BMPi input BJT Bias

[antonis]

Hi again guys (and ladies, of course..)

Trying to increse the input impedance of a classic Big Muff Π by increasing the value of the resistor of the bias voltage divider (R4) I realized that the first stage amplifier bjt base voltage is settled to about 1,54 Volts (or I am totally out of bias calculation.. )

If the above is right then the amplification is not symmetrical and if I consider of a "normal" guitar pick-up amplitude (about 50-60mV) then the "negative" waveform will start to distort in a gain greater than 30...
The "positive" waveform will normaly NOT distorted because the overall gain is 150 (R5/R6) so it's amplification hardly reaches 7.5V (which is the "safe" area for undistorted amplification..)

I just wonder if my thoughs are correct and (if they are) would result in any problem if I raise the value of R4 to result in symmetrical biasing (4.5V), raising simultaneusly the input impedance of the circuit..??
@edit: Propably I'll have to trim the gain (R5/R6) to about 90...

P.S.
Plz have mercy on me if I put dummie questions but I'm not familirized neither with advanced electronics nor with embeding stompboxes circuits..

[GibsonGM]

Why do you want to increase the input impedance?

[antonis]

Why do you want to increase the input impedance?

Because  I think that an impedance of less than 100k should be a problem of "tone sucking"...

[Davelectro]

I don't think tone sucking is an issue here. This is an over-the-top distortion/fuzz, not a mild overdrive. It's all about "ruining" your signal. It doesn't even have to clean up well when you roll back your guitar volume.

Anyway, this might be useful:

http://www.electrosmash.com/big-muff-pi-analysis

[antonis]

This is an over-the-top distortion/fuzz, not a mild overdrive. It's all about "ruining" your signal.

That's the point..

I'm trying to restrict distortion at two clipping stages, so I want to have a "clean" signal (boosted but undistorted) after the first amplification stage..


Propably I'll manage to "destroy" the BMPi sound..

[midwayfair]

I think you're better off replacing the entire input stage with something other than a BJT amplifier if you want a clean signal with high input impedance. Use the MOSFET booster or something like that instead and solder a cap in parallel with R6. You'll also have to make the 500pF cap much, much smaller. Right now it cuts everything down to 677Hz, which is pretty dark. You will probably need to increase or add treble cuts later in the distortion circuit to make up for removing this one.

[antonis]

I think you're better off replacing the entire input stage with something other than a BJT amplifier if you want a clean signal with high input impedance. Use the MOSFET booster or something like that instead and solder a cap in parallel with R6. You'll also have to make the 500pF cap much, much smaller. Right now it cuts everything down to 677Hz, which is pretty dark. You will probably need to increase or add treble cuts later in the distortion circuit to make up for removing this one.

Thx very much..!!!

As I keep dealing with vintage circuits I realize that there is allmost nothing to improve without loosing the "feeling" of the pedal..

(but they are a very good area for experiment..!!)

[Davelectro]

Anything you do will change the original circuit's feel in some way.

I think you're better off replacing the entire input stage with something other than a BJT amplifier if you want a clean signal with high input impedance.

I agree. But if you want really clean and don't feel comfortable dealing with transistor biasing just use an opamp. As easy as it gets.

[Gus]

Why do you want to increase the input impedance?

Because  I think that an impedance of less than 100k should be a problem of "tone sucking"...

The input resistance is not 100K it is less than that.  R3 is a bias and feedback resistor.  The base input is an imperfect inverting amp node.  39 K is in series with the guitar into this node

[GibsonGM]

I agree. But if you want really clean and don't feel comfortable dealing with transistor biasing just use an opamp. As easy as it gets.

This ^^^   I don't think you can get high input Z easier. You could also just omit stage 1 and use a buffer, no?

[PRR]

> a problem of "tone sucking"...

The BMP distorts. Distorted Highs are just painful. Most distortion techniques "need" some "treble loss" or "tone suck" to stay musical.

As for "tone suck"... the 500pFd appears to high-cut at *700Hz*. Again it is probably not that simple. But clearly this thing is *supposed* to modify frequency response (suck tone). If this were some forgotten un-loved box, so what? However various BMPs have been sold and used for 40+(?) years. My first guess is that what it is, works.

However.

There is only so much you can do with a single transistor. And as Gus points out, R4 is only part of the story.

Sim...

The gain is 8 for low freqs, -3dB at 1KHz, down to unity at 8KHz.

The input impedance, stock, is 42K below 200Hz and 38K above 2KHz.

Depending on guitar Volume pot setting, a ~~40K input is likely to cause additional loss above around 1KHz.

I suspect this is how Mike liked it.

> result in any problem if I....

What problem do you fear? Drummer gets louder? Bartender cuts you off? Your guitar gets stuck on polka-tunes? (If you are a polka artist, on thrash metal.)

Just change the resistor and see/hear what happens.

[EATyourGuitar]

I did some spice sim of this (BMP Q1 buffer) today and I noticed that you really can't change anything to get more headroom and less distortion for guitar level. it just works pretty darn good as is. you can get much flatter frequency response with 100pf but I would not put this clean buffer anywhere near my muff.

[bool]

... but that Pink Floyd guitar dude has a multiple pretty darn full-range clean buffers in front of his muff ... he must be weird or something.

But back to the impedance thing, the loading will also change the guitar "adsr" not only the frequency response. So the thing is multi-faceted and if you're into sludgy stuff, you probably won't like Hi-z, and vice-versa if you want to impress girls who take shower regularly, you will probably want to raise the Z a bit. (imho)

[antonis]

I agree. But if you want really clean and don't feel comfortable dealing with transistor biasing just use an opamp. As easy as it gets.

This ^^^   I don't think you can get high input Z easier. You could also just omit stage 1 and use a buffer, no?

Propably you can substitute all BMPi BJT stages with Op-Amp respectives but I feel that you get a totally "different" sounding pedal..

[antonis]

What problem do you fear? Drummer gets louder? Bartender cuts you off? Your guitar gets stuck on polka-tunes? (If you are a polka artist, on thrash metal.)
Just change the resistor and see/hear what happens.

As I stated in previous posts, I'm new on pedals function embending (not even a guitar player - more likely a guitar scratcher ) and I just want to understand the reason for "ear change" by "item change"..

(maybe I should had study in depth physics and electronics A LONG TIME ago - but this was not included in my priorities..)

[antonis]

if you want to impress girls who take shower regularly, you will probably want to raise the Z a bit. (imho)

So no need for candles and a bottle (or maybe 2 or 3) of wine..

[bool]

so go for the beer drinking ladies

[teemuk]

Propably you can substitute all BMPi BJT stages with Op-Amp respectives but I feel that you get a totally "different" sounding pedal..

Not really too different. Most active parts of Big Muff design are highly negative feedback based: feedback largely dictates stage gain, clipping, etc. The device characteristics are largely minimized due to negative feedback, so in the end the difference from running a BJT in feedback loop vs. opamp in feedback loop isn't all that great. For example: How much gain does a stage have: it's defined by the feedback loop. When will it clip: It's defined by the diode part in the feedback loop. Both apply equally with BJT and opamp. You get the picture...

Passive circuits remain the same so in the end we have largely effects of lower open loop gain of a single BJT vs. ridiculously high open loop gain of an opamp. Probably these account for slight impedance mismatches, which could even fall below usual component tolerance. There are quite a many Big Muff versions that have differences in component values that affect open loop gain, but these do not sound radically different from each other because the active stages within the unit are largely controlled by closed feedback loops. The sound and response of BJT and opamp version change a little bit but it's going to be far away from "totally different" change.

[antonis]

Propably you can substitute all BMPi BJT stages with Op-Amp respectives but I feel that you get a totally "different" sounding pedal..

Not really too different. Most active parts of Big Muff design are highly negative feedback based: feedback largely dictates stage gain, clipping, etc. The device characteristics are largely minimized due to negative feedback, so in the end the difference from running a BJT in feedback loop vs. opamp in feedback loop isn't all that great. For example: How much gain does a stage have: it's defined by the feedback loop. When will it clip: It's defined by the diode part in the feedback loop. Both apply equally with BJT and opamp. You get the picture...
Passive circuits remain the same so in the end we have largely effects of lower open loop gain of a single BJT vs. ridiculously high open loop gain of an opamp. Probably these account for slight impedance mismatches, which could even fall below usual component tolerance. There are quite a many Big Muff versions that have differences in component values that affect open loop gain, but these do not sound radically different from each other because the active stages within the unit are largely controlled by closed feedback loops. The sound and response of BJT and opamp version change a little bit but it's going to be far away from "totally different" change.

Nice and clear...!!!

[Thecomedian]

... but that Pink Floyd guitar dude has a multiple pretty darn full-range clean buffers in front of his muff ... he must be weird or something.

But back to the impedance thing, the loading will also change the guitar "adsr" not only the frequency response. So the thing is multi-faceted and if you're into sludgy stuff, you probably won't like Hi-z, and vice-versa if you want to impress girls who take shower regularly, you will probably want to raise the Z a bit. (imho)

In general I'm a fan of the idea of preserving all frequencies through equipment and being able to change the response intentionally by hand, rather than having pedals do it and having a lack of control in the process. It might be that you just want to turn those highs down anyway, and the FR of a pedal might just be the right kind of FR, whereas if you go to your EQ setup and tone it down from there, the curve may end up not being the same.

If I wanted a sound that's like guitar -> BMpi -> Tube amp, the simplest and cheapest way to do that is with those three things hooked up.