Home Forums General DIY Pedal Discussion 18VDC Operation for circuits Reply To: 18VDC Operation for circuits

#3051
Barry
Keymaster

To help I am copy and pasting Tonmann’s original article from the old site here. Unfortunately the excellent Images he had provided from his Scope are no longer available which really helped understand his points. Keep that in mind when reading.

Courtesy of Tonmann:

The gain for op amp and transistor circuits have nothing to do with the power supply, both are set by circuit components.

Distortion is where the output signal is not a faithful reproduction (but usually just a larger amplitude) of the input signal; while clipping can be considered as distortion, distortion is not necessarily clipping:

This is distortion in a transistor circuit which was done by reducing the base voltage. The input signal (green) is perfectly symmetrical above and below the 0V line, the output signal is not symmetrical – the positive going part of the signal is flattened and has a lower amplitude than the negative going part of the signal.

This is clipping in a transistor circuit which was done by increasing the gain of the transistor stage (bypass capacitor across the emitter resistor).

Notice that the signal peaks do not reach the power supply voltages (9V and 0V), this is due to the internal workings of the transistor.

I’ve put in the above information so you can see what I mean when I say that op amps don’t distort,at least not in the sense of the first diagram, but they do clip as in the second diagram.  Of course you can make an op amp distort by adding extra components such as diodes to the circuit.

The level at which op amps clip is dependent on the power supply, consider the following diagram where the power supply is 9V with the output biased to 4.5V and a “standard” op amp is used:

Again notice that due to the internal workings of the op amp the signal never reaches the power supply rails (9V and 0V) but fall about 1.5V short.

Doing the simple math it should be plain to see that the maximum signal level before clipping occurs is about 3V.

If you increase the power supply the maximum signal level before clipping occurs also increases e.g. for a 12V supply you would get roughly a 4.5V signal.

Main Point:

A 3V signal is much too hot for guitar amplifiers or other effects pedals – why would any one want to increase the power supply to get a higher clip free signal when it would have to be attenuated by a larger amount to feed an amplifier?