# How can I minimise the power consumption of this crossover clipping circuit?

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.

simulate this circuit – Schematic created using CircuitLab

Here is an example output:

## 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).

• Running the simulation on your circuit and putting a current measurement on the collector of Q1 I'm getting 650 uA peak. Where are you seeing 10 mA? Jun 2, 2019 at 21:34
• I get larger currents when I increase the resistor between transistor bases, which I use to reduce the clipping distortion. Jun 2, 2019 at 21:48
• @loudnoises What exactly is this supposed to drive? What's the range on the load? How much power (worst case) must be delivered to the load?
– jonk
Jun 2, 2019 at 21:54
• Do you actually need the current gain? What happens if you disconnect both collectors and just use the transistor BE junctions as diodes? Jun 2, 2019 at 21:56
• Do you just want a deadzone? Jun 2, 2019 at 22:38

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:

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:

With current draw:

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.

• Sounds like you have a plan. I was thinking along the lines of a circuit that could drive anything from, say, 32 Ohms up to many k Ohms, and provide consistent cross-over distortion since the circuit actually measured it and used feedback to control for it under any load situation. In the process, I'd have had to discuss class-AB in lots of detail. The circuit was way more complex than this, though. So I'm actually glad to see this works for you.
– jonk
Jun 3, 2019 at 15:49
• @jonk you've got me intrigued now how you would measure cross-over distortion in a real-time feedback scenario now. I think I'll have to go get up to speed on Class AB designs! Thanks for the thought. Jun 3, 2019 at 16:04
• The technique is to observe a part in the circuit (a grounded resistor, for example.) The voltage across it stays fairly flat and elevated vs signal input if there's no cross-over. Integrating that with a filter yields a maximum value when there's no cross-over. With cross-over, the area diminishes. Here's a picture. The top plot shows the amount of cross-over with a sine wave. The bottom plot shows the measurement point I'm discussing. The green line is for the no cross-over case. Red is "moderate" and gray is "deep." I like puzzles as you presented!
– jonk
Jun 3, 2019 at 16:13
• I should have said "red" is when it just starts to move into the early stages of cross-over.
– jonk
Jun 3, 2019 at 16:33