Guitar Preamp


So far this is only a very basic guitar preamp with just the more essential features, but may become something more complex eventually when I try a few ideas. There are slightly different requirements depending on the type of pickup. The 'humbucker' dual coil types have generally a higher output voltage, and have a higher recommended load impedance, typically 500k or more, and this is the only type I have been using for initial tests. The lower output single coil types can be used with the same design, but it may then be a good idea to add some gain in the input stage. With a high output type I decided to stick to a unity gain input buffer to avoid overload problems. The buffer needs to provide a high input impedance and low noise, and also I wanted it to operate from a single 9V battery. The OPA2134 dual op-amp appears to be a good choice, and can also provide the second stage.

The second stage needs to perform a few different functions. First is frequency equalisation. Guitar outputs tend have excessive bass, and some correction is needed, but of course this is a matter of taste and so an adjustable bass cut filter is needed. Another function is adjustable clipping. Unlike hi-fi sound amplification where we aim for minimum distortion and want to avoid clipping, guitar amplifiers are expected to contribute to the sound. In a live performance at high sound levels the actual clipping of the power amplifier may give the wanted effect, but for practice use with either a domestic hi-fi system or a headphone amplifier something less drastic is needed, and adjustable clipping at a few hundred mV level can be applied using diodes. There are many variations published, for example those at GM-Arts and Elliot Sound Products.
Most of these use symmetric clipping using a back to back pair of diodes, but asymmetric clipping should add some even harmonics, and may be useful, so an adjustable level of asymmetric clipping is added. This can have one unwanted consequence, which is the addition of a DC component to the signal, so we then need an output coupling capacitor to filter out low bass and DC. I had a few old OA5 germanium diodes, and this type are sometimes preferred on the grounds that they give softer clipping, so these are tried first. Silicon types such as 1N914 should work ok, giving sharper clipping.

To make the preamp portable I decided on battery operation, and fortunately the chosen op-amp has reasonably low current demand and sufficient output with only a +/- 4.5V supply. To convert a single 9V battery to a dual supply we can use a simple potential divider and smoothing capacitors, but it is easy to also make the divider operate as a battery voltage indicator by adding two LEDs in series with each resistor. The ones I used are 3mm Red LED type HLMP-1301. These drop about 1.7V each when lit, so if the battery voltage drops to about 4 times this, i.e. 6.8V, then the LEDs become very dim, and we know the battery needs replacing or recharging. I found very little audible deterioration down to that point. With the preamp in a case one of the LEDs can be mounted on the control panel where it is easily visible.

The first example tried is shown next:

The first 2k lin control on the left is the bass cut filter giving -3dB level adjustable from 500Hz down to 70Hz. The other 2k adjusts the symmetry of the clipping, giving adjustment from equal clipping levels up to maybe a two to one ratio, but this depends on signal level. The 100k log control operates as a volume control when at low resistance, but at higher levels clipping takes place and the control is then changing how heavy the clipping is.


The asymmetric clipping proved to be disappointing, there was some audible effect but not enough to convince me this was a useful addition. What does appear to be useful is a low pass filter to reduce the high frequency components added by the clipping, and also an output volume control so that headphones can be driven direct. The next example was tried, reverting to symmetric clipping, and there is now an adjustable output filter for use when connected to an amplifier, but this also works well enough as a volume control when the output is connected to typical 32R headphones such as the Sennheiser PX100s. The 22uF output capacitor should be increased to 100uF for driving headphones, and should preferably be a non-polarised type. It could actually be omitted, I only included it for the asymmetric clipping to eliminate the resulting DC output, but it also gives some protection to the headphones in case of fault conditions, so I prefer to keep it.

Clearly there are many possible variations, and further experiment is needed to find improvements. On the plus side the preamp is very low noise, I was unable to hear any circuit noise in normal operation, and battery life is very good, I used a rechargeable PP3 size battery, and expect it to last for around 20 hours on a single charge.
I only intended to make one of these for my son to practice his guitar, so I only built it on Veroboard (perforated copper strip board) so I don't have a PCB design for it in its present form.

Other effects to try include compression, reverb, and tremolo, but maybe I will just copy some of these from the Elliot Sound Products musical instrument project list.


References

OPA2134 Data, this is the dual version of the jfet opamp OPA134.

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