Quick Question.

Hello, is this the proper way to wire the active x-over with volume pot and amp ?.
Do I need the first buffer? if yes, 1 or 2 ( with or without resistors. ).?
Volume pot location?.
Thank you !

Bruno .


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The initial buffer is only needed if the input is from a high impedance source.

The volume pot logically goes before the crossover, but you might want to consider whether each crossover section also needs its own volume preset for adjusting relative levels between the drivers.

If you omit the input buffer then one of the crossover channels is DC-coupled which might not be a good idea - so replacing the input buffer with a coupling capacitor is worth considering.

There ought to be EMI suppression on the input of an audio amp unless you are happy with mobile phone break-though and the like. 560pF ceramic to ground mounted direct on the input connector (with short leads) is a reasonable approach initially.

The 10k+10k divider in the Sallen Key filter feedback sections is rather high valued - although the TL072 itself is probably the main source of noise - reduce these in proportion perhaps (but don't overload the opamp outputs). 1k+1k divider is probably the lowest values for TL072, and 2k happens to be the design load for many opamps.

If you use an input buffer the volume pot can be quite low in value, 2k or 5k, without an input buffer 10k is the minimum value you should use to present the conventional input impedance of 10k to 50k for audio signals. For noise reasons I'd suggest a maximum of 10k also. 10k is very commonly available too which is useful.
No deep reason, 10k is a very common kind of value for opamp circuitry. More modern low noise design favours values more like 1k partly due to more systems being powered +/-5V or +/-2.5V rather than the +/-18V of a classic mixing desk.

Or put another way the input noise of a 5532 is equivalent to the Johnson noise of a ~1k resistor - using ~10k means the resistors dominate the noise performance rather than the silicon.