Electronic – How to minimise the power consumption of this crossover clipping circuit

circuit-designclass-b-amplifierdistortion

I am trying to produce a circuit that produces crossover clipping distortion for musical purposes (to sound like a crappy old transistor radio). The problem I am running into (among a few, but to limit the scope) is that to observe clipping distortion, the circuit draws a ton of current to drive the BJTs and load. My intention for the circuit is to run it in a guitar pedal off a 9 V battery (though boosted to 18 V for op-amp headroom), but this seems implausible with the current design.

Is there an efficient way to emulate crossover distortion?

Circuit

The current circuit used is a simplistic Class-B amplifier. As R4 increase the bias improves, moving it towards Class-AB and reducing the distortion. This will be parameterised with a potentiometer so that musicians can dial in the amount.

schematic

simulate this circuit – Schematic created using CircuitLab

Here is an example output:
enter image description here

Design constraints

The three design constraints I am dealing with are:

  1. Keeping resistances relatively high so that the highpass filters formed with C1 and C2 keep a low cutoff frequency, allowing wide-band signals to pass through. Ideally the cutoffs should be beneath 20 Hz.

  2. R3 and R2 can be left equal for simplicity.

  3. The ratio between R3/R2 and R4 is one factor that determines the amount of crossover clipping

  4. The second clipping factor is the load, R1, as it decreases the distortion increases.

Problem

Overall the circuit draws nearly 10 mA of current. For comparison, I have two Sallen-Key op-amp filters before it which each draw less than 1 mA each (ideal LTspice simulation, mostly quiescent). Do I just have to suck it up and suggest that users keep a draw full of batteries to use? (Or more likely a wall wart).

Best Answer

Following @DaveTweed 's comment, a low-power cross clipper has become obvious to me. There are different issues with the circuit now: cross-clipping is heavily dependent upon input amplitude, so if the signal is loud the relative distortion is low, a low-amplitude signal will be dominated by the distortion, if it passes at all. It would be nice to figure out a way of tracking the distortion based on input amplitude...

Circuit

Removing the collector connections of the NPN and PNP BJTs, the circuit becomes:

schematic

simulate this circuit – Schematic created using CircuitLab

Values for R1 - R4 have been adjusted to provide an optimal range of clipping distortion. By increasing R1, capacitors C1 and C2 can be dropped, meaning I can probably use nicer cap dialectrics like film instead of electrolytics.

Behaviour

Changing the value of R4 from 10 Ω to 100 kΩ (as it might be with a potentiometer) wit 10 kΩ steps yields the following behaviour:

enter image description here

With current draw:

enter image description here

So a max current draw of less than 50 µA for a 1 Vpp signal, I'm quite happy with it! I still probably don't understand Class-B / AB amplifier design very well, but this accomplishes my design goals. Thank you to everyone for their helpful comments, and @DaveTweed if you want to submit an answer I will mark it as correct, since this answer simply follows on from your direction.