variable AC power supply from D-class amplifier - thoughts ? - Page 1
Forum section: electronics / beginners

author: xani
author_rank: Frequent Contributor
timestamp: April 21, 2020, 04:38:08 pm
content: So I'm in need of small variable AC power supply (or rather I'm tired of just digging random transformers out of the parts bin every time I need to test something) and I had a thought - the cheapo $8 D-class amplifier modules basically have everything required. All it needs is signal source and maybe some feedback if I will eventually want to have current limiting on it. Is there anything obvious that I'm missing ?

author: Shock
author_rank: Super Contributor
timestamp: April 21, 2020, 04:40:06 pm
content: A nice set of matching impedance loudspeakers?

author: bob91343
author_rank: Super Contributor
timestamp: April 21, 2020, 06:18:04 pm
content: Many years ago I needed to test some equipment over the power line range from 50 - 400 Hz. We didn't have a proper source so I dug out an old 300 Watt Kenwood stereo amplifier and drove it with an audio oscillator and used a step up transformer at the output.It worked great. One of my colleagues freaked out and said he was angry that he didn't think of that first.

author: xani
author_rank: Frequent Contributor
timestamp: April 21, 2020, 08:37:00 pm
content: As extra it makes it easy to inject noise into power supply and test its performance

author: ArthurDent
author_rank: Super Contributor
timestamp: April 21, 2020, 10:37:33 pm
content: A few years ago I bought a variable frequency A.C. supply cheap because no one else bid on it. It's good from 0-240 volts, 50-400 Hz, 200VA, and voltage regulated. Nice small self contained bench type supply.

author: 16bitanalogue
author_rank: Regular Contributor
timestamp: April 21, 2020, 10:43:32 pm
content: If you don't need the bandwidth. The LC filters will certainly greatly reduce what's beyond the audio range.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: April 22, 2020, 09:01:51 am
content: Quote from: xani on April 21, 2020, 04:38:08 pmi'm (ab)using a QUAD 405 as variable AC power source, in conjunction with a programmable signal generator, but would prefer the solution you are suggesting in terms of simplicity.however, two things come to mind that may spoil the party:1. what happens in case of a failure in that AC source? for example, will the D.U.T. survive an eventual DC input in that case?2. Class D half bridge topologies can be prone to overvoltage failures due to something called "bus pumping". what about the modules you have in mind? can they handle it?nice idea, though. keep us posted!

author: xani
author_rank: Frequent Contributor
timestamp: April 22, 2020, 10:02:29 am
content: Quote from: Le_Bassiste on April 22, 2020, 09:01:51 amDUTs are pretty much "analog circuits accepting AC because they use symmetric supply", so probably ?. I did thought about eventually adding 1:1 transformer at the end but that's maybe in second iteration. The chip (supposedly) used by module is TPA3116D2 and it does have DC voltage protection.Of course, seller lies about its output power ("150W" when chip's datasheet says 100W max in mono to 2 ohm load) but I only need way less power than that.Quotefull bridge, and at least from datasheet is has plenty in terms of protection (DC voltage, short circuit, temperature)QuoteI did some testing with random D amp I had (only 15W so not enough voltage for my needs) and it seemed to work reasonably well. My plan is to roughly calibrate gain, drop it in a box with some micro to generate the signal. Still wondering whether I should go to an effort of adding a current limiting or just set it on my bench power supply and hope for best.

author: David Hess
author_rank: Super Contributor
timestamp: April 22, 2020, 08:16:05 pm
content: Quote from: xani on April 21, 2020, 04:38:08 pmIt also requires an LC output filter if not already present.