Tube Amps / Music Electronics
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|3/6/1998 8:42 AM|
|R.G.||Re: Fuzz Face for the people!|
>Input impedance of FF is about hfe*25/Ic. If
>hfe=84 and Ic .25 mA, it's about 8.4k,
If only that were the case. As I understand it, the voltage feedback connection actually reduces the input impedance even more, as the voltage at the collector is 180 degrees out of phase and much larger. The effective input impedance of the VF stage is probably under 1K. I have a text somewhere that describes how to figure a number, but it's supposed to be even lower than the input impedance for a grounded emitter transistor stage with no feedback, which you correctly quote.
Adding an emitter follower adds enough current drive to drive the input as a voltage however los the input impedance is, and as a result can sucessfully drive the first transistor into hard saturation, which is otherwise impossible with only a guitar pickup. This causes the input to lose that soft mushy saturation that sounds so good, and makes the sound harsher - as you correctly note.
If you want to buffer the input, you need to put some impedance in series between the buffer and the input of the FF circuit proper. This impedance should be between 10K and 47K, in the range of impedances for pickups.
Diddling the value of the collector resistor is still useful to tailor the conduction current for special situations. As you note, this would change the saturation characteristics and the tone of the FF.
|3/6/1998 8:51 AM|
If only that were the case. As I understand it, the voltage feedback connection actually reduces the input impedance even more, as the voltage at the collector is 180 degrees out of phase and much larger. The effective input impedance of the VF stage is probably under 1K.
I wrote a reply to my own post of the same subject, but isn't it so that the bypass cap changes things, as the feedback voltage no more strictly follows that of at the collector of Q1 due to the charging/discharging of the 22uF cap and the internal emitter resistance of Q2?
|3/6/1998 9:32 AM|
To the extent that it successfully bypasses all of the signal. This does happen at full drive settings. At other settings, you still have the 100K feedback path lowering the input impedance, as soon as you get a few hundred ohms off the max setting. In that case, the input impedance goes down as you back off on the drive control.
One of the difficulties in understanding the FF is that there are two feedback loops tied up right at that one component, the 100K feedback resistor, and they affect both AC and DC situations.
Another way to look at the FF is that the thing is a feedback amp with Rf determined by the 100K/1K/22uF and Ri being the source impedance of the guitar pickup/volume control/cord capacitance.
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