base blocking resistor

[Jneely88]

Hello

When a design shows a resistor following the input in say a fuzz or distortion to the base of a transistor. Is this for blocking? Like a grid stopper on a tube?

Thanks
Jeff

[R.G.]

Maybe. It very, very much depends on the rest of the circuit. Can you post an example?

[teemuk]

It could be a "RC" circuit, resistor working in combination with base capacitance....

...or it could be there to limit excessive base current, a protective addition.

That's already two basic applications for a resistor so agreed; very much depends on rest of the circuit.

[Jneely88]

Actually I will post the schematic tonight. Essentially its a transistor muff fuzz with a jfet buffer. Coming off the output cap of the buffer is a 100k pot that then goes into the first stage of the muff fuzz...it does have an input cap. The problem is when the pot is wide open there is oscillation and the tone becomes shrill. Thin. Treble loaded. Just a hair back and it's fine

I will post a link schematic tonight when I get in.

[GibsonGM]

Yes, you can add a base resistor to deal with that. Low-value, of course...could be 4.7k?   

I would start small and go up if needed.  I hate when pots make that happen...

[Jneely88]

http://i1268.photobucket.com/albums/jj567/jeffreyneely88/new-od-fuzz_zps1bbiibuc.png

Here is a link to the drawing

[GibsonGM]

I think the purpose of the 150R resistor at the input should be doing that job, no??   

You also have the 4.7k there?


I don't think you'd have any problems if you increased that 150R to something higher. Even 1K, maybe...it's good to try things like this, and you will cause NO harm to the transistors.

[Jneely88]

The 4.7k is just to keep the pot from hitting zero volume. The issue is the higher I raise the 150R the less gain I have overall.

[Jneely88]

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[Jneely88]

My question is also...the buffer is suppose to be unity gain. So why does it do this? I was under the assumtion that unity was equal to the signal going in. When I run it without the buffer I don't have this issue.

[Keppy]

Actually I will post the schematic tonight. Essentially its a transistor muff fuzz with a jfet buffer. Coming off the output cap of the buffer is a 100k pot that then goes into the first stage of the muff fuzz...it does have an input cap. The problem is when the pot is wide open there is oscillation and the tone becomes shrill. Thin. Treble loaded. Just a hair back and it's fine

I will post a link schematic tonight when I get in.

If you move R15 to the other side of the gain pot, it will divide the signal down a little (VERY little) without increasing the parts count. This might get you the "just a hair back" that you need.

C3 is a very small coupling cap for such a low impedance (150R plus whatever Q1 adds). The impedance is lowest when the gain is maxed, so it will tend to be shrill under those conditions. Increasing C3 will darken the fuzz at high settings.

My question is also...the buffer is suppose to be unity gain. So why does it do this? I was under the assumtion that unity was equal to the signal going in. When I run it without the buffer I don't have this issue.

The Muff Fuzz circuit (Fuzz Face variant, low input impedance) loads down your guitar signal causing loss of signal voltage, particularly in the high frequencies. Buffering reduces/prevents this by providing unity VOLTAGE gain but lots of CURRENT gain. Running into high impedances, buffers affect the sound very little because there was very little signal loss to begin with. Running into low impedances, they affect the sound a lot. Search for Fuzz Faces and buffered pedals for a more in-depth explanation of this effect.

Also, removing the buffer probably kills the oscillation just by allowing some signal loss at the input.

[Jneely88]

Thanks Keppy

I was just emailing a reply saying I was thinking it was a impedance issue. I will try both of those.

[Jneely88]

Nope no change bummer. went up to .22 uf for the cap and moved r15 and tried up to 3 k. I have the same issue on the bread board and
i bought the muff fuzz pcb from GGG and added a small pref board for the buffer. same issue each set up. So I am pretty sure its the circuit not layout.

[Jneely88]

found this link. http://www.diystompboxes.com/analogalchemy/sch/antiquity.html

I used a 1n4148 and the oscillation is gone..

[GibsonGM]

found this link. http://www.diystompboxes.com/analogalchemy/sch/antiquity.html

I used a 1n4148 and the oscillation is gone..

Hmm, interesting - I'd like to know why this occurs.  I've had the same thing happen and chaulked it up to some kind of coupling causing the interaction (maybe a bigger power supply cap or something would help?? Dunno).

Now I'm curious why the diode helps...

[Jneely88]

Maybe RG knows. I am guessing blocking current.

[R.G.]

The fuzz-face style two-transistor amplifier has some unusual characteristics. One of these is that it has quite a low input impedance. The impedance presented to the input signal is just the Shockley resistance of the first transistor's base-emitter, as modified by any feedback. In this case, the feedback in negative, which reduces the input impedance of an overall inverting stage like this.

This is actually the origin of the "harshness" of fuzz face circuits when driven by buffers and the preference of many musicians for putting fuzz face circuits first in their chain, right after the guitar. The electric guitar's output is fairly high impedance and much higher than the input impedance of the fuzz face circuit. So the *guitar* impedance acts as a current limiter, stopping the ff circuit from being so grossly overdriven, and also making the circuit's output sound much more controllable by rolling off the guitar's volume pot. The pot not only reduces signal level, it inserts a resistance of up to 1/4 of the volume pot in series with the signal going to the cable. So you get a complex mix of changing input impedances and change in tone with the change of impedance of the guitar signal driving a current into the input.

Putting a resistor in series with the input of a ff circuit is one method of keeping the maximum effective gain at least bounded. This can make it less harsh, or make it stop oscillating. This principle has been re-"discovered" many times. As you might expect, the actual changes this causes depend a lot on the transistors in the ff, and the resistor/capacitor components there as well. A low value resistor, even 150R, may cause the input impedance of the circuit to be drastically larger than it would otherwise be.

This has all kinds of implications, as you might guess.

The diode in series with the input is probably there as a variable impedance. Maybe. Have to ask Joe what he intended. That diode is very marginally turned on by the Vbe of the first transistor, but at a much lower current, and its signal impedance varies a lot depending on how much current is flowing through it. Big input signals will change its impedance a lot, causing a lot of half-cycle by half-cycle variation in impedance and in output signal.

[Jneely88]

Thanks RG.

[Jneely88]

So the stock transistor muff fuzz had 100k input impedence?

[Jneely88]

So my option is really use a large limiter resistor and increase the circuit gain down the line. The diode is bad news...if I understand