Gainclone amplifier improvements - Page 1
Forum section: electronics / projects

author: clockbiscuit
author_rank: Contributor
timestamp: September 27, 2016, 02:24:28 am
content: Ahoy EEVblag community members!I have built a stereo amplifier from kitset parts, it is nothing special at all really - uses two of the monolithic amplifier chips (LM3876) for a 50W rated stereo output. This is coupled with a Jaycar preamplifier kit module. Nothing ground-breaking and probably done to death. But I have a couple of issues with this arrangement that I am hoping someone can help me with because surely many of you have stumbled into the exact same problem at some point.I get significant interference (line buzz) introduced through the preamplifier power cable. This is a 12V line running from the power amplifier board, it is noticeably louder when the cable is pulled near to the toroidal transformer powering the amp. My current fix is to tack this cable up and away from the toroid, but this is pretty "how ya doing" and I would prefer to have some kind of actual decoupling. I cant seem to measure this noise using an oscilloscope but I guess it is going to change both the 12v power and ground line levels? The preamp itself has several filter capacitors on the 12V input, and I can only assume the module was designed by someone more competent than myself so perhaps this is not the right approach to take. I was thinking maybe I want some sort of ferrite choke... but would this work? how do I design for either a ferrite or a capacitor if i cannot see the noise I am trying to eliminate?Also - I get a pretty bad pop when turning the system on. I have been thinking about experimenting with the LM3876 mute pin... but I am not sure if this pin acts in the right part of the circuit internally to the LM3876. Any good advice on this would be greatly appreciated.Regards

author: Berni
author_rank: Super Contributor
timestamp: September 27, 2016, 05:37:14 am
content: D class amplifiers have worse supply noise rejection than AB class so they tend to need a clean supply voltage. The way you connect grounds in amplifiers is usually pretty important in order to keep the noise away.As for turn on pop, there is supposed to be a capacitor on its mute pin for that very reason. It takes a while for it to charge up via the resistor so it keeps the amplifier muted for a short period after turn on. Some amplifiers can even have turn off pops if the supply ramps down in a way it does not like, those are harder to fix.

author: clockbiscuit
author_rank: Contributor
timestamp: September 27, 2016, 07:24:14 am
content: Berni, the grounding is pretty decent and i have also tried running some thick straps to items that were not explicitly re-grounded just to try and see if it would make any difference. Unfortunately it didn't.There is a cap on the mute pin in the design i followed. The schematic looks like this:http://www.dumpt.com/img/viewer.php?file=pwt3bkq4edxueuzdas9f.jpg(cant seem to get uploads to work here?)Mute is on pin 8 and mute should be off when that pin is pulled to the amplifier negative rail voltage - you replace link LK1 with a switch. The 22uF capacitor should need to charge up when power is applied, perhaps this is not taking long enough. I guess I should be able to measure this with the digital scope on single-shot, but isnt the mute activated by current draw not voltage? could be tricky to know when it is actually being activated...

author: Berni
author_rank: Super Contributor
timestamp: September 27, 2016, 10:21:24 am
content: Try checking the voltage on your power rail with a scope or if you don't have one a multimeter set to AC volts.And yes according to the datasheet the mute pin is current activated, but it needs some voltage before it starts to conduct current out of it (About 2.6V) so the capacitor to ground delays it. You may need to increase the value of that capacitor if you have the amp configured to have a frequency range that reaches deep in to the low frequencies since it takes longer to settle in. As a quick test disconnect the mute pin, power it up and then manually connect it after a second or so and there should be no pop, perhaps there might still be a slight click.

author: bobaruni
author_rank: Regular Contributor
timestamp: September 27, 2016, 01:29:37 pm
content: Quote from: clockbiscuit on September 27, 2016, 02:24:28 amPutting the power supply or input cable of a pre-amplifier near the power transformer will always introduce hum.Tucking the offending cable away is a good idea, shortening it if possible and using a thick gauge wire might help.Sometimes the orientation of a wire can null out the hum, try mounting the wire on a different angle to null the hum out.Also, best practice is to use a common ground or star formation for grounding everything at a single point to prevent hum loops due to eddy currents.Try putting small capacitors (maybe 10nF MKT or greencap 100V) across each diode in the bridge rectifier, this will prevent some diode switching noise being radiated by the toroid.Read more info about reducing diode switching noise herehttp://www.hagtech.com/pdf/snubber.pdfIt's very hard to diagnose without hearing it and seeing the schematic and how you have wired and laid it out.

author: Kevman
author_rank: Regular Contributor
timestamp: September 27, 2016, 01:41:33 pm
content: The LM3876 is not Class D.What's the preamp for? A Microphone? The LM3876 should have plenty of gain alone for any line level uses.Microphone preamps, though, will be very sensitive by nature. And what's wrong with fixing the issue with rerouting? Better to not introduce noise at all then to try and filter it out once its already there, no?

author: Berni
author_rank: Super Contributor
timestamp: September 27, 2016, 03:44:37 pm
content: Quote from: Kevman on September 27, 2016, 01:41:33 pmSorry about that i got mislead that googles number one hit for the partnumber is Texases website that has " Analog Input Class-D | High-Power Audio Amplifiers" in the title. Had another look at the datasheet and it is indeed an AB class.

author: Audioguru
author_rank: Super Contributor
timestamp: September 28, 2016, 02:54:20 am
content: You have an LM3876 but you are using the click and pop muting parts for a different LM3886 amplifier.You should download the datasheet for the amplifier you are using and follow it.