How can I Double 24VDC? - Page 1
Forum section: electronics / beginners

author: lesabb
author_rank: Contributor
timestamp: March 26, 2015, 07:15:04 pm
content: Hello all, I came across some cheap 24VDC switching supplies like the one on the picture. What I need is to know if the 24V out can be doubled to feed phantom power to microphone and use the 24V to power the preamp circuitry? I don't think I can use the usual rectifier/doubler scheme here? Any suggestions? The mic will draw ~20mA and the pre-amp about 200mA.L

author: TimFox
author_rank: Super Contributor
timestamp: March 26, 2015, 07:20:21 pm
content: No easy way to double the DC output, but if they are cheap enough you could connect two in series and drive the 24V preamp from the connection between the two devices.I have a severe prejudice against using switching power supplies on low-noise analog circuits such as microphone preamps. It is possible to filter the switching garbage from the outputs, but that is a separate project that needs careful attention.

author: lesabb
author_rank: Contributor
timestamp: March 26, 2015, 07:31:09 pm
content: I had no idea they had more garbage coming out than linear supplies?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: March 26, 2015, 07:32:12 pm
content: That power supply looks like it could be used for spot welding, way too much for such a small load. You sure the supply will work with such a small load?"I had no idea they had more garbage coming out than linear supplies?"Yes, almost guaranteed by design.

author: TimFox
author_rank: Super Contributor
timestamp: March 26, 2015, 07:33:24 pm
content: Look at the DC output with an AC-coupled oscilloscope. Switching power supplies need to switch at relatively high frequencies (10 to 100 kHz typical) and there will probably be spikes on the DC related to the switching waveform.

author: Zero999
author_rank: Super Contributor
timestamp: March 26, 2015, 08:40:25 pm
content: A boost converter or charge pump would work but it needs to be extremely well filtered.http://www.ti.com/lit/ds/symlink/lm5002.pdf

author: lesabb
author_rank: Contributor
timestamp: March 27, 2015, 02:58:32 pm
content: Hero, thanks a lot! This is what I was hoping for. So looking at figure 23 of the data sheet, what would need to be "well" filtered? Extra Caps on the out?L

author: Monkeh
author_rank: Super Contributor
timestamp: March 27, 2015, 03:01:36 pm
content: By the time you're done farting around with that huge lump, a boost converter, and filtering, you may as well use a small transformer.

author: katzohki
author_rank: Frequent Contributor
timestamp: March 27, 2015, 03:04:37 pm
content: Yes, caps on the output. As others have mentioned, switchers are very high in noise.

author: Seekonk
author_rank: Super Contributor
timestamp: March 27, 2015, 03:05:15 pm
content: Seems you were posting before about copying another design and are not in the US. Too bad, I have three 48V wall warts I would let go for cheap.

author: flynwill
author_rank: Regular Contributor
timestamp: March 27, 2015, 03:16:11 pm
content: Or just buy an inexpensive Mic preamp that will do everything (Including the phantom power for the mic) all in one box.Alternately, even though 48 Volts is the normal spec, a lot of phantom powered microphones will work fine on much lower voltages -- you might try powering your mic from 24 Volts and see what happens.

author: wraper
author_rank: Unknown
timestamp: March 27, 2015, 03:21:18 pm
content: IMO it is easier to just use another PSU or better to build a linear PSU instead.

author: Cliff Matthews
author_rank: Unknown
timestamp: March 27, 2015, 05:07:04 pm
content: Try a 48vdc wall wart. At this voltage, look for those used on home voip phones (where users are less likely to have a PoE switch).About $15 - if you search you'll find - eg:http://www.amazon.com/UpBright%C2%AE-Global-Adapter-Phones-Aironet/dp/B00N2JIEXQhttp://www.allnt.ca/21982-Nortel-11XX-12XX-200X-Power-Supply.htmlFrom the output, add a separate current limit resistor (2.2k??) with basic L/C filter if you hear any SMPS artifacts.Transformer type (bigger, heavier, but still very cheap - $14 from ebay)http://www.pcliquidations.com/p11864-telrad-avanti-connegy-iphttp://www.ebay.ca/itm/like/201234872706?clk_rvr_id=804271102192&item=201234872706&lgeo=1&vectorid=229529&rmvSB=trueFor the pre-amp, drop 10 to 15v with a 5w zener diode and let a heat-sinked LM7824 do the rest (with a 2200uf/35v capacitor on the output to keep the audio loose and punchy).