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Virtual ground supply help
Virtual ground supply help
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Old 13th June 2018, 04:37 PM   #1
MAAC0 is offline MAAC0  Portugal
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Default Virtual ground supply help

Ok folks
You know all those little boxes for PA use like Promatch, Samson S Convert, Wireless mic receivers etc..
All of those usually use a 12V 500mA to 1A single wall-wart supply.
Assuming a virtual ground for powering a couple of op-amps, I'm left with -+6V for rail Voltage. +4dBu is 1.23V so I have some margin.
DJ consoles are usually powered from USB 5V. Perhaps they feature a step up converter somehow.
I don't have access to exotic chips like rail splitters and OPA series op-amps unless I import them.
I need a simple schematic for about 100 mA rail current. Assuming 4*25mA Op-amps, perferrably regulated.
Can anyone point me in the right direction ?
Thank You.
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Old 14th June 2018, 11:09 AM   #2
FauxFrench is offline FauxFrench  France
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Hi MAACO,

I am surprised no members have responded here. Many took part in a discussion on virtual grounds sometime ago.
For the moment you will have to do with me.

To make a virtual ground with components at hand, I will suggest you:
either 2pcs LM358 (4 OP-AMPs) connected in buffer configuration, with 47Ohm in series with each output before a common (virtual ground) connection point with 1000uF-2200uF capacitors to each rail and the OP-AMPs biased at the inputs by a voltage divider of two 10K resistors,
OR,
A push-pull stage biased just below conduction, using BD139/140 and 2K2 base resistors to each rail and three 1N4148 diodes between the two base pins. Again two 1000uF-2200uF capacitors to each rail from the emitters connection point. You better use 10Ohm resistors in series with the collectors to limit the transistor currents during charging of the capacitors.

Is that sufficient and adequate for your use?

Last edited by FauxFrench; 14th June 2018 at 11:15 AM.
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Old 14th June 2018, 04:26 PM   #3
MAAC0 is offline MAAC0  Portugal
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I will try the fist one. I've seen here on electronics shops DC / DC converters also, but I'm afraid of the noise and don't know the current they can deliver. I will ask.








Thanks.
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Old 14th June 2018, 05:35 PM   #4
OlegSh is offline OlegSh
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Virtual ground supply help
Some time ago I designed a dual polarity PSU based on industrial grade DC-DC converter with low noise LDO regs on each rail and HF filtering on DC-DC converter input and outputs. The result is shown here. The schematic and design process is documented earlier in that thread. The PSU is very "quiet" so DC-DC converter based PSUs can be made suitable for audio with a bit of care. Such power supplies solve several problems at once. They reduce weight and size as well as help avoiding dealing with mains circuits by simply using a ready made suitable power adapter. I use high current USB charger to power mine for example.

Regards,
Oleg
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Old 15th June 2018, 04:43 AM   #5
nigelwright7557 is offline nigelwright7557  United Kingdom
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If you have a positive rail you can make a negative rail with an ICL7660 and a couple of capacitors.

I did that on a USB powered oscilloscope and it worked very well.
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Old 15th June 2018, 06:43 AM   #6
FauxFrench is offline FauxFrench  France
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Quote:
Originally Posted by nigelwright7557 View Post
If you have a positive rail you can make a negative rail with an ICL7660 and a couple of capacitors.

I did that on a USB powered oscilloscope and it worked very well.
Isn't the problem that ICL7660 is rated for up to 10V input voltage with an extended version "ICL7660A" handling up to 12V (just on the limit)?
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Old 15th June 2018, 07:05 AM   #7
nigelwright7557 is offline nigelwright7557  United Kingdom
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I have used it for -12 volts with no problems.
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Old 15th June 2018, 11:28 PM   #8
MAAC0 is offline MAAC0  Portugal
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The problem is getting those exotic chips...
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