Vulcan Rat Creation

[Ben Lyman]

Some recent threads rekindled my interest in Joe Davisson's circuits, the first thing I threw on the board was his original Vulcan and it sounded great.
Lots of range from sparkly to saturated, tons of extra volume available. I decided the first thing to add was a simple Rat Filter, then some extra clipping diodes on a switch.
I also added a resistor (5K6 R20) to keep the distortion control from going all the way off.

Finally some tone changing filter caps to block some unwanted noise and radio stations and these are the caps in question.
C11 (2n2) helped block unwanted extra noise coming in and changed the tone in a nice way but is there another way to do the same thing here? What have I done here? It sounds good, so is it good?

C7 (47p) blocks out the radio station and doesn't seem to have any affect on the overall tone, but it works anywhere between C2 and C4. Is there any "best" place to put it?


All diodes are 1N914 and transistors are 2N5088

[Kipper4]

Looks great Ben.
Can't wait to hear it.
Is it still at the breadboard stage?
There's a few tricks you could try maybe.
Lookie here

https://dl.dropboxusercontent.com/u/7464107/Bogner%20Ecstasy%20schematic.pdf

A jfet xtc emulation schematic.

From this post


http://www.diystompboxes.com/smfforum/index.php?topic=86151.0


Look at the values C1
C3 is a neat trick too

Just throwing it out there.

[Ben Lyman]

Cool, thanks Rich. I liked this one so much that it went from BB to completed pedal in 2 days. I still have a BB layout going to tinker with as well.

This circuit has a great range of tones and it's very sensitive/dynamic/whatever.. sounds like a tube amp is what I'm saying I guess.

I thought about the SWTC for this but it has so much expendable volume that the RAT Filter is just fine and it fits the layout easier.

I will try to make a vid when I get a chance

[Kipper4]

Cool man.
I used a variation of this in my B52 emu

http://www.muzique.com/lab/tone3.htm


There's a demo here

http://www.diystompboxes.com/smfforum/index.php?topic=111869.msg1030599#msg1030599

[PRR]

> C11 (2n2) ...What have I done here?

Loaded-down the guitar. The effect will be *different* for every guitar setting, every guitar, or any non-guitar sources.

I suggest a smaller value Q1 C to ground, where it is isolated from the input.

> C7 (47p) blocks out the radio station and doesn't seem to have any affect on the overall tone, but it works anywhere between C2 and C4.

The whole C2-C4 network should be small and tight. No long wires! If unavoidable, shield them. If you use a small "C11" where I suggested, you may not need a little cap here. If you feel you do, R6 wiper may be the best place, and nothing wrong with ~~50pFd. However if you get radio on breadboard, you really should not get it when properly boxed.

[Ben Lyman]

Thanks Paul

"I suggest a smaller value Q1 C to ground"

Sorry, I'm not sure I follow. Is it:
remove C11, put small cap where I show it here as C15

[stonerbox]

Thanks Paul

"I suggest a smaller value Q1 C to ground"

Sorry, I'm not sure I follow. Is it:
remove C11, put small cap where I show it here as C15

I interpreted it as replacing the C3.

[Ben Lyman]

Ah yes, of course. thanks stonerbox, that makes sense.
In the meantime, I simply removed my C11 "Loading Cap" and called it a day. 
Here's a long boring demo. First part is Strat into pedal into 1w TDA2822 amp with 2x12" cab; Second part is with my PT2399 delay, SD-1 clone, Really Cheap Compressor and Wah Wah. And Green Ringer.

[Ben Lyman]

I was experimenting on the breadboard with this and tried a bipolar buffer in front, it's the 5th one down this AMZ page: http://www.muzique.com/lab/buffers.htm

I didn't notice any change in the pedal, can anyone tell me if there is any advantage to adding it? It says the input impedance is low but I also moved my 2M2 pull down resistor onto the buffer input, does that help? 

It's like this right now:

[antonis]

I was experimenting on the breadboard with this and tried a bipolar buffer in front, it's the 5th one down this AMZ page: http://www.muzique.com/lab/buffers.htm
It says the input impedance is low but I also moved my 2M2 pull down resistor onto the buffer input, does that help? 

It actually says that the Input impedance is lower compared to Fets..!!
Imput impedance for BJT is the parallel combination of biasing resistor(s) and Emitter resistor times beta..
(about 100k in your buffer..)
For Fets it's just the biasing resistor(s) which is (are) much bigger than the respective BJT because of practically "current independence"..

I didn't notice any change in the pedal

Just calculate the In-Out impedances before and after buffer placement..

[Ben Lyman]

Thanks Antonis! So probably around 390K(ish) input impedance, maybe acceptable or a little low. I can raise it to about 1M easily then, just change the emitter R to 10k, right?

So my 2M2 doesn't really have any bearing on the input impedance, I guess?

Input impedance of the original design without buffer looks more appealing, does it not?

[thermionix]

I'm still learning this stuff, so I could be way wrong, but I think your input impedance will always be lower than R22 (220k) in your last schematic.  I don't think the pulldown has much effect in that setup, because it's so much higher.  You could raise R21 and R22 to 2M2 if you wanted to approach a 1M input impedance.  I think.

[Ben Lyman]

You are probably right, but..
    for BJT is the parallel combination of biasing resistor(s) and Emitter resistor times beta..

Hmm.. so I always just look at the lowest R in the equation and figure. "about that but a bit less" so.. I currently have 3K9 emitter R.. times that by 100= (app)390K

This all brings me back to wondering: Is it even worth adding a buffer to this particular circuit? I think the original input impedance it probably fine but I also wonder if a buffer does more than I know of, like help with adding another pedal in front.. a wah wah.. or using different guitars.. or.. ?

[antonis]

<So my 2M2 doesn't really have any bearing on the input impedance, I guess?>
Right.. 

<Input impedance of the original design without buffer looks more appealing, does it not?>
It does.. 

[antonis]

You are probably right, but..
    for BJT is the parallel combination of biasing resistor(s) and Emitter resistor times beta..
Hmm.. so I always just look at the lowest R in the equation and figure. "about that but a bit less" so..

Actually, lower than the lowest one...!! 
And in your circuit the 3k9 x 100 is the biggest one..

<Is it even worth adding a buffer to this particular circuit?>
Maths can answer your query but I think your ears already verified it...

[antonis]

but I think your input impedance will always be lower than R22 (220k) in your last schematic.

It actually will be lower than the half of R22 or R21..
(220k)//(220K)//(3k9x100+25R*)
*B-E junction resistance for 1mA Emitter current - which is negligible here..

P.S.
Many people tend to only count on the resistor(s) leading to circuit's GND and forget than DC supply must also considered GND for AC signals..

[Ben Lyman]

Yes, I guess my ears have verified it but since i'm on a mini learning streak..
umm, why is my 3K9 the biggest? not the 2x 220k ?
Maybe I misunderstood your equation. Is it parallel value of all 3 R's times 100?
Or is it emitter R(100) in parallel with base R's?
i.e. 1/220k + 1/220k + 1/3K9(100) divided by 1 = 85K8

[Ben Lyman]

oops, never mind. You answered as I was asking  Thanks!

[antonis]

I've just answered to thermionix on that.. 

edit: You've read it..

@ Ben: Imagine the base of the transistor as a node with all the resistors incoming there and outcoming to the same point (which is GND)..

[Ben Lyman]

Cool, thanks again antonis, I can imagine that