Who makes

Excellent responses by Bruce and Bombaywalla.

Bob R. adds a good point also. During the 1980s and 1990s Bob Carver designed some Carver Corporation and later Sunfire Corporation solid state amplifiers that were intended to emulate tube amplifier characteristics to some approximation. That was accomplished in part by putting a resistor in series with the output, as Bob mentioned.

One of Carver's earlier such attempts, during the 1980s, involved configuring an amplifier such that its "transfer function" (the relation between its output and its input) approximately matched that of a well regarded high end tube amp, at least when driving a specific speaker load. As demonstrated by electronically "subtracting" the output of one amp from the output of the other, with both amps receiving identical inputs, and measuring and listening to the residue. That ended up provoking a good deal of controversy, of course.

I can't recall any other solid state amps that can be said to approximate power paradigm characteristics. I recall reading that Nelson Pass, and perhaps some others, have created some designs that essentially act as current sources, having VERY high output impedances (e.g., 25 ohms). But those would not maintain an approximation of constant power delivery as a function of load impedance either, because their output voltage would vary dramatically as load impedance varies. (Power = voltage times current, oversimplifying slightly).

Regarding the McIntosh solid state amps that use autoformers at their outputs, those have very solid state-like output impedances (e.g., 0.2 ohms or less). The autoformers appear to be used to make life easier on the output devices when faced with low impedance loads, by making those loads appear to the output devices as higher impedances. Thereby reducing distortion, and probably also increasing power capability into low impedances relative to what it would otherwise be.

Best regards,
-- Al

The Nelson Pass amps I referred to earlier that act essentially as current sources and therefore have very high output impedance are listed at his "First Watt" site here. (His amps that are marketed under the Pass Labs name are of course voltage paradigm amps, as Bruce indicated). The ones I was referring to, which I suppose could be called "current paradigm" amps, are the F1, F1J, F2, and F2J, which are indicated as no longer being produced. The literature provided at the site indicates that they are of course only suitable for use with a limited number of speakers, primarily those using just one driver and providing high efficiency.

What may be more interesting, though, are the currently produced SIT-1 and SIT-2 models, which I hadn't been aware of previously. Based on a quick look at their descriptions, specs, and manuals, those would appear to be true solid state power paradigm designs, although their power ratings are limited to SET territory at 10 watts.

Best regards,
-- Al

12-02-13: Bifwynne
Al or Ralph, if you can simplify or clarify what I just said, I'm sure I and the other readers would appreciate your edits.

You said it all clearly and well as far as I am concerned, Bruce, aside from a minor miswording that I suspect was inadvertent: As you probably realize, damping factor is not measured in ohms. Since it is the ratio of two impedances, it has no units.

Best regards,
-- Al

So tube amps have more current on tap than ss?

Not as a general rule; more often than not the opposite would be true. It goes without saying that generalizations are not likely to be meaningful if drawn based on a comparison between a $349 amplifier and amplifiers that are in a VASTLY different league in terms of performance, quality, and price.

... what about musical dynamic peaks? That's what I'm, mistakenly(?) trying to get at. If you play a song at 90 dB with 110dB musical peaks like rim shots, don't you need the current to give that dynamic range for the 10 milliseconds?

Good response by Bombaywalla, of course, to which I'll add some further specifics.

I took a look at the specs of the Zamp, and it appears that what Ralph (Atmasphere) surmised about the 12 amp current spec is correct -- it most likely represents how much current the amplifier can supply into a short circuit (zero ohms) for a miniscule amount of time. Also, I would infer that the reference to 12 amps "peak" probably means "peak" not only in the sense of maximum, but also in the sense of being distinguished from RMS, which is the form in which the voltages and currents corresponding to maximum continuous power ratings are defined. For the sinusoidal waveforms upon which these numbers are based, RMS current equals peak current divided by the square root of 2, so on an RMS basis the maximum current rating is only about 8.5 amps.

In any event, what is important to realize is that the specified peak current is unlikely to ever be available to a real world speaker load, because for a reasonable load impedance the amplifier will not be able to supply the voltage corresponding to that current times that impedance, and it will not be able to supply the power corresponding to that current squared times that impedance.

What I think you are really asking about in this question is what is referred to as dynamic headroom, meaning the ability of the amplifier to deliver greater amounts of power to a speaker for brief amounts of time than its specified continuous maximum rating.

Dynamic headroom is often unspecified, and when it is specified there is often no indication of the amount of time the power increase can be sustained for, so comparing that spec for different amplifiers is usually not very meaningful. Also, having more dynamic headroom is not necessarily a positive attribute. It can be looked at in two ways: The amplifier is ABLE to deliver more power for a short time than it can deliver continuously, or it is UNABLE to continuously deliver an amount of power that is close to what it can deliver for a short period of time. Some of the world's best amplifiers have essentially zero dynamic headroom.

My impression is that typical dynamic headroom numbers range from zero to a few db, and rarely if ever exceed or even reach perhaps 6 db. 6 db corresponds to a four-fold increase in power, and would raise the sound pressure level heard by the listener by no more than 6 db, and perhaps somewhat less due to "thermal compression" in the speaker.

A number that generally says more about the robustness of a solid state (but not tube) amplifier than all of the foregoing is how closely its 4 ohm continuous power rating approaches being double its 8 ohm rating. The two ratings for the Zamp are 45 and 60 watts, which may be a better ratio than most other amplifiers in its price class have (many of which do not even have a 4 ohm rating), but does not approach the factor of 2 that many multi-kilobuck solid state amplifiers can achieve.

Regards,
-- Al

Hi Bruce,

Kudos on your interest in the technical aspects of this stuff. Most people's eyes just glaze over :-)

Regarding the 1040 joule energy storage capacity of your amp's power supply (that number perhaps being particularly appropriate considering your occupation :-)): What would happen without adequate energy storage is that during musical passages requiring lots of energy, especially high volume bass transients, the amount of ripple on the DC output voltages of the power supply would increase significantly, and since it is that DC which powers the audio circuits, the result would probably be some degree of contamination of the audio signal.

Now, is 1040 joules overkill for a 150 watt amplifier, that presumably has well designed audio signal pathways? I don't know. Particularly in audio, it seems that the line demarcating good conservative design (i.e., design that provides comfortable margins relative to the expected needs) and overkill can be very blurry.

Perhaps Ralph or Bombaywalla will have some additional thoughts on your question, but that's the best I can do on it.

Best regards,
-- Al

Ralph, thanks for the thorough explanations. I too was having some difficulty understanding some of this, but after reading your two posts on the subject a couple of times I think I follow what you are saying.

What I'm still not quite seeing, though, is the SIGNIFICANCE, at the system level, of the distinction you are drawing between acting like a voltage source as a result of feedback and truly having low output impedance. Putting aside the effects that feedback may have on amplifier characteristics such as distortion, gain, and bandwidth, and PROVIDED that the amplifier is operated within the limits of its current, voltage, power, and thermal capabilities (whatever they may be), if we consider the amplifier to be a black box of unknown makeup, wouldn't it behave in the same manner with respect to its interaction with the speaker regardless of whether it achieved a given "output impedance" (as the term is commonly understood) with or without feedback?

And if so, isn't it reasonable to think of feedback as resulting in the amplifier having lower output impedance, as long as it is operated within the limits of its capabilities?

Best regards,
-- Al

Bombaywalla, your analysis of the 1040 joule number looks reasonable to me. Obviously the 10 amp number is just a rough guess, but even if it is off significantly the bottom line conclusion would undoubtedly still stand.

On the output impedance issue, I suspect that you didn't see Ralph's last post before submitting your last post, the two posts having appeared at around the same time. Given that clarification I think we are all on the same page technically, and what Ralph is essentially saying is that equating voltage source characteristics resulting from feedback with reduced output impedance, besides not being technically precise, reinforces lack of recognition of the tradeoffs that are involved. Given that interpretation, I see no issues.

Best regards,
-- Al

Hi Bruce,

Starting with your last question first:

If I am finally starting to get it, does that mean that not all SS amps are true constant voltage sources if power doesn't double down if impedance halves, and the opposite being the case if impedance doubles??

An important distinction needs to be kept in mind between the amp's MAXIMUM power ratings doubling down, and the behavior it will have when operated within those maximum limits. All or nearly all solid state amps having feedback WILL double the amount of power delivered into a halved impedance as long as they are operated within those maximum power limits. (In saying that, I'm oversimplifying a bit by putting aside the effects of phase angle variations). Although, of course, in doing so their distortion performance may suffer.

Are you saying that a tube amp that uses negative feedback is able to adjust the amp's voltage output to compensate for impedance swings in such a way that power (i.e., watts, or volts x amps) approximates the amount of power (watts) a true constant voltage source SS amp would make at a given reference impedance and frequency, assuming that the SS amp was operating within its specs.

Feedback would cause the tube amp's output voltage to vary LESS as a function of load impedance variation than would otherwise be the case. In that respect, its behavior would come closer to the behavior of a solid state amp than would otherwise be the case.

If that is correct, then how should I interpret John Atkinson's bench specs of the Ref 150 when he says that output (???) regulation is +/- .4 db off the 4 ohm taps (output impedance being .55 ohms) and +/- .8 db off the 8 ohm taps (output impedance being about 1.1 ohms)??? Is he speaking about output voltage or output watts?? Based on Ralph's explanation, if JA is referring to watts, then presumably voltage must be swinging all over the place to match the power output of a voltage source amp.

As I see it the only conflict between Ralph's point and JA's statements would concern JA's use of the term "output impedance." Ralph's point is that that term is misleading with respect to what is really going on inside the amplifier, but, as Ralph indicated in his last response, when considering the amplifier/speaker interactions that result from varying speaker impedance there is no conflict.

The basic point is that the feedback that is designed into your amp REDUCES the amount by which the output voltage of your amp varies as a function of load impedance variation, thereby changing its behavior in the direction of being more solid state-like. An ideal voltage source would have regulation of +/- 0 db. An amp that is similar to yours but uses no feedback would have a regulation number that is significantly greater than +/- 0.8 db.

I also assume that an amp's output impedance number may have much to do with the number of turns on the output trannies' secondary windings. But here again, I assume negative feedback is also at play, further reducing measured output impedance.

True, if we define "output impedance" in the sense JA (and others) use that term.

I surmise the "constant voltage source" paradigm breaks down with tube amps because in the end, as Ralph has said many times, a speaker's SPL is a function of the power (in watts) being pushed into the speaker circuits at a given frequency. So, the NF servo circuit is telling the amp to do "ramming speed" (ala Ben Hur) with **voltage** in those cases where speaker impedance is low, and the opposite when speaker impedance is higher.

It doesn't break down, it's just that even with feedback your amp doesn't hold output voltage AS CONSTANT as nearly all solid state amps would (if operated within their limits), under the same conditions of load impedance variation. Therefore while your particular tube amp, operated within its limits, would increase power delivery as load impedance decreases, it would not do so to the same extent that a solid state amp would. It's a matter of degree.

Best,
-- Al

Hi Bruce,

I would think that he is measuring voltage, since it would be more practical to do that, and since the majority of speakers conform to the voltage paradigm.

BTW, this article may be of interest. It describes the standard simulated loudspeaker load he uses for those measurements.

Best,
-- Al