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Old 15th November 2019, 02:46 PM   #11
Mark Tillotson is offline Mark Tillotson
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You can't use a standard series voltage regulator to establish a virtual ground, regulators cannot sink current.


You need an R/RC divider and opamp buffer (perhaps several buffers). This is totally standard practice and you'll see this everywhere.
However its a bit of a hack, a large piece of equipment like this should have true +/-12V supply rails.

Last edited by Mark Tillotson; 15th November 2019 at 02:49 PM.
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Old 15th November 2019, 03:07 PM   #12
Douglas Blake is offline Douglas Blake  Canada
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Quote:
Originally Posted by Mark Tillotson View Post
You can't use a standard series voltage regulator to establish a virtual ground, regulators cannot sink current.
Good point, thanks for posting it.

Yet, I used to see this in my service work, all over the place. Generally the 7812 would be tied to a relatively large capacitor, or several distributed around the board. It pretty much keeps the caps charged while the caps handle the source/sink duties. The 12 volts then acts as a reference voltage, not a true ground.

You are correct that the only true virtual ground is a voltage divider, not a regulator.
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Old 15th November 2019, 10:43 PM   #13
MarcelvdG is offline MarcelvdG  Netherlands
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It will work reliably when you just add a resistor from the 7812 output to ground such that it always has to source at least 5 mA. The current sinking is then done by the resistor and the output decoupling capacitors. Mind you, the noise level is probably a lot higher than with a capacitively filtered voltage divider with or without buffer.
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Old 16th November 2019, 12:46 AM   #14
oon_the_kid is offline oon_the_kid  Malaysia
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Quote:
Originally Posted by JCRAcoustics View Post
Hi All,



I am trying to power the op-amps in my active crossover (image attached) using the single 24V DC supply for the rest of the board. I initially just had a potential divider but this is insufficient as it can't supply enough current without the virtual ground voltage dropping. Smaller resistance values can be used but this results in large heat dissipation.



I've read that an op-amp can deal with this issue however am unsure how to determine how much current they can supply from the data sheet. I'm also unsure of what value on the datasheet states how much current the op-amps in the active crossover require.



I've also read that if I use a virtual ground it can be an issue that I'm powering other components on the board with the same DC supply but I am unsure of why this is or how to rectify the issue.

Does anyone have a solution to this power supply issue and can direct me on how to understand the current requirements?



Thanks!
I think there are some errors in your circuit. C111 and C113 is connected in series. Similarly R70 and R72. You may need to add a capacitor at the final output unless you are very sure the next stage already has one. Generally it is a good idea to put capacitor before the last 2 resistor as it will load your op amp in DC output. But 100k should be fine .You may not need to add in virtual ground at every stage for low pass filter.

Careful in your design as a single supply rail circuits can go boop during turn on and can damage speakers as your virtual ground goes from 0 to half Vcc.

Oon
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Old 16th November 2019, 04:56 PM   #15
JCRAcoustics is offline JCRAcoustics  United Kingdom
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Hi Douglas, thanks for the suggestions.
Would the capacitors in my design such as C115 not act as coupling capacitors? Not entirely sure where I would need to place these caps if there are some missing.
I was looking into regulators but because the power supply to this board may vary (10-24V) I need to guarantee the virtual ground will be half the supply in each instance so fell on the potential divider.
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Old 16th November 2019, 06:02 PM   #16
stratus46 is offline stratus46  United States
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Quote:
Originally Posted by JCRAcoustics View Post
Hi Douglas, thanks for the suggestions.
Would the capacitors in my design such as C115 not act as coupling capacitors? Not entirely sure where I would need to place these caps if there are some missing.
I was looking into regulators but because the power supply to this board may vary (10-24V) I need to guarantee the virtual ground will be half the supply in each instance so fell on the potential divider.
Your life would be SO much easier if you just built a dual (+/-12 Volt) power supply. You have 10
opamps that have a quiescent load of less than 4mA each so a trivial 100 mA supply would deliver
2 times what you need.

The single supply version WILL have a turn on thump as all those caps get charged up so you will
need to add power on mute just to avoid the thump. You REALLY can't have that into the tweeter.

Someone mentioned resistors and caps in series. That is certainly easier than finding exact 2:1
component values to achieve Butterworth response. If you use surface mount it will cost almost
no PC Board space. Just remember caps in series don't add, they divide by 2.

I have done virtual grounds when absolutely necessary (in my car 30 years ago) but under
protest as the added parts are MORE than a dual supply. If I was doing opamps in the
car today I'd add a DC-DC converter to get the negative supply.

That solution could work for you too. If you're worried about switching noise use a +/- 15 volt
supply with linear +/-12 regulators to clean it up. I know with certainty that this approach works
well as I'm using an 8 Volt switcher with a 5 Volt regulator to supply my 5.1 optical DAC on the TV.
The improvement was well over 10dB.

DigiKey has +/- 15 Volt DC-DC converters less than $15.

G

Last edited by stratus46; 16th November 2019 at 06:06 PM.
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Old 17th November 2019, 03:26 AM   #17
Douglas Blake is offline Douglas Blake  Canada
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Quote:
Originally Posted by JCRAcoustics View Post
Hi Douglas, thanks for the suggestions.
Would the capacitors in my design such as C115 not act as coupling capacitors? Not entirely sure where I would need to place these caps if there are some missing.
I was referring to the series chains in each filter with nothing between them... for example: R70 and R72 ... these could be combined into a single 22k resistor. C111 and C113 ... can also be combined into a single .047 uf capacitor. Done across all filters, this would reduce your parts count and thus the risk of interference and failure substantially.

The caps such as C115 and C119 do have a connection between them, so you can't combine them.

Quote:
I was looking into regulators but because the power supply to this board may vary (10-24V) I need to guarantee the virtual ground will be half the supply in each instance so fell on the potential divider.
If the voltage is variable, yes, you would have to use a voltage divider rather than a regulator. A simple solution for this, as others suggested, is to use an opamp as a low impedance buffer.
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Old 17th November 2019, 06:13 AM   #18
Douglas Blake is offline Douglas Blake  Canada
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Quote:
Originally Posted by JCRAcoustics View Post
Hi Douglas, thanks for the suggestions.
I hope you don't mind one more question...

I've been studying your drawing a bit and I'm wondering ... are you trying to implement balanced (xlr) audio?
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Old 17th November 2019, 12:24 PM   #19
JCRAcoustics is offline JCRAcoustics  United Kingdom
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I was using them in series as it makes matching the resistors/capacitors easier. The crossover is designed for differential audio signals coming from a RN-52 BT audio module. Each of the high/low lines are fed to their own TPA3116D2 Amplifier.
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Old 17th November 2019, 01:40 PM   #20
Mark Tillotson is offline Mark Tillotson
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Using a single resistor value makes it easier to get good matching between the R and 2R sections, since you only need one value of precision resistor - reducing the BOM complexity reduces manufacture cost too.


From a tight layout perspective paralleling is more convenient than series strings, ie use 22k and 22k||22k instead of 11k+11k and 11k.
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