Question about equalizer

[Pajarico]

Hi,

I've built a 3-band equalizer based on 741 op-amps. It's composed of three stages (you can see them from left to right in scheme adjunted):
1- Pass-band filters
2- Amplification/attenuation (x10/x.1 adjusting the potentiometers)
3- Adder (sums the three bands)

The top op-amps are the low band (20-200Hz)
The middle op-amps are the middle band (200-2kHz)
The bottom op-amps are the high band (2k-20kHz)

There is a strange phenomenon. Everything that follows is for an 100Hz input signal which should be only amplificated by the top part of the circuitry. However...

-If I have a high gain of the low band and move the potentiometer of the high band nothing happens at the output. The high band doesn't affect it (this is normal, right?).

-If I have an attenuation of 0.1 at the low band and move the potentiometer of the high band the output gets very affected...

Why?

I can expect some influence from the middle band over the low and high bands because they are adjacents and there is the 20dB/decade slope and all that. But how can two sepparated bands affect each other?? Even more: how can the low band get affected ONLY when its gain is 0.1??

BTW This is for a class assignment and the teacher said it's normal, but I can't figure it out. I supposed there was some limitation on the input voltage of the adder so I only noticed the effects of the high band gain when the low band was at minimum, or the signal at the output of the low band amplifier was somewhat going backwards thru the amplifier of the high band?? But that doesn't quite explain why it only happens when the low band is at 0.1...

Thanks for your reading.

 

[audioguru2]

I will assume that the 100Hz input is a pure sine-wave without any harmonics.

The low frequency section has a gain of 0.1.
The high frequency section has a gain of 10.1.
So the high frequency section has 101 times more gain than the low frequency section.

Calculate how much of the 100Hz signal is at the output of the high frequency filter. Then it gets amplified.

 

[Pajarico]

Thanks.

I will assume that the 100Hz input is a pure sine-wave without any harmonics.

Yes, it's a sine wave 1Vpp, forgot to mention... I don't know how to tell if it has harmonics (I've a very vague idea of this). It comes from a function generator and in the oscilloscope seemed clean.

Calculate how much of the 100Hz signal is at the output of the high frequency filter. Then it gets amplified.

The point is that even if the 100Hz signal gets to the output of the high band filter that doesn't explain why this only happens with the low frequency band at 0.1, does it? I might be missing something.

I've to do some measurements today because I don't have an oscilloscope at home... Will se how much of the 100Hz signal gets across the high band pass filter.

Regards.

 

[audioguru2]

I mentioned a "pure" sine-wave because distorted sine-waves from function generators have 0.5% to 5% of harmonic distortion. If the harmonics are at 5% then ithey will be amplified a lot by the high frequency filter.

Calculate how much 100Hz signal is at the output of the high frequency filter:
1) You know the frequency that its highpass filter starts at (2kHz).
2) You know that the slope is 20dB/decade.
3) Maybe you don't know that 20db is 1/10th or 10 times the voltage and that 6dB is 1/2 or twice the voltage. Maybe you don't know than an octave is half or 2 times the frequency.
4) With the high frequency attenuator/gain amplifier turned up then it has 101 times more gain than the low frequency filter.

EDIT: I had the slopes wrong, it is corrected.
        I also had the octave multiplication wrong, it is corrected.

Try the same test but use a 1kHz input singnal. Turn down the gain of the mid-frequency section. You will see that the low frequency section and the high frequency section affect 1kHz a lot.

Try it with a 10kHz input signal. Turn down the gain of the high frequency section. You will see that the gain of the low frequency section affects 10kHz.