<Community Design Project> Boutique Mosfet Madness

polarbearfx · Post by **polarbearfx** » 13 Nov 2007, 15:05

" This circuit contains absolutely no IC's and features mosfets for a natural amp tone that was previously not possible with your favorite op amp circuits"

These kind of statements
seem to keep popping up.
Is there any truth at all in
this blanket statement?

alteredsounds · Post by **alteredsounds** » 13 Nov 2007, 16:04

Dont care how it is worded, the only thing with a natural amp tone is a natural amp. End of.

DougH · Post by **DougH** » 13 Nov 2007, 17:17

This is true.

Nothing wrong with 'pedal sound' but an amp sounds like an amp and a pedal sounds like a pedal.

Let's just generi-cize that statement above, since we see it so much in ads:

" This circuit contains absolutely no <components I know no nothing about, don't understand, or don't like based on any number of irrational fears> and features <the components I do like, do understand how to use, or like based on any number of irrational reasons> for a <holy grail as defined by someone> tone that was previously not possible (because I just found out about it for the first time yesterday) with your favorite <whatever currently is used and works because I have to convince you that *I* just made an important discovery> circuits"

There.

Instant ad-copy for any of you budding booteekers. Just fill in the blanks appropriately for your situation and brace yourself for your first million in sales.

Brian M · Post by **Brian M** » 13 Nov 2007, 18:41

I think there may be some truth to it, but at the same time it comes down to the details of the circuit.

If you design a circuit with any type of amplfier, be it mosfets, opamps, tubes (provided you can make them work in the same voltage range) The amplifier produces gain, but if you design the rest of the circuit to sound the same... same filtering, same controls, you are going to end up with a fairly similar sounding circuit.

That said, in general there are some differences in sound. Let's not forget that we can't HEAR a datasheet. Granted, the components only do what they are supposed to do, but different parts DO sound different in many cases, although the differences can be a bit exagerated in some cases.
All other variables being the same, my personal experience is as follows.

Opamps- easy to work with in general, very consistant, and can produce a lot of gain in a single stage. Normally If I'm using opamps I like 1n4001's or 1n914's as ground clippers. I don't usually like feedback loop clipping as much, and i especially dont like asymetrical feedback loop clipping. Rail clipping sounds different on different opamps, but in general i dont find it very plesant. Although most opamps have a class AB output, both sides are not always equal.... not just in close they will get to the rails. If you set up a feedback loop clipper, and watch it on an oscilloscope, you can see little "spikes" at the beginning of the edge of each square wave. On some opamps its fairly symetrical, but others are a bit more un-even. I'm guessing that this is caused by the opamps slewrate not being consistant.

Mosfet- To be honest, I have not played around with mosfets that much, but here is my input on a common source type amplifier. If you don't squash the output with diodes, and just let the mosfet swing as far as it can (provided there is enough gain, and the headroom is low enouhg) the signal will clip hard as it turns on, but will have a more rounded shape as it turns off. This is a pretty significant difference in comparison to opamps. Also if you've ever put looked at a preamp tube output on a scope it looks quite similar.

That said, once you have a bunch of cascaded stages of either tubes or mosfets, it does get more and more squared off on the edges.... but I'd still say it sounds a bit different than opamps.... especially since quite often there is a lot of filtering that goes on between stages with cascaded gain circuits, whereas with opamps, in most pedals the gain is produced all at one stage. sometimes this will lead to a bit more of a compromise between intermodulation, and low end frequency response.

Now, let's not forget that preamps are only part of the tube sound... and hey, tubes are only part of it as well. I think most would agree that the reactions between the output tubes, transformer and speaker probably contribute the most to what we call "tube tone"

I've seen some tube amp on the net that doesnt use an output transformer. I've never tried it, so I dont have personal experience, and can only speculate. My guess is that while it may sound really good, it's not going to have "that tube sound" IMHO... of course it would probably be significantly less backbreaking to carry around than tube amp

lautmaschine · Post by **lautmaschine** » 13 Nov 2007, 18:41

I agree. Try playing through your average distortion or overdrive pedal directly into your board, and it's not going to sound anything like a decent amp.

I do, however, think some pedals are more atuned to blending or contributing to the overall amps sound than others. Some of my favourites are indeed FET based, but I've also heard some good IC-based examples...

Seishin · Post by **Seishin** » 13 Nov 2007, 18:42

Depends on how they are used... Like a diode clamp? It's still a clamp lopping off the waveform. Cascaded making gain... That is something interesting. It doesn't sound exactly like a toob amp but it is a heck of a lot closer than diode clamps are.

snail · Post by **snail** » 13 Nov 2007, 19:00

Let's start a religion:

Mosfet Saves!

Donnerbox · Post by **Donnerbox** » 13 Nov 2007, 20:38

polarbearfx wrote:" This circuit contains absolutely no IC's and features mosfets for a natural amp tone that was previously not possible with your favorite op amp circuits"

These kind of statements
seem to keep popping up.
Is there any truth at all in
this blanket statement?

Why not.

It says natural amp tone - not TUBE amp tone....

If you were playing thru a mosvalve amp a mosfet distortion would probably sound pretty natural ...

Brian M · Post by **Brian M** » 13 Nov 2007, 20:52

snail wrote:Let's start a religion:

Mosfet Saves!

would that make opamps the overlords of hell? Just currious. could the LM741 be satan?

and where would mosfet based opamps fit in to all this.... some un-holy melding of good and evil... Internal conflict always makes for good mythology.

Skreddy · Post by **Skreddy** » 14 Nov 2007, 17:16

I've got an idea. How 'bout we collaborate on an effect idea.

How 'bout something like mosfet input (for high impedance and bandwidth), a fet (or maybe two) for some mushy/crunchy/midrangy/tubey tones, and then a tone stack of some sort followed by a bjt gain-recovery and master volume.

Want to fit it into a small box? That will determine basically how many knobs to use on your tone stack, assuming you don't want to go all miniature like ZVex.

Simple project. Who wants to start drawing a schemo?

Challenge: just use either product application notes or else your own common sense or intuition or 20+ year old schematics on how to set things up like bias and gains and cap values.

Who wants to go first? Start with whichever block (mosfet, fet, tone stack, volume recovery) you want; we'll cobble something together.

frank.clarke · Post by **frank.clarke** » 14 Nov 2007, 17:25

polarbearfx wrote:" This circuit contains absolutely no IC's and features mosfets for a natural amp tone that was previously not possible with your favorite op amp circuits"

These kind of statements
seem to keep popping up.
Is there any truth at all in
this blanket statement?

It is 100% true. "your favourite opamp circuits" is undefined, so that makes it true for a start.
"natural amp tone", Natural MOSFET amp tone, why would you assume some other kind of amp?
Of course, if the circuit contained no mosfets, or contained opamps, it would be false.

snail · Post by **snail** » 14 Nov 2007, 17:57

Brian M wrote:and where would mosfet based opamps fit in to all this.... some un-holy melding of good and evil... Internal conflict always makes for good mythology.

well that would be Damien of course, as seen on Omen! Son of the evil one and an earth dweller!

Brian M · Post by **Brian M** » 14 Nov 2007, 18:15

snail wrote:
Brian M wrote:and where would mosfet based opamps fit in to all this.... some un-holy melding of good and evil... Internal conflict always makes for good mythology.
well that would be Damien of course, as seen on Omen! Son of the evil one and an earth dweller!

hmmmm i thought the earthlings would all be passives

celadine · Post by **celadine** » 14 Nov 2007, 20:53

How 'bout we collaborate on an effect idea.

Hrm. IMHO, checking out the secret, mind-boggling boutique effects is more fun!

Skreddy · Post by **Skreddy** » 14 Nov 2007, 21:29

Here's a first pass, with no values. Many ideas possible depending on how many knobs you want...

madbean · Post by **madbean** » 14 Nov 2007, 22:23

I got something down on paper this afternoon, as well. It's and SHO with mosfets set up for hard clipping at the output connected to a variable bias voltage (a la OCD) ((actually, I don't know how well that will work in this setup)), then to a volume control, static low-pass filtering, vairable high-pass, then a single jFet stage with fixed (low) gain and finally a SWTC for a bit of tone control. No recovery stage needed! Then a master volume.

So, 5 knobs in all - Pre-Gain (or Drive), Gain, Bass, Tone, and Master. Maybe a switch to bypass the diodes depending on how they work out.

jrc4558 · Post by **jrc4558** » 15 Nov 2007, 03:50

Brian M wrote:
snail wrote:Let's start a religion:

Mosfet Saves!
would that make opamps the overlords of hell? Just currious. could the LM741 be satan?

and where would mosfet based opamps fit in to all this.... some un-holy melding of good and evil... Internal conflict always makes for good mythology.

No, the satan then should be LM301!

Requires external compensation and noise figues are through the roof!

jrc4558 · Post by **jrc4558** » 15 Nov 2007, 04:19

Skreddy wrote:Here's a first pass, with no values. Many ideas possible depending on how many knobs you want...

You will need to bias the gates of T2 and T3. A simple resistor to ground would do.

Brian M · Post by **Brian M** » 15 Nov 2007, 06:14

Skreddy wrote:Here's a first pass, with no values. Many ideas possible depending on how many knobs you want...

If you remove C10 you might be able to get away with out the biasing resistors on the output transistor. Of coures there may be a mild scratchyness as you turn the tone control... of course we already have some of that mojo with the pot on the mosfet.

Brian M · Post by **Brian M** » 15 Nov 2007, 06:16

jrc4558 wrote:
Skreddy wrote:Here's a first pass, with no values. Many ideas possible depending on how many knobs you want...

You will need to bias the gates of T2 and T3. A simple resistor to ground would do.

j201's will sometimes self bias... but yeah, resistors to ground will allow you to filter some low end and reduce intermodulation... probably the biggest advantage of cascaded gain stages.