Audio Probe - Page 1
Forum section: electronics / testgear

author: retrotubeguy
author_rank: Contributor
timestamp: September 01, 2015, 11:40:55 pm
content: Hi, I am building an audio probe that I plan to use to poke around inside amplifiers to trace signal flow. My plan was to inject a sine wave say 400 HZ to KHz and listen at key points with the probe connected to a battery powered amplified speaker. This way I can keep my eyes on the amp instead of the scope. Just for preliminary trouble shooting. I will have a polypropylene axial cap 2KV/600VAC Rated in series with the probe, then a shielded cable to the battery amp. I may also include an audio isolation transformer. My concern is AC mains or High AC secondary voltage of the power transformer. In the event that the probe comes in contact with any of these higher ac voltages, is there a simple way to protect against this while still allowing the relatively small audio signal thru to the battery amp. I would like to reduce the risk of fire,explosion,damage,personal injury. I can think of a few elaborate ways to do this but I am after something simple possibly to fit inside the probe casing.Thanks.

author: barry14
author_rank: Regular Contributor
timestamp: September 02, 2015, 12:18:45 am
content: You are proposing what used to be called a Signal Tracer. These were designed to inject both audio and RF signals into radios for troubleshooting. Heathkit, EICO and KnightKit made such instruments. Usually there are several for sale on Ebay. You might just hunt up a manual for one of these to see how they are constructed and what, if any, safety features are included.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: September 02, 2015, 12:23:15 am
content: Connect the battery-amp Ground to the Chassis of the Device you're checking and put a Crowbar with a threshold of 50V in the input-circuitry.If you accidentally probe high voltages, the power is dissipated through the chassis and therefore Protecting Earth, tripping the FI in your fuse-box.You could also use opto-coupler to connect a battery-powered input-stage to a electrcally isolated second battery-powered Amp-Stage connected to the headphones. Idk how much opto-coupler will distort the signal though.

author: commongrounder
author_rank: Frequent Contributor
timestamp: September 02, 2015, 03:01:22 pm
content: I've put together a few of these signal tracers over the years, using various discrete and IC based amplifiers. You can protect the amp by inserting a series resistor (possibly 10k ohms or so, depending on the input impedance of the amp) followed by back-to-back zener diodes, to signal ground, of a voltage value that would clamp the input below the maximum peak voltage allowed on the amp input stage. I often used 3.3 volt 1-watt zeners. There might be some leakage and distortion, but we are not looking for High Fidelity reproduction here, just a presence of signal. The DC blocking capacitor is a good idea. You could get by with a pretty small value, which might help block some line frequency AC from getting in as well.One last piece of advice. Don't use something like this to probe "hot chassis" (i.e., transformerless) devices without using an AC isolation transformer. You would run a serious risk of direct bodily contact with the AC mains line, and deadly shock.Good luck with your project!

author: t9crepair
author_rank: Newbie
timestamp: September 08, 2015, 03:02:15 am
content: Something like this might do the trick:http://www.qrpkits.com/tracerinjector.html