Audio Amplifier input stages - help needed

[Ducados]

I have built a small amplifier based on actual designs suggested by the datasheet of the TDA 2004 (8 watt stereo). This works fine with my guitar effects pedals, It's surprisingly loud, clear and with little distortion.

I then built a stereo preamplifier based on the TLA071 opamp, again with a design included in the datasheet (gain 10:1, non inverting input). The preamp also works fine, hardwired to the other amplifier with an MP3 as a source

The problem is that when I plug in both input jacks, the sound quality deteriorates, especially the Roland effects pedals. I also noticed a small reduction in sound quality when I implemented the volume controls. Do I need to make the inputs
"invisible" to each other or would the problem most likely be impedance?

Any advice appreciated.

 

[betwixt]

A schematic would help.

It is possible to make them 'invisible' to each other by using what's called a 'virtual
ground'. That means the point where the two inputs are combined effectively has no signal on it so nothing from one input can influence the other. Of course, it isn't a real ground and the reason that nothing seems to be there is that an equal and opposite signal is also being added. That opposing signal is what you tap into to drive your amplifier. In reality, it is very simple and consists of nothing more than an inverting amplifier stage with feedback applied to the virtual ground point. If you look at audio mixer schematics you should see the principle in use.

Brian.

 

[Audioguru]

The "gain of ten" circuit shown on the datasheet of the Texas Instruments TL071 opamp has an extremely low input impedance inverting amplifier. Maybe a non-inverting amplifier with a higher input impedance will fix the problem.