Adapting Gain Circuits to Discrete Versions

[aziltz]

I've read about making pedal versions of amps by replacing the tube gain stages with FET's and jFets, etc...

Are there any notes or rules of thumb for doing this sort of thing?  I want to make an FET version of my Tech21 Double Drive (it uses opamps and I have a schematic...)

Any suggestions or help would be greatly appreciated!

[John Lyons]

It depends on what you are going for.
What are your reasons for making a discrete version?
You can roughly make an op amp with discrete transistors with "Diode Compression op amp" in the projects section.
Will it sound like the original Tech 21 circuit, most likely not.

john

[aziltz]

well, i have a subdecay liquid sunshine, which uses cascaded jFets for its gain, and i really dig its sound and feel.

The Double Drive uses cascaded opamp stages that are mismatched to introduce different harmonics, with their own gain control.  I wanted to give this a shot with jFets or similar instead, as well as using better components for less noise, eliminating the buffer, and adding true bypass in a smaller box.

It's basically an experiment i want to try out.  See how it sounds, see what I learn.

[PerroGrande]

Check out some of the projects at runoffgroove.com.  You might be especially interested in the article that explains the Fetzer Valve.

[aziltz]

yeah, that and OLC pedals/kits were my inspiration.  actually, that fetzer valve article is very helpful. 

are there any notes on opamp gain stages --> fets?  i'll check that diode compression thing mentioned earlier.

[John Lyons]

Ok, fair enough.
One thing to keep in mind is that inside each opamp there are a few transistors for each gain section.
So it gets a bit messy since you would be trading one IC for 10 transistors, several resistors a few caps.
I know what you are trying to do but the IC to transistor swap isn't a simple affair. The gains are different as well
so you would have to cut some levels here and there as well as do some filtering...
More than anything you would need to redesign the circuit a good bit to get it to work in a similar way.
They are very different so it's hard to know where to start.

I don't mean to discourage you...

EDIT:
ROG and some of the projects at OLC (which are taken from ROG) are based on tube circuits not IC circuits.
With tubes it's more of a part for part swap although there are some changes to be made.

john

[aziltz]

Ok, fair enough.
One thing to keep in mind is that inside each opamp there are a few transistors for each gain section.
So it gets a bit messy since you would be trading one IC for 10 transistors, several resistors a few caps.
I know what you are trying to do but the IC to transistor swap isn't a simple affair. The gains are different as well
so you would have to cut some levels here and there as well as do some filtering...
More than anything you would need to redesign the circuit a good bit to get it to work in a similar way.
They are very different so it's hard to know where to start.

I don't mean to discourage you...

EDIT:
ROG and some of the projects at OLC (which are taken from ROG) are based on tube circuits not IC circuits.
With tubes it's more of a part for part swap although there are some changes to be made.

john

right thanks.  i'm not discouraged, thanks for the advice.

it looks like, at the very basic level, i can duplicate the double drive using the fet diode-compression op-amp, http://www.diystompboxes.com/analogalchemy/sch/diodeopamp.html

i think, simultaneously, i'll build a liquid sunshine and study its mojo as well and see what makes those gain stages so special, and see about combining them...


thanks again.