preamp distortion vs. poweramp distortion

[Doug H]

Personally I think this FET emulation business is a bit of a dead end for players who don't play to an A/D converter ... I've noticed from soundclips and from building a few that these circuits tend to sound the same  - granted Doug's Meteor still sounds unique to me (it can be finnicky in front of a blaring amp - maybe the filter stage at the end of these circuits needs to be buffered for more uniform response) ... I suspect that a loss of character can happen when cascadeding too many FET stages, fewer seems better to me - not surprising, the same thing happens in tube distortion chains ...

Then again, your meteor is a lot different than mine. You don't have the freq-dependent attenuation between the 1st and 2nd stage. And you did some things to the output that make it sound quite a bit different as well. Throw in the differences between whatever amps we are using and it's anyone's guess what it really sounds like. Mine makes my SS amp sound like heaven but with other amps it can sound blase' if useable at all. It's pretty finicky as you mention. But I think there is only so much you can do with pedals as far as "amp emulation" is concerned anyway.

Doug

[WGTP]

To sort of summarize what it appears we are saying here, is that there are maybe up to a dozen (12) places in the signal chain where signal altering "distortion" occurs.  There are a variedty of electro and electro mechanical phenomenon that can produce this effect.  Most circuits in amps anyway, were originally designed to try and avoid these effects.   :lol:   Within each stage:

It can be in the form of adding harmonics, the content of which can be symmetrical or asymetrical, which can vary with the level of gain, frequency, etc.

equalization, consisting of various center frequencies and Q's or resonance, which can vary with gain, frequency, etc.

compression, with different attack, sustain, decay and release which can vary with....

and some other things that may not have been mentioned and i don't understand, complex impedance for instance and EMF

Basically we are driving things to the edge of there designed limits and into an alternate universe where strange things happen  

[Eb7+9]

... into an alternate universe where strange things happen   8)

Let's go !!

Been thinking about all this mess over the weekend and changed my outplook a little about all this - maybe some forms are within grasp ... I did Doug's Meteor with slight alterations and so Bryan's Odie - both with performance that differed a bit from the respective clips - me thinks pickups play a big role in these circuits not to mention a potentially large variation in FET specs ... nonetheless these circuits have shown displays of tubeyness to some degree - I agree with your sentiment ThickPresence, let's see if we can build on that ...

First a little background, Eric Pritchard who was in EDN 10 years ago announcing his tube-emulation patent, later confided in me that his circuit primarily consisted of a good squaring circuit and a compressor ... fair enough, that sent me on a quest that lead to my Harmonic Multiplier pedal - at 5-10% mix levels you'd swear you hear something somewhat like a "dry" Bassman ... this is merely playing with the tonal definition of tube circuits and ignores the dynamic side, which is where circuits get their "feel" from and this should be part of the picture if a good emulation is to happen ... the tonal view holds that tube circuits primarily enhance 2nd harmonic coloration ... of course this is not exactly true of the pentode output tube stage when it is overdriven, whether push-pull or single ended ...

So let's see how we can introduce some magnetic dynamics in the picture - for now, say we hold off on the requirement of driving a speaker (maybe that's where we should stop - after all, we are building stompboxes) We've got some great preamp ideas, so let's leave power supply sag and speaker modeling filters aside for a moment so we can focus on pentode/transformer mojo ...

FET's operating at low voltages are good at emulating the plate driving point impedance of Triodes in part because the pinch-off characteristics of FETs at these voltages offers a very low Ro ... Bipolar transistors on the other hand have much flatter curves in pinch-off region, even at low voltages - and this fact can be used to mimmick the nature of Pentodes and Beam-Tetrodes ... hence, we should be using bipolar devices to drive a small coil of some sort (preferably one that saturates very quickly) - it makes sense that saturation limits should be scaled down as in every other waveshapping department ... since single-ended circuits tend to compress and distort a lot more than push-pull circuits (for a given power tube and output power load) let's go with basing our strategy on a Fender Champ or the Garnet Herzog which was specifically designed to operate as a stompbox ...

... to be continued ... jc

[Gary]

Great read!

[Paul Marossy]

I remember reading a paper somewhere that stated that the only place where tubes really matter is in the output stage. If that is true, then it seems reasonable that power tube distortion would sound better to most people. Preamp distortion just doesn't sound that great all by itself. And as it has been said, you can't just pick out one thing as being the reason why, it's a very interactive combination between the preamp, power tubes, output transformer, filter caps, etc., etc.

Here's that article I refer to above, published by Fender in 1981. It's interesting reading. http://www.milbert.com/TVTIEGA.pdf

BTW, IMO EL84s have a nice breakup, as I am finding out with my Matchless Spitfire clone.