Understanding this schematic

[sine_wave_ninja]

So I've taken some electronic classes but never really got into circuit design.  I just ordered this drive kit from diy pedals and was wondering if anyone could give me some insight on what everything does.  I understand that the transistor is going to act as an amp and give it the distortion, but I don't understand the diode or the capacitors in parallel connected to the tone knob.  Any help would be much appreciated.


http://www.diyguitarpedals.com.au/shop/boms/Ember_drive_schematic.pdf

[Buzz]

The diode is for reverse polarity protection.

[Buzz]

It should give you a clean boost, I cant see any way it's going to add distortion to the signal.

Don't take my word for it though. Someone a lot smarter will be along shortly.

[smallbearelec]

I understand that the transistor is going to act as an amp and give it the distortion, but I don't understand the diode or the capacitors in parallel connected to the tone knob..

Buzz is correct....no distortion here. It's a plain common-emitter amplifier like the old E-H LPB-1. Yes, the diode is to protect the circuit against reverse polarity of power. The parallel capacitors: Notice that C1 is 10 times the value of C2? As the resistance of the tone pot is decreased, the combined capacitance of C1 + C2 looks larger. By the rules, this will pass more low frequencies, so you'll hear progressively more low-end in the output. This "faux variable capacitor" has become a staple, particularly for Rangemasters. See figure 25 in this article:

http://diy.smallbearelec.com/HowTos/BreadboardGeDarlingtonRMs/BreadboardGeDarlingtonRMs.htm

[Gus]

for the first posts link
base to +9 bias resistor?
grounded emitter?
250k volume with a 4.7k collector resistor?

it will load the guitar hfe x re (at the collector current) and will change with input level

look in the book "The Art of Electronics" about circuits like this. Look for what happens with a triangle wave input
what is its  temp stability?
Some other simple gain stages have have C to B bias

It should distort

[chumbox]

It's basically a modified Em Drive with the input/gain pot removed and a tone knob added instead.  Being that the design is almost identical, with your guitar volume on 10 I would be expecting it to distort very mildly in a similar fizzy manner but not distort below 6-7 on your guitar volume.

Good build to get you started on.

[Transmogrifox]

Gus is correct.  It will distort if your guitar output is max.  The tone knob needs to be max on the low side.

Also as Gus mentioned, there is significant loading presented by the emitter-base interface.  Wikipedia has a good summary of the small-signal hybrid pi model:
https://en.wikipedia.org/wiki/Hybrid-pi_model

Using this I approximated small signal input resistance of about 1.87k. 

The following is true when you drive it with a buffer:
With tone knob cranked, that gives you a cut-off of 8.5 kHz.  This means it is a treble booster with a 6 dB per octave slope from 8.5 kHz on down, so because clean electric guitar doesn't have much above 6 kHz, it just re-weights the frequency spectrum with a 6 dB per octave slope with more emphasis on the treble end.

Cranking the tone knob the other way (such that you have equivalent to 110 nF on the input) you get a cut-off at about 770 Hz.  This is just about perfect for a mild overdrive.  If you drive this with a buffered bypass pedal or just a buffer you will get an audible mild overdrive. 

Now if you connect your guitar to it directly, all of this changes.  The gain goes down because this pedal loads down the guitar, but the frequency response goes lower as you roll in more series resistance from your guitar pot. This will be interactive with your guitar volume and tone controls.  The more you roll back the volume or tone, the more low frequency content will come through. 

That said, your guitar pickups and output volume and tone controls are an important part of this circuit that is not drawn.  What you use to drive this makes all the difference in how it sounds.  That's part of the beauty of it:  it adds another dimension to the tone and volume controls on your guitar.

[PRR]

> I approximated small signal input resistance of about 1.87k.

I get ~~6K.

Assumptions: hFE=200, Ie=0.9mA, iIE about 30 Ohms, so hIB about 30*200= 6,000 Ohms.

This shifts your "8.5KHz" down to like 2.7KHz, which makes a bit more sense (as you say, e-guitar is far down at such high frequencies, and it makes little sense to chop more than you "need" to).

Fully agree that it is VERY sensitive to what drives it. Response with a naked guitar will be quite different than from a buffer. Funky.

[Transmogrifox]

> I approximated small signal input resistance of about 1.87k.

I get ~~6K.

Assumptions: hFE=200, Ie=0.9mA, iIE about 30 Ohms, so hIB about 30*200= 6,000 Ohms.

My assumptions,
hFE=350, Ic = 1 mA, Vt = 25 mV
Using hybrid pi model,
rpi =hFE*Vt/Ic =350*25mV/1 mA =   8.75k

Using your assumptions I get 5.56k using my method (rpi model, Sedra/Smith), so I think I did the math wrong somewhere.  Thanks for correcting this.

In either case it shows how much variation there can be even between even 2N5088's over the specified range.  I think my assumption was more in line with 2N5089's but I suppose that even points out how much a difference transistors make to how this circuit will sound.  The differences between, say, a 2N3094 and a 2N5089 will be very audible in this configuration.