DC to AC converter, 12VDC to 16VAC @ 1A

Hello one and all, this is my first post to these forums!

I have a new tube amplifier on the way:
**broken link removed**

And unfortunately, it requires 16VAC at 50Hz or 60Hz and 1A.

As I am planning to use this for a portable microphone setup, I was hoping I'd be able to power it from a 12VDC 50Wh power source (a universal Li-ion 12V/9V/6V/3V powerpack to be precise).

I have spent several days scouring the net for circuits to efficiently convert the power from 12VDC to 16VAC, especially while maintaining a pure sine wave (which as far as I'm aware is vital for good preamplifiers) and while skipping and intermediate 120V/240V stage.

Any and all help would be much appreciated, what I thought would be an readily available product has really turned out to be very rare!

Kind Regards
James

PS: My current plans are to use a pure sine wave oscillator amplified by MOSFETS to interrupt two voltage doubled 12V lines (one for +24V and one for -24V) which would give me the +/-22.7V range I need and eliminate the need for a transformer... but thats where my poor electronic knowledge fails me, for in the schematic world, I would burn toast.

I suggest you first find out it's actual requirements (as opposed to what adaptor comes with it) - you may be able to power it from DC.

But why are you wanting to power such a strange and expensive device for a portable microphone?, I can never see the advantage of wanting to add distortion and lower the quality of the signal?.

Well because this microphone setup will mainly be recording vocals, tube amplification is often a requisite for a certain sound, right down to the particular tubes you use (this amp also does have the option for solid state amplification, hence it being dp = dual pathways).

But why not just use another amp? Well there are some other considerations. First, the the Rode NT1-A mic I use is well matched to this amplifier. Secondly, the mic requires 'Phantom Power' which is a 48VDC@10ma supply added to the XLR connector to 'power' the mic (a rare thing in all battery-operated amp circles). Lastly, all the other recordings will be done on this amplifier, so for consistancy of sound across the whole album, it is necessary to use this amp.

I sincerely doubt there is any way past the 16VAC requirement (as the need for AC power seems common to all tube amplifiers), but I would be glad to be proved wrong!

Thanks!

The bluetube may take all of the AC and turn it into DC except possibly for the tube filament. It probably has an internal transformer that generates the higher voltage needed by the tube plates. It may or may not be all that sensitive to a standard inverter. Experimenting is one way to find out.

My thought is to purchase a low power inverter and use a transformer to get down to 16 vac. The transformer will do some filtering by itself. While this isn't a do-it-yourself approach entirely it is a way.

If you are willing to use Nigel's approach and dig in to the unit - you'll likely find the high voltage (higher than 12 volts) requirement to be very small in terms of milliamps. That might make your project a little easier. Amateur radio operators were building solid state 12 vdc source, high voltage power supplies back when tube equipment was mainstream - you might do some research there for ideas. An alternative might be a handful of 9 volt batteries in series - 10 of them get you 90 volts - 10 more is 180 vdc. Not elegant but for a few milliamps it might be enough.

Heh, people really go a long way to avoid DC to AC circuits! Although I am happy to open her up and poke around (you do that to change the tubes anyway), I can not permanently modify the unit to use a different power source as this will change the sound/power requirements when back on mains power. If there are no solutions, I will have to go for the mini-inverter/transformer idea, and I have emailed the company enquiring about the exact internal power requirements.

But there MUST SOMEWHERE be a reasonably simple or already designed circuit to handle low power DC -> low power AC? It seems hard to believe that there isn't a schematic or marketed device that would do something near 12VDC -> 16VAC that could be modified for that exact power range?

But if what I am asking is truly stupid in the electronics world, feel free to tell me that. However, these projects seem to be close:
https://www.aaroncake.net/circuits/inverter.htm
**broken link removed**
Is there not a tweak for these? And yes, I have attempted to contact these people too to see if they have any ideas!

Again, all help appreciated!
James

The 1st inverter project you posted has errors that cause it to blow up its capacitors! It is far too simple to be an inverter.
Most inverters like these simple ones have a square-wave output that makes a nice loud buzz on audio equipment.

There is nothing stupid about your requests or thoughts. 12 vdc to 16 vac, 50 or 60 hz, pure sinewave does not seem all that common to me. Lots of power supplies take in low DC voltages and produce higher voltages - but usually DC after a high frequency switch mode process. 12 vdc to 120 or 220 vac - that's common.

An approach might be to take chemelec's circuit and work your way up from there.

Like just about anything else - a small number of people make their own DC to ? power supplies. Of that small number only a few have done conversions to AC voltage - and only a few of them have posted the results of their efforts.

You might take chemelec's circuit and use a variac to get the voltage you need. We dont yet know about the frequency or waveform from this supply. If the frequency is significantly higher than 60 Hz your equipment may not tolerate it.

Like I said before, you first need to know what you need - and 16V AC is just too vague. A circuit diagram of the unit would be useful?, but valves don't run on AC, I would imagine there may be a small transformer inside that increases the 16V AC to a higher voltage before rectification for the anodes of the valves.

There also must be something to feed their heaters, and 16V is a VERY unlikely voltages - 6.3V or 12.6V is MUCH more likely, and stabilised DC is preferable to AC (reduces hum problems).

Also bear in mind there may well be a split supply to feed opamps?, we just don't know?.

stevez said:

You might take chemelec's circuit and use a variac to get the voltage you need. We dont yet know about the frequency or waveform from this supply.

Click to expand...

Here is a clip from Chemelec's project:

Ok, have been tinkering with the unit. Presonus told me 16V DC would work just fine, but that turned out to be untrue: With 16VDC positive to tip, only the switches work (nothing else lights up), with 16VDC negative to tip, the whole thing lights up like a christmas tree, the VU meters go off the scale and the clipping lights turn on, no matter what the gain. At 12VDC, same deal, just dimmer. At 7.5V DC, clipping lights still on but the lighting is pretty-much off.
Useless
I have however opened her up and taken some photos in the hope that someone might be better equipped to interpret the circuit than I. The amplifier section seems to just rectify the AC with capacitors straight back to DC (on the right) but thats all I can tell.

**broken link removed**
**broken link removed**
**broken link removed**

If the links don't work (403 Forbidden), just select the title bar and press enter again. Its stupid, but google needs to know you browsed to the photos from a google site...

I have made a voltage doubled battery circuit that will give me any voltage up to 24V, and I want to use that to make 16VAC. My best idea so far is to have a circuit that feeds the unit 22.7VDC, but switches the polarity every 50th of a second. Then I would solder in a capacitor or two to 'smooth out' the resulting square wave.

My second best idea is to use a MOSFET as a 'power amplifier' and a tone generator (50Hz sine wave) to create the power pattern I need. However, the exact wiring of such a circuit is beyond me, and I'm worried that for 50%+ of the load time, the mosfet is going to be my main resistive load, and so thats going to kill the efficiency of said circuit?

Any and all feedback appreciated guys!

When I try to look at your pictures, I am told that it is forbidden.
Not much help there.

JimB

Sorry JimB, just select the title bar and press enter if you get that 403 forbidden message. For some reason, google needs to know you came to the image from a google site...

Why do you post your pics over there and not attach and upload them to here?

koopa said:

I have however opened her up and taken some photos in the hope that someone might be better equipped to interpret the circuit than I. The amplifier section seems to just rectify the AC with capacitors straight back to DC (on the right) but thats all I can tell.

Click to expand...

Looking at the circuit board, those diodes and capacitors on the right probably form a voltage doubler to get a bipolar power supply.

koopa said:

My second best idea is to use a MOSFET as a 'power amplifier' and a tone generator (50Hz sine wave) to create the power pattern I need. However, the exact wiring of such a circuit is beyond me, and I'm worried that for 50%+ of the load time, the mosfet is going to be my main resistive load, and so thats going to kill the efficiency of said circuit?

Click to expand...

If I'm right then a simple 22V >50Hz square wave is all you need. The on-board capacitors should provide adiquate filtering to remove the harmonics. Note I said >50Hz, this is because higher frequencies should also work, but don't go silly as the doides won't be able to switch fast enough. You might be able to get away with just over 20kHz so you can't hear it but there agian the RF harmonics might cause interference.

There are many ways to get an AC 22V from 12V squarewave: You could build a DC-DC converter to convert 12V to 22VDC then use an h-bridge driver connected to an oscillator. You could build a 12V h-bridge with oscillator connected to 12V and connect the amp to the output via a 1:1.83 transformer. You could try building a push-pull blocking oscillator with a transformer and a couple of transistors.

One advantage of using a higher frequency is the transformer can be smaller but you don't want to go too high as it can cause problems.

Whoo, thanks so much Hero999, I'm going to go with the H-bridge idea as I have already rigged up the necesarry 24V voltage doubler circuit.

The ideal chip (I think) is the LMD 18200 by National Semiconductor
https://www.electro-tech-online.com/custompdfs/2006/10/R27-18200.pdf

I am currently looking at ways to try and rig up this chip in the simplest possible fashion (my electronics knowledge is only a few steps above sparking batteries for kicks). From what I can understand, I can keep all the inputs static save the direction (forward/reverse) pin which I will have to drive with oscillating circuit (555 timer chip)?

Again, all help appreciated!

PS: As I am doubling 12V, I also have that available, but not the 5V required for logic control?

That'll work just fine.

What are you using as a voltage doubler?

You can get the +5V from 12V by using a simple linear regulator IC like the LM7805 or LM78L05.

The DC DC converter is from Kemo (from Germany) #M029
http://www.kemo-electronic.com/en/module/m029/index.htm

I would be much obliged if you could help me put together a circuit. My last attempt at using 555 chips failed, and I don't have an oscilliscope to check through what I've done wrong.

I shouldn't need 5V after all since the chips are all spec'd to 12V anyway.

I've installed Electronics Workbench Multisim 9, but the LMD18200 isn't included and its internal circuitry seems a little complex to input.

My main difficulty is knowing which pins to ground, which pins to VCC, which pins to 555, and which ones need a high-ohm resistor to keep them from building up an inappropriately significant static charge.

IE:
- Which pins are actually significant to my project (some are only useful for PWM, which I don't think I need) ?
- Do I need any other chips than a 555 and an LMD18200?
- Is there anything I need to add to allow for voltage tolerances?
- Is there anything I need to add to aid efficiency?
- Is there anything I need to add to allow for frequency adjustability?

Thanks!

1) The only pins that are important are the outputs and the direction pin apart for the power supply pins of couse.

2) What you're having problems with the 555? Please post the schematic. The LMD18200 can be subsituted with two p-channel and two p-channel MOSFETs?

3) The voltage shouldn't be too important, if your tube amplifier is well enough designed then it should be able to accept quie a wide power supply range.

4) That will depend on the DC-DC converter, there is nothing you can do about that, except use a higher quality DC-DC converter. But is power consumption really a consideration?

5) The frequency isn't improtant, but it can be adjusted by altering one of the resistors on the 555 (see the datasheet).