Op amp based preamp supply. Charge pump, single vs symmetrical & ripple

[John B]

For your application I'd probably skip the 7660 since it's max input voltage is around 12V depending on the model. Good for a 9V battery powered device though. Yeah, you can do voltage doubling like it says in the datasheet, but it's more hassle and space taken up. Obviously the manufacturer is happy for you to buy 4 chips vs 1.

The boost pin brings the switching frequency to around 60kHz from memory, but its a wideband burst of noise that propagates to the output of the op amps. I have used them some simple designs (a splitter and mixer for smartphone earbuds/mics) where audio quality was not a concern, but haven't put them in studio quality gear yet.

What I was getting at is that because you do not have to adhere to any standard for power, ie 9V DC input or batteries, you can cut a lot of hassle by simply providing enough voltage in the first place, and spend your effort on the signal path instead.

[dazz]

For your application I'd probably skip the 7660 since it's max input voltage is around 12V depending on the model. Good for a 9V battery powered device though. Yeah, you can do voltage doubling like it says in the datasheet, but it's more hassle and space taken up. Obviously the manufacturer is happy for you to buy 4 chips vs 1.

The boost pin brings the switching frequency to around 60kHz from memory, but its a wideband burst of noise that propagates to the output of the op amps. I have used them some simple designs (a splitter and mixer for smartphone earbuds/mics) where audio quality was not a concern, but haven't put them in studio quality gear yet.

What I was getting at is that because you do not have to adhere to any standard for power, ie 9V DC input or batteries, you can cut a lot of hassle by simply providing enough voltage in the first place, and spend your effort on the signal path instead.

I just thought that by not adhering to an almost de facto standard like the 9V supply for guitar pedals I would be limiting the user's options. I was going to put there a 9V input in case you want to implement the power supply in the pcb, or two +16V, -16V for those who prefer to use an external converter, or a dual power supply of their own in which case they don't need to populate the power supply section in my pcb.

Maybe I didn't make myself clear enough, I'm just designing the PCB, not the whole pedal. Well, it's designed to fit in a 1590BB enclosure but it could also go into a larger case with a complete power supply and power amp to make an amp instead of a pedal. Hope that clears that up

[Zero999]

You could build your own power supply but it's a pain. Just use a DC:DC converter module. Some examples are linked below, which have a regulated, isolated output. Additional filtering will probably be necessary to prevent audible noise, due to rectification of the RF in the op-amps. No doubt there are cheaper modules available off ebay, Amazon etc.

https://uk.rs-online.com/web/p/isolated-dc-dc-converters/1714747/
https://docs-emea.rs-online.com/webdocs/168f/0900766b8168f657.pdf

https://uk.farnell.com/xp-power/itw0515s/dc-dc-converter-1w-15v-0-033a/dp/2422847
http://www.farnell.com/datasheets/1847138.pdf?_ga=2.154705917.435636270.1543350785-1037805362.1481052103

Those are just great simple, compact solutions, than you. If I can find those for cheap it's a done deal

What are the budget constraints?

Is just just a one off hobby project? Will it be made and sold commercially in small quantities i.e. 5 or so? Or are you going to make 1000s? It's often cheaper to use a module, once the cost of all of the parts, assembly and board space is taken into account.

Don't use the MC34063, far too much quiescent power wasted.

According to the datasheet the quiescent current is 3mA. 6mA if I was to use two chips... just as much as the opamps themselves, so yeah, not ideal at all I guess.

I'm in the middle of some similar experiments with the 7660s, to see if I can reduce the switching noise. Probably a mix of both input and output filtering.

It's going to be far more complicated than anything implemented in a commercial musical product, like pedals or guitar preamps, but for a DIY application, parts count isn't the problem, its noise and efficiency.

You're going to have to filter the MC34063 anyways.

I'm a little confused with your input power requirements. So you're not going to run it off batteries ever? Always an adapter? If that is the case, simply supply your pedal with a wide enough +/- range in the first place. Either with 2 floating DC adapters or an AC transformer supply. The input can be filtered and linear regulated, both on the + and - rails.

Your dual power supply idea seems like the way to go at first, but you end up spending far too much time, effort and space trying to make it work, not to mention probably about 30% efficient 

Well, I have an adapter cable to plug a 9v battery or two Li-Ion cells to the 3.5mm socket in case I need it but what I was referring to is that I personally hardly ever use anything but a wall wart 9V supply. There's no requirement in the design for that though, If someone else wants to build the pedal and power it with batteries, that should be an option. I'll just provide a 9V/DC input in my board, at least that's the initial plan.

Are you having trouble with noise with those 7660S's? Even using the boost to 30KHz pin? Bummer. I was hoping I wouldn't need to filter my supply if I could get the ripple frequency above 30KHz or so

BTW, I said before that for my application (dual opamp) I would have an almost constant current draw from the supply, which I now think is BS. Wouldn't that be a problem with something like a 7660S since it would sag pretty bad (a couple of volts) if you get a 5mA spike in your current draw?

How about using two 9V batteries or a 12VAC mains adaptor?

A voltage doubler circuit with the 78L08 and 79L08 could provide a +/-8V supply when running off the mains and batteries can give +/-9V directly. It's possible to get sockets with a built-in switch to disconnect the batteries, when running off mains power.

[b_force]

What happened to the  LT1615 , LT3460, LT1316 idea?

[Zero999]

What happened to the  LT1615 , LT3460, LT1316 idea?

Going by the prices of those ICs in Mouser, it was probably deemed too expensive.

I don't see the point in doing hobby electronics on a shoe string budget. If you really want to save money, there's plenty of cheap junk available.

[dazz]

Those are just great simple, compact solutions, than you. If I can find those for cheap it's a done deal

I meant to write "thank you" there, sorry about that.

What are the budget constraints?

Is just just a one off hobby project? Will it be made and sold commercially in small quantities i.e. 5 or so? Or are you going to make 1000s? It's often cheaper to use a module, once the cost of all of the parts, assembly and board space is taken into account.

It's not a commercial product, I'm simply designing the PCB and publishing the gerber files for DIYers, so one-offs typically. Re: budget constraints, well, I don't have a figure in mind, just thought it made sense to use cheap components that are readily available regardless of where you live. The idea of building this thing was posted in a spanish forum by someone else, I believe using components that can only be purchased at mouser or digikey, with shipping costs and all, doesn't make too much sense. There's already a "cheap" solution available here:

http://tagboardeffects.blogspot.com/2014/01/bipolar-voltage-converter.html
https://tagboardeffects.blogspot.com/search?q=3005

How about using two 9V batteries or a 12VAC mains adaptor?

A voltage doubler circuit with the 78L08 and 79L08 could provide a +/-8V supply when running off the mains and batteries can give +/-9V directly. It's possible to get sockets with a built-in switch to disconnect the batteries, when running off mains power.

I'm going to put a 9V input for those who want to use the pcb's supply and also V+ and V- inputs for those who prefer to use an external supply like those you suggest, or one of those cheap ebay +15V/-15V converter boards. The pcb is going to be 80x40 mm anyway because it's meant to be fitted in a 1590BB enclosure with pcb mounted pots like these Since I need those pcb dimensions to get proper spacing between pots (see pic 1 for initial pcb design) I have plenty room to include the optional power supply.

I think I might be able to make it work with +9/-9 but spec said that it wouldn't sound the same as running it at +15/-15:

PS: if you run the opamps from +-9V, as you suggested, the sound would change, not due to headroom or noise considerations but because the opamp itself is  operating from lower rails. This effect is not so marked in newer opamps, but more so with the older opamps.

What happened to the  LT1615 , LT3460, LT1316 idea?

Going by the prices of those ICs in Mouser, it was probably deemed too expensive. I don't see the point in doing hobby electronics on a shoe string budget. If you really want to save money, there's plenty of cheap junk available.

Point taken. But it's not just about the money, smd components complicate things for DIYers.

[Zero999]

Since you want a symmetrical power supply, then how about using a boost converter to get 24V to 30V, then a rail splitter, such as the TLE2426 to get the 0V point?
http://www.ti.com/lit/ds/symlink/tle2426.pdf

[dazz]

Since you want a symmetrical power supply, then how about using a boost converter to get 24V to 30V, then a rail splitter, such as the TLE2426 to get the 0V point?
http://www.ti.com/lit/ds/symlink/tle2426.pdf

According tho this I can do pretty much the same thing with an opamp as a voltage follower, is that right? I'll try both anyway. I've ordered 7660s, 2577's, and MC34063's to try for the boost converters/inverters in different configurations. Will also try 7815s, 7915s, 7824s to regulate and filter the rails when needed, pi filters, and I'll get a few flyback transformers to try this: https://www.hobby-hour.com/electronics/flyback-regulator-lm2577.php.

But yeah, I think the simplest approach is boost converter + filter + rail splitter. I already have a preamp based on TL082's with a virtual ground, not even a rail splitter, just a resistor divider, and it works great as long as the supply rail is properly filtered (using a 7809 for that one)

At the end of the day, as always, the ultimate objective is to learn and I have plenty things to try already.

[T3sl4co1l]

If you're going to the trouble of a boost converter, you might as well do a buck-boost or SEPIC or the like, with a coupled inductor, so you get everything in one stage without any fumbling with rails.

Tim

[dazz]

If you're going to the trouble of a boost converter, you might as well do a buck-boost or SEPIC or the like, with a coupled inductor, so you get everything in one stage without any fumbling with rails.

Tim

I had no idea that such a thing existed, thank you.

[dazz]

I've been trying the MC34063 today to build a negative rail. It works great... except for the current draw. I know you guys said it's an old chip and very inefficient, but come on... the thing is drawing almost 300mA to convert 9V to -9V! I must be missing something 

[dazz]

Success! I eventually managed to pull this off with an MC34063 based boost converter and some LC filters. Works a treat!

Attached pics of the schematic and the pcb. Thanks everyone for your help