Observations on the Muff circuit and new guy tweaking

[plexi12000]

i've built a few triangle versions now. one was slammin' right out of the starting gate.  the others were in the "eh, so-so" category.

so, i figured i'd play around with component values and try to get them were i really liked them.  on his website, Kit Rae says, the muff is a "highly interactive circuit" and

"switching this to do that" might not work as desired.  Yeah....he's definately right. -lol   i could change a couple things and sometimes, not always, but sometimes the

differences was hardly noticeable.  even after some radical value changes.

after tons of changes and no great results, i just put them all back to "stock"....and just started swapping transistors. And THAT is what made the "so-so" into

pretty darn good. not all the dang parts swapping.

i had two that were pretty boomy and loose on the low end. (with what i'd call, i guess, medium gain transistors? 300-500hfe)  if i'd put in say a 5089 or 5088....boom,

i was in a nice neighborhood. huge improvement.  pretty cool, hey, i'm happy.

i'm sure this is no news for the experienced/engineers out there....but just sharing info. for conversation. 

[duck_arse]

well, now it behooves you to put those 'eh' transistors back in, and do some voltage readings. then, when you put the 'boom' trannies back in, we can compare the voltage readings you're going to both take and post. them of.


(what's the sound of 'one guy tweaking'?) (further, if he tweaks in the forest, how loud does it need to be before anyone can hear it?)

[LightSoundGeometry]

you have to buy top transistors .after reading on here, I wont buy from places like Ebay ..only a shop like Small bear or similar. I dont care about how high his prices are I rather pay the extra 5-10 dollars and know what I am getting.

is there a correct collector voltage or is it merely to taste?

[teemuk]

on his website, Kit Rae says, the muff is a "highly interactive circuit" and

"switching this to do that" might not work as desired.  Yeah....he's definately right. -lol   i could change a couple things and sometimes, not always, but sometimes the

differences was hardly noticeable.  even after some radical value changes.

Well, if you think of it, the classic Big Muff circuit relies on gain stages enclosed in local feedback loops.

This means that as long as open loop gain of the stage is high enough, closed loop gain, and properties of the feedback loop, largely define the operation.

Meaning, if the transistor in the circuit has enough of gain, and you end up biasing it about right, the operation won't be drastically different even if you swap transistors and components beacue the feedback loop keeps it in tight control.

Those stages essentially behave very much like inverting opamps and you can largely ignore device variation and their individual biasing, as long as it ends up being in the "ballpark". It won't affect operation drastically. Someone even made an experiment with making a Pi clone by substituting the discrete transistor stages with opamps (not talking about OpAmp muff, it's a different design altogether). Gain and frequency responses of both versions were about identical. That's how much the feedback loop design counts.

So, the design is actually very uninteractive to all kinds of changes, as long as those changes don't involve feedback loops or circuits coupling the stages to each other.

Which means I don't really agree with Kit Rae about this being "highly interactive" circuit. It's pretty much a good example of how to design transistor circuits that are less interactive.

I do find much more sense in following comment, which is also a Kit Rae website quote:

Spread across thirty-five caps and resistors, it may seem mind boggling to keep up with how each of these these may affect the sound, but looking at the circuit architecture, only a few of these have a major impact on the tone. The rest can have values that vary slightly and not influence the tone in any great way. The ones I have found that make the most difference in sculpting out unique tone differences are the Feedback/Filter Caps, Clipping/Blocking Caps, and the Tone Stack Filters. These really define what makess one version different from another more than anything else.

(http://www.kitrae.net/music/big_muff_guts.html#Circuit)

And not surprising, these are either components -outside- the local feedback loops (e.g. coupling, tonestack), or components defining the operation of the closed loops (feedback circuit).

...And then there's this:

This current gain should really not affect the circuit much because the bias for the amount of gain the circuit will have is set by the resistors coming from the collector and emitter of each transistor, but it does seem to have an affect on certain aspects of the tone if the hFE is very low. Too too low and it may not have enough gain for the clipping sections to work properly and may be weak or dark sounding. Lower gain tranys are said to be smoother sounding than higher gain ones, and higher gain tranys are said to influence the mids and pick response, although actually hearing the diferences is not an easy thing. ...  The only real difference I noticed was the transistor in the last stage. Different transistors and different hFE values seemed to alter the sound here, but they were subtle differences.

Which again makes sense: Device choice really matters in this circuit only when the gain of said devices happens to be too low (resulting into low open loop gain). In practice, you have components baising the device (and you can use these to acoount for device variation) but all this biasing merely affects to providing proper open loop conditions. The closed feedback loop will ultimately define the closed loop gain, that is, the stage gain. BUT a feedback loop system can not work effectively if there isn't enough difference between open loop and closed loop gains. ...So as long as the device gain is "high enough", and biasing conditions about in the ballpark the closed loop keeps things under its tight control and what happens inside that loop has lesser importance.

[Gus]

Measure what happens with the last stage when you try different transistors in that stage. 

[plexi12000]

aw man, Ducky.......that's like, a lot of work, buddy.  hahahaaha! -jk

ok, so you mean take a volt reading from each E,B and C?   what will that teach me?  -thats a serious question...not being a smart alec! lol

[plexi12000]

teemuk---  thanks for responding too.  i wish i knew what the hell you were talking about! hahahaa

seriously though....after reading your comment over a few times...i think i understand what your driving at.  i love when i get these "technical" comments!


---if you wouldnt mind, coupel questions.... 1) "feedback loop" is that in each clipping stage? with the 500pf cap and 470k resistor?

2) is it possible to explain, in simple terms, what "open loop gain" and "closed loop gain" is?  thank you 

[duck_arse]

if you take the bad voltages and compare them to the good voltages (and reference them to your transistor hFE's) you may well find the ballparks teemuk mentions. and if there is differences in your readings, you'll have some numbers to aim at in future tunings of the "badd" transistors.

[teemuk]

It's a complicated topic...

This is a "crude" insight to it:
http://www.learnabout-electronics.org/Amplifiers/amplifiers31.php
Take note on the closing sentences:

This means that the gain no longer relies on the variable, temperature dependent and non-linear gain characteristics of the transistor, but on a minimal two resistor network that has a linear temperature coefficient and an easily predicted β value.

Anyway, when we take a basic Muff circuit...

....and inspect the gain stage architecture we can see they're very much akin to generic common emitter amp:

In basic form, resistor divider at base circuit, along with emitter resistance, set up DC bias conditions of the stage. If transistor's gain is reasonably high voltage gain of the amp is approximately collector resistance Rc divided by emitter resistance Re.

That would be "open loop" condition. In practice, the gain would depend on other parameters as well and wouldn't end up being that predictable. That's bad for consistency.

A textbook common emitter amp is pretty much the final stage of the Big Muff circuit.

In Big Muff circuit, however, there are also gain stages 1, 2 and 3 that implement feedback from the collector circuit to base circuit.

We can crudely simplify the idea to following illustration:

We have an inverting amplifier, it can be an opamp, or, in all its simplicity, it can be a single device, like a bipolar transistor. In a transistor, collector serves as the output, which is also inverted in respect to base signal, thus base acts as the inverting input. Emitter is the non-inverting input of the transistor.

You can now see how in these gain stages of a Big Muff circuit the circuitry between collector and base forms one part of the feedback circuit, "Rf", the other, "Rin" is that at the base circuit, as according to scheme portrayed above.

Configured this way, the feedback loop dictates final stage gain and hence we have a concept of "closed loop" gain. Stage gain is therefore largely defined by Rf / Rin, instead of Rc / Re. Since closed loop gain is purposefully lower than open loop gain stage gain becomes largely dictated by the closed feedback loop. That's more consistent.

In Big Muff feedback circuit we generally have this resistive divider along with "bypass" caps that decrease gain towards higher frequencies. Stages two and three also incorporate clipping diodes. Whenever collector-base voltage swings to about +/- 0.5V the diodes turn conductive and limit the gain, causing signal clipping. This configuration also pretty much eliminates any clipping that would be inherent to the gain stage itself. The diodes simply limit before it. Thus we have another factor of design consistency.

Yep, the final stage is simply reflecting the issues of a "simple" common emitter amp circuit. It's much harder to design for consistent results and performance, you may have to tweak it much more at component and device level. In other words, the circuit is more interactive to component changes affecting bias, gain, etc. than the ones incorporating negative feedback. Hence it has more more variation in performance with different transistors and more "evident" effects from component value changes overall.

[Gus]

Also look at the different versions of the effect schematic.  Look at the last stage resistor values

The last stage can be set up for different gains
How it loads the tone control (resistors and beta/hfe)
Output resistance
highpass filter

http://www.diystompboxes.com/smfforum/index.php?topic=105257.msg1019336#msg1019336

[teemuk]

Each time you transistor swap in a gneric common emitter circuit, it tends to require some tweaks to overall bias and component values. Otherwise you end up with different gain and collector voltage values, which has effects on overall sensitivity to overdrive and asymmetry of ovedrive clipping. Naturally stages where output signal swing is not "clamped" by limiting devices (e.g. diode clipping) are more profound to show such effects.