Re: Experience with Smith Micro?

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Most, should I say all, have large negative feedback. Gain these days is cheap. Negative feedback removes distortion arising from nonlinearity of the power components in the amplifier.

In particular, voltage feedback makes amplifiers look like low impedance sources. Speaker wire resistance adds to the impedance of the effective source driving the speaker. Even so, unless there are long runs of relatively small speaker wire, that increase in source impedance is small. Monster cable is an expensive and unnecessary way of minimizing the effective source impedance.

Negative current feedback increases source impedance to where the amplifier becomes a constant current source. Any natural vibrations of the speaker will not be damped under those circumstances.

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I think that you cannot judge fidelity of running water because there is no standard for it. Water in bathtub is going to sound different that in yours.

Resistive filters for crossover makes no sense. It is reactive components or their electronic equivalents that actually separates the bands of audio from one another. Resistive filter by itself can give equivalent signals out but no frequency response modification.

With enough negative feedback, there is no reason why amplifiers cannot be perfect reproducers of their inputs as far as human ears can determine.

In the end, it will be transducers, microphones, speakers, headphones, and listening environments, that would be the big distorters.

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Technically, there is MORE skin effect for audio frequencies above 60Hz than at power frequencies. Solid monster cable would be very susceptible to it because of smaller skin depths. Monster cable, I believe, is litz wire so that the skin effect is tiny Even if not tiny, the effect would not show up for ordinary listening because the resistance and inductive reactance at important audio frequencies would be to small to notice.

At power frequencies. maximum conductor diameter is set by the economics of wasted copper (or aluminum) if you make them too large. Monster cable would be too large if stranded without insulation.

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If the impedance of my phones varied from 20 to 170 ohms depending on frequency, I could say a 220-ohm resistor in series had the effect of negative current feedback. With my resistor network, the amplifier saw more than 20 ohms, but the phones saw 1.8 ohms, meaning 99.2% less negative current feedback than from the standard resistor.

The sound of water running into a bathtub is like white noise. A sound-effects man could probably simulate it in different ways. The sound I heard was trickling or dripping. It was familiar in real life, but I hadn't known it could come over a radio so clearly.

We're talking about crossing over from positive to negative in a push-pull amplifier. A feedback resistor sounds like the way to mitigate it.

I do not fully understand this point.

The impedance phones present at its terminals is from a combination of factors. There is the actual impedance of the internal driving network (voice coil) with the transducer blocked as modified by the loading of the diaphragm and the acoustic load of the air it drives. Adding large series resistance R turns a voltage feedback amplifier into a current source. That is, for an output voltage V the current through your phones approaches V/R as R increases no matter what the impedance of the phones are. It is the Norton equivalent resistance of the amplifier and the attached R. This is not negative current feedback because the current flowing in the output has no effect on the signal that ends up getting amplified.

Depending upon the details of the phones, constant current drive may or may not end up giving improved performance over constant voltage drive. One thing the series resistance will do is prevent blowout of a low impedance voice coil when a loud passage occurs.

Because a watt or two of sound is pretty loud in confined regions, a practical tradeoff is to have high power amplifiers with low efficiency speakers if the dissipation in the speaker circuitry improves (flattens) frequency response.

I'm not used to the phrase "negative current feedback." I supposed it could be used to protect the power transistors in an amplifier: if more output current were sensed, the signal would be reduced in the preamplifier.

If you put a resistor in series with a loudspeaker and the signal voltage coming from the amplifier were constant, and the speaker impedance changed, the voltage at the speaker would vary inversely with the current. That sounds like the effect of negative current feedback.

I have connected a sine-wave generator to an amp to check loudspeakers and phones. All were louder at certain frequencies. Current was minimal at those same frequencies. In other words, they produced sound more efficiently at resonance. Negative current feedback would mean increased voltage at resonant points, making them harsher and more mechanical sounding.

That observation led me to make a low-impedance source for phones, as is normal for loudspeakers. I was rewarded.

Consider an amplifier with high gain that has two input terminals for signal input. If a small fraction of the output 1/m of the output is fed back to subtract from the signal going into the amplifier terminals, the output would be about m times the input, irrespective of the actual high gain of the amplifier. That also means that the output voltage will be held almost constant even with load changes--low impedance output.

If feedback is voltage from a sampling resister Rf times the current flowing in the load, then the output current would be almost Vin * Rf irrespective of load impedance.

That is, like in an operational amplifier. the actual input to the real amplifier is near zero. This assumes that the amplifier is not driven into saturation by using extreme loads.

That is true. Speaker cones driving air have resonance, series and parallel ones. The feedback signal can be a combination of voltage and current feedback. Such resonances can be from the room as well as the speaker suspension.

If what you're sampling is output current (I), it will increase if load impedance decreases. So I/m, the negative feedback, will increase. Because it's negative feedback, the gain will vary in proportion to load impedance.

If the amplifier gain is constant but you put resistance in series with a load whose impedance varies, the load will also see a voltage that varies with its impedance. In this case, voltage will be approximately output voltage times load impedance divided by the sum of load impedance and series resistance. (It won't be exactly that unless the phase of the load impedance is zero.)