Step up voltage?

[EuphAural]

I asked this over at DIYSB, but I'd like to see what you guys say over here too:

For my internal bass preamp, I would like to use a small rechargeable battery, like the one in a phone or something of the sort. The problem is that those tend to be 3V7, 9V is standard for most guitar circuits. So I'd need to raise the voltage. Perhaps higher than 9V, since it's being stepped up anyways.

Now, there are many ways to do this. I could probably fit a switchmode supply in there. Or there are those voltage multiplier ICs. But, being weird, I'd like to use a linear supply. If I use an oscillator to generate a sine wave, couldn't I just treat that as similar to the mains? A transformer could step up the voltage followed by rectification and filtering, or I could use some of those diode voltage multipliers. I think it would also double as a safer way to practice mains wiring, since there is less voltage and current there.

What do you think? I suppose it's crazy, but it's still doable, isn't it?

[eschertron]

One way to do it is use rechargeable 9v batteries. That's what I do.

As far as your inverter/step up/rectifier design, go ahead and engineer it. I think you'll find it will take up a lot of space (in your bass cavity?) and require a DC source that can provide capacity equal to or greater than your target storage device (the ubiquitous 9v battery). A small 'coin' type cell will be running out of juice quite fast. But please don't let my naysaying stop you. Innovation always comes from experimentation.

edit: I'm intrigued by your 'safer way to practice mains wiring'. What exactly do you mean by that?

[Mick Bailey]

You could use a blocking oscillator circuit with a small inductor DIY wound onto a small ferrite rod to step up your voltage. There's a balance between frequency and output current, but you should be able to get it to run ultrasonic to eliminate whine. If your oscillation frequency isn't high enough your pickups will pick up the radiated magnetic field from any inductor as audio. You may need to put a simple low pass filter on the input of your preamp - sometimes step up circuits can induce sum and difference artefacts in the pickup output.

It depends on your pickups, where your voltage generator is located, and the axis of your inductor core.

[Enzo]

Rechargeable batteries come in various voltages and currents. Cell phone batteries may be 3.7v but all that means is... don't use a cell phone battery.

Want 9v? I agree with eschertron, just go to any Radio Shack or other battery source and get a 9v rechargeable battery. Presumably you intended to mount a cell phone type battery, then have a changing jack on the guitar. SO now you have a 9v battery with a charging jack on the guitar. Plug a 9v charger into it instead of a cell phone charger.


I also did not follow the second part. You intended to make a switchmode to do....what? Take the 3.7v battery and kick it up to 9v? A DC-DC converter I'd call that. That seems way more complicated than it needs to be.

If you intend to make a power oscillator to make a sine wave to run through a transformer to step up the voltage to then rectify and filter, well.... whew. Start with what would you power all this circuitry with? There is no free lunch, you still have to power your preamp, but now all that other conversion stuff as well.

I absolutely have no idea what the mains has to do with it, unless you have a power cord coming out of your guitar.

[EuphAural]

Yeah, I figure I could just go the easy rechargeable 9V way and just mess with this for fun. I'd think it a safe way to practice mains wiring, as using a power oscillator means lower power than a wall socket. If I made a mistake, there wouldn't be 120V to worry about, just whatever that oscillator puts out. It doesn't mean that I can throw caution to the wind of course. Just that I'll get a nasty shock instead of a fatal one in the event of an error. 9V is a painful surprise, 120V is a paunful death. There's no safety earth though, which could be an issue.

[Enzo]

I am still unclear, but I fear you may be a little confused. 9v is neither painful nor dangerous, you can pick up a 9v battery and touch it all you want. Touching it to your tongue is uncomfortable, but is the old-time way to check the things for charge.


Wiring is wiring, whether it has 9v or 90v running through it. Practice at wiring is good, of course, but we don't solder any different for mains voltage.

If you are stuck on cell phone batteries, just run three in series.

[EuphAural]

Yeah, after hearing 'Don't work on mains wiring unless you are qualified!" a bunch, I figured I should probably refrain from doing so until I've had some practice. Working on and testing a power supply that runs off of 9V seems like better practice. So I'd like to work at a lower potential until I feel comfortable enough to work on stuff that connects to the mains.

I'm not set on phone batteries, but I do like the idea of using my phone charger for my bass. I think I might have to go with DC-DC or switchmode though. After looking into it, linear supplies are a bit wasteful. I guessed something like 60 mA draw. That'd kill a battery fast.

Well, thanks for the advice and suggestions. Second opinions are good to learn extra isues an idea may have.

[Enzo]

Anything you do other than a straight battery power is going to be "wasteful." Every circuit has its inefficiencies. A switcher may be a lot more efficient than a linear supply, but it still has losses. A battery provides energy. You can design for a battery that does the job, or you can design for some other battery and add a bunch of stuff between it and the circuit. And all that stuff will be taking bites from the energy of the battery. The battery will be providing the same energy to the circuit, PLUS running the other conversion stuff.

[EuphAural]

I agree. It's the degree of waste that matters though. I mean, stepping it up means more current draw too because power in = power out. But if I'm going to have circuitry between battery and preamp, it might as well be the more efficient kind. Linear is about 50% or so, right? A boost converter is about 90% I think, so that'll help keep draw low. I figure the preamp itself will be around 6 mA at maybe 15V. Which would be 30 mA at around 3V.

Perhaps a rechargeable battery at the voltage I want is better. For some reason though I consider ordering online more effort than building a workaround. Don't know why. Probably because building stuff is fun?

[Enzo]

I suppose, but again, a 9v rechargeable battery is easy to find. I have a couple Eveready ones on my bench as I write. Radio Shack sells them too. Home depot sells them. If you build a switcher, will you have all the parts on hand, or will you be ordering them online?

I can't think offhand of a linear supply that boosts voltage.

Find a switcher IC you want to use, make the thing, and tell us how it works out. I have no idea what average draw is on a cell phone, so what battery life would we predict from a cell phone battery at 30ma?

[km6xz]

It is really easy now to boost to 9 volts with either a buck/boost DC-DC converter IC and 6-7 other cheap parts or buy a module for the correct DC in and out.
How about looking up a LM2698 boost regulator chip, add 6 other common small parts for less than $3 plus labor and have a nice clean 9 volts out with any input voltage from 2-3.9 volts.
Actually it can have a much wider range of in and out voltage but I just did a model of it and calculated the values using those voltage in and out. Due to the low current needs the efficiency will only be 83-85%. If you needed more current the efficiency would increase, up to about 93% you pulled 220ma from it.

[EuphAural]

I thought those voltage doubler supplies increased voltage in a linear supply? I've seen a fee that run the AC through a series of diodes to boost it. I found a few schematics by looking up 'diode voltage multiplier'. I haven't tried them though, so I can't be sure they'd work.

Looking at average capacity (1000 mAh) in a few of those, I got about 22 hours of time with my estimated current draw. Assuming an hour or less a day, that's a little under a month. I've heard of 9Vs lasting the better part of a year in some preamps, though I guess mine draws a bit more than most.

The boost converter is looking better and better though. Thanks for the part #s km6xz. I might still try putting one together though, just because I like the hard way.

[Steve Conner]

There is no "linear" way to do this. Linear regulators can only drop voltage, not boost it. Also, the diode multipliers you saw require AC to work. You need to convert the DC from the battery to AC, step it up and then rectify it back to DC again.

For low currents, the most efficient way is a charge pump like this.
9V battery replacement

[EuphAural]

You know, maybe I'm using the wrong word for it. I thought 'linear' in the context of power supplies meant 'conventional', ie transformer-rectifier-filter. I guess that's the wrong word for it.

As to inverting and rectifying again, that was the plan. Isn't that how most boost converters work too? They chop it up and rectify again? The charge pump method sounds good for efficiency. There are some that let you set frequency too, right? I've heard that some add some whine to the supply because their frequency is too low.

[km6xz]

Most operate at minimum frequency of about 80khz but many operate in the 1-3 Mhz range. It is much easier to filter 80khz than 60hz noise.
Go to Converter (Integrated Switch) - Step-Up (Boost) Converter - LM2698 - TI.com Texas Instrament's web site and take a look at the really cool, cheap and easy to build around buck/boost converters. There is a WebBench simulator on the right side where you can dial in your input, output and current requirements and it will calculate passive part values and show you a schematic, in this case only 6 parts. The resulting supply will work over a wide range, stay cool, have high efficiency and be a lot cheaper and smaller than a transformer. They calculate the BOM at just over $2 but that is in 1k lots. Figure twice that for your build of one. You probably only need to buy two parts, the inductor and the chip.
Have fun