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This question might be rather generic, but seems like I'm not able to find a proper answer out there.

Following is an active filter circuit from PCM4104 evaluation module, along with differential to single-ended output conversion:

active filter circuit from PCM4104 evaluation module

Now, the amplifier board which I want this circuit to connect to has following diagram for BTL mode:

Wiring diagram for the BTL module

As you can see, Vin channel 1 needs to be inverted, while Vin channel 2 not.

Question: How can I adapt active filter circuit above to invert the phase for channel 1? Something tells me it's not enough to just flip the input connections of opamp, swapping + and -. Or should I really go for additional inverting buffer?

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2 Answers 2

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If the source is truly differential flipping the inputs should invert the signal, so you could use two of this circuit, one feeding channel 1, one feeding channel 2, and reverse the input connections on one of them.Differential Amps out of phase Diff Amps out of phase plot

Or you can use one of your circuits and use an inverting amplifier to invert it's output. Differential Amp with inverter Diff amp with inverter plot

Either will give you two signals 180\$^\circ\$ out of phase. The second one shows a little higher output level because there is half as much load on the source.

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  • \$\begingroup\$ The reason why I don't want to have extra inverting buffer on one of the channels - tiny phase shift (which of course I can avoid by adding a voltage follower buffer on another channel) plus extra added noise. But is my concern even noticeable? Will I see the difference on thd+n measurements? \$\endgroup\$ Jul 15, 2023 at 18:37
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    \$\begingroup\$ @AndrejsCainikovs Funny, I was going to mention that, was just checking the simulations to see if I could see the affects of it first. The first circuit should match the two signals better, I don't know how much difference that would make in the real world though. \$\endgroup\$
    – GodJihyo
    Jul 15, 2023 at 18:47
  • \$\begingroup\$ Thanks! Small question - I see you have added 100k pull-down on output in your simulations. Is this just to make the simulation work, or should I consider this in my circuit? \$\endgroup\$ Jul 15, 2023 at 18:54
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    \$\begingroup\$ @AndrejsCainikovs I just added them so the outputs are terminated in the simulation. You don't need them in your circuit. \$\endgroup\$
    – GodJihyo
    Jul 15, 2023 at 22:15
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You can flip the inputs, but adding an inverting buffer is more cost efficient.

If you are looking to increase power you should add another power stage with less than 100 ohms ouput impedance (R57).

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  • \$\begingroup\$ Can you please elaborate more on adding an inverting buffer is more cost efficient? Also, in my circuit I'm dropping R57 - is this reasonable? Excerpt from amplifier board datasheet: The input impedance of the signal input section is minimum 8k over the audio bandwidth, which is an acceptable loading condition for pre-amps, active crossover outputs etc. \$\endgroup\$ Jul 15, 2023 at 18:33
  • \$\begingroup\$ adding an inverting buffer is more cost efficient? By that I mean that you can take the output signal of your the circuit you posted and putting that into the input, so you could do with less components than with the differential configuration. \$\endgroup\$
    – Ralph
    Jul 15, 2023 at 19:07
  • \$\begingroup\$ Dropping R57 wouldn't help much as NE5534 you are using in the differential amp is not going to be able to provide much current, especially if you want to stay within spec, as the test condition is with 600 ohm load according to datasheet. \$\endgroup\$
    – Ralph
    Jul 15, 2023 at 19:08
  • \$\begingroup\$ The quotation you posted from amp board datasheet is only referring to input impedance, meaning that the input impedance is ok for many types of outputs. \$\endgroup\$
    – Ralph
    Jul 15, 2023 at 19:10
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    \$\begingroup\$ Looks like the circuit I meant is already in the other answer. \$\endgroup\$
    – Ralph
    Jul 15, 2023 at 19:36

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