Switching DC/DC-Converters

I’m planning to assemble a little laptop-free mobile studio, simply because I hate laptops. Or DAWs for that matter. And I wanna play in the sun =)

Everything from Ambika to midi keyboard runs on 9v, lead batteries are available at 6V or 12V. Would a cheap but somewhat efficient DC/DC converter work or devastate the audio signals? I’m not too proficient in electronis, hence my question.
I believe i read something about 120mv Vpp output…was it ripple? Something like that. But that was on some weird chinese modules with unrealistic high efficiency mentioned in the datasheet, so the ripple or whatever it is will probably be slightly higher.

I’d recommend something in the middle of the line for any “cheap chinese knockoff product X”, where X is a DC/DC converter in this case - Usually middle of the line is vastly better than bottom end, yet not that inferior to top end…

Anyhow, the Mutable-Instruments line of products is very resilient towards ripple in my experience, I have been running a shruthi off a extremely cheap chinese 220v to 12v adapter, and it worked perfectly fine, so there shouldn’t be any problems with noise :slight_smile:

Ah, ty once more! Sounds good, I’ll try that!
I’m a little careful after building a speaker with an actual high quality converter where, no matter the capacitor, that high pitch beep was deafening…

Any modern converter will work at frequencies even Batman couldnt hear. Badly price isnt any indicator for quality here, so maybe the schematics of your “high quality” convertor were high quality 10 years ago…

I just bought some DCDCs for converting 12V to 5V for just 4,65€ (incl shipping) / each directly from HK and i cant measure any ripple whatsoever. And i have quite a nice Scope. Another Example would be Mutable Instruments 5V converter which is also cheap (and highly recommendable if you havent already a 5V Rail in your Eurorack) and with unmeasurable ripples.

Id suggest anyway some NiMh batteries, just for the weight :wink:

Since you mentioned the Ambika, note that it runs on 9V AC

Duh, right NiMh! I suppose pretty much any appliance will forgive me the 0,6-1V Übervoltage? Or should I convert those either way?

And regarding the speaker…that tiny little bit of ripple went through a 40W amp, I guess that played a role :x

Edit: 9V AC? Well, crap. Do inverters exist for that voltage? I suppose an easy square wave won’t suffice :?

Humm, when I mentioned cheap chinese powersupplies I was thinking more of the 12v wallwart I got for something like 2-3bucks… 4.65€ is already waaaaay above that, considering the difference in stuff needed for a DC/DC converter vs. AC/DC… Anyhoot, you get the point…
(Since we’re coming up with impractical solutions, might I suggest a car battery? I have three of them in the garage :wink: )

Well, disregard all that DC/DC stuff, thanks to larsens reminder my problem now is to invert 9/12VDC to (I guess perfect sine) AC. Or at least to pulsating DC.

If you don’t mind a bit of hacking/modding, you could get back to your DC idea… It shouldn’t be that difficult to mod Ambika to run on 12V DC - using a pair of LT1054 to power the voicecards (you’ll need more than 1 since the total current draw of a fully loaded unit exceeds 100mA… so this will mean cutting a trace somewhere and powering two or 3 voiecards with a different -8V as the one available from the mobo). I thought about it for a while during design and ruled it out for cost reasons…

Ha, like I said way up there I’m not very proficient at electronics. In fact I suck hard. Willing to give a try though…
Am I mistaken or do I not need to touch any of the voice cards? Pretty much just IC5 and IC6, yes?
Would help if I could figure what that mysterious VCC looks like; it’s all theorycrafting as my Ambika isn’t even here, let alone assembled yet.

Except for those 2 ICs I mentioned, the mobo is all DC?

And is there any circuit simulator you could recommend? Without one this would be just black magic to me.

As far as I know spice is the recommended model for simulating circuits, a popular form of spice is LtSpice, although I have to admit that I find that more black magic than circuits… :smiley:

What do you mean by figuring out what VCC looks like?

The amplitude…heck I don’t even know if any pulsating voltages remain after those 2 regulators and capacitors. And the -8V ultimately confuse me; I have never dealt with negative voltages before. More so since the LT1054 reads -5V.

If the voice cards already are or can be supplied with DC, I don’t see why the Ambika would be built around AC? I don’t know about the Ambikas power consumption; let’s say I’ll need 5 LT1054…21€ - is that the only reason why DC has been ruled out? Not complaining, just trying to understand (…why any appliance without a motor or heater would use AC).

Edit: finally figured how to use LTSpice’ unfriendly interface. Apparently the capacitors are so huge that about 8V DC is all that remains after C4/C12? Now I don’t understand why I would have to mod my Ambika at all (with LT1054). To me it looks like it’ll happily accept DC, although 9V might be a bit high. Am I wrong?

It’ll happily accept DC but then you’ll only have one rail operational…I assume the blue trace in your simulation was mean to be the neg rail?

Edit: Duh, now that you mention it, my schematic is wrong. I’ve never been into puzzle games and this is turning into one >.<
Well, now I see what the LT1054 is meant for. Could I simply replace the 7908 with a handful LT1054? Am I right if I assume VCC is simply a flat 5V and VEE are flat -5V? No fancy oscillation whatsoever?

In that case I think I’ll omit the whole voltage regulation part on the pcb and I’ll more than glady get rid of all the AC hassle, lol. Just how many LT1054 are needed? How much power do 6 voice cards consume? I’m thinking about just etching another board with GND, 3,3V, 5V, and -5V out.
Hard to tell how much space I can save without a board in my hands, I’ll just hope I can stick my pcb ontop of the regulation part on the mobo.

Here’s also the proper graph, you’re absolutely right. (Colors unlike previous graph :P)
I feel somewhat enlightened.

The LT1054 mirrors a positive supply voltage into a positive and a negative. Following it you put a 780x on the positive and a 790x on the negative supply voltages to regulate them down to 5 or 8V. Check out the schematics for the Shruthi filter boards, there you have the simplest form with just+5V and -5V. When you understand that one, look at the Anushri for a version with a bit more bells and whistles. Then look at the schematic for the Ambika and figure put how it gets the pos and neg supply voltages from the AC supply, and finally figure out how you would use an LT1054 to do that staring with just a positive supply.

The chip you have to get rid off is IC5. You have to remove the diode rectifier at the input, and the larger rectification caps.

VCC is a steady +8V

The mobo is all DC. Audio circuits are always running with DC.

> I don’t see why the Ambika would be built around AC?

Because it needs a voltage below ground, and because for a professional musical application you will get your power from the grid anyway and your input will be AC to start with. A DC wallwart or battery will give you a positive voltage. How to get a negative voltage form there? You’ll need something like a LT1054 which is expensive and ridiculously inefficient (100mA maximum output). You make it sound like it should be the “normal” solution but nobody really does that. What you suggest would be:

In the wall-wart:

  • 220V AC > transformer> 15V AC > rectifier> 12V DC

On the Ambika mobo:

  • 12V DC > LT2940> 8V DC
  • 12V DC > LT1054 #1> 7908 -> -8V DC #1
  • 12V DC > LT1054 #2> 7908 -> -8V DC #2
  • 12V DC > 7805> 5V DC

What the current designs does is:

  • 220V AC > transformer> 9V AC (wallwart)

On the Ambika mobo:

  • 9V AC > positive rectifier> 10V pseudo-DC
  • 9V AC > negative rectifier> -10V pseudo-DC
  • 10V pseudo-DC > LT2940> 8V DC
  • 10V pseudo-DC> 7908 -> -8V DC
  • 10V pseudo-DC > 7805> 5V DC

Aye, right on. If I knew the Anushri uses a LT1054 I wouldn’t of had this trouble.
Also, how about LT1617, could I use it instead of a LT1054? Seems to be able to supply ~300mA http://cds.linear.com/docs/en/datasheet/16171f.pdf

Besides, I didn’t want to make this sound to be the “normal” solution, it’s just the first time I come across AC voltages below 50V. My Virus for example also takes 12V DC and I wonder why that is. They sell their unit with a PSU along with it, they surely could have made it run on AC aswell. Since that’s apparently the cheaper solution, why didn’t they? Is that a benefit of using a DSP, that you simply don’t need a negative rail?

@V’cent
We thought about running a Modular from 2 Car batteries - we planed to do a Set of Cables for the Cigarette lighters, so you can plug 2 Cars together to get ±12V. Luckily we had no Plugs available…

> it’s just the first time I come across AC voltages below 50V

If you take apart any piece of audio equipment with a mains power chord, the first thing you’ll find is a step-down transformer bringing down the input to 12V, 15V or maybe 24V AC. I use an external wall-wart because I don’t want to sell kits that are directly connected to mains power.

The Virus does not need a negative rail because it contains no analog circuitry. It’s probably DC all the way long, with an AC coupling cap just after the codec.

Another approach is to use a virtual ground. I believe this is what is used on DSI products, and on the A4. This works well but it has constraints on board layout that did not allow this to work well for Ambika.

LT1617 is in a SOT23 package and requires external inductors. Do you really want that?