Ground Loop Isolator Circuit for Sound - Page 1
Forum section: electronics / projects

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: June 16, 2014, 09:41:44 am
content: Hallo guys and girl,I finally decided to make myself useful in the forum.From time to time I release youtube videos about electronics or other stuff that I like, but sometimes you need to work from yourPC / Laptop to make a video. With the integrated microphones of my laptop being rather inefficient for producing a pleasant sound,I borrowed a Behringer mixer and microphone from a friend. Unfortunately every time I connected it to the laptop (either input/output),it would become noisy. Reading around the internet and after some research I found out that it´s a grounding problem and a groundloop isolator was needed. Unfortunately everything that has the word audio in it, and it is not a horrible Chinese product is expensive.So, I said to myself... Why not design one myself! And I did!!!I have no interest in earning money from it so I release it here open hardwareHere´s the specs:Input ~3Vpk-pk (MAX)Output ~3.5Vpk-pk (MAX)Flat baseband from 10Hz to 20kHz with ~1.5db of gain.I have NOT built it yet and not tested it in real world (only sim.), but should work nicely.Another thing. I don´t know how lineal 4n27 is. I biased it at 5mA and it´s working pretty much likea class A amplifier. Another good thing is that the design is working from a single 5V rail when youwant to experiment with it!I´ve built everything in just 4 - 5 hours so if I missed anything please have mercy!I hope you like it and that someone might find it useful!Best Regards, LefterisThessaloniki, GreeceP.S. If there is someone who is willing to build it, I will upload the schematics and board. The project is built using Eagle 6.3.0..

author: nickm
author_rank: Regular Contributor
timestamp: June 16, 2014, 01:58:36 pm
content: You can get "high linearity" optocouplers which have two identical outputs, one of which you take your feedback from to compensate for the non-linearities. LOC110 is an example. Did you consider any other design topologies? I'm curious because I too have been thinking about breaking ground loops in long runs of cable.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: June 16, 2014, 02:58:42 pm
content: To tell you the truth, the above circuit was a 4a.m. brain fart cause of insomnia.I´ve seen that kind of optocouplers before, like the LOC110. Vishay has something similar as far as I remember.According to the datasheet of 4n27 I am indeed working in the linear range. I am also driving the led with current andnot with voltage, that should make a difference. Furthermore I am biasing the LED at 5mA with a max swing amplitudeof 4mA. Between 1mA and 10mA the graph seems linear enough. Also the sim showed promising results.Also, I really only use it for PC - mixer and mixer - PC interconnection. The voltage output of the PC is 2Vpk-pk? Somethinglike that, maybe less. And I think that the mixer has a 2.5Vpk-pk voltage output max, something like that. That meansthat the LED is going to operate in the 2mA - 8mA range. It should be fairly linear at that range.I have to build the circuit first and test it, to see firstly if it eliminates the hum, and secondly if it sounds right. If it soundsright, it´s going to be pretty much ok. I´m not looking for studio performance.I´ll try this one for the moment, see how well it goes and then I´ll probably try one of this high linearity optocouplers. It´sworth a shot, although compensating the feedback loop seems a little bit scary :-PBest Regards, LefterisThessaloniki, Greece

author: HomerA
author_rank: Contributor
timestamp: January 22, 2015, 12:36:40 am
content: Hi there,i am just wondering if this circuit worked for you or not? I fancy making one.

author: moffy
author_rank: Super Contributor
timestamp: January 25, 2015, 03:07:09 am
content: You could use a high quality isolating transformer such as: M0705, M0706 or M0707 from:www.altronics.com.au.They are very linear and have good bandwidth.