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Old 1st January 2015, 06:18 PM   #1
Stozzer is offline Stozzer  United Kingdom
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Default Op amp band pass filter connection problems

**picture edited to show correct voltages**

Hey All,

I have built a backpack speaker system that uses an Sure STA 508 4 channel class D amp and it works great.
However i am trying to get the most out of a mid bass speaker by using a band pass filter made of an op amp.
I am having problems getting my head around the connections to ground, as the amp runs on 24v and the preamp and op amp runs on 12v.
I have a split rail power supply made from LM317s and a LM337 which take the 24v and turn it into -6, Gnd and +6v which powers both the op amp and the preamp.
The op amp on its own works fine when i test it with a line source and headphones using the split rail power supply.
The problem arises when i connect it into the amp.
Can someone comment on the ground connections in the picture?
Please excuse the awfull paint picture!
Click the image to open in full size.

Last edited by Stozzer; 1st January 2015 at 07:58 PM. Reason: voltages on picture did not make sense
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Old 1st January 2015, 06:24 PM   #2
Mooly is offline Mooly  United Kingdom
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Just clarify something...

The main power amp you have drawn as connecting across 24 volts (two series connected batteries) but the power amp shows a plus and minus 24 volts. Conventionally that would imply a plus 24 volt, then a ground, then a minus 24 volt rail.

As you say it works I'm assuming it actually runs on a single 24 volt rail.

Its important we know this. Does the point you have marked -24 volts on the power amp actually connect internally to the point marked ground on the power amp ?
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Old 1st January 2015, 06:26 PM   #3
Stozzer is offline Stozzer  United Kingdom
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This is the schematic of the bandpass filter.
Click the image to open in full size.
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Old 1st January 2015, 06:42 PM   #4
rayma is online now rayma  United States
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Quote:
Originally Posted by Stozzer View Post
This is the schematic of the bandpass filter.
It appears that you are trying to have two different grounds, at different potentials.

One terminal of the amp, probably the -24V terminal, is connected internally to ground.

Check with an Ohm meter.

Your split power supply cannot work this way, since its ground is 12V above the amp's ground.

The easiest way to make this work is to run the amp only on the upper 12V battery, with the common battery connection grounded.
THen the lower battery is used for the negative supply.

Last edited by rayma; 1st January 2015 at 06:55 PM.
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Old 1st January 2015, 06:45 PM   #5
Mooly is offline Mooly  United Kingdom
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OK

I was trying to get an idea of how it was all configured with the above questions because as drawn its not really correct... so we really do need to know so that we can be sure nothing goes pop

Also... assuming normal linear regulators (7812, 7912 etc) then you can not get -/+ 12 volts from just two 12 volt batteries. The regulators would need be to a special isolated type to work as are wanting to show.

There are to many inconsistencies as it stands to advise you. Do you see

The power amp as drawn needs a split -/+24 volt supply, 48 volts in total. That is what you have drawn. But without the ground. We need to build on that and work out what you have.
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Old 1st January 2015, 07:09 PM   #6
Stozzer is offline Stozzer  United Kingdom
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Apoligies, i will correct the picture.
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Old 1st January 2015, 07:18 PM   #7
Stozzer is offline Stozzer  United Kingdom
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I have corrected the image to show the voltages.
The power amp can take 0-30V DC, i am running it from the two 12v batteries. The ground from the two batteries is displayed to show it is availble, although i havent connected it to anything.
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Old 1st January 2015, 07:31 PM   #8
Mooly is offline Mooly  United Kingdom
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OK, so the amp uses 24 volts. That makes sense and it means the lower negative battery terminal is really "ground" on the power amp.

Question....

Have you made the opamp circuits yourself ? Is it easily alterable or is it on a premade PCB ?
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Old 1st January 2015, 07:37 PM   #9
Stozzer is offline Stozzer  United Kingdom
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i have made this myself. It is very alterable if need be.
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Old 1st January 2015, 07:43 PM   #10
Mooly is offline Mooly  United Kingdom
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Excellent !

What this is going to come down to is this. You will either need to convert the opamp filter to run on a single 24 volt rail (not a split rail) or if that isn't possible you would need a simple to use DC/DC convertor giving an isolated dual rail.

As drawn its dead easy to rig the opamp for single rail. You would then get rid of the regulators (not needed) and AC couple the opamp output to the main amp input.

The problem you have is wanting two grounds that are going to be at different voltages... not possible as you have it drawn.

All we do is this.

1/ Couple the power amp up as shown running on 24 volts.

2/ The front end preamp you have marked as running on 12 volts will either need a single 7812 type regulator feeding it (the reg input coming from the plus 24 volts) or you could power it from the junction of the batteries (so it runs off the lower one).

3/ That leaves your opamp. You need to add a capacitor (say 10uf) to the output together with a resistor (say 100k) from the output side of the cap to ground.

4/ To convert the opamp to single rail all you do is connect the non inverting input to 12 volts. That is best derived from a zener diode.

I'll draw the essentials for you.
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