About positive/negative grounding

[ricothetroll]

Hi,

Looking at the schematic of the Fuzz Face and the schematic of the Fuzz Factory, I just can't figure out why Fuzz Face is positive ground and Fuzz Factory is negative ground. Basically, if you remove the input booster of the Fuzz Factory, those are the same circuits. The only difference I see then is that the output pot of the Fuzz Face is referred to the (+) side of the battery, and the Fuzz Factory's one is referred to the (-), that allows interconnections with other pedals without shorting the PSU, when AC powered.

I'm sure there are reasons for it but I just can't figure them out ! Can someone explain those to me ?

That leads to this question : why not reffering the output pot of the Fuzz Face to the (-) instead of the (+) to avoid the PSU shorting while interconnecting with other pedals ? The rest of the circuit is DC isolated with coupling caps so - from my ignorant opinion - this shouldn't be a problem, right ?

Schematics :
Fuzz Face : http://www.geofex.com/Article_Folders/fuzzface/fffram.htm
Fuzz Factory : http://basicaudio.net/Fuzz%20factory%20schem.PNG

Best regards.

Eric

[petemoore]

  Either the base has Neg or positive doping, reverse polarity on transistors...is'nt fuzzface.
  Both NPN and GE transistors are around [whether you want to start new searches for NPN Ge's is another matter] and can be used in appropriate FF circuits: PNP/Positive Gnd, or NPN Negative Ground.
  At this point it is worth a look at Geo: +/- 9v from one battery, using a MaX1044 chip to invert the power.
  Having used batteries, MAX1o44's [they work but you might find cheaper/better chip from searching here/there], and NPN Si and NPN Ge Fuzzfaces running on regulated power sources...it's pretty easy to get any kind of FF up, running, and being compatible in a chain of other 'normal' pedals [NPN/Neg. Gnd.].

[brett]

Hi

The rest of the circuit is DC isolated with coupling caps so - from my ignorant opinion - this shouldn't be a problem, right ?

Almost.  As well as DC isolation, the power rail and ground must be AC coupled (VERY well AC coupled, given the high gain, so that the output doesn't back-track to the input) . A battery alone has plenty of DC and AC resistance, so supplementary coupling is required if you want to do this. (e.g. a big electro cap -100uF, a small electro cap -1uF, and a small film cap - 0.01uF)

In the past it was a pain to DC decouple and AC couple for neg ground, and good quality NPN transistors weren't common when the FuzzFace was born, so it was PNP and pos ground. PNPs were the best cheap transistors until the early 1970s. By that late 1970s NPN and Si were dominant in terms of quality and price, especially in western Europe and the US. However, some high quality Ge PNP devices were made tfrom the mid 1970s to the late 1980s, especially in Japan, eastern Europe and Russia.
cheers

[ricothetroll]

Hi,

Thanx a lot for your answers ! I'm actually trying more to understand what's going on than to find a solution for a particular build.

Almost.  As well as DC isolation, the power rail and ground must be AC coupled (VERY well AC coupled, given the high gain, so that the output doesn't back-track to the input) . A battery alone has plenty of DC and AC resistance, so supplementary coupling is required if you want to do this. (e.g. a big electro cap -100uF, a small electro cap -1uF, and a small film cap - 0.01uF)

So if I'm understanding things well, what makes the difference between ground and power rail while running on a battery is that the ground is generally connected to a small AC/DC resistance "source" (please forgive me if I don't use the right words - and be welcome to correct me as well) as an amp's ground for example. So for AC concerns, ground and power supply must be linked to avoid any voltage to be created due to the currents flowing through it. Am I right ?

Best regards.

Eric

[petemoore]

  Ground, usually under your feet, has a water pipe to which all your electronic equipment should be referenced [lamps and other hardware just isolate the AC, insulating the lamps voltage potential from humans and other voltage references [to prevent shocks and sparks, shorting out].
    So we have a reference voltage now of 0.0vdc, although there is no absolutely perfect voltage, if the connection to the water pipe actually hits moisture, and there's no lightning in the area, it stays close enough to a voltage reference that we call it 'ground reference voltage = 0.0vdc''.  Though we strive for a more perfect ground, perfect ground doesn't exist when current flows, ohm doesn't allow it, electrons move, electromagentic forces are sensed by any wires. I'm not sure what 'ground' is referenced to since we're on a watery dirtball in space, the sun influences our DC voltage potential here at earth.
   A battery has a 'floating voltage potential'.
   A circuit requires 2 poles, one more positive than the other, a wire across the poles of the battery completes a heater circuit.
   To amplify AC, a DC reference voltage is often used [most guitar gear]. The reference voltage is connected to say one side of a magnetic pickup coil [HB or SC] the other side will have an analog of the string movement in the form of rising and falling DC potentials...when the string goes one way the voltage drops, when the string moves the other way a rising DC level 'follows' the string, creating an electronic analof of the string movement, a wave-form
   
 

[petemoore]

  A "floating voltage'' is what a battery has before it is connected to a circuit, the universe would be 99.9999etc. % oblivious to this voltage potential until it becomes a circuit.
   A circuit could be a heater, a wire across a charged battery's +/_ poles, current flows through the wire.
  We want 'ground' to be a voltage that is 'stiff/constant DC', very often 0.0vdc. There is no perfect ground according to Ohm.
   If a pipe is stuck far into the earth to contact moist dirt or water, we call it 'ground', and reference everything to this constant 0.0vdc. it is a solid, steady constant 'rock to push or pull from'.  A non-circuit or floating voltage is like waving your arms and legs around in space with nothing to push against, there is no foreward/backward pressure...you need a solid reference point to connect to before the body can be pushed or pulled from anything. Sorry, but that's the best analogy I can come up with for 'floating voltage' ='s 'absence of a reference voltage.
  You can take a pickup and connect to a circuit with no ground and get signal which can then be amplified. If there were no electrons flying around it'd be noise free, unfortunately we resort to ground-shield-wiring to protect sensative signal wires from being bombarded by electrons [I'll call 'em 'outside electrons' as in not motivated by the pickup coil...probably from telephone, CRT or flourescent lights].
         

[ricothetroll]

Thanx, Peter !
Even though I was familiar with this un-perfect voltage reference issue, that's a brilliant explanation !
Performing an ultimate research, I finally found a post from John Greene that demystifies that negative ground oscillation problem (there's so many posts about Fuzz Faces around here) :

I think I know why negative ground PNP fuzzfaces oscillate. Let's see if I can explain it clearly enough......

I'll treat the transistors as simple switches in this scenario.

If we start with Q1 switched 'off' the following happens:
Q1's collector is low bringing Q2's base low. This turns on Q2 bringing its collector "high" (as high as it can get). This means the voltage at Q2's emitter drops. The base of Q1 is connected to the emitter of Q2 through a 100K resistor. So the 2.2uF input cap starts getting discharged through the 100K. It has a little trouble with this as the current coming out of the base keeps adding some current. Once the base of Q1 is low enough to switch it 'on', Q1s collector goes high. This switches off Q2 thus raising the voltage on its emitter and now the 2.2uF capacitor starts charging, this is a little easier because now the base current is helping you. Because of this the gain of the input transistor plays a role as well because it contributes current to the base. The lower the gain of the input transistor, the more current that flows out its base for a fixed amount of emitter current. Under normal conditions, this process converges and you have stable bias.

Now introduce an imperfect battery that has internal resistance. When Q2 switches 'on' it is dumping current through a (roughly) 8.5K load. This causes a slight drop in battery voltage. The battery voltage appears across the path of Q1 emitter -> Q1 base -> 2.2uF cap. The problem with that is when Q2 switches on, it reduces the battery voltage a small amount. The 2.2uF cap is going to do its best to not let the voltage across it change so it brings the base of Q1 even more positive which switches it completely off. Now it has to wait for the 100K resistor to discharge the cap before it can switch back on again. This causes Q2 to suddenly switch 'off' which increases the battery voltage slightly and the 2.2uF on the input of Q1 will pull the base down a little more negative keeping it saturated until the 100K can charge it back up again. It's this reaction of the input 2.2uF cap to power sag that causes the oscillation. You can short the input at the 2.2uF cap and if the short is connected to battery negative, it will oscillate, if you short it to battery positive, it won't.

If the input 2.2uF cap is returned to the same voltage as its emitter, it no longer has this differential voltage across it because of battery sag. Thus no oscillation. If the power source is a low enough impedance, then the sag is eliminated which also stops the oscillation. But it has to be a really low impedance voltage source. It only takes a few ohms of internal resistance to provide enough sag to cause it to oscillate due to the low Base-Emitter voltage drop of Germanium transistors.

So what can be done? Big, low ESR, capacitors would lower in the source impedance AC wise but you would need a lot of capacitance to overcome the 100K/2.2uF time constant of 220 mSec. So you may only succeed in lowering the frequency of the oscillation, but not eliminating it. Or you could run it from a good stiff power supply. Unfortunately this forces the signal into the base of Q1 to return -through- the power supply and this can introduce a lot of hiss even with a really good (quiet) power supply. Also, A big caveat of this is the well known fact that Duane Allman preferred the sound of a Fuzz Face with batteries on the verge of dying. Implying that this sensitivity to the internal resistance of the power source provides pleasing results in a positive ground configuration. So fix the power source problem, lose mojo. The best solution is probably just don't mess with it. Put up with the fact you can't run it on the same power supply as your negative ground pedals. And even then, it probably isn't going to sound as good as it will running from a battery. It's just the price you have to pay for good tone.

--john

http://www.diystompboxes.com/smfforum/index.php?topic=89944.0

Best regards.

Eric