Simple Drive

[Gus]

After reading this post http://www.diystompboxes.com/smfforum/index.php?topic=103683.0and others with a simple gain stages I simmed something for the forum
A simple one transistor gain stage I used a 2n2222 because it often matches well from sim to real builds I have made
R3 is both part of the bias and gain control of the circuit
To the left of R7 is a simple guitar cable sim omit in build.



It is a circuit close to the Electra

[slacker]

Looks good, I'll try and breadboard it and report back.

[Gus]

Try a .047uf and smaller for C1

The reasons for the 2n2222 and collector and emitter resistor values are to get a lower value of a C to B bias resistor so you can have more control of the gain by using a lower value series input gain control resistor.

This type circuit with a higher hfe transistor will need a higher value bias resistor with is also part of the gain you can use higher series input resistance but there is a noise issue.

Open loop gain of the circuit and the feedback/bias divided by the series input resistor(s) are part of what sets the gain  This circuit operates like an inverting opamp circuit, however the gain stage has limited gain and the opamp has a lot more gain.

[tca]

I've been playing with this type of circuit but with different BJT's and similar to Ursa Minor but using a 1k resistor and a 10u cap from emitter to ground. You can also put 2 or even 3 in series to get some overdrive/fuzz sounds. For 2 or more stages I've used lower gain devices and R1 from 10k to 100k. In this case you should also use a big cap 2000u from +9V to ground.

@slacker
Also, and this is the fun part, you can power this things with a 1.5V battery.

Cheers.

[Gus]

I am not a fan of this type circuit.  I adjusted values of this circuit to give a good range of gain and output drive to a 100K volume control.
It was a reaction to a thread that someone was trying to build an EM drive and I made the mistake to look at the schematic. Then I looked at the COT 50 schematic

A circuit  to try would be one like the first stage of the BMP for a simple gain stage make the
33k input a fixed resistor and variable resistor
and/or
use the same bias setup and change resistor values
and/or
ground the emitter
and/or
and a emitter gain control
and/or
...

[tca]

...
and/or
...

ah, but the *real* problem is all in the details.  One thing is knowing the path and other thing walking the path ...

Cheers.

[Bill Mountain]

I have come full circle with circuit ideas and I'm back to single transistor overdrives.  I like the Ursa Minor but it gets all "tinny"* sounding when preceded with a buffer.  Would this design have the same effects?  Maybe I need to try something bootstrapped.  Would this have enough gain to reliably clip LED's in the feedback path?

*Meaning a severe loss of bass and an increase in highs when a buffer is in front of the circuit.

[Bill Mountain]

Hopefully someone smarter than me will be along eventually to point me in the right direction so I guess I should ask a few more questions that have been bugging me.  I have seen circuits like this that have R3 or R1 split into 2 separate resistors with a cap to ground in the middle.  I've tried searching and I can't find any examples at the moment.

What kind of effect this would have on the circuit?

[nocentelli]

http://www.jiggawoo.eclipse.co.uk/guitarhq/Circuitsnippets/snippets.html

Quite a few of Tim Escobedo's BJT based designs have two base-collector bias resistors in series with a large-ish cap to ground in the middle: I am also curious as to the effect this has.

[Bill Mountain]

http://www.jiggawoo.eclipse.co.uk/guitarhq/Circuitsnippets/snippets.html

Quite a few of Tim Escobedo's BJT based designs have two base-collector bias resistors in series with a large-ish cap to ground in the middle: I am also curious as to the effect this has.

My simple guess is some sort of HPF.

Here is another example:

[Bill Mountain]

http://www.jiggawoo.eclipse.co.uk/guitarhq/Circuitsnippets/snippets.html

Quite a few of Tim Escobedo's BJT based designs have two base-collector bias resistors in series with a large-ish cap to ground in the middle: I am also curious as to the effect this has.

Looking at his circuits you see it in all sorts of places.

[nocentelli]

That voodoo bass looks like a classic low noise biasing, with the 47k/47k providing the 1/2 supply vref, but the 2M providing this vref to the transistor base instead of an opamp non-inverting input as I am accustomed to seeing it: That kind of voltage divider always has a large cap. I was think something more like the square wave shaper -

http://www.jiggawoo.eclipse.co.uk/guitarhq/Circuitsnippets/swsh.gif 

[Bill Mountain]

That voodoo bass looks like a classic low noise biasing, with the 47k/47k providing the 1/2 supply vref, but the 2M providing this vref to the transistor base instead of an opamp non-inverting input as I am accustomed to seeing it: That kind of voltage divider always has a large cap. I was think something more like the square wave shaper -

http://www.jiggawoo.eclipse.co.uk/guitarhq/Circuitsnippets/swsh.gif 

Me to.  Like I mentioned I'd seen it done a couple of different locations on a transistor.  I figured they had different purposes.  You showed one thing I mentioned so I tried to find a schematic that showed the other thing I mentioned.

[Gus]

Hopefully someone smarter than me will be along eventually to point me in the right direction so I guess I should ask a few more questions that have been bugging me.  I have seen circuits like this that have R3 or R1 split into 2 separate resistors with a cap to ground in the middle.  I've tried searching and I can't find any examples at the moment.

What kind of effect this would have on the circuit?

Are you posting about two resistor from C to B with a cap to ground in the middle?  If so it is simple, it removes most of the signal feedback and acts mostly as a DC bias resistor.
If you make the cap a large vale (RC math) the two resistors do no audio EQ and is not part of the signal feedback. if you make the cap smaller it works with the resistors and starts to reduce the gain as the cap becomes less effective(RC math)

If you look at may post often I call the C to B resistor a feedback/bias resistor it reduces the gain and lowers the Z at the base node

[Gus]

A better drive three more parts

R3 makes a difference
I made this grounded emitter you can adjust it for an emitter resistor etc.
C1 was selected to roll the bass off with gain
R11, C5 help select the gain range
Make R6 a volume control
Adjust R2 R11 C5 for different hfe transistors
guitar and cable sim to the left of R12 omit in build
misspelled emitter in sim

[mac]

I built a very similar circuit, a single hfe<100 transistor, rc:10k, rfeedback:1M, cout:103, cin:472-682.
What I got is a mid booster, the freq response curve is similar to a gaussian bell function.
The small in cap interacts with the guitar coil forming a bandpass filter. Some resistance in the way and I have Gus curves.

mac

[Gus]

Change a few values and use a bass sim
A bass drive
Note how the gain setting changes the curves 

[Bill Mountain]

Well now.  I guess'll I have to breadboard this one too.

[Gus]

Just wired up the bass drive version on  perf board to check DC voltages.

DC voltages are close to the sim voltages I added a shockley diode between the battery and a 100uf cap
I measured 4.6VDC at the collector.  LT spice 2N2222 model seems to match with the build.

Next I add the controls and box it up and test it with a bass.

[Gus]

While waiting for the paint to dry on the box I wired it up with no bypass switch to test it.

Using a MIM J bass with CS 60's pickups it seems the 100k gain might be better as a 50K.  I liked the sound at 1/2 rotation(50K) to 0 ohms so using the 33K and .068uf cap might be fine without the gain control

The grounded emitter and EQ (1000pf and 470K) and interaction with the bass seems to make the bottom a little "bigger"