Non-Clipping Amplifier

[ph_christensen]

I have been working on an amp design, using an opamp to compare the output signal to the input signal and create a feedback signal to correct it. the crazy thing is regardless of what load I put on the amp It never clips, just increases the current, has anyone ever seen anything in a design like this. I simulated it at 8 ohms and got 140v pp and 185 watts output, when I replaced the load with a 1ohm resistor i got 140v pp and almost 1500 watts with no clipping. Has anyone ever seen somthing like this used before?

[wg_ski]

There are a lot of things you can simulate in Spice, but never build in the real world. Simulators use ideal power supplies (voltage souces) and smoke never comes out of a transistor model when you overload it. And the amp *would* clip eventually, if you tried to put out more voltage than those ideal sources are capable of. Even at 8 ohms.

If you had a big enough transformer and enough output transistors in parallel, 1500 watts at 1 ohm is very doable.

[teemuk]

Simulation results always require some common-sense reality checking. The described behaviour simply doesn’t sound realistic. Anyway, the overall idea is not that far-fetched: It is a rather common technique to compare the input and output signals and use the comparator’s output in driving a VCA or some other input signal-attenuating element. You can conveniently take these signals from the inputs of the differential stage. This design actually works (as clipping prevention) but you must note that besides clipping it will also trigger to things like phase shift or slewing of the amplifier - or any other anomaly that creates unequal differential signals. Commonly circuits like these tend to “overreact” at higher frequencies due to aforementioned reasons.

[ph_christensen]

I understand that there are alot of thing that you can simulate but dont actuly work in the real world.

but when I simulate the amp using typical global feedback, it starts to clip with a two ohm load, when I use the feedback with the opamp circuit it never clips, because it reduces the gain to keep the signal exactly 5 volts under the rail voltage when the signal is at its peak poit. I was more intrested in the never clipping, not so much the output power.

[poynton]

If it is limiting the gain to prevent clipping, then surely it must be compressing the input signal i.e. it is non-linear and therefore useless for audio.

[ph_christensen]

heres how it works, I removed all of the global feedback, and got a square wave on the output, took the output and ran it through a voltage devider to get the same peak to peak voltage square wave as the input, used the opamp to find the diffrence of the two and then applied that as feedback into the voltage amplifier, to recreate the input, just 180 degrees out of phase. so it takes the amplified square wave and subtracts the inverted input to make an amplified sine wave. as long as the input voltage never goes above the designed level it never clips.

[teemuk]

Originally posted by poynton
If it is limiting the gain to prevent clipping, then surely it must be compressing the input signal i.e. it is non-linear and therefore useless for audio.

I truly disagree with this statement. Clipping is the ultimate form of compression and all audio amplifiers clip (at least in some form) if overdriven. Are you saying that all amplifiers are useless for audio?

It's just a matter of choosing in which form the evident non-linearity takes place. Gradual compression ("soft clipping") is preferred by many over the sudden and harsh "flat topping" clipping (which may also create other annoyances like "rail sticking", oscillation etc.) Consider, for example, the popularity of tube amplifiers and the use of limiter/compressor solutions in PA and instrument amplifiers. Are you saying that these are useless for audio?

[poynton]

Originally posted by teemuk


I truly disagree with this statement. Clipping is the ultimate form of compression and all audio amplifiers clip (at least in some form) if overdriven. Are you saying that all amplifiers are useless for audio?

It's just a matter of choosing in which form the evident non-linearity takes place. Gradual compression ("soft clipping") is preferred by many over the sudden and harsh "flat topping" clipping (which may also create other annoyances like "rail sticking", oscillation etc.) Consider, for example, the popularity of tube amplifiers and the use of limiter/compressor solutions in PA and instrument amplifiers. Are you saying that these are useless for audio?

Do not take remarks out of context.


The use of limiters/compressor solutions in PA amplifiers and also non-linear instrumentation amplifiers is not what is being discussed here and represents a "special case" scenario.

Originally posted by ph_christensen
.... when I use the feedback with the opamp circuit it never clips, because it reduces the gain to keep the signal exactly 5 volts under the rail voltage when the signal is at its peak poit. ........

Your simulation does not clip because it assumes a limitless current supply and as you state " keeps the output voltage exactly 5 volts under the rail voltage " by reducing the gain.

Therefore, overall it is non-linear. Works fine on a simulated sinewave input but would be horrible with music.

[richie00boy]

Originally posted by ph_christensen
as long as the input voltage never goes above the designed level it never clips.

And that is exactly how you prevent a normal audio amp from clipping as well.

Originally posted by wg_ski
There are a lot of things you can simulate in Spice, but never build in the real world. Simulators use ideal power supplies (voltage souces) and smoke never comes out of a transistor model when you overload it. And the amp *would* clip eventually, if you tried to put out more voltage than those ideal sources are capable of. Even at 8 ohms.

If you had a big enough transformer and enough output transistors in parallel, 1500 watts at 1 ohm is very doable.

Totally agree.

[ph_christensen]

Your simulation does not clip because it assumes a limitless current supply and as you state " keeps the output voltage exactly 5 volts under the rail voltage " by reducing the gain.

Say you have 75 volt rails, with a 1 vpp 1khz input signal you get a 140 vpp 1khz sine wave output regardless of the current flowing. that sounds pretty linear to me. I understad that in a simulation you have limitless current.

if you simulate an amp as you change the load resistance, you will see clipping, all I am saying is no matter what load I put on it it dose not clip. design an amp capable of 1500w at 1 ohm, and then put a 1/4 ohm load on it and in the simulation I bet you it will clip.