UREI 1620 EQ Mod

[smilan]

Hi, I would like to modify the EQ section of the 1620 mixer and make it a 3 band EQ.
I have two questions:
1. How to choose the right values for the EQ section to create the right cuts for each EQ band ?
2. Would it be better to design the mid section like the treble section (with two resistors on both sides of the pot and a cap on the wiper) or like the bass section (with two capacitors on both sides of the pot and a resistor on the wiper)?

 

[abbey road d enfer]

• 1. How to choose the right values for the EQ section to create the right cuts for each EQ band ?

It's quite difficult because its interactive. I've done the calculations a long time ago when I thought everything could be done with math.
IMO there are two practical solutions:

1. Tral and error. I would start with 2.2nF everywhere. You need a little understanding of how these circuits work. The caps across the pot determine the lower frequency of the bandpass, te cap going to the inverting input of the opamp determines the higher frequency of the bandpass.
2. Simulate in Spice; that's what I would do, since I'm fluent enough with LTspice, and lazy with a soldering iron.

2. Would it be better to design the mid section like the treble section (with two resistors on both sides of the pot and a cap on the wiper) or like the bass section (with two capacitors on both sides of the pot and a resistor on the wiper)?

See attachment for what you need to add.
Note that the two caps across the pot could be replaced with a single one (not connected to the wiper) for a slightly different response.

 

[smilan]

It's quite difficult because its interactive. I've done the calculations a long time ago when I thought everything could be done with math.
IMO there are two practical solutions:

1. Tral and error. I would start with 2.2nF everywhere. You need a little understanding of how these circuits work. The caps across the pot determine the lower frequency of the bandpass, te cap going to the inverting input of the opamp determines the higher frequency of the bandpass.
2. Simulate in Spice; that's what I would do, since I'm fluent enough with LTspice, and lazy with a soldering iron.

See attachment for what you need to add.
Note that the two caps across the pot could be replaced with a single one (not connected to the wiper) for a slightly different response.

Thank you very much!
I'll look in to it, LTspice seems like something I need to get for those kind of simulations.