Friday, 2 September 2016

Klon Centaur 3v3 Clone: LTspice analysis


Klon Centaur is one of the most famous, controversial, loved and talk-about guitar pedals and also one of the most cloned and copied ones, probably because it's quite expensive. Basically it's a Boost / Overdrive pedal with a gain, a treble and a volume control. It's subtly mixes the original signal with an amplified version of the signal that also goes through a germanium clipping section, before everything gets again amplified and filtered by the tone section that can considerably reinforce the mids.

Klon Centaur guitar boost/overdrive pedal
The first dual opamp section is powered between GND and 9V, but the second dual opamp section is powered between -9V and 18V to have more headroom before the opamps saturate. The circuit has a lot of gain and the signal that can get into the guitar power amp is huge, to breakout the guitar amp preamp section.

The main reason for making this project is to verify this idea I will expose here:

The output of most guitar pick-ups is usually several hundred millivolts peak-to-peak, 1Vpp is already a loud signal for a guitar amp. At the same time, guitar amps and specially valve amps are biased in a way that 2Vpp already saturates the input.Why then a guitar pedal should need more than 2Vpp or 3Vpp of headroom since those levels would be already enough to saturate the input and "breaking-up" the amp?

On the other hand, "breaking-up" or saturating the signal inside the pedal gives more control than "breaking-up" the guitar amp, because each model of guitar amp may have different input headroom and hence saturate earlier or later than others.

9V battery voltage is the "standard power" on guitar pedals, but that does not mean that the electronic circuits inside the pedal must be powered at 9V, they could be powered at a lower voltage and still have enough headroom for feeding the guitar amp.

Nowadays there exist very good audio opamps with very low harmonic distortion that can be powered at lower voltages and have rail-to-rail inputs and outputs.

If an efficient step-down converter is used, battery charge could be maximized and guitar pedal could have the same performance even at the end of the life of the battery. At a lower power we also could expect less power consumption and longer battery life. There exist cheap, efficient and stable step-down converters with a small footprint, they just need a small inductor, input and output capacitor and a couple of resistors to set the output voltage. Switching frequencies can be very high, so that any ripple in the power supply will not be audible, and additional LC filtering can be added to minimize ripple and noise.

With all that in mind I decided to do a Klon Centaur "3.3V clone" pedal.

Klon Centaur LTSpice analysis

I will start by doing a LTSpice simulation of the original circuit, and then I will try to implement a 3.3V circuit that tries to match the response of the original circuit.
Klon Centaur LTspice schematics

Klon Centaur Frequency response. Gain =0-10. Treble = 10
First we do a frequency response analysis between 10 Hz and 20kHz with treble set to 10 and varying gain from 0 to 10. Klon Centaur produces a high gain to small signal from 47dB @ 810Hz up to 75dB @ 674Hz reinforcing mids with peaks around 670 - 810Hz. Gains are much smaller at low and high frequencies.
Klon Centaur Frequency response. Gain =10. Treble = 0 - 10
Then we do a frequency response analysis between 10 Hz and 20kHz with gain set at 10 and treble varying from 0 to 10. Klon Centaur amplifies the signal at mids with a peak around 650Hz from 52 dB to 75 dB.
Klon Centaur Time response. Gain = 0-10. Treble = 10
Now we do a time response analysis during 25ms (only 6ms are shown) with gain between 0 to 10 and treble set to 10 using four 300mVpp sinewaves at different frequencies: 82Hz, 440Hz, 2kHz and 4kHz. The output obtained is a large amplitude signal more than 24Vpp that saturates the op-amps between -12 and 12V.
Klon Centaur Time response. Gain = 10. Treble = 0-10
Finally we do a time response analysis during 25ms (only 6ms are shown) with gain set to 10 and treble between 0 and 10 using the same four 300mVpp sinewaves. The output obtained only saturates the op-amps for high treble values.

Klone Centaur 3V3 clone LTSpice analysis

The Klone Centaur circuit is adapted to 3.3V using 3.3V rail-to-rail op-amps and passive values have been changed to provide a similar level of saturation, which means lower level of gain.
Klon Centaur 3V3 clone LTspice schematics
Klon 3V3 clone Frequency response. Gain =0-10. Treble = 10
Again we do a frequency response analysis between 10 Hz and 20kHz with treble set to 10 and varying gain from 0 to 10. Klon 3V3 clone produces a high gain to small signal from 20dB @ 1kHz up to 39dB @ 970Hz reinforcing mids with peaks around 970 - 1kHz. Gains are much smaller at low and high frequencies.
Klon 3V3 clone Frequency response. Gain =10. Treble = 0-10
Then we do a frequency response analysis between 10 Hz and 20kHz with gain set at 10 and treble varying from 0 to 10. Klon 3V3 clone amplifies the signal at mids with a peak around between 735 Hz to 970Hz from 21 dB to 39 dB.
Klon 3V3 clone time response. Gain = 0-10. Treble = 10
Now we do a time response analysis during 25ms (only 6ms are shown) with gain between 0 to 10 and treble set to 10 using four 300mVpp sinewaves at different frequencies: 82Hz, 440Hz, 2kHz and 4kHz. The output obtained is a large amplitude signal, but in this case op-amps saturate between -1.65 and 1.65V for a maximum signal of 3.3Vpp.
Klon 3V3 clone time response. Gain = 10. Treble = 0-10
Finally we do a time response analysis during 25ms (only 6ms are shown) with gain set to 10 and treble between 0 and 10 using the same four 300mVpp sinewaves. The output obtained only saturates the op-amps for high treble values but again maximum signal level is 3.3Vpp.

As we can see, frequency response gain has been reduced from a maximum of 75dB to 39dB, 36dB less in order to obtain a similar level of signal saturation in the time domain. The signal that will get to the guitar amp will be different in both pedals, specially at high gains, Klone Centaur pedal provides large signals of 24Vpp, while the 3v3 clone will provide 3.3Vpp signals at most. Will it sound very different? The only way to really know it is doing a real implementation of the effect.
Stay tuned for next post on effect practical implementation.

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