Help me understand electra distortion?

[bunglenutter]

Hi everyone,

I've decided to try the Electra Distortion (http://s457.photobucket.com/user/wood4630/media/electra_sch.jpg.html) as my first pedal build. I would just like to get a better understanding of how the circuit does what it does, so am I right in saying it works like this:

- The transistor is biased via the 2M2 resistor so that when the input signal goes into its lower/negative part of the cycle it's pushed up a bit into the positive voltage range and the transistor can function correctly.

- During the positive part of the input signal's cycle, the voltage across the transistor drops as current flows more freely from C to E. During the negative part, the voltage across the transistor increases.

- The voltage then going through the output cap is then also dropping and rising, with any DC components (bias?) being filtered out by it.

- The signal at this point then has a positive and negative part of the cycle again, albeit amplified due to the supply voltage (or is it the gain of the transistor?), as well as in opposite phase to the original signal.

- During the positive part of the cycle at this point, once the voltage rises above the forward voltage of the 1N34A it becomes forward-biased and the current can pass through it easily, meaning that at the output the signal is clipped at that point until the signal falls below the forward voltage of the diode.

- The opposite happens during the negative part of the cycle regarding the 1N4148 diode.

Please correct me if I'm wrong - I have other questions but this is the most important to me, as I really want to understand what's happening step-by-step.

Any advice will be appreciated!

[bluebunny]

Yep, x6.

amplified due to the supply voltage (or is it the gain of the transistor?)

I like to think of it as the power supply is being passed to the output, modulated by the action of the transistor (or: amplifier = audio-modulated power supply).

And welcome! 

[smallbearelec]

When you get down to breadboarding the circuit (as you should before you commit to solder), you'll be able to measure the DC voltage at the Collector with and without the Base bias. Then the whole business of setting the DC operating point of the transistor should be clearer. If you remove the 2.2 meg resistor, you'll see the voltage at the Collector go up to almost the supply voltage. If you sub a 1 meg resistor with a 1 meg pot in series for the 2.2 meg, you can watch the Collector voltage rise and fall with the changes in the Base bias. Usually, the bias is set to put the Collector at half the supply voltage so that you get maximum "swing" when an input signal is amplified.

A couple of links:

http://diy.smallbearelec.com/HowTos/Breadboarding/BreadboardIntro.htm
http://diy.smallbearelec.com/HowTos/BreadboardUrsaMinor/BreadboardUrsaMinor.htm

The first one covers the basics of using a breadboard, and the second is a "do-it-this-way" setup of my take on the Electra. While I have a commercial interest in that I sell the parts, they are all generic and available from numerous sources. Welcome to the Forum, and enjoy making noises.

[bunglenutter]

And welcome! 

Thanks for your reply, I'm a bit more at ease now!

When you get down to breadboarding the circuit (as you should before you commit to solder), you'll be able to measure the DC voltage at the Collector with and without the Base bias. Then the whole business of setting the DC operating point of the transistor should be clearer. If you remove the 2.2 meg resistor, you'll see the voltage at the Collector go up to almost the supply voltage. If you sub a 1 meg resistor with a 1 meg pot in series for the 2.2 meg, you can watch the Collector voltage rise and fall with the changes in the Base bias. Usually, the bias is set to put the Collector at half the supply voltage so that you get maximum "swing" when an input signal is amplified.

So the two resistor values (47k and 2M2) have been selected to provide a base voltage of 4.5v? If so, is there a calculation to work this out?

[Transmogrifox]

So the two resistor values (47k and 2M2) have been selected to provide a base voltage of 4.5v? If so, is there a calculation to work this out?

There is, but it depends largely upon a parameter called "Beta" (in other places referenced hFE) that varies significantly between manufacturing processes, and a lot between transistors.

In general it will turn out fine using the component values in the schematic because all things considered it turns out "good enough for rock 'n' roll".

Here's the calculation, though:
Ic : collector current
Ib : current into base
Ie : current from emitter

Ic = beta* Ib
Ie = Ic + Ib

Because of the way this is biased in the electra, the current in the 47k collector resistor will be the same as the current in the 680 ohm emitter resistor.

Start with the collector resistor --> here you want a 4.5V drop:
4.5/47k = 95.74 uA

Then find the voltage at the 680 ohm in the emitter:
Ve = 680*95.74u = 65.11 mV

Now we do some guesswork:
Assume base to emitter voltage is about 0.6V.  Without that assumption you have to iteratively solve a logarithmic function and you may as well use a circuit simulation software.  0.6V is pretty close.

Because of the 0.6V approximation we're using, I look at the 65 mV at the emitter voltage and think "this is pretty much insignificant".  And, the current is down around 100 uA, so it's likely on the low side of .6, so let us assume voltage at transistor base is:
Vbase = 0.6V.

Here's where hFE (beta) comes into play.
You have 4.5V at the collector (Vc)
You have 0.6V at the base (Vb)

Rcb = (Vc-Vb)/Ib = (4.5-0.6)/(Ic/beta))

From the datasheet we see hFE for this device ranges anywhere from 30 to 300 -- now you see why all assumptions and guesswork really are good enough for rock 'n' roll. 

Usually I estimate a 3904 transistor with beta = 200:
(4.5-0.6)/(95 uA/200) = 8.2 Meg

To get to the resistor actually selected (2.2 M), you assume hFE = 50.

Does this matter?  Probably not very much.  There is a convenient little interaction between emitter and that 680 ohm resistor that helps balance things out.

Now having seen hFE varies between 30 and 300 on this device, you may understand why you'll measure any range of voltages here, and why it might be a good idea to breadboard it before committing to solder.

On the other hand, all you need is enough positive and negative swing to push over the 0.7V drop on the diodes.  It's going to distort one way or another...and something about the name "electra distortion" tells me that is the point

You have picked a perfect first project.  The Electra is a really great simple circuit to learn the basics.

Enjoy your venture out into DIY stompboxes.

[bunglenutter]

Enjoy your venture out into DIY stompboxes.

Thanks, that's a lot to take in! I can follow the schematic easily enough and I will definitely breadboard it, but there's a part of me that wants to know where all the resistor values come from and doesn't like the 'black box' element to putting it together.

It was bound to be complicated of course!

[brianq]

FYI this circuit is good for mods,different diodes,caps,etc. good beginner project,I started on this one also.

[Transmogrifox]

.. but there's a part of me that wants to know where all the resistor values come from ...

That's a good part of you to have.  Taking the time to satisfy that inner nagging and learn this stuff will really pay off as you take on bigger projects.  Partly because you can make modifications in meaningful ways to customize something to your own tastes and your own setup (guitar/amp/cabinet).  The knowledge also is a great tool when you are trying to track down an error in your build (like solder bridges, cold solder joints or a wrong component).  

If you know how it's supposed to work and you know enough to imagine reasons it might not be working you can usually figure out what is wrong.

Specifically with the Electra, a really bad bias condition might result in somewhat splatty sound with poor sustain.  I don't think the Electra could get that bad with the component values as designed, but it would certainly sound bad like that if there was a bad connection.

[ElectricDruid]

There's one other detail that no-one's mentioned yet, but which turns up in a lot of fuzz/distortion/overdrive circuits - the asymmetrical clipping.

The two diodes aren't the same. One's a silicon diode (1N4148) with a voltage drop of about 0.6-0.7V, and the other is a germanium diode (1N34A) with a voltage drop of about half that (The other major difference between them is the price!). This means that the positive and negative peaks of the waveform get clipped at different levels, and that produces a mixture of odd and even harmonics, instead of the all-odd harmonics that you'd get if the diodes were the same and the clipping was symmetrical.

HTH,
Tom

[bunglenutter]

I've seen batches of 1N34A's on sale on eBay for little cost, but they come from China or Hong Kong, and when you look at the more local sellers they sell them for a lot more so there's something not quite right with the Chinese ones maybe?

[bunglenutter]

Specifically with the Electra, a really bad bias condition might result in somewhat splatty sound with poor sustain.  I don't think the Electra could get that bad with the component values as designed, but it would certainly sound bad like that if there was a bad connection.

I'm sorry about the continuous questions but I'm still having some issues wrapping my head around getting 4.5V at the collector and how you work out the resistances to get this.

Say you had no signal coming in to the input. This means that you would want to have 4.5V at the collector. Knowing this, how then do you know that you need a 47k resistor at the collector and a 680 resistor at the emitter? Do you treat it as a voltage divider where you add the collector-emitter resistance to the 680 resistor and use that number in conjunction with the 47k resistor to do the (R2/R2+R1)*V_in calculation? Presumably then the collector-emitter resistance plus the 680 will be 47k?

Am I close?

[Transmogrifox]

Specifically with the Electra, a really bad bias condition might result in somewhat splatty sound with poor sustain.  I don't think the Electra could get that bad with the component values as designed, but it would certainly sound bad like that if there was a bad connection.

I'm sorry about the continuous questions but I'm still having some issues wrapping my head around getting 4.5V at the collector and how you work out the resistances to get this.

Say you had no signal coming in to the input. This means that you would want to have 4.5V at the collector. Knowing this, how then do you know that you need a 47k resistor at the collector and a 680 resistor at the emitter? Do you treat it as a voltage divider where you add the collector-emitter resistance to the 680 resistor and use that number in conjunction with the 47k resistor to do the (R2/R2+R1)*V_in calculation? Presumably then the collector-emitter resistance plus the 680 will be 47k?

Am I close?

You are exactly right about wanting to see something close to 4.5V at the collector when there is no signal present.

Some of the choice about the 680 ohm and 47k is just a matter of the designer's own mark in parts selection.  At the same time, there are some constraints that would cause most similar designs to come up with something close to this combination.

The first thing in a distortion circuit, especially a single stage transistor is a need for lots of gain.  To get gain you need a resistor in the emitter that is much smaller than the resistor in the collector.  While getting to the real gain of the circuit is probably a little more than I should try to explain now, you can approximate it pretty closely in a configuration such as this:
R2/R1 where R1 = 680, and R2 = 47k in this case.  

So the Electra gain on first pass seems to be a little less than 100.  

This brings to up the next constraint:  Output loading.

You may think, well if I want more gain, I just turn the 47k into a 470k....Now the thing you need to consider is you don't know what might be connected after this, and you have a pot to ground.  All the impedances in parallel with the 47k resistor combine to make an AC load at audio frequencies that turns into a smaller resistor than you thought.  Make that resistor too big and then every amp or every pedal you connect it to will make it sound different because the input impedance of the next circuit becomes a major contributor to the amount of gain the circuit has.

I would say a rule of thumb generally would be to make this resistor at least 1/10th of the expected load impedance.  As it is most guitar amps and FX are 470k or higher, so 47k makes sense.  True, it's parallel with the output volume pot -- but because you know the pot's value and you know it always will be there, you can count on it in your gain calculations and the output load probably won't change the sound of this pedal too much as long as it's several hundred k-ohms.

Another one of the constraints is input impedance.  You really don't want to just connect the emitter to ground and get max gain from the BJT because this looks like a relatively low impedance to your guitar.  (as an aside, your guitar is an electric power generator, however small that power may be.  A low input impedance means more demand for power from the guitar pickups.  Power from the guitar pickups comes literally from the strings.  A significant electrical load works as an electromagnetic brake on your guitar strings and damps the motion to a point where it begins to do what some call "tone sucking").

As a rough approximation, you can multiply the emitter resistance by "beta" to get the equivalent input impedance for the BJT.  In this case
200*680 = 136 k,
which is in parallel with the 2.2 Meg
136k*2.2M/(136k+2.2M) = 128k

You wouldn't want to go much lower than that, but you wouldn't want to make the 47k in the collector much higher, so it's a bit of a balance between giving acceptable input impedance upstream to your guitar output while having enough drive to keep up the gain down the line.

This is all a somewhat general approximation of it all, but gives you the idea about what would go into the decisions about the emitter and collector resistors.

Obviously at some point the designer of the Electra had to say "this is all the gain she's got".  In other words, that person would not think they could make a heavy metal high gain distortion with a single BJT amplifier, but it was noticed it could be used to make a nice little overdrive.

[bunglenutter]

Thanks for taking the time to give me all that info - I think I will have to mull on all of this for a while before I get it, but it looks as though there's a lot happening in this little circuit and one thing depends on another most of the time. I appreciate it.

[Transmogrifox]

...one thing depends on another most of the time.

This is the fun with analog circuits. 

One could solve the input and output impedance problems with input and output buffers, but then that requires yet two more circuit stages.  A clever design like the Electra comes with a price of some potentially significant interaction with other things, but generally it plays nice with most amps and FX and it is simple.

Again, you are really on a good track trying to understand this one.  This basic circuit topology is such a bread-and-butter building block for many more complex circuits that you will have covered a pretty broad class or circuits by the time you have your mind around Electra.  You will definitely have a good idea for what makes an overdrive pedal work.

I just did a little search for basic transistor tutorials, and I found one here that briefly skimming over it appears to have quite a lot to digest, but you might glean quite a bit of useful stuff from it:
http://www.sentex.ca/~mec1995/tutorial/xtor/xtor.html

[brianq]

These values come from an actual Electra distortion module that came in my LP style Electra from early mid 70's
4.7k instead of 47k
470R instead of 680R
1N260 & 1N4001 for diodes
Socket some medium gain transistors, remember these they were imported from japan.
Mine had an unmarked transistor with a yellow dot.
You really have to own one of these guitars to get the real effect,  I think this circuit was really taylored around their special design pickups. Super Magnaflux I believe,known for their extended sustain.

[digitalzombie]

OMFG. Please disregard all of this. I just realized the 2N2907A is a PNP.

I know this is an old thread but I have a different question about this circuit that I'm sure could expand my knowledge into future projects. It has to do with trying different transistors.

I have mine on the breadboard right now and it's the same parts as the OP's schemo with the exception of a 10K pot in place of the 680R at the Emitter.

Using the stock 2N3904 it sounds awesome. Since this is such a simple circuit I thought I'd use this platform to demo the small collection of NPN transistors I have stocked up on in the past few months and take note of their characteristics. Dropping in my first subject - a 2n2907A - immediately taught me there's a lot more to it than just dropping in what you have lying around.

With the 10K pot cranked I get fizzy, lo-fi and gated sounds. Absolutely awful. I decided to check voltage against both to see what good sounding vs. bad sounding looked like. Unfortunately my DMM is limited to reading <2V, or <200V with one decimal place

With the 10K pot turned all the way down;
2N3904: C=2.3, B=1.554, E=1.019.
2n2907A: C=9.0, B=8.1, E=0.049.

So, logic tells me the bad sound is coming from too much voltage to the 2N2907A's collector and I need to add resistance to bring that down. Here are both datasheets.
2N2907A
2N3904

Problem is, as a beginner I don't know what I'm looking for on these datasheets! I've tried doing a bunch of reading over the last couple weeks regarding transistor in's and out's, but I tend to glaze over after a while and hardly any of it sticks. There's just so much info out there and hardly any of it has to do with specifically what we're using them for.

Can anyone tell me, just by looking at the datasheets, is there a way I could have seen ahead of time that this transistor wouldn't have worked in this circuit without changing the values of the biasing circuits? Or maybe even be able to tell what it would sound like just by looking at the numbers? I really want to learn before I go building a bunch of stuff just to build it.

[Transmogrifox]

If you're using a battery you can just swap the + and - leads with a 2907 and make it work.  This would turn it into a "positive ground" circuit, which is fine as long as you aren't mixing positive ground and negative ground pedals on the same power supply.

[digitalzombie]

If you're using a battery you can just swap the + and - leads with a 2907 and make it work.  This would turn it into a "positive ground" circuit, which is fine as long as you aren't mixing positive ground and negative ground pedals on the same power supply.

I'll have to try this one day!