Standard Hemmo Fuzz problem

[lazerphea]

Hi all!
As suggested by R.G., I'll re-post my problem here.
I'm new to this great board, and I started lurking the "What does that bit there do?" thread a couple of weeks ago. Thanks to all the infos i found there I managed to build (over my breadboard Smiley ) the Booster, the OpAmp buffer and the Distortion+ with satisfying results; I then moved on to the Hemmo Fuzz, even thought I don't like the effect very much, I thought that could be a nice exercise to practice a bit. The schematic  and the breadboard layout follows:




Note: the pinout of the two 2n3904 seems to be E-C-B, front facing the flat side, according to my multimeter (i.e., I can read the hfe value, about 150, only if I place them that way in the socket)... 

Now the problems: the circuit outputs a huge signal that saturates my amp as soon as I lightly touch my guitar's strings...
The only mods I did to the original circuit have been the substitution of the 150k and 18k resistors (R3 and R2) with some resistors in series, because I didn't have the right values at the time.
I then started to debug the circuit, and I found out one strange thing: when I measure the 330k Resistor (R1) out of the circuit, my multimeter reads the correct value, but when I put it in place, it drops to 121K and I don't know why, because I take the measure exactly at its pins... could this have something to do with the fact that it's a 0.5W resistor and not a 0.4W one?
Another thing: I measure 2.60V at the Q1 base and 2V at the emitter... can someone help me please? Smiley

[oskar]

A finnish fuzz unit?! 

Measuring resistance in a circuit is tricky. When you measure resistance, the DVM puts out a tiny current. This will follow any path possible in your circuit back to the DVM, so other paths are possible than over R1. It will also interact with voltage sources so you need to disconnect the battery.

Can't you just put in a potentiometer(volume pot) at the output?

ps. I think C2 on the schematic should be turned the other way, not?

[lazerphea]

A finnish fuzz unit?! 

Yeah, seem so!

Measuring resistance in a circuit is tricky. When you measure resistance, the DVM puts out a tiny current. This will follow any path possible in your circuit back to the DVM, so other paths are possible than over R1. It will also interact with voltage sources so you need to disconnect the battery.

Ok, got it!

Can't you just put in a potentiometer(volume pot) at the output?

I put a 100k trimmer and tweaked it so that the output signal doesn't clip anymore, but the sound is awful... terrible response while I play... :\
I think the problem must be with the transistors, but dunno what to do...

ps. I think C2 on the schematic should be turned the other way, not?

I omitted C2 because it is used when you switch to the booster's signal path... I just built the fuzz
Oh, and I forgot to orient the capacitors in the breadboard layout, bu they're correct in my circuit!
Thanks for the reply!

[PerroGrande]

Hi Lazer,

Your results don't surprise me.

First off, the emitter and base voltages you list for Q1 seem to match up with expectation based on calculation and the results I got with a circuit simulator.  So in this stage of things, all appears to be working properly.

This circuit has two stages, both of which work in a similar fashion.

Stage 1 sets a DC operating point of about 2v on the emitter (~2.8 volts on the base via the divider, converges to about ~2.65 volts when base current is considered). This sets the emitter current at 200uA (2V through 10K).

In turn, since the Hfe (beta) of the transistor is large, we can assume that the collector current is the same, which means we have an expected operating point of the collector of about 5.4 volts.  Why?  Because there is a 3.6 volt drop through the collector resistor (200uA through an 18K resistor), so 9v - 3.6v = 5.4v.  So our DC operating points look promising.  Were it not for C3, we would expect a very tame voltage gain of 1.8 (Collector resistor / emitter resistor). However, C3 "bypasses" the emitter resistor.

At DC, C3 isn't really an issue -- it looks like an open connection.  However, at signal frequencies, C3 gets into the works.  In fact, at signal frequencies it looks like a less resistive path to ground than the resistor.  The two components (R4 and C3) are in parallel with each other, so the lower "resistance" dominates.  Using this bypass capacitor is a common way to increase gain while maintaining a stable DC operating point.

In this case, at 20 Hz, C3 looks like an 800 Ohm resistor, dropping to 80 ohms by 200 Hz.  Even at 20Hz, the voltage gain becomes effectively 18000/750 = 24, increasing as frequency increases. (In the above calculator, the 750 comes from the parallel resistance of the 10K resistor and the 800 Ohm impedance of the capacitor).

This is more than enough to overdrive not only the next stage (which works very much like the first stage), but your amp as well.

The point is that this stage is designed to work like this -- as is the one that follows it.  I completely expect the behavior you're experiencing.

Many circuits use a potentiometer in the emitter circuit to vary the gain, but this one has both stages bypassed all the time.  There is nothing to prevent the "balls-to-the-wall" gain that you're experiencing.

Just for kicks, temporarily remove C3 and C5 from the circuit (lift one leg on each).  You should end up with a very timid clean boost.  If you want to tweak around with the circuit a bit, try replacing the 10K emitter resistors in both stages with a trim pot or another potentiometer.  Connect the "outside" leads of the trim pot the way the existing resistor is connected (one end to ground, one end to the emitter).  Wire the bypass capacitor to the wiper (+ side of the cap to the wiper, the other side to ground) instead of directly to the emitter...  This will allow you to tweak the gain of each stage so that you can see if the circuit will produce a suitable sound.   

Something like:



However, it sounds to me like you've constructed a perfectly working circuit that you don't like...  Kinda like me cooking most types of fish... People tell me I do it well, but I can't stand most fish, so no matter what, it isn't going to taste good to me!

[lazerphea]

Little update: I tried to build only the booster path, just to see that it doesn't boost... :\ Is there a chance I fried the transistors in some way, maybe measuring them the wrong way with the DVM?

[oskar]

http://www.fairchildsemi.com/ds/2N/2N3904.pdf
http://www.nxp.com/acrobat_download/datasheets/2N3904_4.pdf
Check the pinout...

[lazerphea]

Hi Lazer,

Your results don't surprise me.
(cut)

Hi Perro! Thanks a lot for the accurate and interesting explaination! I now have much clearer how transistors work! So I'll go for the trimmers option (shiz... I only have 100K trimmers... :\) and then I'll let you know!
Thanks so muck, I go and try to cook this fish the best way I can!

[lazerphea]

http://www.fairchildsemi.com/ds/2N/2N3904.pdf
http://www.nxp.com/acrobat_download/datasheets/2N3904_4.pdf
Check the pinout...

Thank you Oskar, I've already downloaded these datasheets; what confused me is that my DVM doesn't read an hfe value if I follow that pinout... :\

[lazerphea]

Last update: I did what Perro suggested and I put a 10K pot instead of the 10K resistor... now the circuit outputs a signal of -14db if I hot low E, -45db with high E... I declare my two 3904 dead.
At least I can play with my Distortion+...

[PerroGrande]

I'm not sure your transistors are fried...

What is the voltage at the Emitter, Collector, and Base of Q1 now?  (Measure it with no input).

[lazerphea]

I'm not sure your transistors are fried...

What is the voltage at the Emitter, Collector, and Base of Q1 now?  (Measure it with no input).

I switched the 18K resistor with 2 10K resistors in series, R3 with  a 100K + 10K + 41K (actually a 47K but I get 41K with my DVM...) ,and substituted R4 with a 10K pot.

Switching between three different 3904, I get different values:

E: 2.93V
B: 3.25V
C: 2.50V

E: 2.62V
B: 2.92V
C: 3.50V

E: 3.23V
B: 3.86V
C: 3.43V

What is goin on here???

[lazerphea]

Something is changing...
Forgot the last posts, I did different stupid wiring error (hey, I'm still a n00b )!
I'm now sure about the pinout of my 3904, it is E-C-B front facing the flat side; using the 10K pot I can modulate the gain, but it's still not enough. Anyway, Is C3-R4 a low-pass filter? Because I see that what cause clipping problems are the low freqs (E and A strings), while the high freqs are merely amplified (if they're amplified at all). Maybe I could raise the corner frequency a bit (10K resistor + 0.1uF cap)?

[PerroGrande]

Well good!  We're making progress here! 

The R3/C4 combination do effect the frequency response.  However, they do not form a low pass filter in this particular configuration.

At DC, C4 is essentially an "open" circuit (remember the old adage that capacitors "block DC and pass AC").  The higher the frequency, the lower the impedance of C4 -- because Zc = 1 / 2*pi*freq*C.

As the frequency climbs, the gain of this circuit actually increases because C4's impedance decreases -- thus causing it to dominate the 10K resistor.  Because the gain of a common emitter amplifier with emitter generation is G = Rc / Re (the ratio of the collector and emitter resistances), as the capacitor's impedance drops the gain rises. 

The reason this "trick" is widely used is very simple.  To get the necessary gain, Re is frequently forced to be very small.  For a variety of reasons, this is not a good idea.  So we use a more stable DC bias point and crank up the gain via bypassing.

If you're losing highs, there is something else killing them -- not R3/C4.   Check the wiring of the second stage carefully.  It may be drawing way too much current from the first stage and creating problems...

[lazerphea]

Well good!  We're making progress here! 

The R3/C4 combination do effect the frequency response.  However, they do not form a low pass filter in this particular configuration.

At DC, C4 is essentially an "open" circuit (remember the old adage that capacitors "block DC and pass AC").  The higher the frequency, the lower the impedance of C4 -- because Zc = 1 / 2*pi*freq*C.

As the frequency climbs, the gain of this circuit actually increases because C4's impedance decreases -- thus causing it to dominate the 10K resistor.  Because the gain of a common emitter amplifier with emitter generation is G = Rc / Re (the ratio of the collector and emitter resistances), as the capacitor's impedance drops the gain rises. 

The reason this "trick" is widely used is very simple.  To get the necessary gain, Re is frequently forced to be very small.  For a variety of reasons, this is not a good idea.  So we use a more stable DC bias point and crank up the gain via bypassing.

If you're losing highs, there is something else killing them -- not R3/C4.   Check the wiring of the second stage carefully.  It may be drawing way too much current from the first stage and creating problems...

Thanks for your patience man! I'll destroy and re-do everything tomorrow morning, and let you know the progresses!! 
Thanks again!

[PerroGrande]

I don't know that a complete re-build is necessary.  However, if you choose to do so, here is something to try...

Build only the components surrounding Q1.  Get the first stage working well and then try adding the second one.

I would expect the following quiescent (input grounded) voltages on the transistors:

Q1:  (without Q2 connected)

E: ~2v
B: ~2.6v
C: ~5.4v

Q1:  (with Q2 connected)

E: ~2v
B: ~2.6v
C: ~5.2v

Q2:

E: ~4.5V
B: ~5.2V
C: ~4.7V

[oskar]

EDIT... (   I wrote something about transistors here....    - deleted!     )

[lazerphea]

I don't know that a complete re-build is necessary.  However, if you choose to do so, here is something to try...

Build only the components surrounding Q1.  Get the first stage working well and then try adding the second one.

I would expect the following quiescent (input grounded) voltages on the transistors:

Q1:  (without Q2 connected)

E: ~2v
B: ~2.6v
C: ~5.4v

Q1:  (with Q2 connected)

E: ~2v
B: ~2.6v
C: ~5.2v

Q2:

E: ~4.5V
B: ~5.2V
C: ~4.7V

Yeeeah!!! I get almost the same values you listed!
So... this is the correct circuit... and it is unusable for musical purposes... uhm...
Well, it was a nice exercise anyway!! Thanks for the support, perro!!!!

[lazerphea]

Here's the sound sample:

Hemmo fuzz

I lowered a lot my pre output volume to record it; the first part is made with the guitar's volume knob almost closed, while the second part is with full volume.
And here's the Distortion+ clip:

Dist+... truely not that close to the original sound... Mxr Distortion+! :p

[arma61]

And here's the Distortion+ clip:

Dist+... truely not that close to the original sound... Mxr Distortion+! :p

Oh God! , I wish I could make mine sounds like your  !!! also pro playing there, I've visited you site, great stuff there !


Armando

[PerroGrande]

+1 on the excellent playing!  Very good!

The circuit sounds about like I'd expect it to sound based on the gain, the devices involved, and the resulting waveforms.  So I'd say this is a case of a flawless build, but a circuit that just isn't to your liking. 

When I reach this point (and trust me, it happens ALL the time -- especially with my own designs), I always have to weigh the options;  do I set it aside and forget about it, or do I try to mess with it and get it to behave more the way I want?

A lot depends on the physical ease of making modifications.  On the breadboard, stuff is easy to move around and tinker with.  Less so for vero or strip builds, and less still for a PCB (although that hasn't stopped me).  Of course, the underlying design itself will create some inherent limitations.  Sometimes the amount of "tinkering" exceeds the effort of a complete re-design/re-build, and then it is no longer worth it.