SS Amplifier with automatic gain control

[teemuk]

In looking for this one, I stumbled on one patent for using diodes in the feedback path of an amplifier to cause distortion - filed in the mid 80s! Feedback clippers were standard items long before that. I can't fathom how this got through, other than perhaps the patent examiners were completely clueless about the music industry.

They usually are.

For example, long before that there were German patents covering the use of (soft) clipping diodes in the feedback path of a solid-state power amplifier (a scheme of which's existence many "designers" still seem to be in total denial). The same patent also presented an intentional DC bias drift scheme (also for the power amp) which gradually shifted from symmetric clipping (odd harmonics) towards asymmetric one (even harmonics). SLM (Crate & Ampeg), by the way, was somehow able to get a US patent for almost a carbon copy of that latter scheme about 20 years later. For their defense, they probably even weren't aware that such thing had already been invented.

Yes, the examiners (who supposedly should be "skilled in art") are mostly totally clueless, not to mention the entire U.S. system of patenting being very often totally idiotic as proven by wonderful "inventions" like these:
http://www.freepatentsonline.com/crazy.html

... I still like to surf that stuff to occasionally stumble into the few real gems, though.

[tca]

> Peavey patented a variable resistor in series with clipping diodes to soften the clipping in the 90s, the uniqueness apparently being using a dual-section pot to mitigate the change in apparent volume that causes. I almost can't imagine something more "obvious to one skilled in the art". Well, OK, using two resistors as a voltage divider, or a resistor to limit or sense current, maybe. But still.

Can you point to that patent?

Thanks.

[R.G.]

I'll try to find it again. I stumbled onto it in trying to find the Vox clipper patent, and in doing that I bumbled around through which patent references what other ones. It was a multi-forked path.

I'll try to recreatr it.

[tca]

^ Never mind, I thought you were saying a varying (voltage control) resistor.

Cheers.

[R.G.]

Sorry - it was a dual-section pot, not a voltage variable resistor.

[Rob Strand]

Check out some of the schematics of Peavey and Fender Bass amplifiers.

[tca]

I've drop the AGC at the input and used it to control the bias of the first PNP transistor... similar (a little less) audible output but with different distortion, a kind of duty cycle modulation. More tests needed.

[Mark Hammer]

Check out some of the schematics of Peavey and Fender Bass amplifiers.

Crate also uses feedback from the power-amp chips (frequently TDA20xx type) to Vactrols positioned just ahead of the power stage in their lower-power SS bass amps.

Given this, and Rob's comment, I'd imagine that handling the big transients of basses is often an issue for "underpowered" SS bass amps.

[tca]

For example, long before that there were German patents covering the use of (soft) clipping diodes in the feedback path of a solid-state power amplifier (a scheme of which's existence many "designers" still seem to be in total denial).

Do you have the ref. for that patent? Thanks.

[Davelectro]

The little Vox Busker uses diode clipping in the feedback loop of a TDA2030. I´ve never heard one of those, though. Are they any good?

[teemuk]

Do you have the ref. for that patent? Thanks.

It's Deutsch patent No. 3118042, Verstärker zur Nachbildung des Betriebsverhaltens von Röhrendstufen, Dynacord Electronic und Gerätebau GmbH & Co KG

[tca]

The little Vox Busker uses diode clipping in the feedback loop of a TDA2030. I´ve never heard one of those, though. Are they any good?

I guess it just works as a limiter not as a compressor, probably similar to two diodes in a negative feedback of an opamp.

[amptramp]

There is a rather interesting limiter that I have seen used as a noise limiter for communications receivers.  It uses a grossly overcompensated LM101 op amp stage as an inverting amplifier.  When a spike comes through, the large compensation capacitor holds the slew rate to a value that keeps the amplifier from responding, but the signal is amplified normally for any signal with a slew rate that can be accommodated with the capacitor chosen.  To deselect limiting, just take out the bulk of the compensation and leave only the necessary compensation.

Another idea would be a log amp which does cycle-by-cyle limiting using the instantaneous value of the signal input.  A diode clipper is almost the same over certain input ranges, but true log amps are available.

[tca]

I've been testing this AGC with chip amps and it also works nicely... of course. My idea of using a class A amplifier was to decrease the damping factor to a value near a tube amp damping (correcting the frequency response) allowing the speaker characteristics to appear. The same can be done using current feedback with class AB but *now* using batteries! And so back to the bench...