funny statements on output imedance from article

[Ptron]

A few times in this article http://sound.westhost.com/instamps.htm , the author says something to this effect...

Many of the requirements for guitar amps also apply to bass amplifiers...A high output impedance and high dynamic output are also desirable features.

and...

From the very first guitar amplifiers I ever built using transistor output stages, I used current feedback to increase the output impedance of the amp....Regrettably, this technique does little to improve dynamic output. Output impedance of my amps was typically 200 Ohms.

Any clue what this guy is talking about? That's news to me. (ok, lotsa things are news to me but anyway) As I understand it, anything other than an output impedance that matches the speaker reduces the efficiency/power output. 200 ohms seems crazy. What don't I get here?

Other than that, the article seems to be reasonable to me.The site from which it comes is full of great, knowledgable articles that dispell a lot of myths and voodoo.
http://sound.westhost.com/articles.htm

[Enzo]

You don't get that it is a solid state amp. Tube amps have a transformer, and it transforms the impedance of the tubes to the speaker, so the speaker needs to match the winding. The transformer itself doesn't have an impedance per se. It only has a relationship between primary and secondary. If you change the impedance on the primAry side - such as using different type tubes - then the secondary will show a differing impedance as well. REmember how you have to move the impedance down a notch when you pull half the power tubes? The transformer didn't change, only the primary impedance changed.

But that was tubes. SOlid state amps put a voltage on the output terminals, and acts as a current source to the speakers. There is no inherent impedance to "match." The lower the speaker impedance, the more current it will draw at that output voltage. AMps will have a lower impedance limit - 4 ohms, 2 ohms, whatever - but you can use as high an impedance as you like.

I never thought about it, but I was under the impression that lower output impedance was preferred on a SS amp. WHy add impedance in the way of the load?

[tbryanh]

This is an excellent article.

The amplifier damps the speaker. The lower the output impedance of the amplifier, the more the amplifier damps the speaker.

An example of damping can be seen using a tuning fork.

Mount the base of the fork to an empty wooden box, and hit the fork with a screwdriver or other small object.

The fork vibrates at its resonant frequency, and the note it produces is relatively loud and lasts a relatively long time. The fork is undamped.

Now squeeze your fingers on the trunk of the fork, and hit the fork again. You get the same note, but is is not as loud and it does not last as long. The fork is about 50% damped.

Now squeeze your fingers where the trunk branches out to the prongs of the fork, and hit the fork again. You get no sound at all. The fork is 100% damped.

How gently flick your finger on the cone of your speaker while the speaker is out of the cabinet and has its magnet end laying on top of the wooden box. You will hear a soft tone. This is the reasonant frequency of the speaker. Unlike the tuning fork, the speaker is already damped quite a bit; i.e. the tone is soft and does not last very long.

For hifi speakers, no resonace is desired, but the resonance of guitar speakers is deliberately made by the manufactuer. It colors the sound of the guitar alot. The ribs in the cone color the sound too, but this is probably a separate issue.

Now put the speaker back in its enclosure and flick the cone again. The tone will probably be louder and last longer because the cabinet probably works better than the wooden box.

Now put fiberglass in the cabinet and flick the cone again. The tone is not as loud, and it does not last as long. The fiberglass damped the speaker.

OK, so the big question is how does the amp damp the speaker? Well, the speaker converts the electrical energy applied to its coil into mechanical energy at its cone. We knew that already. But the speaker also converts the mechanical energy applied to its cone into electrical energy at its coil. Ahah, the speaker generates electricity! Since the amp is connected to the speaker, the amp absorbs the electricity the speaker generates.

If the amp absorbs all the electricity the speaker generates, the speaker is completely damped by the amp. The amount of electricity the amp absorbs is determined by the impedance of the amp. The higher the impedance of the amp, the less the amp damps the speaker.

If the impedance of the amp is too low, the speaker is damped too much, and the resonance of the speaker cannot color the sound. The sound is flat, and we don't like it.

If the impedance of the amp is too high, the amp does not damp the speaker enough, and the speaker colors the sound too much. The sound is boomy and muddy, and we don't like it.

So the trick is to damp somewhere in between to produce a nice colorful sound.

So when is mechanical energy being applied to the speaker to make it generate electricity? It happens when the signal from the amp reaches its maximum amount and starts decreasing. The speaker has momentum and continues to move at the same speed even though the amp is providing less power to it. The more power the amp can absorb from the speaker at this point, the sooner the speaker slows down.

It is true that maximum power transfer occurs when the amp impedance matches the speaker impedance, but this condition apparently damps the speaker too much. So we trade power for color. Anyway, it takes 10 times as much power to sound twice as loud, so color does not cost that much.

[Steve Conner]

Hi ptron,

It's difficult to explain "what you don't get". I guess Rod Elliot has had a formal EE education, and he can happily throw around words like "output impedance" and assume we all know exactly what he means. But if you haven't sat through 4 semesters of circuit analysis classes, you won't be singing from the same song sheet.

Broadly, the conclusion is pretty much what tbryanh said. What Rod Elliot describes is a circuit trick to raise the output impedance of a solid-state amp to make it damp the speaker the same way as a tube one. However, this impedance is faked up electronically: it's not a real resistor and doesn't waste any power.

An amplifier's output impedance is generally NOT the same thing as the required load impedance for optimum power output. It may be so for triode tubes under certain circumstances, but not generally. A pentode amp with no negative feedback can have an output impedance in the 20s to hundreds of ohms, while still delivering its best power into an 8 ohm load.

Solid-state amplifiers have an output impedance approaching zero, though as I said above, this does not imply that they function optimally when loaded with nearly zero ohms.

You measure output impedance by firing the amplifier up with a signal generator into a dummy load, measuring the output voltage, then changing the dummy load resistance a little and measuring the output voltage again. You then run the potential divider equation backwards to figure out what value the fictitious "resistor" inside the amplifier is. What do I mean by "fictitious resistor?" See below:

http://hyperphysics.phy-astr.gsu.edu.../thevenin.html

FWIW, I have experimented with a tube amp driving a speaker directly, A/B'd against the same tube amp driving a dummy load resistor, attenuator, solid-state power amp, and speaker. There wasn't really any great difference.

Finally, as a fun experiment, try tapping the cone of a woofer on your stereo with the power amp turned off, then try again with the amp turned on. Sound different? Why?

[tbryanh]

Finally, as a fun experiment, try tapping the cone of a woofer on your stereo with the power amp turned off, then try again with the amp turned on. Sound different? Why?

It will sound different. When the amp is off, the amp is not damping the speaker. When the amp is on, the amp is damping the speaker.

[Ptron]

Well I'll be. Speaker damping is one of those terms that I've heard from time to time but never really delved in to. Nice explanation, tbryanh. I'll buy that.

But that leaves me with more questions. Take this little sequence from the article:

In a valve amp, the source impedance may be as high as 200 ohms for a nominal 8 ohm output.

That's from the original article from 1980. He then adds this recent correction:

The source impedance of valve amplifiers is generally lower than that quoted above, and in fact it will generally match the speaker impedance (assuming no feedback is applied). I appear to have goofed in the original article.

but then follows with this also recently added statement:

From the very first guitar amplifiers I ever built using transistor output stages, I used current feedback to increase the output impedance of the amp. This restores much of the "valve sound".... Output impedance of my amps was typically 200 Ohms.

Now this statement seems to conflict with the previous one unless:
- most tube amps do have negative feedback, and maybe this does bring the output impedance that the speaker sees up in 200ohm range?? or
- This simulates some other tube amp effect that he's not elaborating on.

Steve, I have an associates degree in electronics, would you believe? I know, I know. It's nowhere near an EE degree but we did spend a lot of time on Thevenin equivalent circuits so I do understand what the fictitious resistor is but I'm still confused here. If you include the feedback in the equivalent circuit (and this may be the key to the whole misunderstanding) Then maximum power transfer happens when the output impedance = load impedance. How can it be any other way??

Is it possible that output impedance as discussed is pre-feedback?

Or maybe it has something to do with the mechanical energy of the speaker (which would necessarily affect the speaker's impedance as seen by the amp, I would think) but I'm just pulling stuff out of the air now.

[tbryanh]

1. If I have an amp with 8 ohm and 16 ohm taps, and I connect an 8 ohm speaker to the 16 ohm tap, is the speaker damped the same amount as when it is connected to the 8 ohm tap? Why or why not?

2. Suppose I have two identical amps, and each amp has 8 ohm and 16 ohm taps, and suppose I have two speakers that are identical in every way except one is 8 ohms, and the other is 16 ohms. If I connect the 8 ohm speaker to the 8 ohm tap of one of the amps, and I connect the 16 ohm speaker to the 16 ohm tap of the other amp, are both speakers damped the same amount? Why or why not?

[mooreamps]

I read the article. I can't say I personally agree with all of it. But I guess he's entitled to his opinions.
-g

[teemuk]

The more you increase the output impedance of an amplifier the more it interacts with the non-linear speaker impedance, thus shaping the amplifier's frequency response. Another article from the same site (ESP) may shed a lot of light on this topic:
http://sound.westhost.com/project56.htm

[Steve Conner]

Hi guys,

First, the maximum power transfer theorem. EEs seem to love this for some reason, but we need to drive a stake through its heart today.

The two main problems are that it assumes the resistances to be linear, and that it assumes you actually *want* the maximum power! Let's take the example of a solid-state power amp that uses IRFP460s in the output stage. What is the output impedance really? I can give you several answers.

First of all, there is the small-signal impedance which is relevant to speaker damping. This is a function of the whole amp design, including the negative feedback loop. A typical solid-state amp might have a damping factor of 400 at 1kHz, so the output impedance is 8/400 = 0.02 of an ohm.

Now, what is the impedance that gives us maximum power transfer? Well, maybe it's equal to the impedance we calculated above, 0.02 ohms. Hey, this screwdriver has a resistance of about 0.02 ohms, so I'll just throw it across the speaker terminals. With a 50V B+ I should get 1250 amps out and 15.6kW RMS. Bang! Oh, I guess that was wrong. Let's get some fresh MOSFETs, pick pieces of the old ones out of our foreheads, and try again.

Maybe it has something to do with the actual resistances in the circuit, then. An IRFP460 turned fully on has a Rds(on) of 0.27 of an ohm. And let's say we used a 0.47 ohm emitter resistor. There is a total of 0.74 ohms of actual resistance between the speaker and B+, which might be 50V. So naive application of the MPT theorem suggests an optimum speaker load of 0.74 ohms now, and a resulting power output of 422W RMS.

Of course, trying to get 420W into .7 ohms from two TO-247 packaged transistors in a linear amp will result in instant destruction. If we look at the IRFP460 datasheet, the required peak current of 135A is well over the safe absolute maximum. Bang again! So we wipe MOSFET guts off our safety goggles that we wore this time round, slap a label next to the speaker terminals saying: "4 ohms minimum load", and goodbye maximum power transfer. The situation is similar for high powered tube amps.


Now let's look at the effect of feedback on these two output impedances. The first one, the small signal one that determines the damping factor, is what gets changed. Negative voltage feedback lowers it, and negative current feedback raises it.

In a pentode or beam tetrode amp, it's hundreds of ohms, so negative voltage feedback is mandatory to lower it. However, even with as much feedback as you can apply without going unstable, it's still several ohms for the kinds of circuits in MI use.

In a SS amp, it's a few fractions of an ohm even with no feedback, so we use current feedback to raise it to mimic the above pentode or beam tetrode amp with cheap transformers and modest NFB.

UL gets down to maybe 8-10 ohms without NFB, and triode lower still, which is why audiophools who refuse to use negative feedback prefer them to pure pentodes or beam tetrodes. If only the poor fools realised that UL is just another kind of feedback. ;-)

The second impedance we discussed, the minimum load for safe operation, is not affected by any amount of tampering with negative feedback. To change it you need to swap out transformers, add more tubes or power transistors, etc.

[tbryanh]

1. If I have an amp with 8 ohm and 16 ohm taps, and I connect an 8 ohm speaker to the 16 ohm tap, is the speaker damped the same amount as when it is connected to the 8 ohm tap? Why or why not?

2. Suppose I have two identical amps, and each amp has 8 ohm and 16 ohm taps, and suppose I have two speakers that are identical in every way except one is 8 ohms, and the other is 16 ohms. If I connect the 8 ohm speaker to the 8 ohm tap of one of the amps, and I connect the 16 ohm speaker to the 16 ohm tap of the other amp, are both speakers damped the same amount? Why or why not?

No takers?

[Ptron]

No takers?

Well, let see...I guess it would depend on whether the mechanically generated emf from the speaker changes with it's impedance. I've never really thought about what is changed in speaker construction to produce speakers of different impedances. If it's the # of coils (I have no idea if it is) Then I suppose a 16ohm speaker would generate twice the emf of an 8ohm.

I'll take a guess that it is and say:

1) It's damped more on the 16ohm tap. Higher output impedance for the same load = greater damping

2) Same amount of damping. Change in mechanically generated emf cancel cange in damping from output impedance

[Ptron]

1) It's damped more on the 16ohm tap. Higher output impedance for the same load = greater damping

WTF? I really should read over stuff before I post it. Just looked back at this thread and noticed I somehow said this completely bass akwards. I'll try again.

It's damped less on the 16ohm tap. Higher output impedance for the same load = less damping

[J M Fahey]

The amp you name is solid state. Itīs "real" impedance is itīs internal resistance as a generator, (guess 1 or 2 ohms) but itīs irrelevant to sound, because itīs always used with very heavy feedback, which changes all. The usual type is voltage feedback, which lowers output impedance dramatically (to less than 0.1 ohm usually) which means heavy damping, "dry" bass, typical of whatīs usually called transistor sound. In this particular case, "current" feedback is used, which does the opposite, rises virtual output impedance, lowers damping, and, hopefully, gets some flavor of tube sound. By the way, itīs whatīs commercially called Valvestate.

[Earl]

All right....first off, it's called - Maximum Transducer Gain equation .. 10log K^2*4Rs/Rl
where K = Vl / Vs

geez... :-)

As mentioned, this is mainly for matching sources and loads that are 'high' in value...more or less just a voltage divider equation. In very low source resistance/impedance amps, there's very little voltage change at the output regardless of what's hanging off of it. The higher the source resistance/impedance, the more effect it will have on the voltage division, and hence, changing the E in the E^2/R=P.

Don't understand the math behind the NFB and it's effect on this.

Oh, you can do the speaker damping 'trick' by jumpering the terminals on a raw speaker together with a clip lead.