Custom audio mixer/selector wiring help


Apr 22, 2017
Hello Community!

I have started a project and am at a stage that I would like my plans verified before I invest in the parts needed to complete the project. Here are the notes...

I am looking to build an audio selector which allows me to independently control (through analog dials) my audio source, output and mic source. Additionally one of the audio source will be a mix of two audio sources. I have been reading around and have come up with the following diagram. I was hoping someone could validate its not a terrible idea and also answer these questions three...

1) Are the resistors the correct value for the job/ placed correctly? I have no interest in an active system and do not want to have to supply power to this box.

2) I dont quite understand how the grounding works and am concerned that with two different audio signals combined (computer and iphone) the ground will loop (?) How can I prevent this, preferably without a ground loop isolator.

3) Do I need any shielding inside the enclosure to prevent feedback?

Thanks in advance for any feedback on this!



Overall it is good to see that you are doing some pre-planning and working things out.

I am not an audiophile by any means but there are some very good ones who are contributors within this forum. Hopefully a couple of them will have suggestions and ideas to post. Likely to be some very interesting discussions....

The 4.7 resistors I still wonder about. Resistors limit current and generate heat. They may also contribute to noise but not sure about the end results within your circuits. There is quite a bit of difference between your project and the Instructable project being referenced.

I would be concerned about the USB powered speakers. Some USB ports may not be up to the requirements.

Ground loops are a definite concern: If you google "ground loops" you will find numerous links - text and video.

For example:

Look for links that are current and applicable to home audio systems.

Are you familar with breadboarding? I recommend that you take the time to breadboard (and test) the individual circuits working your way up to the full design. Again many "how - to" or other demo links online.

Besides if you are interested in electronics you will need a breadboard and probably a multi-meter as well.

Always good to test before soldering. And to be able make some measurements - may catch an error that would damage one of your devices.

Also evolve your plan. You basically have a picture diagram. Move towards a more formal schematic diagram. Not an easy task per se but a schematic diagram will bring out more detail and perhaps flag some other relevant issue. There are a variety of software applications (free and otherwise) to help with that.

But just using graph paper, a ruler, and a pencil works just as well. Plus an eraser :)

Sketch out your circuit, make copies, and modify on the copies.

The idea is to learn more and gain enough understanding to where you are fully comfortable with your plan.

And wait for the arrival of Mr. Murphy.....



Just three immediate questions - there may be additional questions from other people.

However, just to "get started"....

1) What is the purpose of the resistors and how did you come up with the sizing?

2) It appears that you are hardwiring the computer audio and Iphone dock audio together. Is that the "[one] audio source will be a mix of two audio sources"? Your second question indicates that to be the case as does the diagram. Are the computer and the iPhone dock sharing an electrical outlet? What about power to the Auxillary Source?

3) What USB port is going to be used to provide power to the USB powered speakers? The computer perhaps



Apr 22, 2017
Hey Ralston18, thanks for your response! here are some answers for you.

Ive been looking around online and am under the impression resistors are required when mixing two audio sources together to prevent feedback. I read about it first here HERE (Link to instructables)and later confirmed the use of resistors on other peoples projects. The values have been different from project to project but this one (in link) uses the same ones I do and was the closest example to what I am looking to accomplish with these two lines

Yes, so basically I would have 3 options for Source signal... 1) Computer AND iphone mixed 2) just iphone 3) auxillary. I am guessing this is the setup but please correct me if it is wrong. I am going off the project linked above.

A fine question which i had not considered. The iphone dock will be usb powered from the computer now that I think about it. AUX is yet to be determined and do not have any plans for these ports currently. Added them incase in the future I have something to add into this scheme.

Yes! USB speakers will be computer powered for sure.

Thanks again for your help - look forward to your reply.

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