TDA2050 as "false stereo" amplifier

[cibi]

HI everybody,

I am making a simple amplifier with TDA2050. The particoularity is
that i want my amplifier to mix together left and right channel of
the source, so to amplify them togrther.

To mix source channel i use a regoular dual opamp, each channel in
non inverting configuration, then I use TDA2050 in inverting
configuration, (as a simple mixer). (See picture in attachment).

Practically i split the 22uF-680ohm branch from inverting input to
ground of classical configuration into two branches with such values
to make things equivalent.

The matter is that the configuration doesn't seem to be stable.
Does anyone know why? Has anyone got any possible idea to reach my
intent in a different way?

Thank you very much.

[sreten]

Hi,

You don't need the op-amps. Connect the TDA2050 in shunt
feedback mode with two input resistors, one for each channel.

/sreten.

[cibi]

OK, sreten, I will try, but the reason for the two opamps is the following: without the opamps, the 1 meter cable I use becomes part of the negative feedback net. Its parasitic components may bring to low stability; my fear is that it may oscillate or couple with external signals.
I will let you know.

Tahnks, Cibi

[richie00boy]

Are you using a split or single supply? As you have DC coupled speaker I assume split supply, so then why are you biasing the TDA non-inverting input not at ground? Also your op-amps have no DC reference on their non-inverting inputs, and they are driving too low an impedance, they don't like to see less than 2k.

[cibi]

OK, richie00boy, you're right. The picture i posted is just a draft made with paint to show the principle.

I use a 12V single power supply. It comes from a PC silver box (so it's switching). The speaker has a 1000 uF decoupling capacitor, (not present in the schematic), and the opamps have a 100K - 100K partition in non inverting input for DC polarization and a 10uF capacitor for signal coupling.

Is all this important?

Thank you, Angelo

[richie00boy]

OK that sounds alright. Can you better describe what instability you are experiencing?

[cibi]

1. I have much more noise than I have with the "normal" single side configuration, (that I also built myself);
2. The speaker makes something like "shots" with a frequency of about once per second.

[richie00boy]

Have you tried a 12V car battery instead of SMPS to see if it still does it?