P01-119A mic power amp with AGC cutoff for de-essing. - Page 1
Forum section: electronics / projects

author: asgard
author_rank: Regular Contributor
timestamp: May 18, 2013, 07:02:06 pm
content: Here is an adaptation of the "overload" circuit from the EEVblog recent topic. It would be really neat if the forum could handle EAGLE files directly, but I know Dave does not want to fall into that particular rabbit hole (uses XML and all that).

author: asgard
author_rank: Regular Contributor
timestamp: May 19, 2013, 03:31:27 am
content: O.K. Trap for young players. negative linear regulators have the filter caps "upside down". Dohh.

author: codeboy2k
author_rank: Super Contributor
timestamp: May 19, 2013, 03:47:20 am
content: Quote from: asgard on May 19, 2013, 03:31:27 amelectrolytics and tantalum caps just like to have the + side on the more positive side than the "-" sideActually, your whole power section there is wacky.. Don't lie, you drew this up after a keg partyYou've got the input of the 7905 grounded, and the ground of said regulator is floating.Plus the battery stack of the negative rail is upside down too, it should have it's positive terminal grounded, and it's negative terminal input to the 7905 regulator.

author: asgard
author_rank: Regular Contributor
timestamp: May 19, 2013, 05:07:37 am
content: Oh, do be kindTry this on for size.

author: codeboy2k
author_rank: Super Contributor
timestamp: May 19, 2013, 08:22:28 pm
content: much better

author: asgard
author_rank: Regular Contributor
timestamp: May 19, 2013, 11:09:34 pm
content: Here is a board layout which I have worked out, but I hoped that Dave or somebody can check me on the functional parts of this whole thing. Mostly, I wanted to get verification that I have the right idea on the NFET switch for reducing the gain for a period when the thresholds are reached in the window detector.This is a 37mm x 60mm board. The flood pours are turned off so the bottom side can be seen better.

author: Hideki
author_rank: Frequent Contributor
timestamp: May 20, 2013, 12:22:07 am
content: I wouldn't call this a de-esser. It doesn't even have a highpass or bandpass filter to pick out the S sounds, so it will turn down the volume of everything above the threshold.Are you sure the time it takes to switch is slow enough to avoid clicks or pops?The OPA134 output will have a strong negative DC offset, since R14 goes to the negative supply. Maybe your weird offset adjustment is meant to counteract that? I doubt it will work.Instead of the MOSFET, you may want to use a JFET to ground, like you see in many dynamics processors.The voltage regulators aren't really required, unless some of the opamps can't handle 18 volts.You connected the input signal to the + input of both comparators, so this won't work. Watch Dave's video againLM393 has an open collector output, but you haven't added a pull-up resistor for the outside window comparator.The microphone input isn't going to work either. IN1 and IN2 are shorted to the same node and have no ground reference.It would be helpful if all the opamps looked like opamps in the schematic, instead of just being rectangles.

author: asgard
author_rank: Regular Contributor
timestamp: May 20, 2013, 07:44:57 am
content: Here is a respin. I have gone into the silly default Eagle libraries and fixed up the components for the Microchip and TI opamps. Instead of tying the OPA134 to -5V the tie is to the ground reference, so the nulling offset trimmer should be more effective. I have corrected the oversight about the comparator inputs. I have also respun for a quad device instead of two dual devices. Makes the layout much better. The regulators are still needed (for now) as the calculation is for the window based on splitting the voltage rails to +/- 2.5V. One of the things I want to do in the future is to use a charge pump to produce a -3.3V supply split off the BLiVIT (P01-109 - see next July Elektor for that) output of 3.3V and recalculate the window comparator dividers. Then I can eliminate the linear regulators and run off a wireless LiFePO4 cell. I will think some more about the output pull-up. The speed of the output is only about 200-300KHz but an RC stage in front of the MOSFET gate might be able to ease up the speed of the switch. I am guessing a 100ms ramp to the gate will be enough.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 20, 2013, 09:03:21 am
content: Quote from: Hideki on May 20, 2013, 12:22:07 amA "de-esser"?I assume you are referring to a sibilant reduction circuit?

author: Hideki
author_rank: Frequent Contributor
timestamp: May 20, 2013, 09:27:26 am
content: Yes. Like it or not, a device that does de-essing is often called a de-esserSome background info:http://en.wikipedia.org/wiki/De-essingGood old analog version:http://spl.info/index.php?id=188&L=1Plugins are more common these days:http://www.waves.com/Content.aspx?id=325

author: asgard
author_rank: Regular Contributor
timestamp: May 21, 2013, 04:35:11 am
content: I have run some more numbers, which for one thing, allow me to eliminate the regulators and the comparator resistor divider. The new values for R3, R5 is 1K, while R4, R6 is 7.2K. This puts the window setpoints to 1.1V. The typical electret mic AC excursion is about 50mV, which when fed through the pre-amp stage with gain 23 should result in about 1.15V swings at the comparator inputs. I have managed to lay out the whole works into about 40mm x 40 mm board area, although I use 50x50 so the usual logos etc will fit.

author: asgard
author_rank: Regular Contributor
timestamp: May 22, 2013, 08:16:26 pm
content: Here is a 3D realization of the circuit. I managed to get it all in a 40mm x 40mm board after all.

author: Hideki
author_rank: Frequent Contributor
timestamp: May 22, 2013, 08:50:04 pm
content: Yes. it looks good in 3D and all that, but shouldn't you try to design a working circuit before finishing the layout?

author: asgard
author_rank: Regular Contributor
timestamp: May 22, 2013, 08:52:42 pm
content: Well, of course I am, but since I have the tools lying around...