[2653] There Is No Sag in FX Pedals

Date: January 29th, 2020 | Comments : none | Categories: DIY.

There is no sag in guitar effects pedals. Sorry.

I should start off by defining what sag means with respect to audio circuits.

Sag is the drop in the power supply voltage when an amplifier input is hit with a sudden strong signal. It is most often found in tube amplifiers and is caused by resistance from the tube rectifier and the resistors that are used to filter ripple out of the dc voltage in the B+ voltage line going to the power amp. A large input causes a sudden need for more current in the power stage in order to meet the demand imposed by the big signal.

Ohms law says that current though a resistance will have a voltage drop across that resistance, so the available voltage at the output tubes will decrease as the current increases. The voltage on the power tubes sags (decreases) momentarily until the signal strength decays and the amp’s power supply can once again provide the needed current/voltage.

Sag is most obvious in amps with the volume cranked up. The effect may be small and most often is a feel noticed by the player rather than an audible effect. It is detected as a sudden compression in response to picking with a moderately slow decay.

There are several reason that there is no sag effect in guitar pedals:

1. Guitar pedals run at very low currents, usually on the order of a few millamps.
2. The current draw in most pedals does not vary much at all from idle to max signal. If there is no change in current draw, there can be no sag.
3. Effects pedals usually have a big value capacitor as a filter on the dc voltage input. This capacitor acts as a reservoir that can supply power for a sudden demand when a strong audio signal hits the circuit. The cap will smooth out minor fluctuations in voltage/current.
4. If the pedal is powered by a wall adapter, then it will have plenty of amps on taps that are more than enough to deal with changes in current. A 1700 ma. regulated wall adapter is not going to have any trouble keeping up with the demand for power from any fx pedal, especially since most pedals are very low current.
5. Tube amps most often have unregulated power supplies and the voltage can fluctuate (sag) depending on how much current is demanded from the power supply because of the lack of regulation. Most pedalboards are using regulated power supplies that can compensate for changes in current draw, and will maintain a steady voltage (no sag). The regulation circuit in the dc adapter is rapidly altering its internal feedback to deal with the changing demands on its output so that it keeps a steady voltage.

A lot of players say that they can feel a sag coming from a pedal, usually a distortion or booster, and it is a real effect… just not sag. A strong signal into a guitar pedal will peak the audio and it is possible for the pedal to run out of voltage swing if it amplifies the signal by a significant amount. When the output transistors or op amps can no longer swing the signal to the extremes that are dictated by the circuit design, the peaks of the audio will be rounded over or even clipped on strong signals. The transition from clean (or slightly distorted) signal to a clipped signal introduces a form of peak limiting and compression, which is what the player has detected. It’s just not sag, as the term is used in relation to amplifiers.

There is no sag in guitar effects pedals… but there may be compression that sounds similar.


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