\$
    – tarmogr
    Dec 14, 2020 at 20:00
  • \$\begingroup\$ Issues I see with this circuit: 1) Why are there 2 opamps present? I think IC7D and R23 are not needed. To get more gain you can make R17 = 20 kohm. 2) I do not see how VGND12V is created. \$\endgroup\$
    – Bimpelrekkie
    Dec 14, 2020 at 20:01
  • \$\begingroup\$ @Bimpelrekkie 1) , 2) I updated the schematic, there was a mistake. \$\endgroup\$
    – tarmogr
    Dec 14, 2020 at 20:14
  • \$\begingroup\$ Why are R38 & R37 so low (1k)? IC7D has trouble driving a 1k load. Try increasing these two resistors to 3.9k. \$\endgroup\$
    – glen_geek
    Dec 14, 2020 at 21:36

1 Answer 1

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The datasheet unfortunately does not give separate specs for TL074P. It gives them for TL074, TL074A, ...B, H, and M, but no P. However, if we look at the table 6.18 p17, we can glean some useful information. That table applies to TL074C with a supply of +/-15V.

\$V_{OM}\$ is the maximum peak voltage output swing. They give values \$V_{OM}\$ for various \$R_L\$ values.

When \$R_L\$ is greater than 2K\$\Omega\$, but less than 10k\$\Omega\$, they only guarantee \$V_{OM}\$ to be 10V. Sad but true. That is a full 5V below the +15V rail.

Your supply is 24V, with a virtual ground, so we will call it +/- 12V. Slightly different scenario, but you can see that if the spec only guarantees 10V with a +15V supply, it wouldn't be surprising if loading your op amp with less than 10K\$\Omega\$ \$R_L\$ at +/- 12V supply would clip at 8.5-9.0V. :-(

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  • \$\begingroup\$ I replaced the TL072 chip with OPA1644, this gets me the swing voltage of about 21 Vp-p @24VDC. Much better, but expensive too. \$\endgroup\$
    – tarmogr
    Jan 21, 2021 at 13:44

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