Filterless class D amplifier - does it need filter? - Page 1
Forum section: electronics / beginners

author: rosko
author_rank: Newbie
timestamp: July 30, 2014, 05:28:35 am
content: This is my first post here, so - Hello everybody!I want to make neckloop, which uses piece of wire and power resistor, not actually a speaker(which inductance is big than that on the neckloop's piece of wire).So the amp is thisTEXAS INSTRUMENTS - TPA2005D1DGNor thisTEXAS INSTRUMENTS - LM4667MMThese two are available on my local store.Does it need filter on the output, or the internal oscillator will do the work for me and compensate?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 30, 2014, 05:40:39 am
content: Neckloop, for driving hearing aids? Not sure a class D amp would be a good idea there...For radio interference purposes, I can't imagine it'll do very well without a filter though. Walk past an AM radio, and your private music, now all distorted and gross, blasts through the speaker...Tim

author: rosko
author_rank: Newbie
timestamp: July 30, 2014, 04:57:46 pm
content: Yes, exactly for hearing aids.So what you'll suggest - to add filter or to walk around for class AB?And can you recommend some wattage, I think that ~800mW will do the job for my neckloop project, but don't know..

author: David_AVD
author_rank: Super Contributor
timestamp: July 30, 2014, 09:18:04 pm
content: I think you'll have to do some experimenting with this project. Is this a one-off ?

author: rosko
author_rank: Newbie
timestamp: July 31, 2014, 04:49:26 am
content: Probably not, but if I fail, it'll be one-off projectThink that class D will be good idea, because they don't have so many THD as class AB, but designing an output filter will cost more money and I think it'll have power loss in it, which don't justify class D for my project.Thanks!

author: David_AVD
author_rank: Super Contributor
timestamp: July 31, 2014, 06:36:58 am
content: I suspect that a lot of amplifier chips won't be happy driving into what's essentially a short circuit (the neck loop).Getting really low THD figures shouldn't be really high on your list. Getting a reasonable frequency response out of the neck loop is more important.

author: rosko
author_rank: Newbie
timestamp: July 31, 2014, 07:45:49 am
content: No, there's no short circuit, it'll have a power resistor in series with the wire, otherwise there's no amp I think that can drive 0.01 ohm or so load..

author: David_AVD
author_rank: Super Contributor
timestamp: July 31, 2014, 09:04:59 am
content: What you want is current drive, not voltage drive.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 31, 2014, 09:35:30 am
content: Quote from: David_AVD on July 31, 2014, 09:04:59 amNot necessarily. The active bits are probably sensitive to EMF. Although, not that you'll get much EMF out of a couple turns of wire, at low frequency: DCR will dominate (and it will be current driven after all!).At any rate, the difference is merely an equalization, tedious but not really a fundamental problem kinda thing..Tim

author: SeanB
author_rank: Super Contributor
timestamp: July 31, 2014, 07:44:32 pm
content: You probably will be able to get by with well under 1W of drive in a class AB amplifier, and you can use a low supply voltage as well with a low impedance. Class D will have bad EMI issues with hearing aids in any case. Most of the hearing aids are fine with a low power input from a phone voice coil, and this is in most cases under 100mW of power as it is driven by the line side. You could even use a standard opamp as driver with a little care. I have seen some that used a 741 as driver with little else.

author: rosko
author_rank: Newbie
timestamp: July 31, 2014, 08:13:15 pm
content: Ok, thank you very much for the straight-forward answer.I will probably go for class AB, think that ~200 mAh Li-Ion battery will do the job for around 3 hours , but that's another story...Now another question (little aside of the topic) comes and it is - what and how long piece of wire I need ?

author: SeanB
author_rank: Super Contributor
timestamp: July 31, 2014, 08:35:11 pm
content: You probably will need around 10-20 turns on the neck loop, so either you will have to have a multipole connector somewhere or make a loop that fits over your head. Flexible multicore 18AWG cable with 12 cores used for alarm wiring will probably be a good initial try. 12 cores will be a good first try.

author: VK5RC
author_rank: Unknown
timestamp: August 01, 2014, 12:54:32 am
content: The loop will work BUT impedance matching the loop to the power amp will be the problem, search magnetic loop antennas, you also get higher voltages than usual and for a personal device tricky. I recall D type are very dependant on the impedance match of the load, quite often the load is the filter, the person's neck will change the loading significantly I would suspect.

author: rosko
author_rank: Newbie
timestamp: August 01, 2014, 05:29:21 am
content: Here are shown somechineese neckloopswhich work and only use one turn around the neck, but the cable is multicore insulated inside from each other. Does this matter?Ok, without designing one and trying it on my own, nothing will happen.Thanks for the replies guys!

author: jakeypoo
author_rank: Regular Contributor
timestamp: August 01, 2014, 01:48:20 pm
content: Class D amps are preferred in battery powered circuits because they will draw less power.As for your filtering question, You can usually get away with a ferrite + capacitor to squash the nasty HF. Much less expensive and smaller than a LC filter.Lastly, I gotta agree with everyone else, use multiple loops. You could use something like speaker wire and make a few loops, or use a multi-conductor cable and wire it up so it forms a multi-turn loop.

author: bgsteiner
author_rank: Contributor
timestamp: August 02, 2014, 12:48:32 am
content: What about a class T I have one that has a 15W output that only draws about 200ma @ 12v at full power

author: rosko
author_rank: Newbie
timestamp: August 04, 2014, 05:11:45 am
content: Quote from: jakeypoo on August 01, 2014, 01:48:20 pmHm, so that's why chinese have that multi-core isolated each other cable... they've created multiple turns just with one cable.@bgsteiner - I will have a single cell Li-Ion battery @ 3.7v, so I must place an amp that is suitable for that voltage and current consumption...