help me out with this tone control

[soggybag]

This is a snippet I took from another project. In the original the first op-amp was inverting and the other two were on-inverting. I switched these around so the second two op-amps are now inverting and the first is non-inverting. I wondering how this will effect the two tone controls? The upper pot is Low pass and the lower is a high pass filter. Will these still work the same going in to an inverting stage?

[earthtonesaudio]

Why'd you change it?

[soggybag]

Thanks for the reply.

This section needs to be inverting. The last stage, not shown, is also inverting. This would give a non-inverting signal path.

The original had an inverting input followed by the two tone controls which were non-inverting. I figure a non-inverting input is a better choice. Which meant that I had to make the two buffers following the tone controls inverting to keep this section inverting.

What I was really wondering was if the two 10K resistors, at the input and feedback loop, would effect the tone controls preceding them?

If not then everything should be good. Otherwise I what should I do to set up the tone controls so they will work with inverting op-amps following them?

[earthtonesaudio]

The reason I asked is this:

To maximize the "buffer" quality of the amps following each tone control, the input impedance should be very high.  With the component values shown, I'd say 10k is rather on the lower end of medium, and will load the tone controls a bit.

Of course, to make the input impedance higher you just increase the resistor value, but then you have to increase the feedback resistor value to match, and you end up with some very high value resistors which can mean more noise.  Same goes for a non-inverting buffer, but in that case you only have one resistor at the input (and you don't need one at all for the lowpass side, as it's directly coupled) versus two required for each inverting amp.

If you want the output inverted (and assuming you don't want a Baxandall-type stage for both bass and treble controls), I'd suggest returning to the inverter-at-input configuration and using non-inverting buffers after each tone control.  Your input impedance is only 10k, so to get the gain of 6 as shown you only need a 60k resistor, which would mean less resistor-induced noise even than the way you have it now.  Of course, C8 will then need to be made larger to keep the same response, but not by much.

Also it's good to remember that the phase of the signal after the tone controls is not going to be exactly equal to the phase of the original signal that went in.  So with heavy equalization, the utility of preserving the polarity, in general, becomes less important.

[soggybag]

Thanks for the reply, that was very informative.

If the phase doesn't matter so much, maybe it would be good to make all stages non-inverting?

The next stage, not shown here, is a NE570. I need to limit input here to something no greater than 140uA or 2.8V. I had the idea that with an inverting stage before the NE570 I could more easily adjust the level of the signal going into it.

[earthtonesaudio]

I'm not familiar with the NE570 compander chip, but if it's a matter of getting levels right, inverting vs. non-inverting doesn't make any difference. 

Polarity and phase issues only really start to matter when you have multiple copies of the same signal mixed together, such as a close-miked speaker AND a room mic going into a mixer.  The ability to flip the polarity of one source is handy because sometimes (not always) the cancellation/reinforcement effects can be undesirable.  But in the majority of cases like these, you're using mixers that already have polarity switches on them. 

However, you do have the right idea.  In general it is good practice to keep the polarity roughly the same at the output as it is on the input.  You can't anticipate every possible scenario, so it just makes life a little easier.

[soggybag]

The NE570 starts to distort when the input signal is greater than +-140uA/+-2.8V. My idea was to adjust the gain of the tone buffers to try and keep the signal within this range. I guess I could also adjust this at the input buffer also. 

I have been trying to figure out the best way to keep the signal within this +-140uA/+-2.8V limit. My guess is that a guitar signal at it's initial peak might be about this limit. I seem to remember someones guess at 5V.  From there I seem to remember 1V.