Suggestions for converting 12V DC to 9 V AC 60 Hz - Page 1
Forum section: electronics / projects

author: EofN
author_rank: Contributor
timestamp: May 20, 2025, 04:46:28 pm
content: Greetings,As stated in the title, I'm looking for ideas on how to convert a 12V DC source into a 9V AC / 60Hz (approx) / 2 amp max, output.I'm wondering if this can be done with discreet components, or if there is an off the shelf IC that could be my starting point.Many thanks for your time in reading and / or responding to my first post here!

author: PGPG
author_rank: Frequent Contributor
timestamp: May 20, 2025, 05:33:29 pm
content: Give some more info:Do AC have to be sinus or can be square?How precise 60Hz have to be?Why you need AC (will not it be then converted back to DC)?

author: EofN
author_rank: Contributor
timestamp: May 20, 2025, 05:50:27 pm
content: The waveform does not have to be a perfect sinewave, but I would want to low pass filter any square components to make it round - ish. It is for powering vintage audio gear that requires AC.

author: EofN
author_rank: Contributor
timestamp: May 20, 2025, 05:52:27 pm
content: The waveform could be between 55 - 65 Hz, not precise.

author: EofN
author_rank: Contributor
timestamp: May 20, 2025, 05:56:18 pm
content: My initial design idea would be a simple oscillator, driving a simple transistor amp, with no regard to distortion, and possibly a low pass filter on the output. I don't want to end up with a massive heatsink, or use any expensive components or anything crazy like that.

author: EofN
author_rank: Contributor
timestamp: May 20, 2025, 05:59:25 pm
content: I've searched all over the internet, but have only been able to find standard transformers that go from 120V AC to 9V 2A. They cost about $20 if I buy them in bulk. I noticed that there are a glut of high amperage 12V DC power supplies, and thought it could be possible to use them with an add-on adapter.

author: Zero999
author_rank: Super Contributor
timestamp: May 20, 2025, 06:09:35 pm
content: Quote from: EofN on May 20, 2025, 05:50:27 pmI very much doubt the wave shape matters.Have you looked at the schematic? It probably just converts it to DC anyway. If it needs AC then it'll be to drive a voltage doubler.A 12VAC square wave has similar peak voltage as a 9V sine wave, just use an H-bridge and an oscillator.Here's a self-oscillating MOSFET H-bridge, complete with inrush limiting and inductive clamping, to protect the MOSFETs.Astable inverter MOSFET bridge.ascQuote from: EofN on May 20, 2025, 05:59:25 pmThat's not hugely expensive.The voltage probably isn't critical. It's fine to go with 10V and more amps is no problem. I found this in Digikey.https://www.digikey.com/en/products/detail/hammond-manufacturing/187D10/2360624

author: David Hess
author_rank: Super Contributor
timestamp: May 20, 2025, 06:10:06 pm
content: 9 volts AC is about 25 volts peak-to-peak, so the 12 volt source needs to be boosted as part of the process. This could take the form of a 60 Hz transformer on the output, but boosting the voltage earlier is probably better.1. Boost 12 volts DC to +/-14 (or whatever) volts DC.2. Convert the higher bipolar DC voltage to AC with an integrated class-D switching amplifier intended for audio.A bipolar supply is required to avoid using an AC coupling capacitor on the output, but maybe this is acceptable?

author: CaptDon
author_rank: Super Contributor
timestamp: May 20, 2025, 06:12:02 pm
content: 9 volts A.C. is kind of standard for many synthesizer keyboards. You should be able to find many power packs on Ebay for under $20. Going from 120VAC to 12VDC and then to 9VAC is too much crap and you won't do it cheaper than just buying a 120VAC/9VAC power brick. Your inverter circuit would need a bunch of filtering and so forth to keep from leaking switching noise into the audio chain. As for an off-the-shelf bare transformer, 9VAC is kind of an oddball. 6.3VAC and 12.6VAC are far more common. The primary is always wound next to the core so you may be able to find a 12.6VAC transformer and remove some winding turns off of the secondary to get what you want. A 9VAC 2Amp rated transformer will actually probably read around 10 to 10.5VAC open circuit.

author: bdunham7
author_rank: Super Contributor
timestamp: May 20, 2025, 06:14:03 pm
content: Quote from: EofN on May 20, 2025, 05:50:27 pmJust because the device is configured to use 9VAC doesn't mean it actually needs it. If it's PSU uses a voltage doubler configuration for a bipolar supply internally, then AC you must have. However, if is just using a full-wave bridge to create a single DC supply, then it may well work just fine on 12V of either polarity. What is the vintage audio gear? Does it happen to be a Thorens turntable?

author: EofN
author_rank: Contributor
timestamp: May 20, 2025, 06:18:34 pm
content: Thanks for the suggestions, however,- Anything other than 9V AC is completely out of the question.- power supplies that supply 9V AC at 2A are extremely rare and cannot be obtained cheaply. Please post a link if you know of one, I would be very happy to find one.

author: CaptDon
author_rank: Super Contributor
timestamp: May 20, 2025, 06:19:17 pm
content: Forget the voltage doubler stuff. A lot of this audio gear hooks one side of the 9VAC to chassis and uses the other side with a pair of diodes to obtain + and - 11 to 12 volts filtered D.C. which then 'may' be regulated back to anything from +/- 5vdc up to +/-9vdc used to power the audio opamp chain in bipolar fashion.

author: themadhippy
author_rank: Super Contributor
timestamp: May 20, 2025, 06:19:47 pm
content: QuoteIs a 240 to 18v tranny any cheaper?

author: EofN
author_rank: Contributor
timestamp: May 20, 2025, 06:24:47 pm
content: Thank you so much for your amazing information, I appreciate it very much.This looks like an interesting path to explore, I like the idea of using mosfets, perhaps with some tweaks here and there, like you said the waveform is not critical. Perhaps it can be tweaked for higher efficiency so that I can use smaller heat sinks.The device in question does indeed convert to 5V, 12V and -12V rails, using voltage doublers, which is why it has to be AC (the device cannot be modified internally)Quote from: Zero999 on May 20, 2025, 06:09:35 pm

author: bdunham7
author_rank: Super Contributor
timestamp: May 20, 2025, 06:28:28 pm
content: Quote from: EofN on May 20, 2025, 06:18:34 pmI'd suggest looking for older used stuff as they probably will be rare these days. Think Goodwill, not Walmart or eBay, not Amazon. I dion't have time to check my private stash at the moment but I might even have one somewhere. 1 to 1.5A will be more common, but this one is 25VA which means about 2.8A.https://www.ebay.com/itm/335178150196

author: CaptDon
author_rank: Super Contributor
timestamp: May 20, 2025, 06:29:16 pm
content: I forget if the keyboard is a Korg or a Yamaha but I bought exactly a 9VAC 2amp power brick for our keyboard player in our band. I got it from Ebay and it may have been $18 with tax and shipping. He is still using it and it is exactly what the keyboard called for. If I wanted to go cheap I would maybe still look for a 12.6VAC 2amp raw transformer. They are very common known as '25VA' size (12.6 X 2 = 25.2VA) and I would unwind a few turns off the secondary. You're just wasting time home brewing an inverter circuit. Are you a serious musician playing out for money? If so, don't bring anything unreliable to a gig. Equipment failures leave a lasting impression on the venue and your fans!! Is $20 or $30 really a problem to buy something off the shelf and rated for the job?

author: CaptDon
author_rank: Super Contributor
timestamp: May 20, 2025, 06:42:48 pm
content: Ebay GE 22-155 alarm transformer 9VAC 25VA $6.35 plus $9 shipping. I bet he would ship several for the same $9 total. Do you need the coaxial power plugs to plug into your audio gear? You will still be doing some soldering etc to finish the project.

author: EofN
author_rank: Contributor
timestamp: May 20, 2025, 06:47:20 pm
content: Yes, I do a lot of soldering! I assemble a lot of custom power connectors. Actually, I restored old audio gear of specific types, and have trouble finding these power supplies. I like the idea of the cheap alarm system supplies, except I don't like how they look, with the exposed terminals. Thanks again for the input!

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 20, 2025, 06:47:59 pm
content: Do you actually need 2A ?Digikey do a 1.3A one :https://www.digikey.co.uk/en/products/detail/ideal-power-ltd/77DA-12-09/15667498

author: EofN
author_rank: Contributor
timestamp: May 20, 2025, 06:49:25 pm
content: Yes, it must be 2A, I tried smaller ones like 1.5 and it gets super hot. And not in a good way.Quote from: mikeselectricstuff on May 20, 2025, 06:47:59 pm

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 20, 2025, 06:50:07 pm
content: UK source/voltage , but they do existhttps://poweradaptorsuk.co.uk/products/2000ma-2a-9v-18va-ac-ac-ac-output-mains-power-adaptor

author: BrianHG
author_rank: Super Contributor
timestamp: May 20, 2025, 06:52:48 pm
content: Quote from: EofN on May 20, 2025, 06:18:34 pmYour stuck if your output needs to be 9vac isolated.For 120vac to 9vac, a number of solutions are possible.For 12dc to 9vac, are you allowed to modulate the GND, IE: full differential output. There may be a disastrous looping ground conflict in your wiring is the 12v source isn't floating. Otherwise, you can only make around 4vac from a 12vdc power source.To make real 9vac from a DC source, you will need a power supply with +13v and -13v output, with a 2 amp capable class D amp, plus a reference 60hz sine wave.This is not for the beginner.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 20, 2025, 06:53:15 pm
content: https://www.ebay.com/itm/372230910429?ac to ac 9v 2 amp adapterhttps://www.ebay.com/itm/122413179451?just to be sure, ask the seller if they are really 9VAC not 9VDC

author: BrianHG
author_rank: Super Contributor
timestamp: May 20, 2025, 06:56:48 pm
content: Ohhh, now we are doing power supplies, ok, here are 3 amp 9vac supplies.And even better, for audio, they are split bobbin supplies:Atari 800 power supply

author: BrianHG
author_rank: Super Contributor
timestamp: May 20, 2025, 06:59:26 pm
content: Even cheaper:Cheaper Atari 800 power supplyJust make sure you are not buying the Atari XL series power supplies which have a DIN connector for their output. They are DC regulated supplies.

author: Buriedcode
author_rank: Super Contributor
timestamp: May 20, 2025, 07:12:04 pm
content: Quote from: David Hess on May 20, 2025, 06:10:06 pmThis is exactly how intended to do it for replacing the heavy digitech RP7 power supply. The prototype worked well, but shelved the project when life took over. As a bonus I made the first stage an isolated bipolar supply, to avoid ptential ground loops (most 12V plug in power supplies are floating anyway so shouldn't be necessary). The result isn't particularly efficient, but its smaller and most importantly - lighter - than the original transformer, with no appreciable increaes in noise on higher gain patches. Also can take between 12 and 24V in.The RP7 uses a mulitplier for generating a higher volage for the tubes, and whilst it does work with a square wave, it appears a lot harder on the caps, so I went for the more difficult sine wave generation istead of throwing a H bridge at it.

author: EofN
author_rank: Contributor
timestamp: May 20, 2025, 07:22:07 pm
content: Sounds like we had the same idea! Did your device produce a lot of heat?Quote from: Buriedcode on May 20, 2025, 07:12:04 pm

author: BrianHG
author_rank: Super Contributor
timestamp: May 20, 2025, 07:56:13 pm
content: Take the 60watt mono version of this amp:3 dollar bridged output amp.Power it with 13vdc. Expect some sine wave distortion with a 12v power supply.(Note that most switchers have an adjustment pot allowing +/-1v adjustment. Or use a 15v switcher.)Use a microcontroller with a built in DAC to generate a 60hz sine wave to the amp's input and you will get a 9vac output.

author: BrianHG
author_rank: Super Contributor
timestamp: May 20, 2025, 08:06:45 pm
content: Remember, be careful of the frame ground throughout your system. If your switcher is frame ground referenced to your AC outlet's ground, and the device you are powering uses the AC input with 1 lead tied to GND chassis GND (IE using the AC to make a +/- supply internally with 2 diodes and 2 caps), you will generate a short circuit as that tiny amp tries to swing both the + and - outputs from 0v to +12v. This can damage your equipment if any other grounds in your system are tied to any wall outlet's ground. You have been warned.USE a 100% isolated, floating source DC supply to be safe. No mains ground connection. Or use an isolated 12v battery.

author: EofN
author_rank: Contributor
timestamp: May 20, 2025, 08:12:26 pm
content: Thank you for your great input,I don't think grounding issues will be a concern, but I'll remember to put on my safety glasses anyway.That small amp is only $1.50 a piece in bulk, what an excellent idea that is.Quote from: BrianHG on May 20, 2025, 08:06:45 pm

author: BrianHG
author_rank: Super Contributor
timestamp: May 20, 2025, 08:30:25 pm
content: Remember, the reason you can get 9vac out from 12vdc in with that amp is because the output is bridged type.This means as 1 output goes toward 12v, the other goes toward 0v. Then swap for the next polarity of the AC cycle. This means each AC output is wired to GND, then +12v.Now, for example, if you are powering an audio mixer or keyboard where 1 of the AC inputs is hard wired to the system ground, meaning the GND on the RCA audio jacks while the other AC power input goes to 2 diodes, 1 generating the +9v, the other generating the -9v. In an isolated world, you are fine.Say your DC 12v supply's GND output is tied to the AC outlet EARTH pin, like in so many modern hardware.During 1 polarity's peak, you mixer/keyboard GND pin on the AC input will be at 0v with reference to the EARTH pin.During the other AC peak, you mixer/keyboard's GND is now 12v above the EARTH.Say you connect your mixer/keyboard to your PC to sample the audio. Now, when driving the mixer/keyboard's GND to +12v wired to your mains EARTH pin while you PC's audio GND input's GND is also wired to your main's EARTH pin, you just wired a giant loop short circuit.(This would also mean, do not wire an oscilloscope probe's GND clip to the amp's speaker output unless the scope or your 12vdc switcher is floating. Again, without the isolation, you will create a short circuit and burn your probe's GND clip, or the amp's output.)

author: EofN
author_rank: Contributor
timestamp: May 20, 2025, 08:38:17 pm
content: Let's say that this was an issue, would there be any simple way to mitigate this, for example an output capacitor? I checked my schematic, and one side of the AC input is indeed connected to ground. The Inputs / Outputs of the device share this ground, so plugging an external device like an amp could connect our ground to the mains ground. That would be an exciting day.

author: BrianHG
author_rank: Super Contributor
timestamp: May 20, 2025, 08:46:49 pm
content: Quote from: EofN on May 20, 2025, 08:38:17 pmNope. The only safe way is an isolated 12vdc supply. (Note that all switchers which don't have an EARTH pin are actually floating)The other choice is a +/- 12vdc supply with a single ended output amp board.But, for your keyboard/mixer AC input, you must make sure that the GND on the AC input now matches the GND on the dual +/-12vDC supply. (Might as well buy a +/-9vdc switcher and hard-wire it into you device skipping the AC input.)Using a 24v supply with a single ended amp with a series cap at the output also isn't a good idea. If your device consumes 2 amps on the + cycle for audio and other circuitry while consuming 0.5amp on the negative cycle just for the audio op-amps, that DC blocking cap will have an unbalanced charge making the internals of your device be powered completely out of whack. Like only actually getting 2-3v on its internal positive supply while getting -16v on it's negative supply.Or, you can add a 9vac to 9vac transformer. But, that will kill the price savings.

author: EofN
author_rank: Contributor
timestamp: May 20, 2025, 08:52:54 pm
content: That's a lot of very good points, in fact this device uses the bulk of it's power for the 5V rail, so that would result in a major current on the positive cycle compared to the negative.I expect it would work for at least five minutes, and then a lot of "why did the magic smoke come out?" moments.

author: BrianHG
author_rank: Super Contributor
timestamp: May 20, 2025, 08:56:40 pm
content: Again, 12v 2 amp wall adapters with 2 power pins are isolated. You will be fine with these.As for the 60hz generator microcontroller, just use a 78L05 or equiv 3.3v regulator for the MCU. Add a series DC blocking cap at the dac output and then a trimpot to adjust the output sine wave voltage going to the amp's input with a parallel LPF cap to smooth out the low resolution MCU DAC's steps making a smooth analog looking sinewave.Something like a 1k series with 0.01uf cap parallel to GND would smooth out a sinewave generator which has ugly steps in it.

author: BrianHG
author_rank: Super Contributor
timestamp: May 20, 2025, 09:08:49 pm
content: With 12vdc in, your peak non-distorted output voltage will be ~8.5vac.This should be fine.If would be like plugging a 120v -> 9vac adapter to a wall outlet with only 115v.Your keyboard/mixer will still run fine, maybe even cooler as it's internals are probably +/-5v anyways.Note that 15v 2amp DC wall warts are as common as the 12v 2amp ones if you need that extra 0.5v.

author: EofN
author_rank: Contributor
timestamp: May 20, 2025, 09:27:55 pm
content: 8.5 volts will be fine, I am going to take all of this information and come up with some prototypes.Thank you again, I have been meaning to join this forum for some years, and now I'm glad that I did.Quote from: BrianHG on May 20, 2025, 09:08:49 pm

author: BrianHG
author_rank: Super Contributor
timestamp: May 20, 2025, 09:32:30 pm
content: Quote from: EofN on May 20, 2025, 09:27:55 pmBuild only 2 or 3 and also get a 15v adapter to try. The amps have a series on resistance not taken into account in my calculation. You might need the extra headroom.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 21, 2025, 12:29:07 am
content: Quote from: BrianHG on May 20, 2025, 07:56:13 pmThat requires coding and debug.It does not need to be especially pure sine, but I see these function generator modules are quite cheap and have 50Hz claimed as a lower limit,https://www.aliexpress.com/item/1005005867790423.htmlThis one has multi turn pots, and includes an amplitude choice, looks better for set-and-forget use. (wider freq range too)https://www.aliexpress.com/item/1005006624364218.htmlhttps://www.aliexpress.com/item/1005006630001609.htmlAddit : in a plastic shroudhttps://www.aliexpress.com/item/1005008472286509.html

author: Buriedcode
author_rank: Super Contributor
timestamp: May 21, 2025, 01:04:09 am
content: Quote from: EofN on May 20, 2025, 07:22:07 pmThe class D stage was fine, but the bipolar supply did get warm. The RP7 only drew about 1.5A peak, we we're talking 12-14W RMS out. The inefficiency was most likely the fact that I knocked it up using what I had in my parts bin, plus a custom wound flyback transformer for a tapped secondary. I have a heavy snubber across the primary switch that probably dissipated a fair bit of energy, nothing was overly optimised. As I said, I got it "working OK" and haven't bothered to build a half decent working unit - I don't take the old digitech gigging anyway (its almost 30 years old!) so it wasn't as urgent as I initially thought.Edit: Forgot to add - it really is a lot of hassle to replace a box that is essentially a plug, lead and transformer, but it was big and heavy, and I wanted an excuse to build a true DC - AC converter at low voltage.