2BJTE - 2 BJT triode emulator

[tca]

Hi,
    I would like to share with you my latest experiment. It is a two transistor "booster" that has a triode type behavior. Here is the schematic



and a sound sample.

Of course it needs a proper buffer and it is meant to be overdriven, and of course a decent guitar player! The sound sample was recorded with a SS amp and I've used a OD-3 Boss overdrive to boost the input signal with null drive.

The inspiration came from Jack Orman's Muffer and from Tim Escobedo's bootstrap for gain circuits (made by a suggestion of David Edgar, a forum member). This is by far the most simple circuit that I've build with this type of response and there were a few in my breadboard the last month!

Comments/ suggestions?

Cheers.

[kingswayguitar]

Comments:
I really love the sound

[tca]

Added a pre at the input... lets see how that goes (have to check frequency responce).

[Cliff Schecht]

I'm curious how close to a 3/2 (triode) law this circuit actually gets. Just from the looks of the voltage divider feeding the first stage (in the two transistor design) and the resultant waveforms, it really just looks like a slightly mis-biased BJT amplifier, not that this is a bad idea for obtaining distortion. I've had BJT designs distort in a similar manner when you set the bias point lower than 1/2VCC (or a point near this) and it's pretty neat sounding. If you go too far south you start to get a really interesting octave up effect (in this case raising the 470k input resistor or dropping the 100k resistor value a bit).

[tca]

Hi Cliff,
you are absolutely right it is just a slightly mis-biased BJT amplifier! But that was my idea. As in the case of the triode emulation with fets, you just have to fix the working point such that a good amount of 2nd harmonics is generated and some higher order too. The bootstrap capacitor reenforces this asymmetric bias.

One can also improve this 2 BJT amplifier by using a 3 BJT design; more working points to fix and adjust and more ways of getting distorted signals.


BTW Jack Orman's Muffer and the various stages of the Big Muff are biased in this asymmetric way.

Thanks for your comment.

[PRR]

Is R2 C2 actually doing anything?

[tca]

Is R2 C2 actually doing anything?

Not really! Just there for stability...

[PRR]

> Just there for stability...

Doesn't add "stability". To do that, the drop must be significant (so a change-in-drop does something significant). Here the drop is 0.017V, insignificant compared to anything else.

It is actually very stable without R2. R6 R8 set the collector near 570K/100K times Vbe, or about 3.6V. R1+R4 drop the difference between this and the supply voltage. It is NOT highly stable against supply voltage... but we can expect 7V-12V supply and it will work fine anywhere in there (we may often run with 8.8V-9.5V so supply variation is not an issue).

The collector voltage does drop with temperature. Mostly the 0.002V/degC of Vbe (times the R6 R8 divider), but also a rise of hFE when hot which may cancel some Vbe drift. And it won't drift out of operation until the user has fried or frozen.

As for the buffer.... observe that R7 is 1K but the output pot is 100K and the load is presumably higher. There is gobs more current than needed. You could scale all impedances higher. Maybe 50X higher. R7=50K, R1=500K, R8=5Meg R6=22Meg. The suggested BC109 will run fine at 0.01mA. Input impedance would be a decently high ~~300K, instead of the low-low ~~6K it is now. That might allow omitting the buffer. (The buffer shown is only 33K input.)

[tca]

Thanks for the suggestion to scale  scale all impedances higher. Good idea!

[DDD]

Scaling the impedances up in such a circuits is able to produce very interesting sounds.

[tca]

Hi DDD,
did you breadboard it?

[DDD]

Hi tca,
I didn't try your circuit, but I've bredboarded some very close circuits. As per my experince reducing collector current down to 100-50-25 uA (together with scaling resistors) adds audible dynamics and "breathe" to the sound while "softening" to the distortion character.

[tca]

Hi DDD,
thank you for your reply. I've build a few "triode emulators", just to state a few, Fetzer valve, mu amp variations, etc , triodizer, including the one on your post on a "sensationally simple triode", fetron type topologies, etc, and this circuit is the one I like best.

Thanks again.

[DDD]

Well, tca, your circuit seems to be the best because it is simple and doesn't need any special components.  
Did you try scaling the resistors up?

[Davelectro]

tca, is there any benefit over the fetzer approach?

[tca]

Hi DDD, I do get the smoothness that you talk about in the simulator (did not have the time to change the breadboard) by scaling up the values of the resistors specially for lower values of the input voltage. The response changes a lot by scaling these values, I think 50x  as PRR suggested is  probably to much, 5x is enough for getting the "required" smoothness. Scaling the resistors values would also make the boost, that I'm planing to include before, much simpler to design and getting a much larger input impedance.

In what concerns the Fetzer valve and Davelectro question, as far as my experience goes with this type of JFET biasing, it seems that the triode emulation is reasonable good for getting a clean signal, but fails at simulating the overdrive and the non-linear effects of a overdriven triode. But that's very subjective... For instance I do prefer earing a overdriven Fetron type configuration than the Fetzer valve. In case you are questioning about what I mean about a Fetron, here's something to read.

I'll modify the breadboard tomorrow and post the results.

Thank you all.

[DDD]

Hi tca,
Hm-mmm..., I think I should simulate your circuit with R1=56k and R7=36k (non-proportional scaling). Also let's try to redice coupling caps values drastically to avoid "farting" sound at the attack portion of the input signal.
*** Scaling R7 up to 36k may affect on the distortion texture significantly.

[tca]

Hi again,
just scaled all the resistor values 5x, here are my findings: for small values of the input signal one gets a much soft sound but if overdriven the sound becomes very muddy, I've reduced the capacitors down to 47n and this muddiness persists; for small values of the input/output capacitors the signal cleans up and the final volume is small but the mud is still there, like listening to a guitar through  a cell phone.

Probably I'll stick to the initial resisters values.

Cheers.

[Quackzed]

i wonder if it would be possible to incorporate a pot in place of r6/r8 and/or r9/r10 ... maybee not for the entire resistance range, but it might be interesting to play with the bias points of the gain stages... a bit less/more sqush here or there... maybee just a 100k with 2 equal resistances on either side to make up the total resistance and the bases ties to the wiper, so you could play with the non-linearity of each stage a bit... just a thought..
looks good !!! sounds good to!

[tca]

Last night I was thinking about this muddiness that occurs when one scales up the resistors. I've consider this stage, as a toy model,



and done a frequency analysis. If one looks at the wave form at the output no significant different can be seen of course between the this version and the scale up one, but looking at the frequency response of this circuit and the scaled up version one can clearly see that very low harmonics are generated in the scaled up version. This has convinced me about the source of muddiness of the sound that occurs when one scales up the resistors. The question is how to circumvent this phenomenon? Altering the input/output capacitors is not enough, one simple gets a trebled version of the same sound! Need to think more about it. Any suggestions?

Cheers.