Distortion Reduction
We begin by assuming that no one likes hearing distortion and that everyone would like to hear it reduced, which might NOT be a secure first step. Why not? The assumption may be or not be true. Who could possibly like distortion? Try letting your fourteen-year-old set the EQ on your car radio and you will quickly discover who likes diStoRtioN. Or visit an audiophile who has just bought $4,000 worth of subwoofers. DIstortion GLore. Boom. Boom. Boom. A fun-house mirror that made you look 20lbs lighter and ten years younger would prove a best seller to those who don't even own a fun-house. I was about write that some sonic distortions, of course, are universally disavowed, but I am no longer so sure, as no doubt somewhere lives a champion of that distortion.

Just as no new fashion can be so outlandish or silly to be universally disclaimed, no new audio distortion cannot be welcomed enthusiastically by someone. I remember passing a just opened door at an audio show, which poured forth two distinct types of audiophiles. Half of the crowd beamed as if transported, proclaiming that no better sound had ever been heard. The other half dourly damned the grotesque, ludicrous, and incongruous distortion they had just been subjected to in that room. Who was right? Who can adjudicate? One man's sweet is another man's cloying. A total-harmonic distortion analyzer may not be perfect, but at least it isn't subjective.

When negative feedback was invented, reducing distortion was only one of four goals, the other three being consistent-gain, lower output resistance, and age immunity. In fact, I know an audio designer who only uses enough negative feedback to ensure consistent-gain, say 6dB worth between channels. Why doesn't he use more feedback? He finds the lower THD worsens the resulting sound, not improve it.

The super-triode concept uses negative feedback for reasons other than THD reduction; indeed, it might end up actually increasing the distortion that the beefy output amplifier would have produced by itself. The super-triode concept's goal was mirroring of the triode's sonic signature, creating a replica that is bigger and stronger, like a mouse that can roar like a lion. In other words, a super-triode circuit should act like a sonic magnifying glass or electronic lever.

A true distortion-reducing circuit is the clever Sandman class-S topology that exploits the differential nature of a loudspeaker, which only responds to a voltage difference across its terminals.

The above Sandman class-S topology creates a virtual ground out of the bottom amplifiers. On the left we see that if the top power amplifier produces no distortion, then the bottom virtual ground offers no output signal. If, on the other hand, the top amplifier puts out some signal distortion or noise or DC offset, the bottom amplifier puts out the exact same amount of junk signal, with the result being a zero differential signal presented to the speaker, so no sound. (See posts number 20 and number 285 for more details.)

Nested Amplifiers
Another distortion-reduction technique is to cascade two amplifiers: a small, weak, low-distortion signal amplifier and a robust, powerful, hefty amplifier. In itself, the cascade is not enough, what is needed is to nest the bigger power amplifier within the smaller amplifier's negative feedback loop.

One ring to rule them all. One input amplifier to rule the output amplifier. The above circuit offers no signal gain, acting as a unity-gain power buffer. This not the usual setup. Instead, the usual arrangement is to use a low-voltage OpAmp as the input amplifier and a higher-voltage, more-powerful output amplifier, both of which create signal gain. The input amplifier can run on regulated +/-12V power-supply rails, which the beefy output applier runs on unregulated +/-50V power-supply rails. This way, the low-voltage OpAmp need only swing +/-4Vpk, while the beefy amplifier's output swings +/-40Vpk.

A good example of such a design is found in Elecktor Electronics's book 303 Circuits. On pages 152 to 153, titled, "Tuning AF Power Stages," we see the following schematic. (A terrible title, by the way. I wonder how many passed it by due to the clunky title, which might have proved snazzy in the original Danish or French, but clunks and misleads in English translation.)

It looks more complicated than it actually is. The LF356 is the master amplifier and the "PA" amplifier is the slave. Potentiometer P1 sets the DC offset. The input coupling capacitor, Cc, prevents DC from leaking in and capacitor Cb allows all of the LF356's negative feedback to be applied to its DC output voltage. Capacitors, Cx & Cy are fixes, should the combined amplifiers prove unstable. The text explains how amplifier PA should use as much negative feedback as possible, which means that the input OpAmp must provide as much gain as possible. How was possible? The text states that the LF356 was only good about 5V of clean peak voltage swing. Thus, if the maximum output voltage is 100W, then the peak output voltage into an 8-ohm load would be 40V, so the output amplifier would need to amplify a 4Vpk signal into 40Vpk, or a gain of 10 (+20dB). In other words we would set the output amplifier's gain to 10 and the input amplifiers gain to 40, assuming that we expected an input signal of 1Vpk.

Long ago, I discovered in a Bur-Brown data sheets the fallowing schematic that held two OpAmps in a nested-feedback configuration.

The top OpAmp is in charge. The bottom OpAmp is enslaved to the top OpAmp. The result is a substantial reduction in distortion (usually about -20dB better) over the alternative topology show below, while still allowing twice the output current to be delivered into the load impedance.

Moreover, the Bur-Brown master-slave topology offers a substantially lower output impedance, as the two 10-ohm output resistors are enclosed within the top OpAmp's negative feedback loop. The one potential problem is that the enslaved OpAmp must be unity-gain stable, not All OpAmps are.

OpAmps! Are you serious? I would sooner drink fine wine with a straw than listen to OpAmps!

Sure, I know that many are violently opposed to OpAmps. But not all OpAmps are solid-state, as tube-based OpAmps exist; for example, the following circuit.

The single triode defines a non-inverting OpAmp, as the triode is configured as a grounded-grid amplifier, its input signal being imposed upon its cathode. The negative feedback is presented to its grid. The nested power amplifier, which could be tube-based or solid-state, offers a gain of 21, as (20k + 1K)/1k equals 21. The 2k and 100-ohm voltage divider resistors deliver 1/21 of the amplifier's output signal to the triode's grid. In other words, the triode's mu + 1 will set the final gain for this nested amplifier circuit. (A grounded-grid amplifier that is loaded by a constant-current source will yield a gain equal to its amplification factor plus 1.) A 6SN7 triode would deliver a gain of 21, for example; thus, the open-loop gain would equal 21² or 441.

Yes, this is a super-triode design—of sorts. If we used a different voltage-divider ratio, we would get either more or less gain, which means that we would no longer be mirroring the triode's behavior. Ideally, we would want the triode to set the final gain, not the voltage-divider resistor ratio.

The input impedance looks as if it should very low indeed. It isn't, in fact, due to the constant-current load at the plate. Still, the input capacitor will dismay a few readers. Here is a cathode-coupled amplifier instead.

Certainly, many more features and safety devices could be added, but this is the basic topology.

Okay, now that we have warmed mentally, we can move on to the D. Bollen distortion-reducer circuit from the Wireless World magazine from February of 1972, which I mentioned in my last post.

For those of you who still have old computers running Windows XP (32-bit) or any other Windows 32-bit OS, I have setup the download availability of my old old standards: Tube CAD, SE Amp CAD, and Audio Gadgets. The downloads are at the GlassWare-Yahoo store and the price is only $9.95 for each program.

http://glass-ware.stores.yahoo.net/adsoffromgla.html

So many have asked that I had to do it.

WARNING: THESE THREE PROGRAMS WILL NOT RUN UNDER VISTA 64-Bit or WINDOWS 7 & 8 or any other 64-bit OS.

One day, I do plan on remaking all of these programs into 64-bit versions, but it will be a huge ordeal, as programming requires vast chunks of noise-free time, something very rare with children running about. Ideally, I would love to come out with versions that run on iPads and Android-OS tablets.

//JRB