How to compress the audio output on a signal tracer project? And other questions - Page 1
Forum section: electronics / projects

author: t1d
author_rank: Super Contributor
timestamp: May 25, 2022, 10:03:00 am
content: BackgroundThis project is to learn with. Meaning, I do not have particular design goals that I want to meet.I am a hobbyist, with medium skills and a higher-end lab.--------------------------------------------------------------------------------------------------------------------------DIY signal tracer circuits have a common problem... When you go from probing the input of the DUT's amplifier to probing its output, you get blasted with a loud volume. The traditional solution is to ride the signal tracer's volume knob, while probing with the other hand. This is a poor solution, IMO.I have not found an integrated solution, on the signal tracer circuits that I have seen. So, I am wondering if there is a traditional solution? Meaning, a method of compression that produces a set volume on the signal tracer's output, regardless of what is going on with the DUT's signal amplitude.I am also wondering what voltages a signal tracer might need to be capable of reading... The input to an op amp might be as little as 10mV, while the voltages on a tube amp might be hundreds of volts.Is there a well known, commercial signal tracer that I might use as a model for its abilities, functions and circuit? It would be particularly helpful, if the schematic was available, of course...Analog and digital solutions are welcome.Please and thank you, for your help.

author: Benta
author_rank: Super Contributor
timestamp: May 25, 2022, 10:48:56 am
content: Check the ON Semi SA572:https://www.onsemi.com/pdf/datasheet/sa572-d.pdf

author: t1d
author_rank: Super Contributor
timestamp: May 25, 2022, 10:59:52 am
content: Here is a schematic for a design that I built to play with. It might be useful for explaining where to insert the compressor.

author: t1d
author_rank: Super Contributor
timestamp: May 25, 2022, 11:04:48 am
content: Quote from: Benta on May 25, 2022, 10:48:56 amThanks, Benta! Good suggestion. Nice component. $5.17USD @ Mouser. I will surely give it a look.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 25, 2022, 02:35:20 pm
content: The compressor fits in before the gain stage. A 10mV to 100V range will need an 80db of compression.An input attenuator of 40dB with some over voltage protection precedes the compressor. Note that at higher voltages a voltage divider needs to disipate a lot of power. Calculate with care.Just follow the compressor with the much beloved by audiophiles LM386.see attached.

author: t1d
author_rank: Super Contributor
timestamp: May 25, 2022, 03:35:10 pm
content: Quote from: Terry Bites on May 25, 2022, 02:35:20 pmAll of that is very helpful information; thank you, Terry. Any suggestions for an automated attenuator circuit design?

author: t1d
author_rank: Super Contributor
timestamp: May 25, 2022, 03:43:31 pm
content: One idea that occurs to me is that digital voltmeters attenuate their input, so, if I used the same attenuator, I could also read the voltage at the point probed. Hmm... Maybe I could repurpose a cheap DMM, to that end. Remove the battery, add a wall wart input, tap off to the the tracer at the MCU and put the tracer circuit in the battery compartment... Just spit-balling, here...

author: t1d
author_rank: Super Contributor
timestamp: August 30, 2022, 09:40:07 am
content: Quote from: t1d on May 25, 2022, 03:35:10 pmDoh! That was a dumb question, given what you had suggested and the attached Application Note. I missed that the link went to the AN; I thought it went to the 571 data sheet.So, anyway, I am finally getting around to resurrecting this project. I guess I will start by determining which IC would be better for this application, the SA571, or the SA572. Any tips about that would be welcomed and appreciated. Then, I know that I will need everyone's help, integrating the Compandor into the Signal Injector/Tracer circuit. Cheers!