2.1 preamp (Struggling)

[yatesw]

Evening L&G, (Let me know if this is the wrong category)

I have been battling with my OPA2134(or NE5532) pre-amp circuit for a while now, and its nearly there.
The constrains I have are:
-Needs to be produce a 2.1 sound (L & R & Bass) (cut off at around 160Hz)
-Remain on a single supply
-Sound clean
-Simple as possible (Cost and ease to understand)
-A dial for overall volume and a dial for bass volume (Thats it)
-I can't buy an ebay/AliExpress pre made one sadly.

Its source is most likely a aux from a phone or laptop. After this it will be going to and STA540 Class AB amplifier, then to

The questions I would like to be answered are(if you'd be so kind):
1. Does it matter its an inverted wave? Do I really need to add another chip to re-invert the wave in a buffer?
2. Why is my bass signal so out of phase? Can i do this part of the circuit better? Better cut-off?
3. Where would I take the signal to perform an ADC using a uC
-My thinking is that it should be straight away before the first volume pots? (Is this right?)
4.Generally any improvements i could make. (Gain, impedance matching and such) & redirection to anything similar.


I hope this is all okay & I'm looking forward to your replies!! Open to any help!
I've attached my schematic in PDF

Will

 

[millstonemike]

Evening L&G, (Let me know if this is the wrong category)

I have been battling with my OPA2134(or NE5532) pre-amp circuit for a while now, and its nearly there.
The constrains I have are:
-Needs to be produce a 2.1 sound (L & R & Bass) (cut off at around 160Hz)
-Remain on a single supply
-Sound clean
-Simple as possible (Cost and ease to understand)
-A dial for overall volume and a dial for bass volume (Thats it)
-I can't buy an ebay/AliExpress pre made one sadly.

Its source is most likely a aux from a phone or laptop. After this it will be going to and STA540 Class AB amplifier, then to

The questions I would like to be answered are(if you'd be so kind):
1. Does it matter its an inverted wave? Do I really need to add another chip to re-invert the wave in a buffer?
2. Why is my bass signal so out of phase? Can i do this part of the circuit better? Better cut-off?
3. Where would I take the signal to perform an ADC using a uC
-My thinking is that it should be straight away before the first volume pots? (Is this right?)
4.Generally any improvements i could make. (Gain, impedance matching and such) & redirection to anything similar.


I hope this is all okay & I'm looking forward to your replies!! Open to any help!
I've attached my schematic in PDF

Will

For the bass ".1" channel, it seems like you have implemented two 1st order LP filters in series to achieve a 2nd order filter. The standard approach for such a 2nd order LP filter is the Sallen-Key topology. See the attached pic.

You will always alter phase when implementing a filter. For the 2nd order Sallen-Key topology (Q of .707 is Butterworth; Q of 0.5 is Linkwitz-Riley), you will be 180 degrees out of phase with the input signal. You simply swap the +- leads on the sub to correct this phase issue.

Hope that helps.

 

[yatesw]

Thank you for your input, I have even found a brief application note about MBF and sallen-key filters.

And here's a schematic & article on said sallen-key on a single supply




Any other help is greatly appreciated!!

 

[AndrewT]

Are you looking for a satelite/bass crossover?

Then you need to create three filters.
Two high pass filters to feed the left and right signals to the satelites. These would handle the 160Hz to 20kHz part of the stereo signal.
The volume control would be before these filters.

The third filter, also after the volume control would be a low pass filter that is also a summer/adder to add the two stereo signals and then filter them to pass to the bass only speaker.
This can have an optional sensitivity control to adjust the bass to match the stereo signal. The MFB is an excellent low pass filter that also incorporates a summer. One opamp gives you the sum stage AND the 2pole filter stage. This MFB is an inverting stage. The output from the bass only amplifier needs to be inverted compared to the outputs from the stereo amplifiers.

In operation only one dual track audio/log law potentiometer is adjusted to change the voume you hear.
The bass channel sensitivity control is left alone, once you have set up your 2.1 system to "sound right".

All the above on crossover filters is the basic stuff. You will still need to find out what filtering the two sets of speakers need to "sound right".
Read ESP and Linkwitz.