Stupid question number 245: bipolar power

L

LukeFRC

Well-known member
So I’ve built a load of pedals, Mutron clone, Aion’s L4 preamp PPCBs skeptical buffer who have used bipolar power supply’s.

Most others use a monopolar power supply (if that’s what it’s called) +9v and gnd with the vref being fed into the input …

I get that bipolar power supply is how older op amps and ICs worked, and that’s why say Craig Anderson’s designs all use it - and the ubiquity of 9v power means it’s not the standard - but Is there any benefit to bipolar power for anything other than headroom?
 
LukeFRC said:
I get that bipolar power supply is how older op amps and ICs worked,
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They still do.

The purpose of VREF is to "fake" a bi-polar power supply when you don't have one.

Biasing the opamp input at 4.5V creates a "virtual ground", the supply pins are positive and negative relative to the input pins.
The drawback is that we've limited our amplitude to +4.5V / -4.5V (again, assuming an ideal rail-to-rail opamp).

It's generally not the opamp itself that requires a bi-polar supply, it's the circuit.
 
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Ah ok. So the designer would start off with the power supply and then design the circuit - or start off with the circuit and then build a power supply for it?

I realise this is the questions that would be covered in term one lesson one of an electrical engineering course but my google searches have been unfruitful - some of the time I’m trying to hack (in a totally unskilled way) together blocks of circuits to make something interesting - it’s when you have one bit working bipolar and one with VREF I get stuck
 
LukeFRC said:
So I’ve built a load of pedals, Mutron clone, Aion’s L4 preamp PPCBs skeptical buffer who have used bipolar power supply’s.

Most others use a monopolar power supply (if that’s what it’s called) +9v and gnd with the vref being fed into the input …

I get that bipolar power supply is how older op amps and ICs worked, and that’s why say Craig Anderson’s designs all use it - and the ubiquity of 9v power means it’s not the standard - but Is there any benefit to bipolar power for anything other than headroom?
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The question shouldn't be what is requiring a bipolar supply or not because they both do - one using a dual supply, and one using a single supply split in half. Referencing to this halfway point is also a bipolar power supply practice once you decouple DC from the equation.

With a single 9v supply you can get a maximum of +/- 4.5v out if everything is perfect; in practice it's going to be more like +/-3v before clipping. With a dual +/- 9v supply this clean output doubles, which can be important if you don't want any distortion.

Also it's typically much, much easier to lay out PCBs with a bipolar supply, and you can often cut down a couple of components.
 
Chuck D. Bones said:
The audio circuit should dictate the power supply config, not the other way around.
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Stupid Q follow-up - is there common examples of circuits that need +- voltage vs VREF?

And/or where should I be reading up on this to answer them in my own? Google searches show solutions and different power supplies - but less of the whys
 
It's really up to the designer. The Klon has three power supply rails and Vref.
The simplest solution is a single power supply rail and Vref. Takes the fewest parts and usually gets the job done. Explains why 95% of the pedals out there do it this way.
If you need more headroom, then a higher voltage power rail, split rails, or both, are necessary. If you want something higher than 9V, then the choices are:
1. Force the user to supply +18V (or higher).
2. Add a charge pump.
3. Build a power supply that runs on AC voltage and contains a transformer.

Door #1 won't appeal to every player because not everyone has a power block that makes higher than +9V.
The vast majority of designers go for door #2. The downside is charge pumps require more parts and are noisy, which can disturb the audio signal if the designer is not careful.*
Door #3 is rarely used anymore because of size, weight, expense & hum. The original UniVibe ran on AC power.

Let's say you decide on adding a charge pump, then the choices are how much voltage do you need and do you want one rail or two?
The advantage of a split rail is there is no Vref. Opamp bias and signal return all go to ground.

Examples of charge pump designs:
TC Electronics Integrated Preamp
Klon Centaur
OKKO Diablo Mk3
EQD Warden
Aclam Dr. Robert
Maestro FSH-1
Runoff Groove Thunderbird
MI Audio Super Blues Pro
MI audio super Crunch Box v2

A note about headroom. Doubling the power supply voltage adds roughly 6dB of headroom. This isn't much and may not be sufficient. The Integrated Preamp and Thunderbird pump the rail voltage up to approx 25V.

* an example of a "not careful" design is my Stereo Vibe. It contains two Photon boards, each with their own charge pump. The two charge pumps do not run at the same switching freq and the noise from the charge pumps mixes and creates a beat note that is in the audio band. This causes a noticeable squeal. The solution is to either sync the charge pumps or use just one. Charge pump noise can also beat with BBD sampling, so don't say you weren't warned.
 
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Thanks Chuck!

That helps :)

Basically the last few months I’ve built both the frequency interchange filter and the skeptical preamp, and liked the sound of one and the headroom of the other (for bass) - my crazy plan I am half thinking of bodging taking the middle filters of the frequency interchange and sticking it in the middle of the skeptical (omitting two of the three switches) and seeing what it sounds like. I think from my understanding if I sent the VREF on the LPF/HPF to ground it should work
 
will be.
I've mostly ended up using musikdings kits of pedalPCB things - upside is less stuff you don't need, the downside is less stuff to breadboard with so weighing up if its worth it vs building another frequency interchange and feeding it more power (option 1 above)
 
If you're making mods and playing what-if then breadboard is the way to go. And if you are breadboarding, then you need a good supply of parts. There must be some good parts suppliers in the UK. I don't know what it costs to ship stuff to the UK from Tayda, but it's dirt cheap here in the US. At least it used to be. Prices, shipping & import tariffs are a moving target these days. 50 quid will buy a whole lotta stuff from Tayda.
 
FOGHORN LEGHORN.jpeg

AH SAY ah say... an ahh-side:

F' things like a BASS-geetyar or a BassGITTAR-what-with-preample-an'-all...
Ah say, pay attention, son...
Hevving thet-thar +9/-9 fer extra room in the noggin kin be deesire-a-bull, clearly, 'less'n y'all deesire a bull-in-China-shop ear-worm.

Folks afore me here been droppin' pearls of wisdom, much obliged for unscrambling mah grey-matter.
Both sounds are good, ahm tellin' yer.
Horses for courses, Bulls for the Ring or by the Ring, ah say ah say.
 
LukeFRC said:
Stupid Q follow-up - is there common examples of circuits that need +- voltage vs VREF?

And/or where should I be reading up on this to answer them in my own? Google searches show solutions and different power supplies - but less of the whys
Click to expand...
Again - they are literally the same thing in 99% of pedals. Referencing an op-amp to vref instead of ground has the function of creating a bipolar power supply. You can convert between either style by changing vref to GND, or GND to vref (isolated from any other parts of the circuit with a GND reference).

Chuck D. Bones said:
It's really up to the designer. The Klon has three power supply rails and Vref.
The simplest solution is a single power supply rail and Vref. Takes the fewest parts and usually gets the job done. Explains why 95% of the pedals out there do it this way.
If you need more headroom, then a higher voltage power rail, split rails, or both, are necessary. If you want something higher than 9V, then the choices are:
1. Force the user to supply +18V (or higher).
2. Add a charge pump.
3. Build a power supply that runs on AC voltage and contains a transformer.
Click to expand...
These days, DC-DC converters are probably the easiest solution. If you're sticking to 9v input (which is less common), you can get +/-12 or +/-15v output converters for about $10 for 1w (so 33mA/channel @ 15v) or $15 for 2w (67mA/channel @ 15v). 12v input models are even cheaper - 12v to +/-12v is about $5.

In terms of how to lay them out, it's even easier than charge pumps as they're isolated, but they do take up a little bit more room (I think the one I just finished a board with was 20x10x10mm).

These switch miles out of the audio band (so are much, much less noisy than typical charge pumps), typically only require two additional components to meet EMI regs (not required, but should always be done), auto shutdown under fault conditions and most are even regulated with minimal additional filtering caps required.
 
HamishR said:
BTW I don't think either of your questions were stupid at all. That's what this place is for.
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It feels like the kinda thing I *should* have been able to search for and find out!

drew.spriggs said:
Again - they are literally the same thing in 99% of pedals. Referencing an op-amp to vref instead of ground has the function of creating a bipolar power supply. You can convert between either style by changing vref to GND, or GND to vref (isolated from any other parts of the circuit with a GND reference).


These days, DC-DC converters are probably the easiest solution. If you're sticking to 9v input (which is less common), you can get +/-12 or +/-15v output converters for about $10 for 1w (so 33mA/channel @ 15v) or $15 for 2w (67mA/channel @ 15v). 12v input models are even cheaper - 12v to +/-12v is about $5.

In terms of how to lay them out, it's even easier than charge pumps as they're isolated, but they do take up a little bit more room (I think the one I just finished a board with was 20x10x10mm).

These switch miles out of the audio band (so are much, much less noisy than typical charge pumps), typically only require two additional components to meet EMI regs (not required, but should always be done), auto shutdown under fault conditions and most are even regulated with minimal additional filtering caps required.
Click to expand...
The Aion L4 preamp I built used one of these rather than a charge pump. One of the times the much maligned Aion daughter board made sense as it was mounted between the foot switches facing up.
 
drew.spriggs said:
Again - they are literally the same thing in 99% of pedals. Referencing an op-amp to vref instead of ground has the function of creating a bipolar power supply. You can convert between either style by changing vref to GND, or GND to vref (isolated from any other parts of the circuit with a GND reference).


These days, DC-DC converters are probably the easiest solution. If you're sticking to 9v input (which is less common), you can get +/-12 or +/-15v output converters for about $10 for 1w (so 33mA/channel @ 15v) or $15 for 2w (67mA/channel @ 15v). 12v input models are even cheaper - 12v to +/-12v is about $5.

In terms of how to lay them out, it's even easier than charge pumps as they're isolated, but they do take up a little bit more room (I think the one I just finished a board with was 20x10x10mm).

These switch miles out of the audio band (so are much, much less noisy than typical charge pumps), typically only require two additional components to meet EMI regs (not required, but should always be done), auto shutdown under fault conditions and most are even regulated with minimal additional filtering caps required.
Click to expand...

Is the $5 price point for the unregulated version of the DC-DC converters? Is the regulated version worth the $10ish price increase?

How does this compare to an LDO inverter?
 
rossbalch said:
Is the $5 price point for the unregulated version of the DC-DC converters? Is the regulated version worth the $10ish price increase?

How does this compare to an LDO inverter?
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$5 is for unregulated 12v in (as this is a more common commercial voltage range). It's $15 for 2w, regulated, 9v in.

What do you mean by 'LDO inverter'? There are LDO regulators, and inverters - both totally different things.
 
drew.spriggs said:
$5 is for unregulated 12v in (as this is a more common commercial voltage range). It's $15 for 2w, regulated, 9v in.

What do you mean by 'LDO inverter'? There are LDO regulators, and inverters - both totally different things.
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I have seen LDO inverters, looking closer they are more in the 5v range though.

Do you think the regulated outputs are worth it?
 
Google suggests it means “low drop out” and is a step down voltage IC of some kind.
Not going to give me +- 15v from a +9v supply
 
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