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Old June 14th 21, 12:07 PM posted to uk.d-i-y
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Default An electronic question.

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?

--
*He who laughs last, thinks slowest.

Dave Plowman London SW
To e-mail, change noise into sound.

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Old June 14th 21, 12:16 PM posted to uk.d-i-y
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Default An electronic question.

On 14/06/2021 11:07, Dave Plowman (News) wrote:
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
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Old June 14th 21, 12:24 PM posted to uk.d-i-y
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Default An electronic question.

"Dave Plowman (News)" wrote:
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?


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
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Old June 14th 21, 02:21 PM posted to uk.d-i-y
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Default An electronic question.

On 14/06/2021 11:07, Dave Plowman (News) wrote:
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?


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.


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Old June 14th 21, 03:10 PM posted to uk.d-i-y
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Default An electronic question.

In article ,
Fredxx wrote:
On 14/06/2021 11:07, Dave Plowman (News) wrote:
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?


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.

--
*Acupuncture is a jab well done*

Dave Plowman London SW
To e-mail, change noise into sound.


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Old June 14th 21, 03:13 PM posted to uk.d-i-y
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Default An electronic question.

In article ,
Theo wrote:
"Dave Plowman (News)" wrote:
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?


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.


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.

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.


I can see that. Just odd I've never seen it used before.

--
*Succeed, in spite of management *

Dave Plowman London SW
To e-mail, change noise into sound.
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Old June 14th 21, 03:53 PM posted to uk.d-i-y
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Default An electronic question.

"Dave Plowman (News)" wrote:
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

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Old June 14th 21, 05:02 PM posted to uk.d-i-y
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Default An electronic question.

On 14/06/2021 14:10, Dave Plowman (News) wrote:
In article ,
Fredxx wrote:
On 14/06/2021 11:07, Dave Plowman (News) wrote:
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?


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. ;-)


It always "depends" on the detail. :-)

The PS shown is a conventional transformer type with a regulator for each
rail and conventional smoothing.


Are the 2 parallel caps associated with this '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.


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.


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Old June 14th 21, 05:57 PM posted to uk.d-i-y
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Default An electronic question.

Theo wrote:
"Dave Plowman (News)" wrote:
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


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
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Old June 14th 21, 06:55 PM posted to uk.d-i-y
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Default An electronic question.

On 14/06/2021 16:02, Fredxx wrote:
On 14/06/2021 14:10, Dave Plowman (News) wrote:
In article ,
*** Fredxx wrote:
On 14/06/2021 11:07, Dave Plowman (News) wrote:
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?


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. ;-)


It always "depends" on the detail. :-)

The PS shown is a conventional transformer type with a regulator for each
rail and conventional smoothing.


Are the 2 parallel caps associated with this '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.


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.



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



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