Tuneable filter with 14 octave span

I intend to make the juno filter tuneable in software. To do this, I hoped to add a 20 octave range to the CV and just use that. But simulating the filter gave a few surprises:

- when using a 25k input resistor to get 20 oct range, the output of the initial op amp exceeds the rails, so it will clip.

- the maximum cutoff for the filter at the current implementation is around 48kHz

- Below 7-ish Hz the filter acts like a band pass filter so the input is always attenuated, meaning lower frequencies are not possible.

Now, the two last facts are not a big problem. They happen on the filter side of the exponential converter, which is not as temperature sensitive so range hopefully won't change that much with temperature.

The expo converter however will change a lot, meaning the input needed to get a certain output will not stay the same. THIS is where we need to apply tuning relative to temperature.

So what I need to look at is the circuit before the expo converter transistor pair. I need to make sure that THAT can go above and below what is needed for control at 25 degrees celcius. To do this I've started out mapping out what currents into the filter core gives what cutoff. Here is a short table:

2mA = 47.2kHz

1.5mA = 45kHz

1.2mA = 43.5kHz

1.1mA = 42.5kHz

1.0mA = 41.5kHz

0.8mA = 38.6kHz

0.7mA = 36.5kHz

0.6mA = 34.2kHz

0.5mA = 31.3kHz

0.4mA = 27.3kHz

0.3mA = 22.5kHz

0.2mA = 16.3kHz

0.1mA = 8.7kHz

50uA = 4.4kHz

25uA = 2.23kHz

12.5uA = 1.11kHz

6.25uA = 553Hz

3.125uA = 274Hz

1.56uA = 136Hz

781nA = 67.4Hz

390nA = 33Hz

195nA = 16Hz

97.7nA = 7.64Hz

48.8nA= n/a, alltid under -3dB

So... from around 100nA and up to 0.2mA, the tracking seems to follow the rule of doubling the current = double the -3dB cutoff point. After that it falls off a bit. I think that a range of approx 4-40kHz is fine, that's about 13.5 octaves. 

That means that the input range we're looking for is about 100nA to 1.2mA. Next up will be looking at the circuit as it is and see what the input to the expo converter is at these currents. Then I'll see if I can change how the circuit works to allow the input to go lower and higher than this. At a stable 25degrees this won't have any impact (it simply won't work), but it will allow us to correct for changes later. Maybe.

A bit more testing

I did some more measuring and calculations today. First of all, I calculated the necessary resistor to get 1V/oct response. The Juno uses a resistor voltage divider with a 560Ohm tempco at the bottom. To get 1V in = 17.9mV out (17.9mV is the magic number for expo converters at 25C), the top resistor needs to be 31.745k to get perfect tracking. In the current design this is achieved by a 20k multiturn pot combined with a 10k and 8k2 resistor. 

In the 12V version of the filter, we have a few issues.

Without any changes, Iabc tops out at 1.07mA. We need at least 1.16mA to reach 40kHz.

Allowing a 20V input (or rather, changing to a 25k input to get 20 octaves from 5V input) will not work as the cv input opamp output will hit the low power rail (-12v) before going all the way.

So, what can be done:

1) By switching the 10k input resistors on the LM13700 Iabc inputs for 8.2k ones, we get a max Iabc of 1.3mA

2) By decreasing the top resistor in the resistor divider, we can get a response that is lower than 1V oct, for example 0.8V/oct. This means that the range output by the cv input summer can be within the power rails. 

I did some inital trials without calculating too much. For easier reading I use a 0-20V CV input with a 100k input resistor. Swapping R34 with a 5k (reducing the top resistor by 5k) and replacing 10k Iabc resistors with 8.2k, I get a CV AFTER the summer that goes from 6 to -10V (at 15.6V input). The Iabc response goes from 25nA at 0V to 1.3mA at 14V. 150nA, which we know from earlier is about the lowest we need, is output at 2V input. 

20V CV input and around 0.8V/oct tracking. +/-12V filter

Red: CV input, Green: Voltage after summer, shows saturation at around 16V CV. Blue: I_abc, tops out when CV is around 14V.

This means that our useable range is around 12V, from 2V to 14V, and that we have around 2V above and below this. We already have a fairly useable tuning range.

So, what should I do next?

Given 20V in and 100k input res (will be changed to 5V/25k)

1) Find the range we need for 14 octaves (150nA to whatever)

2) Find a gain + resistor divider that puts 3 V in at 150nA and 17V in at whatever is needed for 14 octaves up.

3) Input summer must not saturate, eg. stay within +/-10V output for the full range.

This will be fun!

BUUUT. What about the tempco resistor? Will it still adjust itself correctly when the top resistor is no longer the same? Well, that needs to be checked. 

And for later: At what input audio does the filter start to distort?

EDIT: Input to expo conv: 72mV in = 150nA out, -159mV in = 1.16mA out