Speaker POP in class-d system design - Page 1
Forum section: electronics / projects

author: SpiderElectronics
author_rank: Contributor
timestamp: April 17, 2012, 11:02:38 pm
content: HiI have designed a 20+20W audio amp based around the Texas TPA3123D2http://www.ti.com/lit/ds/slos541c/slos541c.pdfClass D amplifier chip.The chip has logic inputs for MUTE and SHUTDOWN.If I connect the speakers with relatively short cables - say 1 or 2 metres, then the thing works beautifully. However, if I use long (say 20m) cables then there is an annoying POP when the chip comes out of SHUTDOWN.I have tried keeping the MUTE asserted until a while after the SHUTDOWN is removed, but that makes no difference - mainly because in this CLASS-D design the 'Mute' control actually just puts the switching push pull mosfets into a 50-50 duty cycle rather than turning them off.Can anyone suggest why this should be, or have any ideas for a way to get rid of the POP?

author: Lightages
author_rank: Unknown
timestamp: April 18, 2012, 01:28:12 am
content: Really? 20 metres? That is a very long run. The problem with such long runs of speaker wire is that you can end up having a very reactive load in the cables. This reactance is probably causing your "pop". What gauge wire are you using? For 20 metres it should be no less than 10 gauge I would think.

author: chscholz
author_rank: Regular Contributor
timestamp: April 18, 2012, 04:33:36 am
content: Not my field of expertise by a long-shot.It appears to me as is some folks filter pops out in DSP. I helped customers on occasions to build math functions on a LeCroy WaverRunner oscilloscope that implements an A-Weighted filter. Such filter, as I understand it, do just that, filter out pops.Quote from: SpiderElectronics on April 17, 2012, 11:02:38 pm

author: SpiderElectronics
author_rank: Contributor
timestamp: April 18, 2012, 08:15:24 am
content: Thanks for the replies.For the cable gauge - I'm not sure - this report came from an installer - but I have asked him the gauge of cable being used.As for DSP, well there is no DSP anywhere in this system and it happens when asserting/removing the SHUTDOWN signal to the amp.The cables runs are long because the main unit goes in a closet and gets wired out to the whole house. It is a multi source/multi room system making a 4 x 4 matrix.

author: amyk
author_rank: Super Contributor
timestamp: April 18, 2012, 11:01:01 am
content: Curious how the datasheet says"Advanced Power-Off Pop Reduction"and"The patented start-up and shut-down sequences minimize pop noise in the speakers without additional circuitry"Have you tried measuring the outputs with a scope when turning on, both at the amp outputs and at the speakers? How about slowly ramping up the supply voltage when turning on?

author: SpiderElectronics
author_rank: Contributor
timestamp: April 18, 2012, 02:59:48 pm
content: Interestingly... There is very little pop at power-off and power-on, but significant POP when asserting/deasserting the SHUTDOWN logic input.I don't have the ability to change the circuit layout at the moment, as it's in production, so I can't add relays to the speakers connections, or control the ramp up of the main power.

author: Harvs
author_rank: Super Contributor
timestamp: April 20, 2012, 11:58:17 am
content: A typical class AB power amp uses an air core inductor to separate it from speaker cable capacitance at high frequencies, then a zobel network before the inductor to present the amp output a stable hf load to stop it going into oscillation.There's nothing in the datasheet with respect to long speaker cables, which makes me think that they assumed it wouldn't be connected via long cables.Either way, the speaker cable will present a capacitive load to output of the filter. I assume you're using the suggested single ended design in the app section?Have you seen that the real world power output looks to only be about 10W into 4R? The 20W into 4R is with 10%THD, which is an absolute joke. 1% THD doesn't take golden ears to hear the distortion.

author: T4P
author_rank: Super Contributor
timestamp: April 22, 2012, 02:36:21 am
content: Quote from: Harvs on April 20, 2012, 11:58:17 amActually , it does .

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 22, 2012, 04:26:28 am
content: Quote from: Harvs on April 20, 2012, 11:58:17 amNowhere was it mentioned that it was designed for high definition. In an automotive application (for example), the ambient noise would make it impossible to tell the difference between 128k MP3 and FLAC. Even for home use, that chip would probably do surprisingly well since at reasonable listening levels, the RMS power to the speakers is well below 1 watt. Also note that the noise power itself tells little about how objectionable it is - noise that is well spread out (white noise) is far more tolerable than a tone at 2kHz even if the total power is the same.Of course, if you do want HD, it would be a better idea to use some of the newer chips.

author: T4P
author_rank: Super Contributor
timestamp: April 22, 2012, 08:36:25 am
content: Quote from: NiHaoMike on April 22, 2012, 04:26:28 amAnd not some old topology chip , Harvs .

author: Harvs
author_rank: Super Contributor
timestamp: April 22, 2012, 08:37:05 am
content: My apologies, the last part of my post was carelessly worded (I was tired...)I was not trying to say 1% THD+N was too high a distortion level for whatever application this is going to be used for. In fact it may well be very reasonable for say surround sound in the home. Also the last thing I want to do is start a debate about what is audible or not.What I was trying to get at, was the IC manufacturer is possibly being a bit generous putting 20W per channel without putting the 10% THD next to it (which you only find when you read into the AC specs or view the charts.)

author: T4P
author_rank: Super Contributor
timestamp: April 22, 2012, 10:31:51 am
content: That was a practice for new chips of that era and until now , whether class d or not . Actually there are no more Class AB chips that are new because that architecture is just far too old and inefficient already .