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Old 7th October 2008, 10:00 AM   #1
Mordor is offline Mordor  Italy
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Default common ground or not?

I'm building an amp for my subwoofer. Now i'm drawing the schematic of inverter. The question is:
12 volt ground is connected to amp rail ground or not?
If i'snt connected what can i do for insulate the signal input rca ground ?
To get a feedback from rail can i do another small coil on toroid ?

sorry for my bad english, thank
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Old 7th October 2008, 10:15 AM   #2
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The primary and secondary grounds have to be isolated unless you have another way to prevent ground loops (transformer on input or active/balanced input circuit).

If you want to use regulation for the rails and also want an isolated secondary, you can use opto-couplers. It's also possible to use feedback with an isolated secondary (no opto-couplers) but it can lead to instability (noise, oscillation).
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Old 7th October 2008, 11:19 AM   #3
Mordor is offline Mordor  Italy
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mmm ok, optocoupling! But optocoupler have a linear feedback? I need to 0 - 5 volt variable signals ( i use an atmega to drive inverter ).. can you link a schematic with an optocoupler example? thank.
p.s. RCA signal ground must be disconnected or hiZ. Can i put 1k resistor between connector?
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Old 7th October 2008, 11:35 AM   #4
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Quote:
The question is: 12 volt ground is connected to amp rail ground or not?
Most all car amplifiers I have seen have a ~1K resistor between the 12V (primary side) and the amp ground (secondary side).
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Old 7th October 2008, 11:52 AM   #5
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You can get relatively linear output from an opto-coupler but it's somewhat complex. None of the car amplifiers that use opto-couplers for regulation have a linear output. I've only seen that in off-line 12v switching power supplies.

Since the secondary ground and the primary ground will be very close, you could try using an op-amp as a differential amplifier (see attached). The inverting input would go to the secondary ground and the non-inverting input would go to the positive rail.

The inputs to the op-amp would have to have voltage dividers to drop the voltage. You could start with 20k ohms for R1 and R3 and 2k ohms for R2 and R4. This should give you 0-5v for a 50v rail.

The op-amp is to be powered from the 12v supply. You'd have to use an op-amp that can accept ground as an input (LM358 or similar). This would be less likely to have problems when/if the secondary ground fell below the primary ground if it had a split supply (instead of only 0-12v).

You could produce the negative voltage for the op-amp by driving a voltage doubler off of the primary side of the power transformer. A simple Zener shunt regulator could be used to limit the voltage produced by the doubler.
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Old 7th October 2008, 12:08 PM   #6
Mordor is offline Mordor  Italy
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ahm ok! i use op amp with negative feedback to keep low the gain... The 1 k resistor between two ground must be used for close the circuit.
on the amp input signal ground i put 1 k resistor. ok ?
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