40dB Lpad with Variable Volume - Page 1
Forum section: electronics / projects

author: kolbep
author_rank: Frequent Contributor
timestamp: May 06, 2012, 05:30:33 pm
content: Good Day.I am doing videos of concerts, weddings, etc.Before, I used to lug around DVD recorders, and Monitors, Mainly to get clear sound from the Venue Sound System.I have managed to get some MiniDV Casette Cameras, with Microphone Input. Unfortunately I need to pad the line levels of the sound system down to the Mic Level accepted by the camera. I also need to Block the DC voltage that the mic input gives out to power condenser capsules.I have found info on Lpads, with this example on the crown website:http://www.crownaudio.com/kb/entry/11/279/However, I would like to have the 40dB pad, and an adjustment pot in case the Sound System volume is drastically increased (so that I do not overdrive).Attached is my schematic. Please let me know if I am putting the pot in the right area (I am doing it on the Line Level side to prevent as much noise from the pot itself from being amplified).Any other suggestions Analog Gurus?Thanks

author: Lightages
author_rank: Unknown
timestamp: May 06, 2012, 06:00:01 pm
content: First off an "LPAD" is not what you have linked to or drawn. Out of laziness I will point you to this web page to let you see what an LPAD really is. I have not read the entire page to ensure it's accuracy.http://www.colomar.com/Shavano/lpad.htmlWhat you have drawn will work but will not be the best. Just make the pot a 25K or 47K and replace the 10K resistor with the pot. It might also be beneficial to add some kind of back to back zeners to clamp the output of the cable after the capacitor so that your mic input on your camera doesn't see any high voltages that might build up on the capacitor. Maybe something like 3.3V.Edit, maybe something more like 15v to avoid non-linearity.

author: kolbep
author_rank: Frequent Contributor
timestamp: May 07, 2012, 03:28:40 pm
content: Hi Lightages,you say this is not the best.Do you have something that would work better?ThanksP

author: Lightages
author_rank: Unknown
timestamp: May 08, 2012, 12:02:53 am
content: I described what would work. I did not think I needed to make a schematic. Instead of adding the additional pot in parallel with the original circuit, replace the 10K resistor with a 25K or 47K pot so as to be able to adjust the pad. On the output put one 5.1V zener from ground to the output, and another the other way around in parallel with the first.It is that simple.

author: Lightages
author_rank: Unknown
timestamp: May 08, 2012, 12:06:16 am
content: Actually, add the pot in series with the 10K resistor if you want to ensure that you never have less than a 40db pad, and/or substitute a 2k2 for the 10k if you want to be able to pad less than 40db.

author: kolbep
author_rank: Frequent Contributor
timestamp: May 08, 2012, 04:34:28 am
content: Thanks Lightages.I did not mean that you had to do a schematic. I understood your explanation completely fine.I just thought that you were referring to another way to attenuate the signal, that would give better performance than this resistor pad circuit.Thanks for your reply.Peter