TDA8947J, Opamp filters and virtual GND - diyAudio
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Old 11th May 2012, 05:40 PM   #1
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Default TDA8947J, Opamp filters and virtual GND

I'm a noob in the diy electronics field, so this may be elementary, but I just can't seem to figure this out:

In order to make a 2.1 amplifier, I've disassembled a Logitech PC speaker set containing two TDA8947J's, which is a 4-channel amplifier chip. I have successfully built the reference circuit from the datasheet in a simple 2-channel (2.0) configuration using channels 1 and 2. This works like a charm.

Now I want to add some filters, so I can send the highs to channels 1+2, and the lows to 3+4, which can be wired in bridge mode to drive a single subwoofer driver.

Since the opamp-based filter circuits need a split supply (+, - and GND), and since I only have a single supply (Vcc and GND), I've created another opamp circuit that outputs Vcc/2.

I've verified that the filter works as expected using a scope and signal generator.

However, when I pass the input signal through the filter (using the virtual GND and signal ground), the filtered signal is offset by Vcc/2 compared to the supply ground, which is the ground used for the TDA. Apparently the TDA doesn't like this, and an incredibly loud noise, completely drowning out any music, is the result.

So what I'm wondering is: Is the principle of what I try to do sound? I.e: Use a Vcc/2 as virtual ground for the opamp filter and input signal, while still using the power ground as ground for the TDA? If not, how should this be wired?

If you need more info, please let me know.

Thanks in advance,

Jeroen
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Old 12th May 2012, 10:39 AM   #2
AndrewT is offline AndrewT  Scotland
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Create a dual polarity supply by using a pair of half wave rectifiers connected to the transformer secondary. Use these only for the low current opamp power.
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Old 14th May 2012, 08:45 AM   #3
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Doesn't that do the same as a simple voltage divider? In either case, I have a signal ground that sits halfway between my power supply voltage, which seems to be a problem for the TDA.
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Old 14th May 2012, 01:16 PM   #4
AndrewT is offline AndrewT  Scotland
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Why do you bother asking?
Just to argue rather than go and research half wave rectifier?
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Old 14th May 2012, 05:22 PM   #5
jcx is online now jcx  United States
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active supply splitting is fine for the filter circuits

likely the TDA internal DC signal ref doesn't match your active gnd

the TDA shows inputs AC gnded to the negative rail, you should C bypass your Vcc/2 ref to Vneg and keep the blocking caps between your filters and the TDA signal inputs
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Old 14th May 2012, 08:06 PM   #6
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Quote:
Originally Posted by jeroen94704 View Post
However, when I pass the input signal through the filter (using the virtual GND and signal ground), Jeroen
What do you mean by this Jeroen? You are not connecting signal gnd to virtual gnd are you?
The filter input needs to be coupled to the input (which presumably sits at Vcc/2) capacitively.
Then the output of the filter (which sits at Vcc/2) is capacitively coupled to the power chip input (which also sits at around Vcc/2 I presume).
In all these 'transactions', signal ground is the only reference.
If indeed the filter output AND the power input sit at Vcc/2 you can just DC couple filter output to power input, of course.

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Old 18th May 2012, 07:09 PM   #7
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Thanks for the replies everyone. I'm trying to wrap my head around it all, and tried some more things based on your feedback, but no luck so far.

I thought a diagram of how I have things wired up right now might help. It probably something obvious. I hope the diagram makes sense at all . One important thing to note is that the diagram shows a TDA7375, because I can't find the TDA8947J in Eagle. The two are not pin-compatible, but they are functionally close. At least I hope to get the idea across:

Click the image to open in full size.

To answer Janneman's questions:

- By signal ground I mean the ground of the input signal. The SGND pin of the TDA is connected to power GND.
- The TDA is not connected to VCC/2, but is powered by the full VCC and power GND voltage, mainly because there is no way the opamp used to obtain VCC/2 can provide the current needed by the TDA.

Last edited by jeroen94704; 18th May 2012 at 07:14 PM.
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Old 18th May 2012, 09:08 PM   #8
jcx is online now jcx  United States
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the op amp may not like driving the rail Caps on the active gnd Vcc/2 output

I'd remove them, maybe move one to the input R divider center to power gnd
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Old 19th May 2012, 01:34 PM   #9
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The "rail caps", that would be C3 and C4, right? So remove those, and move one of them to "the input R divider center to power gnd". But where is that?
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Old 19th May 2012, 01:54 PM   #10
jcx is online now jcx  United States
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parallel with R4
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