If this is indeed an issue of analog/digital separation there are a number of electrical strategies to overcoming it. The easiest of these is to have a separate ground plane for your analog circuitry from your digital cicuitry and join those ground planes at the common return (i.e. the power input to your board). This is called a star-ground topology. I'm not sure what you're going for with the diodes you mention in your question, but you might also consider filtering the VCC to your analog circuit through a ferrite bead and capacitor, and make sure you have adequate decoupling capacitors near all the VCC pins of your amplifiers and other analog circuits.

Negative Feedback

It gives you:

• better linearity
• lower gain
• even lower output impedance
• opportunity for frequency-dependent gain

As shown, you get DC feedback via R1+R2. But R1/C1 forms a low pass filter for signals coming from the collector and going to the base. Less negative feedback means less reduction in gain for high frerquencies. Cancel out all those double-negatives and you're left with a high-pass filter.

The transistor below corresponds to Q5 in your drawing.

^{simulate this circuit – Schematic created using CircuitLab}