Mixed Gain Stages - Class A and Class B

[Frank Kair]

I was thinking about a overdrive design.

Is it possible to have both class A and class B in a circuit? With a pot for class A gain and class B gain, overall output, tone etc...

Directly coupling the class A amp into the class B amp. My question is, putting a pot in the emitter from the class A transistor will adjust just the level of gain of the class A?
Or it will boost the class B like a pre-gain stage? 

http://www.electronics-tutorials.ws/amplifier/amp21.gif

http://www.electronics-tutorials.ws/amplifier/amp25.gif

I want to have both separated gains, allowing me to blend them, not boost.


Thanks in advance

[Cliff Schecht]

It is possible to have a circuit with both class A and class B amplifiers. I can take the same transistor and bias it for class A, class AB, class B and beyond if I want to go crazy. All this is referring to is the biasing of a single transistor which determines how much of the time the transistor spends on. Class A refers to an amplifier that is always on (also typically indicating center biasing but not always). A class B amplifier on the other hand is biased much colder and only spends 50% of its time on. Class A has the best distortion performance and the worst efficiency, class B has terrible distortion figures but is very efficient.

To be honest you need to be more specific. The schematics you posted did not work. Class A/AB/B can refer to many different topologies. I can take a single transistor and depending on how I bias and what passives are surrounding it I can call it class A, AB, B, C, D, E, F, G, etc. Of course there are more than this now too but many of these techniques are RF specific (or that's the intended application). Push-pull stages can also be class A/AB/B depending on how things are biased.

FWIW I think I get what you are trying to do but don't think you will be very satisfied with a plain Jane class B amplifier. It will sound about like you ran your signal through a rectifier giving you a weird distortion and perhaps a doubling effect. Definitely won't be a very pleasing overdrive, it's a hard clipping that occurs to about half of the signal!


Also cascading one into the other will inevitably tie the two stages together. The trick of taking a signal, running it through two separate distortion circuits and blending them back together is certainly not new. For what you are trying to do I the two distortion stages need to be separated.

Typically you don't adjust a pot in the emitter for gain simply because the emitter is a sensitive node on a transistor and will pick up any scratchiness in the pot. Since the pot has DC on it from the emitter it's almost guaranteed to scratch. A simple volume control before the base of the transistor would act as an effective gain control..

[Frank Kair]

First of all, thanks for the reply!
I am new in Elecronics, but I'm studying it and trying to create and modify designs.
Now I'm dealing with Treble Boosters.

But I had the idea of this overdrive (class A and B)... I was looking for a way to be clearer in my explanation and then I found someone who already did what I was thinking!

http://wp.coldcrafteffects.net/blog/?page_id=19

Now I ask, how the circuit of a pedal like this would look like?

Muuuch better now, right? 

[aziltz]

http://wp.coldcrafteffects.net/blog/?page_id=19

Now I ask, how the circuit of a pedal like this would look like?

Well, its not a new idea, but I'll tell you how I do it (Coldcraft is my brand).  My Cascade circuit uses opamp stages to generate symmetrical clipping in one stage, and asymmetrical clipping in another.  These would create even or odd order harmonics.  It's not the same as discrete Class A, Class AB amplifiers, but I'm not claiming that it is.  It's meant to approximate different tube amp types.

One way to visualize this would be to take the clipping stage of a Boss SD-1 and run it into that of an TS9 Tubescreamer.

[Frank Kair]

Hi, azilts!

So basically, if I use the SD-1 clipping stage before the tube screamer I would sorta get this kind of circuitry...
Interesting.
Congrats for the design, I've watched some videos on youtube, very good sounding pedal!

[aziltz]

Hi, azilts!

So basically, if I use the SD-1 clipping stage before the tube screamer I would sorta get this kind of circuitry...
Interesting.
Congrats for the design, I've watched some videos on youtube, very good sounding pedal!

its not exactly what I do, as fair as filtering and gain levels, but the harmonics/clipping is similar.  My design is NOT a YATS, lol.

[Frank Kair]

I was trying to design a circuit with individual gain controls for two diferent gain stages.

How can I do it? Split the signal in two? But how they mix again?
I mean, I can't just direct couple them because the first would boost the next stage.


Anyone, help??

[NPrescott]

I had a similar idea a while back, mixed gain stages filtered pre-distortion between hi-pass and low-pass. The thread is here, but the take-away is really Mark Hammer's Flexidrive which is, at heart, a lot like what you're talking about. You can set up whatever variety or flavor of distortion that you like and then mix them. As far a splitting the signal and then mixing, I based my ideas on ROG's splitter-blender.

[R O Tiree]

This might help

[J0K3RX]

Some interesting stuff here... all Russian but if you use google chrome browser you can set it to auto translate the whole page...

Look at the JFET Distortion 90xM towards the end of the page... You can use J201's for most of the KP303's
http://www.rusblues.ru/?menu_num=1&subm_num=4&cat_num=4&q=/node/26