Hello. n00b here. Just fiddling around with tone controls in LTspice and was wondering why the output is inverted and what I could do to prevent it.
I tried putting a second inverting buffer before the input, to no avail. Best regards.

 

hi d123,
Please post your LTS asc file.
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here you go

 

The first question is ........ why do You care that the Output is inverted ?

OOps, I just figured it out .......
I suppose You are wanting to measure and Graph the Frequency-Response-Curves.
( otherwise it really wouldn't make any difference that they are out of Phase )

If You already put an Inverting-Op-Amp before the existing Input,
and it didn't put things back in Phase,
then You were simply measuring the wrong Node for your Scope,s "Circuit-Input" display-channel.

The Input-Op-Amp can be reconfigured to be in an Inverting configuration with no changes to Circuit performance other
than possibly the Input-Impedance, which shouldn't be a problem in either case.

I've included my saved version just for your entertainment.
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then You were simply measuring the wrong Node for your Scope,s "Circuit-Input" display-channel.

uh, I'm afraid I don't really understand.
what do you mean by 'node'? there is only one input signal at the 'v in' reference point.
Also, the potentiometer doesn't do anything. where would be the ideal position? at the output?

 

The inversion of the phase is inaudible to the ear, as long as the same inversion is applied to both channels in a stereo system.

 

A "Node" is a specific "point" in a Circuit,
( which in reality includes any conductors connected to that point,
but are not on the opposing side of any Electronic-Component ).

You stated .......... "" I tried putting a second inverting buffer before the input, to no avail. ""
Does this mean that the output was NOT inverted from it's previous state after
placing an additional Inverting-Op-Amp inline with the Input ?

This is an impossibility, unless You are measuring from some place other than the Input.
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This is an impossibility, unless You are measuring from some place other than the Input.
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The probing points are marked with a netlabel as 'in' 'buffer' 'out'. I hope that's visible on the screenshots

 

The inversion of the phase is inaudible to the ear, as long as the same inversion is applied to both channels in a stereo system.

Yes. but I'd still prefer the signal to be not inverted :^)

 

"" I tried putting a second inverting buffer before the input, to no avail. ""

I don't see any second, ( or third ), Inverting-Buffer in any of your provided Schematics.

Are You aware of how easy it is to change
an Op-Amp-Buffer from "Non-Inverting", to "Inverting", and vice-versa ?

Also, You can not only reduce Non-Inverting-Op-Amp-Gain by using a Voltage-Divider on the Input,
but You can also alter the Feedback-Resistors, (or remove them ), to achieve a Gain of "1".

Every Buffer provides an opportunity to add fixed High and Low-Frequency-Filters,
You really should take advantage of these opportunities.

BTW, your Schematic-Photos are extremely small, and therefore, very hard, or sometimes impossible, to read.
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Hi LQC,
This is a tidied version of the original LTS asc file circuit, hope it helps.
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hi d123,
Is this the non-inverting version you are requesting?

E

 

hi d123,
Is this the non-inverting version you are requesting?

Woah. Thanks for going through that effort! Yes, I think this is what I had in mind. The phase is just like the input. Though I don't understand why it needs another opamp for that? My thinking was that an inverting amp before U2 should suffice to bring the singnal back to its original phase.
The power amplifier (LM1875) that I'm planning to use in conjunction with this tone control/preamp wouldn't invert the input, as far as I'm aware.

 

Your inversion problem arises because you have attached the Vin net-node label at two places: to V4 and to R1, thus effectively shorting out U1. Detach it from R1.

 

"" The power amplifier (LM1875) that I'm planning to use in conjunction with this tone control/preamp
wouldn't invert the input, as far as I'm aware. ""

Unless You are trying to discern varying Amplitudes in the Waveforms when the Tone-Control-Pots are turned,
there is no need to be concerned about polarity,
as long as the Left and Right-Channels are constructed in exactly the same manner.

If the Left and Right-Channels are identical,
the Polarity of the Circuit may be flipped any number of times
with absolutely zero detectable change in the Sound-Quality.

Do You have concerns with using these Amplifiers with an existing "Surround-Sound-System ?
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Your inversion problem arises because you have attached the Vin net-node label at two places: to V4 and to R1, thus effectively shorting out U1. Detach it from R1.

Nice catch!

 

Yes. but I'd still prefer the signal to be not inverted :^)

"Prefer" is generally not a valid engineering reason to make a circuit more complex.

 

Please describe why You have concerns about an Inverted-Signal.
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Thanks for all the responses. Sorry for not getting back for so long. Had a lot of stuff coming in between this project.
Also made it a little harder for myself than it should be since I'm controlling the circuit using digital pots and the software part is harder to realize than I anticipated, but it's mostly done.
You convinced me that I shall not have any concerns about signal inversion.

I experimented around with several circuits I found on the net, but FFT analysis doesn't quite show what I expected. I want to use the circuit proposed in this TI document: https://www.ti.com/lit/an/sloa042/s...19575&ref_url=https%3A%2F%2Fwww.google.com%2F
Pots are at 50%, but I don't quite get a proper flat response.

 

Sim of a slightly different Baxandall circuit:
It has only one 47nF (C3) instead of two.

Connecting CB1 and CB2 as you show, distorts the response at the mid settings in my simulation (bottom curves).