50k pot at the input of FF

[yeeshkul]

Guys does the 50k pot modification at the input of Fuzz Face clone cause any extra hiss? If yes - can i help it with a switch 0/50k instead of the pot or something like that?

[petemoore]

  Simple no.
  If the pot is a 2 lug wiring, when fully CCW it should be a direct connection.
  That's the wiring I'd choose for a FF using a guitar pickup as source.
  I thought of putting a resistor in the guitar, right on the output jack to mimic a 9.6 of 10 output volume pot value setting, seems I nearly invariably prefer the volume on the guitar *almost full on or less, this reduces noise and tends to clean up some of the hash I often get when the guitar volume is on 10, unless...I'm going straight into the amp.

[joegagan]

I have been bugged by hiss using a pregain pot on FF type circs for a long time.

Maybe you guys are referring to the type in the fulltone 69, just a series resistor (50Kpot), instead my designs always used a third lug to ground at the front eg. easyface, dinofuzz etc. There is always hiss present when the pregain is less than full cw. A while back puretube suggested I try a conductive plastic pot- this solved the prob but these usually go for around 10 - 15 bucks a pot.

My next experiment involves a rotary switch with 6 or more positions ( different resistors to gd),but I suppose the labor of wiring all that up  compares about equal to the cost of the expensive pot.
The multi position switch forces the user to use 'presets' for gain, less choice than a pot
oh well, time to find a stash of plastic bourns pots at my surplus place

[joegagan]

found the thread where puretube brought this up ( thanks Ton!)

http://www.diystompboxes.com/smfforum/index.php?topic=20062.0

[petemoore]

  Well...could be I wasn't really looking to hear hiss in that pot, and I had some hiss anyway. I hadn't noticed series resistance [I usually only use 47k or so] causing noise increase.
  I use the lug to ground pot on a pregain I know I want to attenuate from being hit to hard by a high gain signal, mostly though opting for a small resistance,  probably 10k - 25k at the FF input.

[yeeshkul]

Yesterday i realised that the electric wiring in a guitar can probably( ) have certain influence on the whole thing.
I found all the pots in my Les Paul are 300k, strange ... they are original but people say 500k are better.

Please can you help me with these questions:

1) i've read somewhere that the input pot in FF is meant to be there for humbuckers/singles/wah-wah adjustment, is that right?
2) is there any article which explains how the output circuit of a guitar works with the input of a stompbox?
3) how do i get the input impedance (resistance) of my FF - can i simply measure the input with MM ( )?

thanks

[GibsonGM]

Hi Yeesh,

You're having good experiences in learning here, dealing with the FF, lol.  Low input impedance of the circuit is what gives it its characteristic tone...and drives everyone nuts who'd like to use a wah with it, and so on!  The only fix I ever found was to try 1K to 10K resistors in series at the input, before the input cap.  Eventually I found one that worked well with my Les Paul (forget exact value).   A pot would be good there to allow adjustment for other guitar (answer to #1).  FF loves single coils.

Ideally, a pedal has high input impedance and low output impedance.  This is related to a simple concept of voltage division that happens when you plug in.  Frequency dependent & involves reactances...but not too bad to visualize in a useful way without going way overly technical.  Read this over at AMZ, great article!  This answers #2.
http://www.muzique.com/lab/imp.htm
We don't want to necessarily transer power to other circuits down our chain; that will make other circuits react with the ones coming before, not good.     

Other than that, experimenting ceaselessly will get you where you want to be, or as close to as possible.  I found that it wasn't worth any more effort to improve the FF (including re-wiring my guitar!!)...it's been around forever, used in its current configuration for 40 years.  In playing with it, you learn about impedance, buffering/tone sucking, what it can and cannot do.

For #3, you're going to have to search.  There are rules of thumb established to guesstimate input impedances of transistor circuits...the common emitter of a FF is 'medium' input impedance (possibly 50 or  75K, IIRC), but it's really hard to get it closer than that due to variances in BJT gain.  Read up about transistor amplifiers and you'll get more info; search the forum too.  I've never really had to think too much about impedance other than som intuitive stuff I learned...you'll know pretty quickly if your Z is too low, due to tone sucking.
Standard opamp inputs are high impedance.   Due to effects within the device, the frequency-dependent nature of impedance and external components, you can't just measure impedance with a MM, sorry!
You'd have to get into reactances and phase angles, and do some vector math    This should help you get started, good luck!   

[yeeshkul]

Thak you GibsonGB!   I'm gonna read the article .

[R.G.]

You could also read "The Technology of the Fuzz Face" at GEO.   

[yeeshkul]

for GGB: that article is exactly what i needed to know!
for R.G.: thanks R.G. i have read that article a few times, i just need to clear up some points

[Gus]

Look up inverting operation of opamps.
Don't think input Z with FFs
think weak opamp inverter circuit with a current summing junction at the input

A perfect opamp in an inverting gain stage  will have 0 ohms input.

It is a summing juction at the - input
setup as an inverting amp

Look close at a FF it is a IMPERFECT inverting amp.  Thats one of the reasons picking transistors is important the effect the openloop gain

guitar to opamp inverter
one side of the guitar grounded the other to the - input
the gain and resulting EQ is do to the changing Z of the guitar at different freqs.
The guitar is the input resistor in the inverting circuit you find in books.

The FF not having alot of gain compared to a opamp cause the first transistor to have  BIG EFFECT on the gain and tone but causing different summing at the input of the FF.

  There are two gain stages in a FF the first transistor collector to input resistor ( the emitter of the 2nd is the same signal but lower Z as if you took the feedback from first transistor collector)  the higher the first transistor gain the better the control of the input junction.  Then the 2nd transistor gain set by the gain control.

Don't think input Z with FFs think weak opamp inverter circuit with a not so good current summing junction at the input

Because the guitar is a inductive resistive capacitve mess connect to the FF and it is part of the feedback network this cause a change in gain vs freq one of the reasons a buffered FF sound different.

The 3 tran  uses a 10k between the buffer and distortion stge as a very CRUDE sim of a guitar

FF sound the best to me right after the guitar with no series resistance to the input.  To much gain tune the feedback drop the 100K use a lower gain transistor.............

[Gus]

http://www.ecircuitcenter.com/Circuits/opitov/opitov.htm

Read this

[yeeshkul]

I read the article on Pedal Impedance. It is actually very good and clear one. There is just one thing which cannot cope with my perception of the things, hehe. I must have missed something.

Imagine we have a guitar - a voltage source with serial impedance Z-out - connected to the input of a classic FF (parallel Zin).
And now, we add a 50k resistor in between of those, ok? The input-dump/wah modification.

My question is: from which point shall i read this shematics?

1) Lets say I i'm looking from the point between the 50k resistor and the FF-Zin to the left - then i see that Z-out of the guitar was empowered by 50k resistor in series -> Z-out gets even bigger than before and Z-in is the same as before (things got even worse). R.G. reads this situation similarly in his FF article saying that the voltage source is becoming more like a current source (a big internal Z) which helps to the tranny to adjust.

2) when i am looking from the point between the guitar Zout and the 50k resistor to the right i see that Zin of the FF was empowered by 50k res (here in series with the rest) so the input impedance of the FF is bigger while the Z-out of the guitar stays the same.

I must have missed something as these two readings go against each other