Triumvirate - purpose of the series EQs

[mark123]

I'm probably going to pick up a Triumvirate PCB so I checked out the PDF. 

Can someone explain the purpose of the series stacked EQs for the Treble, Mid and Bass?

Just wondering.

[Aentons]

Double pole low pass filter with a dual ganged pot.. I think. To separate the low frequency band from the mid band

Edit.. sorry, I didn't look at the doc... no dual gang pots... Looks like trimpots to set the range of the filter

[alanp]

Look at the caps values more carefully. 220nF and 22nF.

[WormBoy]

They second order filters, which are steeper than the more common first order ones. So that should give a better separation between the different bands.

[mark123]

They second order filters, which are steeper than the more common first order ones. So that should give a better separation between the different bands.

Thanks. I'll Google the term.

[mark123]

Googling led me to cascaded first order filters. Is that a better description of this?

[reddesert]

A single RC filter has a cut off slope of 6 dB/octave.  Filters in series multiply, so two 1st order filters have a steeper cut off of 12 dB/octave.

The variable resistors in these are all trimpots. From the schematic, in each place where there are two filters, the second RC pair has R 10x larger and C 10x smaller, so the RC product has the same range, depending on where you set the trimpot of course. Madbean's build document suggests that the reason for this (why they aren't just two pairs of the same values) has to do with the values adding in series. I think another reason for it is that the second filter can load the first, so it's good to have the second filter have a high input impedance.

Another way of making a second-order filter is to use an active circuit such as a Sallen-Key filter. This requires a single op-amp in addition to the two R's and two C's, but avoids the insertion loss that is caused by passive filters/tonestacks.

In the Triumvirate, the passive filters are followed by gain stages, which make up the loss. But you can see that in the Mid section, Peter used a single op-amp as a buffer to separate the Low Mid and High Mid filter pairs.

[mark123]

Excellent. Thanks for the info.