Line Level FET Push Pull circuit

[Kirfew]

Hi everybody, would it be possible to use a circuit somewhat like these: or to "smooth out" the signal from a distorted preamp by cancelling out even-order harmonics (i think?) and maybe also with the NFB loob in the second pic.

[J M Fahey]

Both circuits will basically leave the original signal (clean or distorted) as-is.
Both are unity gain followers.

[Kirfew]

So is there no transformerless way of emulating a push pull stage with fets?

[markr14850]

It's a little unclear what you are requesting. Can you give some more details? It sounds like you're asking for some post-processing circuit which removes distortion.

[loudthud]

A push pull amplifying stage does not magically cancel even harmonics present on the input signal. It just does not create any even harmonics because it tends to distort the incomming signal symetrically.

[Kirfew]

Basically, I'm looking for an easy solid state way to emulate some of the characteristics that push pull power stages have on distorted/loud signals and also be able to include a presence/resonance control in the negative feedback loop. Please forgive me if I come off as an overambitious newbie

[Steve Conner]

An ordinary long-tailed pair of transistors would do the job about as well as anything transformerless could. Or you could overdrive an OTA chip such as the LM13700 or CA3080, which is really just a long-tailed pair inside.

Peavey also have some patents on tube emulation that you may like to check out. I think they have something that emulates bias shift and grid current in the power tubes, or maybe that was Carvin.

The presence and resonance controls in a guitar amp do about half of their work by undamping the speaker. When you turn up the resonance control, you're actually emphasising the cabinet resonance as well as boosting the bass. Needless to say, these controls don't work so well when the speaker is replaced with a resistive load. They become just another kind of tone stack.

My own approach to the problem is this: http://scopeboy.com/scopeblog/wp-con...HybridRev2.pdf A cheap single-ended tube power stage driving a push-pull pair of power transistors directly. It doesn't do away with the output transformer, just moves it one stage back so that it can be smaller and cheaper, while still adding irony to the tone.

It has no NFB at all, but a NFB loop could be taken from the speaker terminal back to the cathode of the EL84 or even the stage before, as Fender did in the Harvard.

[loudthud]

Checkout the link below. You might have to join the forum to see the attachments.

http://www.ssguitar.com/index.php?to...17825#msg17825

[imaradiostar]

You may also want to look at some of the "tube emulation" circuits on runoffgroove.com. There are a few designs there that using varying fet topologies to get where you're headed.

jamie

[teemuk]

So is there no transformerless way of emulating a push pull stage with fets?

Yes there is.

The magic of the push-pull power amp comes from two asymmetrically distorting stages summing together two opposite-phase signals, as an outcome creating a symmetrically distorted signal. Those two signals tend to error correct each other, smoothing out harshest clipping and things like DC offset shifts or "interactive" decrease of headroom due to voltage sag will also introduce plenty of interesting side effects.

Basically, you don't really need a transformer for signal summing; a plain differential amplifier easily suffices and the same substitute is also used in several commercially available amps. At such low current and voltage levels - not to mention practcially mid-range frequencies - you can forget any supposed "transformer saturation" ...and that thing isn't in reality a too glorious effect to begin with. Basically it just messes up your signal and introduces plenty of really harsh overtones to it. Thank goodness it's actually a damn rare phenomenon in amps.

Personally, I see no mentionable value in FETs. You can do pretty much everything they do with other circuitry. Circuitry that's far more generic and predictable from its characteristics. My local component store always packs a bunch of opamps (hardly ever FETs of certaintype) and I never need to buy a ample handful to find just one or two devices that luckily fall within certain tolerance of parameters, nor do I have to mess up with stupid circuits (e.g. bias trimmers, constant current sources) that try to fix problems that I only have when I'm forced to deal with FETs.

Here' my humble take of the idea:

Dynamic clipping threshold control with regulator