Am I doing this correctly? inverting feedback amplifier

[amonte]

I've been messing around with a very simple single stage overdrive design (one transistor pushing two diodes shunt to ground) over the last week or so.  I started out with an LPB-1 style circuit, changed the transistor to a 2N3053 and altered almost all of the values.  My goal was to find something that could produce a decent mild overdrive, but not sound exactly like a tube screamer.

I was poking around the net the other night and found this article:

http://www.aikenamps.com/Equations.htm (scroll down to Single-stage inverting feedback amplifier)

So I tried to do something similar - basically, I tied the second lug of my output cap to a !M pot and tied the other end back the base of the transistor.  I wasn't sure exactly what I could expect from doing this, but what ended up happening was a big cut in gain and a little bit more of an even sound - a bit more tight and focused, not so unruly.  That being said, it did cut a lot of distortion out of the circuit, though I'm thinking if I make other changes (input cap, bypass cap, diode type), I'll be able to get something a little more usable coming out.  I also noticed it seemd to trim or at least tame the high end.

What exactly is going in here?  Can anyone show me another circuit that uses this technique?  SHOULD I use this technique? 

[MartyMart]

Have a look in the Gallery at Erik Vanhansen's "Marsha valve" and "Sixteen" overdrive.
These are based on the ROG "Fetzer valve" and I'm sure will get you closer to what
you're describing, though Fet based ...
The one transistor ( one gain stage) device is tough to get working well, you need
at least another stage or two IMO ....   having said that ......
Check out also Joe's "Easy Drive" circuit for one of the best/simplest OD's out there
more clues in that one for sure

Marty.

[slacker]

I'm not an expert but here's how I think it works. There's a clue in the name of the circuit, the output is inverted or out of phase with the input. So when you feed some of it back into the input the 2 signals cancel each other out which is what gives you the volume drop.

[amonte]

Have a look in the Gallery at Erik Vanhansen's "Marsha valve" and "Sixteen" overdrive.
These are based on the ROG "Fetzer valve" and I'm sure will get you closer to what
you're describing, though Fet based ...
The one transistor ( one gain stage) device is tough to get working well, you need
at least another stage or two IMO ....   having said that ......
Check out also Joe's "Easy Drive" circuit for one of the best/simplest OD's out there
more clues in that one for sure

Marty.

Hi Marty,

I'll take a look at those.

I know it's tough working with a single gain stage, but I guess that's part of what I'm trying to do - get a decent overdrive sound using a single gain stage.  Yup, I've looked at the Easy Drive - in fact, i think I built one a while back - but I'm trying to stay away from diodes in the feedback loop, just to try something different.

It probably seems as if I'm being difficult just for the sake of being difficult, but that's not it.  Rather than just building a circuit straight from a schematic, I wanted to see what I could come up with if I experimented a bit.  Sure, it's all been done before, but I guess I'm hoping to find some magic combination.

[MartyMart]

Well, I've had good "mileage" from the humble Range Master, "don't panic" !
I mean using it a bit more "full range" and NPN as in Fuzz Central's
"Range Blaster"   I like I/O caps of about 0.047uf
You could play with that, I have two that can "break up" quite nice, even
before the boost breaks up the amp input.
Emitter resistor value and the large cap off emitter with Input to
base impedance  etc etc should contribute to some "crunch"  perhaps
RG can shed some "light" on this for you ... ? 
Also reverse diode between base and collector, as in the antiquity fuzz ?

Marty.

[amonte]

I found this as well on the Aiken page:

http://www.aikenamps.com/FeedbackAmp.htm

That definately gives a more detailed description, but I'd be lying if I said I understood it 100%   

Though it definately makes me want to try some of the other variations...