Bandpass filter design

[Tube Radio]

I'm looking to design a bandpass filter using OP-AMPs that is unity gain and will approximate the following filter curve.

I've got +/- 12V to work with and will either use a DPDT relay or switch to bypass the filter when it is not needed.

 

[Indiglo]

Have a play with some filter design tools from TI and Analog Devices.

• https://webench.ti.com/filter-design-tool/filter-type
• https://tools.analog.com/en/filterwizard/

TI asks you to create an account to see the full design whereas Analog Devices provides the complete design.

The Analog Devices version will also provide a LTspice circuit of the filter.

I played around with the Analog Devices Filter Wizard:
These values will get close to the filter response, you may need to vary the values to get the exact response you require.

 

[Tube Radio]

Thanks for that.

I'll bookmark the Analog Devicea filter wizard so I'll have it for future reference.

Any reason I cannot use one dual OP-AMP instead of having the extra OP-AMP sections?

 

[Mark Tillotson]

Any reason I cannot use one dual OP-AMP instead of having the extra OP-AMP sections?

Doubt it, the tool probably assumes one opamp per stage until the schematic drawing part of it which presumably uses a database of parts/pinouts. You'd be able to lose two 100nF decoupling caps with a single dual package.

Its chosen rather old-fashioned high impedance passive values. I'd be tempted to scale the resistors down by 10 and the capacitors up by 10 to improve the noise performance. The AD8676 is an odd choice as its ultra-high DC precision, yet your signal is ac coupled! There'll be cheaper choices I suspect.

 

[steveu]

You may want to add an input buffer because the design assumes a ~zero impedance source, ie another op-amp. The problem with a unity-gain Sallen-Key filter is that you have to use some odd capacitor and resistor values. If can deal with +6dB gain, you can use equal value parts.

 

[steveu]

A 18dB/oct version is actually a bit simpler, ie no 6k5 resistors required, and uses 4 op-amps.

 

[Tube Radio]

Here's the circuit this will be used with.

Is it possible to insert the filter after the first gain stage between pin 1 and the 110k resistor?

I prefer the filter be unity gain as I intend to add a switch to allow the filter to be bypassed.

I could then use a TL082 for the filter stages.

 

[steveu]

Ya. You wouldn't need the input buffer. This is not a very efficient compressor-limiter but maybe it's dressed out the way you want? LDRs are an easy way to avoid distortion, but I prefer a simple JFET circuit, and it could provide attack and recovery time controls, etc. Years ago we perfected a design but I don't see it anywhere today. It was something like this: https://sound-au.com/project67.htm

 

[Tube Radio]

The design was what someone over on the Antique Radio Forums had designed for use with TVs where the commercials are much louder than the normal program.

The audio sounds good and it does what I want.

I'll post a video of the audio in a bit.

The filter having unity gain makes it simpler as I do not have to move the circuit used to flash an LED when the audio is distorted coming out of the first stage. Plus it makes it easy to bypass with just a DPDT switch.

 

[Tube Radio]

Here's a video of the audio from the transmitter that limiter/compressor is used in.

 

[steveu]

Ok, the filter wizard is a good start, but spice allows you to try common component values, and you will notice a pattern that helps making choices.