A few noob quiestions

[swever]

Hello everyone! I'm new here, and new to guitar effect building, although I do have limited experience with electronics, soldering, etc.

Yesterday I have breadboarded a "muff boost"

It works well and I like what it does, however the output level is way too high. So I've started pondering on how to bring it to unity. The first thing I tried was to add a resistor after the "out1" point. Turned out, however, that it not only turns the level down, but also changes the clipping/saturation character (lessens it), as well as makes the timbre much darker, duller. I am not completely sure if all of these side effects actually happen or if its just how my amp (line 6 spider iii 15, clean channel) reacts to the change of input level.

1) I'd be grateful for an explanation on why this happens and what's the theory behind it (preferably in layman's terms).

2) If it's not how my amp reacts, then what would be a proper way to decrease the output level. Do i need a voltage divider here (why or why not)? Is there a way to adjust the transistor behavior, i.e. "how much it amplifies"

3)I currently have three types of trasistors. I have measured their hFE with a lowend multimeter and got these readings:

2n2222a   570
2n3904    490
2n4401    790

These numbers look completely insane to me. Could it be that there should be a decimal point after the first two digits? Or is it just that the multimeter shows random numbers?

ps Excuse my clumsy language, English in not my mother tongue

[duck_arse]

swever - hello, welcome to the forum, and stop worrying about your english. you don't even write with an accent!

if you go to this page, down hal-way where the beavis image is, you will learn probably enough to answer all those questions. if not, come back, ask again.
https://www.talkbass.com/threads/diy-lpb-1-boost-circuit-question.853593/

th resistor you added - what value did you use? and those hFE numbers are somewhat resonable, different meters produce different numbers. you can at least compare one transistor to another, even if the figures are not "right".

also, datasheets.

[PRR]

> Do i need a voltage divider here (why or why not)?

Voltage Dividers are the Audio-person's Best Friend.

First you have not enough gain. You add an amplifier, now you have too much gain. We don't usually rip into an amplifier to change gain. We use a Voltage Divider.

An Adjustable Voltage Divider is sometimes called "the Volume control". Every audio system has one (or more).

In this case, a 100K pot in Volume Control connection is a good thing to try.

Your hFE numbers do seem high, but not by factor of 10. As Stephen says, hFE varies with test current, which means "simple" meters all give different numbers than the specified test-points on the datasheets. Also I think in recent decades all the transistor makers have got their process controlled so they consistently yield hFE on the high side of older specs. A low-hFE part will anger a factory engineer who needs at least a minimum gain, but high hFE rarely brings complaints.

[rutabaga bob]

Hey Alexander - welcome to the forum!

[swever]

Thanks for the warm welcome and useful information, peeps!

if you go to this page, down hal-way where the beavis image is, you will learn probably enough to answer all those questions. if not, come back, ask again.
https://www.talkbass.com/threads/diy-lpb-1-boost-circuit-question.853593/

That's gold! I've built that circuit and tried pottering around with the values, learned and still learning a lot from it! Then tried applying the same principles to adjust the muff boost, with mixed success. That said, I still don't completely comprehend how this circuit works and what role each component plays and, most importantly, their interworking.

th resistor you added - what value did you use?

I cant name the exact values right now, I tried a few.

Voltage Dividers are the Audio-person's Best Friend.

First you have not enough gain. You add an amplifier, now you have too much gain. We don't usually rip into an amplifier to change gain. We use a Voltage Divider.

That makes some sense to me now after I experimented with the circuit. I was not able to adjust the gain in the "muff boost" without altering the clipping behavior. Voltage Divider did seem to do the trick, though I still have a feeling it does not just change the output level. Hope my recording PC gets fixed soon and I will be able to record level matched samples and compare properly. I'd still ask why not hack into an amp in case of a "clean" amp? Isn't it better to change a value or two instead of adding additional components?

Now for a few more noob questions:
1) Why does not just a resistor before the output work? Why does it start to sound low-passed? Why do I need to add a second resistor to ground to make it sound right?
2) In the beavis image, what is the difference between adjusting the R3 and R4? Both seem to adjust the "amplification factor"
3) Why do both of these circuits have their output connected to the transistor's collector? Is not the emitter supposed to be it's "output"?. I know I should just start reading some theory, but would be very grateful for a children's explanation

[balkanizeyou]

3) Why do both of these circuits have their output connected to the transistor's collector? Is not the emitter supposed to be it's "output"?. I know I should just start reading some theory, but would be very grateful for a children's explanation

That's because both of those circuits' job is providing voltage gain - emitter's voltage is "tied" to the input (gate) voltage, because the base-emitter junction behaves like a diode, so emitter voltage is approximately base voltage - 0,6V. No gain at all. Which doesn't mean that this setup doesn't have any benefits - it sets up a pretty high input impedance and low output impedance, so it can be used for example as a buffer for driving low impedance loads (like loudspeakers - that's why you can see it as a part of push-pull AB class amplifier for example). It doesn't have a voltage gain, but it does have current gain 

Anyway, this base-emitter current controls much bigger collector current (that's just how the transistor works), which is sourced through R1. Current passing through R1 is turned into voltage, which appears at the collector's output. Assuming no feedback diodes or resistor, the voltage gain of the common emitter amplifier is minus the ratio of the collector resistor and emitter resistor (the minus sign means that the output signal is inverted)



This expressions makes sense as long as some rules are obeyed - for example, the collector must be more positive than emitter. If the base-emitter current is too big, the collector current may be big enough so that the voltage drop across the collector resistor is bigger than the supply voltage! Of course this is not possible, and also in this case that would mean that the collector voltage is lower than emitter voltage - transistor cannot do that. Instead, the collector goes as close to the emitter as it can (usually within few tenths of volt) and we say that the transistor is saturated, which results in clipping and other weird sounds/behaviour.

To prevent the circuit from this at least a bit, negative feedback is used. Let's assume that there is a big positive input voltage wiggle, which results in big base-emitter current. This causes a big negative voltage wiggle at the collector (remember, the gain is with the minus sign), which in turn is fed back to the base through R2. This lowers the initial voltage wiggle, resulting in lower gain, but better stability and linearity and less transistor distortion. To get the distortion we want, D1 and D2 are used - they are off as long as the collector "wiggle" is smaller than their voltage drop, but when the wiggle reaches the level of ~0,6V, they feed all the voltage above this level back to the base. This results in abrupt "clipping" of the output signal, which is the distortion you hear.

I'd still ask why not hack into an amp in case of a "clean" amp? Isn't it better to change a value or two instead of adding additional components?

That's because the gain is not the only parameter of an amplifier. You also have frequency response, input/output impedances, headroom, amount of distortion etc. It usually isn't easy to change one parameter without changing some of the others, but if you add the big enough volume pot, you're usually able to change the volume without affecting the output impedance all that much.

[swever]

Mikołaj, thanks for a detailed and convoluted answer! I did realise, however, that I lack theretical knowlendge, even to fully understand your post. So I'm off to read some literature!

[balkanizeyou]

No worries, I didn't understand a thing from posts like mine above when I was starting with electronics. If you want some solid basis in electronics and not only building pedals, I recommend "The art of electronics" by Horowitz and Hill, because you don't really need any prior knowledge to understand it and it covers a wide array of topics deep enough to get your feet wet.
Well, I would guess that the 'digital' chapters of the book are a little outdated now, but I never really bothered reading them 

[PRR]

> I would guess that the 'digital' chapters of the book are a little outdated now,

No. The older edition won't tell you how to make an iPad, but IS the type of stuff small-audio uses for switching and blinking.

[Gargaman]

mother tongue

Good name for a band, or album.
Momma Tong maybe..

[swever]

Mikołaj, thanks for the book suggestion!


Meanwhile, I (almost) finished my first build. It's a silicon fuzzface. It's round. My cat seems to like it and so do I. The only thing that it doesn't yet have is an indicator LED.

[balkanizeyou]

You do have a 3PDT switch in there, so adding a led is going to be a cakewalk 
Anyway, cool looking build! I like the fact that it bears some resemblance to the original fuzz face box. I'm not sure whether it'll survive a lot of thumping on stage, but hope it does!

[Gargaman]

^Cool!
that's pretty much how I do my stuff.
Check it out:
Mine tastes more like butter or tuna instead of gums..
You should try some paint work now!

I'm not sure whether it'll survive a lot of thumping on stage, but hope it does!

Yeah, this can be a weak point, but for sage stomping it handles. Try not get drunk and jump over it ( unless you intend to do a quality test)

[swever]

Done!


Not planning to get onstage with this anytime soon. It actually feels sturdy enough to survive gentle home/studio use for some time. If/when it dies, it's gonna migrate to a 1590b that I received today

Gabriel, the one in butter can must sound fat and smooth!
Painting looks like fun, but I think I would leave it for the forthcoming builds. This one looks great unpainted without stickers IMO.