# Switch array (crosspoint) and DC biasing of audio signal

For a project at my university, I am making a guitar effects switcher. Guitar effects work on instrument level audio signal which is AC only. I plan to use ADG2128 analog switch array (https://www.analog.com/media/en/technical-documentation/data-sheets/ADG2128.pdf) for this purpose. I am trying to achieve the following thing:

If I power ADG2128 from a single rail power supply (12V), I need to bias the input audio signal before routing it in the input of the chip, so it does not go in the negative voltage range, in order for the switch array to work.

I have never done something like this before, so my new question is how should the circuit for the DC biasing look like.

I need to add some DC voltage (DC bias) before sending the signal into the array and then remove this offset before sending it to the next effect. What is the proper approach to do this without accidentally filtering the sound or altering it?

• Please see the datasheet where it says, "Absolute Maximum Ratings: Analog Inputs: VSS − 0.3 V to VDD + 0.3 V" – scorpdaddy Apr 22 at 17:21
• Thank you for this comment, I have totally missed this line. I have updated the question as I don't know the proper approach to this. – Klemen Apr 22 at 18:17
• Googling for "ac biasing circuit" may now be helpful. – scorpdaddy Apr 22 at 18:54

The circuit below will provide DC bias for inputs to the switch, and remove DC bias on the outputs.

simulate this circuit – Schematic created using CircuitLab

"in" is any input to the switch. "out" is any output from the switch. The input bias circuit has a corner frequency below 1 Hz. It will pass frequencies above that. The output bias will have a corner frequency that depends on the input impedance of the next device, but will likely be 10k+ so should be ok.

Each input to the switch will need the input biasing below, whether it comes from a guitar, effects pedal, or other device. Each output from the switch will need the series capacitor to prevent the bias from getting to the next pedal, amp, or other device.

The capacitors should be ceramic type, which have no DC voltage polarity. X7R type ceramic has been suggested.

• Thank you for your valuable information and answer :) I have another question. As far as I know most of the effects have ac coupling on the input and do their own DC biasing and then couping on the output. If I connect one effect on the send and return, wouldn't it cancel my coupling of the guitar input signal? If this is true, this most likely means I need to make this circuit not only for in and out, but for each return and send pair, right? – Klemen Apr 22 at 20:47
• That is a good point. The diagram in the question shows the sends looping back directly to the returns. But that isn't incorrect, is it. The sends will actually go to a pedal, and the pedal output go back to the returns. In such case, every input and every output to/from the switch will need bias and decoupling. I've updated the answer to show that. – scorpdaddy Apr 23 at 12:26
• Sorry, I copied the diagram and failed to show this. Thank you for all your help. – Klemen Apr 23 at 13:05