Please explain amplifier output impedance

What is really happening is that we are able to hear what negative feedback does to sound

negative feedback just ensures linearity - it makes sure that the output matches the input. An electrical circuit operates at close to the speed of light...I doubt anyone can hear negative feedback in modern circuitry. Slew rates of good amps are typically 50 volts per micro second or enough to accurately reproduce a signal of well over 100 KHZ without distorting the signal. Since it is accepted that people rarely are able to hear anything above 20 KHZ then it is extremely doubtful that any slight anomalies of this kind of order are actually audible.

Speaker cones will try to keep moving because of inertia, the suspension pulls them back and as they move they induce current (EMF) in the coil which the amp will sense and will dampen by driving the output to match with the input. Provided the drivers have low mass, a high Xmax and a strong magnet then a powerful low output impedance amp should be able to control the driver well. If the driver is low quality, with a weak magnet and a low Xmax then it doesn't matter as much if the high powered amp has a high damping factor as the cones become harder to control as they travel well outside the linear range of the magnetic field...in this case, ultimately, the suspension pulls them back in but unfortunately at this point you have large amounts of audible distortion).

Negative feedback in high levels is quite audible and it doesn't sound good. Most of it can be avoided to a great extent IF the circuit is properly designed and parts are very carefully matched. Most parts aren't matched all that precise and the attention to circuit lay-out ( nominal impedances ) aren't as widely used as one might think.

Voltage source output stages are available. As i've mentioned before, one should be looking at what the amps clip at as impedance varies, NOT the manufacturers rated power specs.

A high damping factor doesn't mean that the amplifier has more control over the speaker. What it does mean is that the variances in impedance that a typical loudspeaker produces as frequency is altered is less likely to modulate the output stage of the amplifier. There are quite a few other factors involved in this equation though, so don't always assume this to be true.

A high damping factor also means that the amp is more likely to act as a voltage source i.e. "double down" so long as the rest of the support circuitry ( primarily the power supply ) can deliver the goods. Many amps simply don't have enough power supply to get the job done as impedances are lowered. This is why i said that one should look at the power at clipping as impedance is varied, as clipping strains the entire amp quite thoroughly.

A very high damping factor is typically found in amps using GOBS of negative feedback and / or negating emitter resistors on the output devices. Gobs of negative feedback makes the amp sound hard and sterile. Think of late 1970's and early 1980's transistor gear that measured very good ( in terms of THD ) but sounded like hell.

The lack of emitter resistors makes the amp far more likely to blow up. When this type of output stage goes down, they typically do massive damage to the speakers. This is besides taking out the entire bank of output devices for that channel, making it more expensive to repair.

The original Phase Linear amplifiers are a prime example of this this type of design. They used quite a bit of negative feedback with no emitter resistors. Damping factor was rated in excess of 1000 into an 8 ohm load. In English, this means that the output impedance of the amp was less than .008 ohms according to the manufacturer.

When these amps "let loose" due to some type of malfunction in the output stage, they would quite typically light speakers on fire. This is how they got the nick-name of "Flame Linears". If using one of these amps, i would proceed with caution for the above reasons. Sean
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Sean's post above is right on but I would like to add a couple important technicalities.

First and most importantly, amplifiers are tested by STATIC means. This means two things: The test signal is continuous and repetitve, e.g., a sine wave or square wave. Secondly, the load is a resistance and not an IMPEDANCE. The difference between the two gets butchered all the time but basically, impedance takes capacitive and inductive effects into account whereas resistance does not. Many times, people use the word "impedance" but if you take a close look, they are actually using an "averaged resistance" at best. This is incorrect use of terminology but it runs rampant, especially here. :)

The sum total is that music is a very dynamic signal that is constantly changing. The speaker's impedance, in most cases, is a ridiculous mess of ups and downs. Combine the two and you get drastically different damping factors, reflected waves and varying slew rates at different points in time AND for different frequencies. None of this information is faithfully represented by the manufacturer's specs.

But as we already know, you have to listen to get a feel for how an amp/speaker combo works - and that some who measure great, fall short in reality due to poor handling of dynamics. Listening is the best feedback on performance you can get because only then are all the real variables taken into account.

The issue of negative global feedback is different than that of negative local feedback. The two are, again, not to be confused. Global feedback puts the entire amp in the loop whereas local feedback is only for the active devices. This latter one is always required for very good stability but the former is optional, depending on the quality of component matching, parasitic inductances, capacitive coupling, type of active devices and layout quality.

I have looked at the output impedance curves of a few amplifiers using an impedance analyzer. In the frequency domain, most of them are very flat but have an inductive rise at high frequencies (>50kHz or so).

Arthur

Negative feedback in high levels is quite audible and it doesn't sound good.

Agreed. A badly designed circuit will sound terrible. Most manufacturers try to avoid building unstable circuitry. However, in the pursuit of ridiculously high damping factor specifications (for marketing purposes), it is certainly possible to build a dangerously unstable circuit. Extremely high amplifier gain will lead to instability and oscillations. This occurs when feedback times open loop gain approaches negative one. In this case, the closed loop gain will approach infinity...which is of course not possible and everything becomes oscillatory, distorted and clipped.

In general, typical SS amplifier circuitry (with negative feedback loops), although very linear when operated within tolerance, are not at all forgiving when they are over-driven; typically when over-driven they sound harsh and then damage speakers fairly quickly. Certainly, high levels of negative feedback are likely to lead more quickly to catastrophic behavior.

Since music is very dynamic, it is relatively easy to over-drive equipment. Some SS gear has built in protection circuitry that is designed to detect and protect equipment from damage.

Arthur: Those are very good points and well worth clarifying. Thanks for taking the time to point them out AND explain them.

I have often said that it is the sum of manufacturer spec's that count more than any individual spec by itself. Even then, most manufacturers don't provide the quantity of spec's that one needs to make such information truly useful.

As to your comment about most amps having a linear output impedance up to appr 50 KHz, that is kind of generous in my experience. Many amps exhibit a noticeable increase in output impedance at or slightly above 10 KHz. How severe this is will depend on the design of the amp. By 50 KHz - 80 KHz or so, performance is starting to suffer quite noticeably. This is why many amps round the leading edge of a 10 KHz square wave. That is, the higher output impedance is part of a bigger problem i.e. limited bandwidth due to the amp being too slow to properly respond. Combine the limited bandwidth / lack of speed with the rising output impedance and you end up with that slightly rounded square wave that you see so often in Stereophile test measurements.

If you think that this sounds "bad", there are REALLY slow / limited bandwidth / higher output impedance amps fail the 1 KHz square wave test. When this type of amp encounters a very fast high energy high frequency transient, most of the attack, definition and duration is lost. This translates into a soft sounding blur, which some people like. This is probably more true with digital recordings and playback, which tends to sound hard, bright and glaring in many systems.

Other than that, this thread could go on and on contrasting various designs and goals. Suffice it to say that there are a LOT of variations that come into play with any / every design. When all is said and done though, the end result is a summary of what the designer / engineer thought was most important. Whether or not you like that product will depend on your own personal preferences and how well that specific component blends with the other gear in your system. As far as i know, there are no spec's to quantify personal preference. Sean
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