Class A Clipper - has this been done before?

[joelap]

Hi everyone, just some circuits I've been messing around with from experimenting on my breadboard and came up with this idea.  We all know the trick of putting a diode pair to ground after the output cap of a booster pedal to give it more grit, but I was wondering if something like this was ever done:



Basically a JFET biased in Class A prior to the diode pair?  I'm sure this isn't a novel concept, and essentially boosting the signal and then clipping to a diode pair is how Tube Screamers work, but I don't think I've ever seen it used in this context.  I used this in a circuit I breadboarded that I based off of a Silver Jubilee / Bogner Extasy schematics.  After the third gain stage I got tired of going from Triode to JFET, and not to mention the 4th JFET stage caused squeeling (when I breadboarded it up).  I used this is a way to give me more gain.  Here's the circuit of that one:



And while I'm at it, here's a fuzz face circuit with my preferred component changes.



Joe

[Dragonfly]

Check threads for Firestarter Overdrive, Firecracker Overdrive, Sparkle Boost, etc, etc....JFet and mosfet clippers can be made to sound very good ! 

[joelap]

Well I may not be the first, but it does sound very good!

[bioroids]

Hi!, I don't see any gain on this stage, it looks like a source follower to me  .

Regards

Miguel

[Gus]

plate to plate

[aron]

I think the output of the FET is around unity right? Does it really amplify the signal much?

[Sir H C]

It actually has loss in this configuration.

[joelap]

It actually has loss in this configuration.

I think the output of the FET is around unity right? Does it really amplify the signal much?

Hi!, I don't see any gain on this stage, it looks like a source follower to me  .

Regards

Miguel

If I take the clipping diodes off, there's a much larger volume increase, obviously so, but also much larger than if I removed those stages altogether.  What gives then?  When I stopped after the third stage and put this on without the diode pair, there was an increase in volume.  Now I'm confused.  Maybe I drew my schematic wrong.  I'm on vacation so I wont have access to my breadboard to check for another week.  Anyone explain to me whats wrong?  Does it have to do with the 100k resistor?

[Dragonfly]

I would think that C11 would need to come off the fets darin to be an amplifier. As is, it looks like a buffer.

[joelap]

I would think that C11 would need to come off the fets darin to be an amplifier. As is, it looks like a buffer.

When I removed C11, the circuit didnt function properly.  The Diodes werent clipping right without C11 in there.. 

I've only had a basic introduction to power amps for one week at the end of my last electronics class, so you'll have to bear with my noob-ness.

[Dragonfly]

I would think that C11 would need to come off the fets darin to be an amplifier. As is, it looks like a buffer.

When I removed C11, the circuit didnt function properly.  The Diodes werent clipping right without C11 in there.. 

I've only had a basic introduction to power amps for one week at the end of my last electronics class, so you'll have to bear with my noob-ness.

Youre misunderstanding me.

C11 AND everything after it should be coming off the Fet drain in order for this to be an amplifier....so....C11, D1, D2 would hook to the Fet drain, not the Fet source. As it sits, its a source follower.

[joelap]

Now thats odd... because I know that the C11 would come off the drain to be an amplifier... but I learned Class A power amplifiers in my class exactly the way I drew it... Only instead of a resistor, we typically used a current mirror.  Now I'm very confused.   So yes, I understand that it would come off the drain to be an amplifier, but thats how I learned Class A Power Amplifiers.

[soulsonic]

In the second schematic, Q1 - Q3 are correct for a standard common-source amplifier, but the last two stages, Q4 & Q5 are source-followers which don't amplify, but are used commonly as impedance transformers when a high input/low output impedance is desired. It's normal to see a source-follower at the end of a circuit to provide a nice low impedance for the output, but I don't see why you should need two in a row.

In my mind, the class A aspect of the circuit should be considered "understood" because any amplifying stage with a single device running "single-ended" should be biased in class A for the most linear response. Of course, if distortion is desired, you could mis-bias it a bit, but for the most part you're going to be working with class A.

[aron]

As it is drawn, it is a (slightly less than unity) buffer with the clipping diodes on the output. As soulsonic said almost all the single transistor amplifiers are usually class A.

[bioroids]

Hi Joe!

It is a source follower, and I believe it is a power amplifier as well (because you have almost the same voltage but can provide more current). But not a voltage amplifier. There's no "boosting" happening there.

There is no way that the signal at the output has more amplitude than at the input, it will be a little less, but with lower output impedance (though the 100k resistor is too big for that anyway).

Regards!

Miguel

EDIT: oops, new replys posted while writing!

[joelap]

OK, so basically, the setup I have now would be a Class A current amplifier, which is what makes it a power amplifier?  Which therefore would not change the amount of the signal that is clipped or volume of the signal.  OK understood. 

Oh, and my purpose for the two of them in a row is to make up for losses in signal strength from adding a tonestack.  In the way it is drawn, the 4th stage could be removed if no tonestack is desired.

One final question.  I got the idea from that by seeing that done in the final stage of a bogner extasy preamp I found on schematicheaven.com ... what purpose would that stage serve in a tube amplifier?  Just to show a low impedance to the Phase Inverter?

Thanks for all the help.  I'm always leary posting things I am not 100% sure about, but in the end I never get the "look at this idiot!" you typically find in other portions of cyberspace.  Much appreciated.

[soulsonic]

You commonly see followers used in tube amps to drive tone control sections. Driving them this way is usually referred to as a "Marshall" or "Tweed Bassman" style tone stack because those are the two styles of amplifiers where they're most known to be used in. Also, many amplifiers use a follower circuit to provide a low impedance for driving an effects loop. Lastly, followers are sometimes used after the phase inverter section if the amp has a large number of output tubes to drive (ie; 6 or more).

Basically, any place a low driving impedance is desired for whatever reason.