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Old 18th February 2015, 08:04 PM   #1
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Wink TDA7377 + Baxandall

Hey guys, I am building an amp with TDA7377 for some old ~35W speakers (First pic). To spice things up a bit, I want to add a 2-band baxandall tone control, but I don't know much about them, only read about how to set the frequency on them.

I am planning to use a circuit similar to the uploaded one (the second one), I understand the blue part, but don't get the red one. So all in all the questions are:
*Why do you need the RED part?
*Is it possible to do a stereo baxandall filter without adding another same looking circuit?

If you know the answers or any good links or anything that would point me at the right direction, I would be very greatful
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File Type: jpg TDA7377.jpg (119.5 KB, 111 views)
File Type: jpg BAXANDALL.jpg (59.0 KB, 110 views)
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Old 18th February 2015, 08:51 PM   #2
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The left part serves three purposes:

1. It corrects the polarity.
2. It buffers the input, making it easier for the signal source to drive it.
3. It generates a relatively large amount of noise.

Regarding 1, you can also do that by swapping your speaker wires.
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Old 18th February 2015, 09:00 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

For accuracy the baxandall circuit needs a low impedance source.
Basically mostly its not needed, but would be from a valve source.

rgds, sreten.
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Old 19th February 2015, 10:54 AM   #4
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Quote:
Originally Posted by matas555 View Post
...the questions are:
*Why do you need the RED part?
*Is it possible to do a stereo baxandall filter without adding another same looking circuit?
<< Is it possible to do a stereo baxandall filter without adding another same looking circuit? >>

Not if you want stereo. To use only one tone control you'd have to mix the left and right channels down into one mono channel, and it would be mono from there on. For stereo the shown circuit is built twice, and R4 and R6 are dual pots.

<< Why do you need the RED part? >>

IC1 is an isolation stage. All circuit designers face the "no telling what they'll connect" problem, so an isolation stage ensures that s/his circuit will work no matter what the input might be.

But on top of that, the component values in tone controls are fairly critical--no surprise there--so the isolation stage prevents other parts of the total circuit from interacting with the tone control and screwing the works. This is commonly done.

At the same time, a Baxandall tone control inverts the audio signal by definition, so the designer made IC1 another inverting stage. This means that the output of the tone control is of the same phase as the input. This might or might not matter in any given application, but it's never a bad idea to cover all bases.
.
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Old 20th February 2015, 06:09 PM   #5
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Thanks a lot Really helped me audio electronics +Level up
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Old 20th February 2015, 06:20 PM   #6
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Quote:
Originally Posted by sreten View Post
Hi,

For accuracy the baxandall circuit needs a low impedance source.
Basically mostly its not needed, but would be from a valve source.

rgds, sreten.
This is the most important reason. Baxandall circuit will not work correctly if driven by a high impedance source.

I always use non inverting buffer in front of a tone circuit. Inverting circuit's gain and frequency response can be affected by a high impedance source. I consider it bad form to put an inverting circuit at the input.

The designer of this circuit wanted to correct the phase. I don't think the phase makes any difference, but universal application (provided by non-inverting input buffer) makes all the difference.
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Old 21st February 2015, 04:46 PM   #7
blu_glo is offline blu_glo  England
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I'll second all of the above comments having recommended similar circuit (red highlighted part shown above) to other people's inverting baxandall tone stages.

The resistor values R1, R2 look a little on the high side. Unless you're deliberately gunning for a high input impedance (= potentially more noise) I'd have 10K, 22K or 47K for each.....

Last edited by blu_glo; 21st February 2015 at 04:58 PM.
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Old 21st February 2015, 05:00 PM   #8
blu_glo is offline blu_glo  England
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ooo - who else is using Altium for schematic/PCB? where did the first (yellow) schematic come from?
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Old 21st February 2015, 05:19 PM   #9
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Hey, I am using Altium Made the first schematic myself
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