An audio circuit I found online and have been playing with has an odd design (to me).
It's op-amp based running off a (separate) +/-15v supply.
There are on board caps across the supply, which is common enough. 10 and
0.1uF in parallel. But instead of going to ground, they are wired across the +/-15v. Does that do the job as well? Or serves a different purpose?
It is probably ok - and it does save two components. Can you post a link to the cct?
PA
Is the opamp powered off those +/- rails? In dual-rail circuits often power is drawn from the + and - rails, and the GND rail acts as a handy halfway reference point but doesn't actually source/sink much current. That means switching transients are taking gulps of current between + and -, and so decoupling caps are placed between + and - to provide them.
If instead of +/-15v you thought of the circuit as 0/+30v with a PNP/NPN stack of transistors between +30v and 0v you might see that even if there's a +15v wire it's largely irrelevant as most of the current is going through the transistor pair.
Theo
I would say it is normal for 0V to be treated as ground and all supply decoupling off that.
I can perhaps understand an instance where you might not want to impose power supply noise/switch/ripple current on the ground rail. It depends on the nature of the power supply.
Thanks for conflicting replies chaps. ;-)
The PS shown is a conventional transformer type with a regulator for each rail and conventional smoothing.
The circuit suggests high quality op-amps and caps (on the audio side) so I doubt it's just to save component count.
Yes. There are a total of 7 opamps. All powered +/- It is an audio filtering device with unbalanced in and out so a good ground important.
I can see that. Just odd I've never seen it used before.
FWIW it's not wrong to use separate capacitors V+ to 0 and 0 to V-, but effectively what you have there is half the capacitance from V+ to V- and double the ESR, so you're adding components to make it worse. It might make sense where there are single-rail loads running between V+ and 0 - for example digital logic, which are more the kind of thing decoupling capacitance is intended for (high frequency switching loads rather than general audio ripple).
Theo
It always "depends" on the detail. :-)
Are the 2 parallel caps associated with this 'conventional smoothing'?
The only explanation I can thing of is the op-amps have very high power supply rejection ratio and the idea is to minimise ground current.
Personally I would have used OV / GND as a PS common and had two caps. There are ways of minimise the injection of PS noise currents.
One thing to be careful of, is this. If you're doing this, the caps should not be dipped tantalum.
+15 ---+----- |+ --- --- | 0 ---+-+--- |+ --- --- | -15 -----+---The problem with the circuit, is the behavior of the power source. Any little bit of reverse bias on a tantalum, sets it up to burst. And the epoxy dipped ones will leave a PCB in a hurry. I had one ricochet off a wall like a bullet. This only happened on my bipolar wired setup (op amp test circuits), like the one above, not on unipolar circuits.
If you're going to do that, ceramic (unpolarized) and maybe an electrolytic will deal with a bit of reverse bias, with less argument.
If you want to do bulk decoupling to reduce ripple on the outside rails, maybe a tantalum would be OK for this. But after my experiences, I don't put tantalum in circuits any more. I still have a few tantalums left, but they stay in that drawer in the parts case.
+15 ---+---------+--- |+ --- --- | 0 ---+-+--- | | | | | -15 -----+-------+----The PSRR of the circuit, helps define how clean your rails need to be. Linear regulators are surprisingly noisy, and opamp PSRR isn't that good at 1MHz. When you need absolutely the lowest noise, supplying power can be a challenge. A switcher at a fixed frequency, followed by several stages of filtering circuits, may give lower overall noise than the usage of linears. That's because, by concentrating all the noise at the one frequency, a more effective filter can be designed to "notch" it out. You don't want variable-frequency switchers, as the noise moves all over the place.
To start with then, running the project off a couple nine volt batteries, and some good-sized electrolytics, might be a way to go. Then there are no linears, and it's just the noise spectrum of a battery (whatever that is).
In the top diagram, the separate bypass on each side of the bipolar supply, I never would have considered that the rail polarity could reverse slightly at shutdown. But the tantalums told me what was happening, in a very effective way. When it goes with a "bang!", it's like it is saying "you idiot, you reversed me!!!". With safer capacitor types, you don't have to worry quite as much.
Paul
Shame you can't point us to the circuits. Even with your extra notes, above, there are still many questions arising - not least if your notes are incorrect. (It does happen!)
Most Op Amps applications are inherently PSU-ripple insensitive.
PA
I managed to find this article, which features different power supply decoupling and some reasoning behind the choices.
They are a design from Elliot Sound Products. But the published schematic doesn't show the supply rail side of the design. You have to buy the PCB to see it.
Now there's a thing. It's a design from ESP I'm referring to. ;-)
Depends is the answer as always. Those will do something but surely there have to be capacitors across each supply half? I'm assuming that the 0v is in fact earth for the circuit. The only time I saw capacitors in the manner you suggest was when the full supply was used as it is as a separate supply to another circuit. One then has to be very careful with your earth! Small caps across split rails are common to stop RF pick up though, that is all I have seen. What exactly is this circuit doing? Normally if its audio bespoke chips can be used that do not in themselves need split supplies. Brian
I had a lot of trouble with some of Uncle Clive Sinclairs monolithic power amps that operated in a bridge config, so the speaker was not really earthed at either end. Worked great until one of power op amps, for in effect that was what they were, popped its clogs and cooked the speakers and firied the other chip.
Those circuits used to pop up in car output stages as I recall. Brian
Assuming that Fredxx has the right link, I think that you should take it with a little pinch of salt. An article originated 2000, referring to a
50+ year old Op Amp design (great advance though it was) is the first hint. Concerns about supply impedance at audio frequencies is another. If the guy was writing about designing with Dynamic Memory, then he'd be closer to the mark. A bit OTT for audio.Incidentally, you attack power supply noise at the power supply. I wonder if the last components in your proposed PSU are - electrolytic and / or other capacitors across the outputs.
At least he isn't trying to argue that no semiconductor device will beat a 12AX7 ;-}
PA
It's comforting to see not everything I say is drivel :-)
I do suggest it is more typical to decouple everything to ground rather than +ve to -ve. And I was trying to think of a possible reason why this configuration might be chosen and not simply to save a couple of components.
I suppose extra decoupling could always be added afterwards if there is any PS noise or a perceived advantage. Without knowing more about the power supply it is difficult to call.
A centre tapped transformer with a full bridge can provide a +ve and -ve outputs, with either a single capacitor or two, as per:
Another reason for having two caps, and decoupling from +ve and -ve to
0V is that many regulators are unstable (and sing or create lots of noise) and specify a certain capacitance with a certain ESR on their output.Without more details this is all lots of speculation.
More than a hint, since transistorss weren't invented then let alone opanps
Concerns about supply impedance at audio frequencies is another.
But you attck nouse ON the power supply wherever its being generated
In 1950, he would be correct
I'd not read that article before.
The PS shown is fairly typical of a regulated analogue type designed for use with audio circuits.
I generally fit 0.1uF ceramic close to each IC between each of the power supply rails and ground.
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