I have a preamp I am about to build and was thinking of adding LP filter to drive some subs.
What do you think about this idea - using a dual triode like a 12AU7, and choosing capacitors to bypass the plate resistors on the sections (which would be cascaded). The first cap size to roll off 6db above 70hz and the second about 90hz.
Which would result in about 80hz fairly low q.
Just spitballin' here.
thanks
What do you think about this idea - using a dual triode like a 12AU7, and choosing capacitors to bypass the plate resistors on the sections (which would be cascaded). The first cap size to roll off 6db above 70hz and the second about 90hz.
Which would result in about 80hz fairly low q.
Just spitballin' here.
thanks
The components scale linearly. You can either increase the caps by 10X in the LF section or the resistors.
Of the two, scaling the caps will result in the lowest noise - which might not matter much in this application.
You can simulate this in LTSpice.
Use a high transconductance tube like the 6DJ8 or 6N6P (increase current here) the filter is quite sensitive to gain, the lower the overall gain the lower the overall Q and the poorer the overall adherence to the desired response characteristics.
A third order butterworth is a good pragmatic choice from the standpoint of maximizing performance with a tube based filter.
Further performance improvements are possible if you replace the 22K resistor with a CCS provided the input impedance of the next stage is fairly high. (100K or higher)
Of the two, scaling the caps will result in the lowest noise - which might not matter much in this application.
You can simulate this in LTSpice.
Use a high transconductance tube like the 6DJ8 or 6N6P (increase current here) the filter is quite sensitive to gain, the lower the overall gain the lower the overall Q and the poorer the overall adherence to the desired response characteristics.
A third order butterworth is a good pragmatic choice from the standpoint of maximizing performance with a tube based filter.
Further performance improvements are possible if you replace the 22K resistor with a CCS provided the input impedance of the next stage is fairly high. (100K or higher)