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previous: Rob W. Hello!I just finish... -- 8/22/1999 5:46 AM

Re: FF: More Treble Wanted!

8/22/1999 1:33 PM
R.G.	Re: FF: More Treble Wanted! Losing treble when you put chords into a distortion device is one of the classic problems with distortion devices. What is happening is that the lower frequency notes pull the signal into clipping one way or the other and the treble notes may not be big enough to pull the signal back into a more linear area, so the higher (and usually smaller) notes get washed out. One way of dealing with this is to not play chords, or to only play simple two note intervals. The other is the way Jack alludes to, the way that is discussed in some depth in "The Technology of the Tube Screamer" at GEO. The tube screamers use a small input cap and other bandwidth limiting to cause a single-time-constant bass cut. This pre-emphasizes the treble into the distortion and gives the usually smaller high notes a chance to be clipped. You can adjust the degree of bass cut by messing with the input cap. The input impedance into a FF is just the junction resistance of the first transistor's base-emitter times its Hfe, which is usually fairly small, Hfe*(25mv/Ie). In the case of the FF where Ie is maybe 1ma, the resistance is probably centered on 1000 to 3000 ohms. With a 2uF input cap, the bass rolloff starts at about 39 Hz, depending on the device. 0.2uF would move it up to about 390Hz +/-, and 0.1uF would put it at 780Hz, about where the Tube Screamers have their input bass breakpoint. There is actually a third way to ensure the treble doesn't get lost. John Greene's post about using MOSFET devices as clipping diodes gives a way to preserve some of the treble content even with large clipping. It's unfortunately not adaptable to the FF, though.

8/22/1999 1:33 PM

R.G.

Re: FF: More Treble Wanted!

Losing treble when you put chords into a distortion device is one of the classic problems with distortion devices. What is happening is that the lower frequency notes pull the signal into clipping one way or the other and the treble notes may not be big enough to pull the signal back into a more linear area, so the higher (and usually smaller) notes get washed out. One way of dealing with this is to not play chords, or to only play simple two note intervals.

The other is the way Jack alludes to, the way that is discussed in some depth in "The Technology of the Tube Screamer" at GEO. The tube screamers use a small input cap and other bandwidth limiting to cause a single-time-constant bass cut. This pre-emphasizes the treble into the distortion and gives the usually smaller high notes a chance to be clipped. You can adjust the degree of bass cut by messing with the input cap.

The input impedance into a FF is just the junction resistance of the first transistor's base-emitter times its Hfe, which is usually fairly small, Hfe*(25mv/Ie). In the case of the FF where Ie is maybe 1ma, the resistance is probably centered on 1000 to 3000 ohms. With a 2uF input cap, the bass rolloff starts at about 39 Hz, depending on the device. 0.2uF would move it up to about 390Hz +/-, and 0.1uF would put it at 780Hz, about where the Tube Screamers have their input bass breakpoint.

There is actually a third way to ensure the treble doesn't get lost. John Greene's post about using MOSFET devices as clipping diodes gives a way to preserve some of the treble content even with large clipping. It's unfortunately not adaptable to the FF, though.

Replies:

AMZ >>a third way... -- 8/22/1999 2:14 PM
Rob W. Thanks very much for the input!!! :... -- 8/23/1999 4:08 AM
Gus R.G. the input Z is even lower I th... -- 8/23/1999 2:03 PM