12v DC to 12V AC  - Page 1
Forum section: electronics / beginners

author: .....
author_rank: Contributor
timestamp: August 19, 2018, 07:18:43 pm
content: i am building and amp project where the preamps need 12v ac and everything else needs 12v dcis there any cheap way for me to convert the 12vDC to the 12vAC required fro the amplifiersi have some 12vDC to 110V DC el wire inverters that i could use to get the power to AC but i dont know how to get it back down to 12V.Jack

author: Ian.M
author_rank: Super Contributor
timestamp: August 19, 2018, 08:27:31 pm
content: No. The odds are the diodes in the preamp PSUs aren't suitable for HF use so you cant simply chop it into a squarewave at >30KHz, + the AC supply may need one side ground referenced, so you cant drive it as a bridge tied load from a H bridge to get the negative half cycles.You also cant generate a 50Hz 12V RMS sinewave to keep the diodes happy as that would need 17V supply rails to get the peak voltage.Therefore you'll need an inverter circuit with a transformer. You cant easily get a self-oscillating resonant Royer ciruit to work at such a low frequency so will need a full inverter controller.It would be much easier to hack the preamps to use + and - DC in & Gnd, then use a boost converter to get 16V to feed their + rail and an inverting buck converter to feed their - rail with 16V (assuming approx 1V of diode drop in their existing PSU circuit).

author: .....
author_rank: Contributor
timestamp: August 19, 2018, 08:32:22 pm
content: thanks for the reply how would i go about converting the preamp to run on dc power instead of acjack

author: .....
author_rank: Contributor
timestamp: August 19, 2018, 08:34:32 pm
content: here is a link to the page that i bought the preamp from there is a picture of the unpopulated board on picture 6https://www.ebay.co.uk/itm/AC-12V-6J1-Valve-Pre-amp-Tube-PreAmplifier-Board-Headphone-Amplifier-Case/292651946401?ssPageName=STRK%3AMEBIDX%3AIT&var=591452656946&_trksid=p2057872.m2749.l2649jack

author: ArthurDent
author_rank: Super Contributor
timestamp: August 19, 2018, 08:35:47 pm
content: If you don't need this to run free of mains power the simpler thing would be to first use a mains to 12VAC transformer for the preamps then next rectify/filter the transformer output to also get 12VDC, or use a seperate 12VDC supply. You may need a regulator as well to stabilize the 12VDC but without knowing more about the application no one can guess.Using the EL drivers are not a good idea.

author: .....
author_rank: Contributor
timestamp: August 19, 2018, 08:39:36 pm
content: im already running 2 12v power supplies into the amp box (one for the cooling system and one for the amps/ premaps) so d rather not use a 3rd ac power supply but it seems like the best option where would you recommend i look to to by onejack

author: Ian.M
author_rank: Super Contributor
timestamp: August 19, 2018, 08:42:27 pm
content: Quote from: ..... on August 19, 2018, 08:32:22 pmOpen it, and sketch a schematic for its PSU circuit as far as the reservoir capacitors so you know how it works. Power it from 12V AC and check voltages, then feed the + and - unregulated rails DC of the correct voltages at the reservoir capacitors. As its a valve amp it may not use a negative supply, and is likely to have a voltage doubler or multiplier circuit to get a HT rail >20V.N.B. it will also need 12V (actually 12.6V) for the valve filament supply.

author: Zero999
author_rank: Super Contributor
timestamp: August 19, 2018, 08:43:52 pm
content: How much current does it draw?Do you have a schematic?It'll probably work off a square wave, which is fairly easy to generate.Don't forget that rectifying 12VAC from a transformer will generate a much higher voltage than 12VDC, a minimum of 15.8V, but probably more like 20V, as small transformers output a significantly higher voltage, when lightly loaded.

author: Ian.M
author_rank: Super Contributor
timestamp: August 19, 2018, 08:46:54 pm
content: It will probably *work* off a squarewave but you are likely to get excessive noise breakthrough of the switching frequency and its odd harmonics as most valve amps don't have great PSRR.

author: .....
author_rank: Contributor
timestamp: August 19, 2018, 08:49:23 pm
content: unfortunately i dont have a schematic i also dont know the current draw do you think it will be easier to buy or built a 12v ac power supplyjack

author: ArthurDent
author_rank: Super Contributor
timestamp: August 19, 2018, 09:00:36 pm
content: It sounds like you already have the 12VDC power supplies for the rest of the system so all you need is the transformer that is rated 12VAC / 1A. They make ones that plug into a wall outlet just like a D.C. power adapter. Check where you bought the amp, they must sell suitable transformers as well.

author: .....
author_rank: Contributor
timestamp: August 19, 2018, 09:05:05 pm
content: thanks for all of the replies i have decided that i am going to buy a 12vac psu that plugs into the mains and just use it for the preampsjack

author: Zero999
author_rank: Super Contributor
timestamp: August 19, 2018, 09:45:09 pm
content: Quote from: Ian.M on August 19, 2018, 08:46:54 pmYes that's a possibility. It can be mitigated somewhat by keeping the rise/fall times reasonably slow, at the cost of efficiency, but I agree, it's a risk.Quote from: ..... on August 19, 2018, 09:05:05 pmYes, all you need is a transformer with a 12V output: it's certainly simpler and better than building a 12VDC to 12VAC inverter.

author: Audioguru
author_rank: Super Contributor
timestamp: August 19, 2018, 09:53:04 pm
content: The no name-brand antique preamp (old vacuum tubes?) has no audio spec's.Its ebay ad says, "It will bring you an unexpected surprise!" Hiss, hum and distortion?

author: james_s
author_rank: Super Contributor
timestamp: August 20, 2018, 03:27:19 am
content: Why do you have all these separate power supplies? You only need one, just a single 12VAC power supply and then a rectifier and regulator for each DC output you want. There is no advantage to having multiple separate supplies, that's just pointless.

author: Ian.M
author_rank: Super Contributor
timestamp: August 20, 2018, 04:10:44 am
content: Quote from: Hero999 on August 19, 2018, 09:45:09 pmIf one's going down the squarewave route, it's probably worth replacing the rectifier diodes in the preamp on-board PSU circuit with fast enough ones to work efficiently at 50KHz to get the switching noise and any of its intermodulation products with the audio signal, well above the audio frequency range. It will probably also need ceramics added across the reservoir caps, if they are electrolytic, to get the impedance @50KHz low enough.Quote from: james_s on August 20, 2018, 03:27:19 amYes, though it may be worth using a boost-buck non-isolated DC-DC converter rather than a linear regulator, with *just* enough bulk capacitance on its input to keep it running through the zero crossings at full load. That will provide secondary side power factor correction and make *far* better use of the transformer's VA rating.

author: james_s
author_rank: Super Contributor
timestamp: August 20, 2018, 04:14:37 am
content: If someone is using a tube amp I would guess they're far enough down the audiophile rabbit hole that a switching regulator is probably off the table. As long as the power level is modest there's nothing wrong with a linear regulator, the wasted energy is probably less than that consumed by the tube heaters.

author: Brumby
author_rank: Unknown
timestamp: August 20, 2018, 07:05:45 am
content: Looking at the silk screen of the underside of the unpopulated PCB, you can see three pairs of pads labelled -20V, +20V and 12.6V.Seems to fit what people have been saying...

author: ArthurDent
author_rank: Super Contributor
timestamp: August 20, 2018, 01:19:58 pm
content: My personal opinion is that all these tube-type preamps benefit the seller much more than the end user. If you check Ebay (US) you will find over 500 listings for this type of low-end preamp. Some designs I’ve seen just have the filaments of the tubes powered so they glow but have a simple transistor circuit to do the actual work, a lot require 12VAC because that is required for the cheap voltage multiplier circuit to get about 28VDC they use for the plate supply and that multiplier won’t work on DC. There are a few that use a DC-DC converter to get about 100VDC for the plates of the tubes and I’d consider these as better, but not good designs. I doubt that you’ll find any of these cheapy designs that have the equalization required for different input devices. The reason most do not include a power supply is that shifts the cost to you and makes for a much lighter package to ship from China.Most of these will kinda work and you can convince yourself that they sound great but if they are checked using audio test equipment I think they will be found lacking. I believe that new MOSFET amps can be made to duplicate the previous tube circuits for softer sounds without the ‘sharp edges’ of the bipolar junction transistor designs and I’d be looking for those designs. As to these cheap amps, you are probably getting what you pay for and a glowing vacuum tube (valve) looks great.

author: james_s
author_rank: Super Contributor
timestamp: August 20, 2018, 03:41:11 pm
content: Oh I agree, but that's beside the point.Tubes are pretty neat, I mean all else aside they look pretty cool. Just because something has a tube in it doesn't mean it's any good though.