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# Calculate IC equalizer frequencies and gain

#### tritonas00

##### Newbie
Hi! I have this diagram:

BA3823LS monolithic, five-point stereo graphic equalizer IC is used. Is there any way to calculate the frequencies (hz) that are defined and how much boost/cut gain (db) is applied in each frequency? If i'm reading this correctly the capacitors (C56, C55, ...) are used for setting the frequencies and the resistances on the side are used to cut/boost those frequencies?

I don't know anything about electronics and all the info i have is https://datasheetspdf.com/pdf-file/233435/Rohm/BA3823LS/1 where application examples of the BA3823LS are shown with the 5 center frequencies and some min/typ/max gain values.

Thanks for your time and sorry if i'm posting this in wrong section.

Ok, about the frequencies i just noticed that the capacitors used in the example application (pdf) are the same as the diagram shown. Show they would probably be the same:

So what remains is the gain (db), how much they boost or cut those frequencies. For example those, if i'm reading this correctly:

Is there an equation to find the db based on those two values?

Your link is to a data sheet which has diagrams where the IC is surrounded by external potentiometers and capacitors. It states that each model number has a different range of adjustment, therefore 'the external component values are different for the same center frequencies.'

Do they use the Sallen-Key topology? Various filter types have their particular formula relating rolloff frequency to R & C values. It allows designing our equalizer based on theory... However in assembling capacitors for our project, it's hard to get them to match for both left & right channel. Tolerances are wide and can yield unexpected center frequencies in defiance of our previous calculations.

A graphic equalizer is a worthy project. Making (or simply simulating) one bandpass filter can give you an idea what to expect from combinations of RC values.

To calculate the circuit, you need internal parameters of the IC, specifically Gyrator circuit resistor values, which are not given in the datasheet.

The product of both resistors values can be estimated according to filter stage center frequency, absolute resistor value defines however min/max gain.

Also output impedance and gain of the buffer amplifier must be known.

Do they use the Sallen-Key topology?

Thanks for the reply! I have really no idea, here is the whole thing actually

Its an amplifier board used in a arranger keyboard. It is a "fixed" type equalizer, so no potentiometers.

I think they might actually tweak 3 frequencies, the service manual of the keyboard states this in the Frequence Response Check routine:

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