Help with Active Crossover

[fglabach]

I have a kinergetics electronic crossover which routes the lower frequencies to my woofer towers.

It uses an Elnanc el2001 70 megahertz op amp input buffer. Then there is a chain of DC coupled bandpass filters that have selectable crossover points.

It also has a op amp at the end of the chain of crossover op amps that supplies a bass boast.

After the bass boast op amp is a 10uf DC blocking cap and then to the output jack.

The input buffer is at plus and minus 12 volt supplies and has no inverting input.

The El2001 had about a 75mv of dc offset measured from the input to ground, which seems rather high.

I have made 2 changes to the circuit.

1. I substituted an el2002 op amp 170 megahertz buffer for the original because the original broke and I could not find the same replacement. It has about twice as high bias current, twice as high bandwidth and slightly lower output impedance. But now I measured about 170mv of DC offset at the input of the buffer amp which I assume is due to the higher bias current. The op amp has a 47k ohm resistor to ground.


2. I substituted a National Semiconductor LM6172 (this is a very high-speed op-amp with bipolar input transistors. Translated, that means this op-amp is hard to use) for the NE5532 as the bass boast op amp. It made a huge difference in bass sound for the better. The app notes say that the slew rate is directly proportional to the bias current because there is a large cap in the op amp that needs the bias current to charge.

3. Remember all of the op amps are DC coupled with isolation resistors only except for the final output cap.

All of the changes appear to have greatly improved the sound.

So here are my questions:

1. Should I be concerned about the higher input offset of the new buffer? Does this affect the bias operating current of all of the other op amps in the chain? How does one know what the bias operating value should be for an op amp? Should I attempt to lower the offset of the buffer? If I reduce the buffer input ground resistor the dc offset drops but this may put too great of a load on the preamp. I could use another resistor to a reference voltage? Should I bother?

2. Should I try to increase the bias current of the Lm6172 to increase the bias current to improve its slew rate performance like the app notes say? The LM6172 is in a very complex bass boast circuit. Is the higher dc offset of the new buffer helping in this regard.

3. The equalizer now appears to have somewhat higher gain then it did in that I can no longer turn the volume all the way up without damaging the woofer towers. This surprises me because the buffer is unity gain and I did not change the gain resistors on any of the op amps.

Any further thoughts?