LPB schematic explanation!

[blakkwater]

Hi folks,

forum newbie here.

I'm trying to build a LPB, but I also plan on eventually making a boost/distortion twin pedal.  Before actually doing that, I figured I have a lot of research to do, and a lot of learning. I'm not a total noob to electronics, I've been modifying guitar electronics for years, so I'm intimately familiar with soldering, and I'm not afraid to tinker.  I guess that's what drove me to deciding to make a pedal of my own (also I modified my DS-1 with the monte allums tri-gain mod with a positive experience).  Thing is, I know very little about electronic circuitry and how various components interact with the signal and with each other.

Lately I've been trying to wrap my head around what individual components in the LPB circuit do, and how I could go about manipulating values to change the sound.

Starting with the explanation of the circuit over at beavisaudio ( http://www.beavisaudio.com/techpages/HIW/hiw1.gif ), I think i understand more or less what happens to the battery's voltage and the guitar signal, but not totally.

I have a few questions which the explanations on beavis did not answer.

1) C1 is the input capacitor.  I understand that it is meant to attenuate certain frequencies.  lower values attenuate more bass where as higher values attenuate treble? the Beavis guide to capacitors says that up to 1uf for the input capacitor gives you the full range of guitar frequencies.  What happens above that?  As part of the high-pass filter, how do R2's values impact the tonal range?

2) Why is the battery connected to the base and collector of the transistor? The way I see it, the positive voltage from the battery goes to R1 then R2 and the Base, and it (in parallel) goes to R3 and then the Collector and C2.  Or am I reading the schematic wrong (due to my abysmal knowledge of electrical theory) and does it actually go to R3 and C2, while a different positive voltage goes from the Collector to C2?

3) How does R3 "convert" the transistor output to an output voltage?

4) How does R4 determine the voltage gain of the transistor if the signal passing through it gets grounded?  Why is that stuff grounded?

5) R1 and R2 form a voltage divider network...but how? What happens to the battery's voltage as it passes through R1 and then R2?  Why is it OK to have a node in between R1 and R2 where the Base is connected?

6) If I wanted a bit more grit in the output signal, could I take the output (just before C2) and put in another gain stage there?  If I did that, would I have to use another voltage divider or could I just take that signal straight to the Base of another transistor?

Cheers and thanks for putting up with my n00bish questions!   

[blakkwater]

Nobody.....?

[Brymus]

Its alot more involved than it looks, I would hate to tell you something that isnt 100% correct(yet here I go  )
Google BJT common emitter stage, and BJT amplifier stage, and also common collector stage, you wont find common base in too many effects so you can skip that one for now.
But first you should read up on Ohm's law and read Gausmarkov's tutorials on resistors and dividers,in fact read all of them if you plan on learning this stuff.

If you want more grit replace the emitter resistor with a 1-2K pot and bypass the wiper to ground with a 1-10uf cap.
Also read the beginers project section,its basically a bootstrapped LPB with the emitter bypassed.

AFAIK after 1uf on your input cap you are full range, the only drop in treble is percieved due to the added bass.
And going higher is only needed for a lower ESR when using elctro caps vs film IIRC.

[blooze_man]

http://www.beavisaudio.com/techpages/HIW/hiw1.gif

[Joe Hart]

http://www.beavisaudio.com/techpages/HIW/hiw1.gif

Awesome! I learned so much in four minutes! But the original poster already saw this and still had questions (as do I)...

Can you use pretty much any transistor (as long as it's the correct type -- like no JFET's or MOSFET's)? I'm asking because I was messing around with the Electra Distortion (very similar to the LPB-1) and found that some transistors worked and some didn't at all. Is there a rule of thumb that I don't know about? Thanks.
-Joe Hart

[blooze_man]

the original poster already saw this

Oops! I guess I should have read the whole post...

[teemuk]

It would take ages to write a comprehensive answer to all those questions. My suggestion is that you buy an analog electronics book that discusses all the 101 in detail. The link is great but also has bunch of minor errors.

For quick "Heureka!" experience, consider the transistor as a variable resistor controlled by the base's signal.

[PRR]

> Is there a rule of thumb that I don't know about?

For 9V battery with 10K resistor in series with the transistor, for Silicon transistor, the main "will it work?" issue is "will it bias-up?"

There's rules of thumb, several.

When a "good" designer does a "good" circuit, it should be very tolerant of the transistor used. However "fuzz" depends on "bad circuit". And not all fuzz designers aim for device tolerance.

BTW: the 2N5133 part shown was cheap and sloppy, hFE could be 60 to 1000. This implies that it should be "tolerant"; yet I suspect it biases "badly" with low-hFE parts.

The LPB plan shown needs hFE over 100; no upper limit. 2N3904, 2N5098 will work 99% for-sure.

As a quick/simple but often wrong rule-o-thumb: compare (this drawing's) R2, R4, and hFE. 43,000/390 is 110. You would like hFE to be several times higher. You want R4*hFE to be greater than R2. However this conflicts with input impedance, gain, and transistor cost (high-hFE used to be very expensive). So many plans "fail this test". Yet a lot of them work.

> It would take ages to write a comprehensive answer to all those questions.

Not even counting the "crunchy" question: it took me years to be confident analyzing transistor circuits. It is not easier than learning to play guitar, which isn't learned in a day, nor through a forum. And it HAS all been written-up before.

While this is transistor, many of your questions are "general amplifier" fodder. This is something you have to master: how to amplify audio. The basics are the same for all devices tube or transistor.

http://www.tubebooks.org/index.html
Inside the Vacuum Tube

One of my new favorites - this is a fabulous book on how tubes work and how to design tube circuits.  this is NOT a college text; rather this is written more at the level of an advanced technician, with not so much math and lots of diagrams that make visualizing what's going on easy.  It even has some 3-D drawings - you'll need anaglyph (red/blue) 3-D glasses to view them.

Download full text with index, 9MB PDF file  http://www.tubebooks.org/Books/rider_inside.pdf