Hello all,
Here are some details of an experimental audio amplifier design I have come up with that I hope will set a precedent for low distortion.
It is essentially a conventional audio power amplifier wrapped up into a conditionally stable 4-th order feedback loop to maximise feedback in the audio frequency spectrum.
A detection circuit that automatically senses whenever the amplifier enters a non-linear region of operation (such as during power up / down or when approaching/entering clipping/saturation) electronically and seamlessly switches the feedback loop into an unconditionally stable, 1st order system for the duration that the amplifier remains in a non-linear region of operation. This maintains stability under all operating conditions (well, that is the plan anyway
)
I actually derived this idea from the Hypex patent by Class-D amplifier designer Bruno Putzeys for a 5-th order self oscillating class D amplifier. See here:
http://www.google.com/patents/US20110068864However Class D doesn’t excite me much and I’d like to have a go at applying the principle to a linear amplifier. Here is a simplified/preliminary schematic:
The majority of the loop gain throughout the audio frequency spectrum and three of the poles of the 4-th order system are provided by the three series-connected integrator stages U2 through U4. There are also three zeros which cause the open loop response to revert to a single pole roll-off as the loop gain goes through 0dB, thus making the system conditionally stable. Here is the fourth-order loop gain and phase response:
Now, the capacitor of each integrator is connected in parallel with an analogue switch. When these three analogue switches are activated, the integrators are turned from integrators into basic inverting stages. When this happens the feedback loop transforms into a 1st order system. Here is the open loop gain and phase response with the switches activated:
The trick to the whole concept of course, is the detection circuit that activates the analogue switches, switching the control loop into a 1st order system whenever the amplifier begins to enter saturation. This is achieved by a window comparator based on IC’s U6 and U7 that effectively monitors the “error voltage” at the virtual earth of the feedback resistor network. Any major +/- voltage deviation here indicates that the amplifier is coming “out of regulation” and this is when the detection circuit activates to switch to a 1st order response. So then, does it work? Well, here is the amplifier clipping at 20 kHz with almost 6dB of overdrive: (EDIT - the schematic has since been modified - see reply#8).
The green trace the amplifiers voltage output, the purple trace is the detector output and the red trace is an amplified version of the virtual earth signal. As can be seen, there isn’t a hint of global loop oscillation as the system automatically reverts from a conditionally stable 4-th order one to an unconditionally stable 1-st order one during the saturation/clipping interval.
And finally, here is the (unsaturated, 4-th order-operating) square wave response at 20 kHz:
Note that the overshoot of the conditionally stable 4th order loop is successfully suppressed by the signal input filter (R18, C7) which implements a pole slightly below the frequency at which the three integrators are zeroed out.
I hope to get a real life experimental prototype built up an operational in the week or so.