It should be noted that Mr Berning is a manufacturer of audio gear. As far as i know, all of his tubed amplifier designs are of a VERY high output impedance, hence his propensity to defend this type of product.
Those that are interested in such subjects may want to look at some "reasonably unbiased" test results as conducted and published by Stereophile on this very subject. The article titled "Questions of impedance interaction" has several different graphs documenting the linearity of four different amplifiers driving three very different speakers.
Remember, these graphs are of the actual amplitude linearity / loading characteristics / frequency response of the amp going into the speaker, NOT the measured response AFTER the speaker output. In effect, you are seeing how various amps deal with various loads, not the actual output of the speakers as driven by those specific amps.
Given the phenomenally distorted / non-linear frequency response / loading characteristics of some of these amps, the output response of the speakers would be even more distorted / non-linear. After all, the speaker can only try to reproduce what is fed into it, and if the amplifier is feeding a phenomenally non-linear signal into the speaker, one can only expect further distortions after the speaker.
As can be seen in the test results, the two low output impedance solid state amps ( Hafler & Aragon ) remain extremely consistent in amplitude output ( linearity of frequency response ), regardless of the load they are presented with. While these would commonly be referred to as a high damping factor design, one can see the benefits of using a very low source output impedance as compared to a higher ( speaker ) load impedance. That's because the difference in impedance between the source and the loudspeaker load acts as an "impedance buffer".
This "impedance buffer" (or "higher damping factor") reduces the potential for the varying impedance that the speaker presents to "load down" or "modulate" the output of the amplifier. This can be seen as output remains quite consistent, regardless of frequency or the impedance that these amps are presented with.
The Melos tubed amp results are kind of tricky. In the text of the article, Mr Norton states the following: "Since the Melos 400 also had a relatively low output impedance for a tube amplifier (at 0.43 ohms at low and mid frequencies, rising to 1.2 ohms at 20kHz, from its 8 ohm tap), I took that opportunity to run some frequency-response measurements using an actual loudspeaker as the load for the amplifier".
While this would seem to be a pretty stable design as far as tubed amps are concerned, and the results somewhat confirm that other than a sloping high frequency response, the results are somewhat misleading. That is, the Melos might have an output impedance ranging from .43 ohms up to 1.2 ohms as frequency varies, but this is on the 8 ohm tap. If you re-read the text pertaining to the actual testing on this amp, the results that Norton chose to publish were based on the 4 ohm tap of this amp.
This is somewhat deceiving and most people aren't technically sharp enough to catch something like this. One has to question why the tests and wording were published in the manner that they were. This is why i said that the test results were "reasonably unbiased". I don't know the reasons of why Mr Norton did or worded things the way that he did, but the text and test results were not consistent pertaining to this one specific brand of product.
Using the approach that Norton did i.e. choosing a lower impedance tap on the output transformer, the transformer itself and the tubed output stage sees less stress. In doing so, there's more of an " impedance buffer" built into this approach, hence the pretty decent output linearity / loading characteristics presented by this amp under this set of test conditions. As a side note, distortion was probably also drastically reduced due to the reduction in output stage modulation by using the tap that he did.
Now, take a look at the results of the tubed Sonic Frontiers amp. This amp has an output impedance of above 3 ohms, resulting in a damping factor of less than 2.7 into an 8 ohm load. As can clearly be seen, the lack of "impedance buffer" or "damping factor" is extremely evident. The amplitude linearity / frequency response into any of these loads is quite poor, especially when compared to the low output impedance SS designs.
As can be seen, the varying impedance of the speaker directly coincides with the varying loading characteristics of the amp. In some cases, the linearity of this amp is inferior to the frequency response of what many would consider even a "decently flat" loudspeaker. Driving the Martin-Logan's, the actual frequency response of the amp looks more like a scenic shot of hills and valleys than a piece of "reference grade audiophile approved" electronics.
With this set of speakers, the Sonic Frontiers amps is down well over -5 dB's by 20 KHz with a massive depression in the warmth, lower midrange and midrange frequencies. All of this with an amplifier that has an output impedance of slightly over 3 ohms. Obviously, this type of situation is an extreme example of why "system synergy" becomes SO important in certain installations.
Obviously, this type of performance puts the Sonic Frontiers ( and other amps like it ) into the "specialty category". That is, they are FAR from being universal in application. This is due to the inability to drive various loads with any reasonable form of amplitude linearity. As a side note, it should be noted that a lack of amplitude linearity is NOT measured as a part of THD, IMD, etc..., but is a measure of "frequency response tolerance". In effect, it is a distortion all to itself.
For sake of reference and comparison, the test results that i've seen for one of Mr Berning's products showed the amp as having an output impedance of appr 5 - 10 ohms or so ( can't remember the specifics ). This could be why he included the following in his response: "In practice, the system frequency response flatness will likely have the similar degrees of deviation for all amplifiers from zero to 10 ohms output impedance".
All i can say to that is "HA HA HA". If such were the case, all amps would sound pretty similar on any given set of speakers because the SYSTEM frequency response would all be equally flat. As was verified in the Stereophile test results, this is obviously NOT true. If such were the case, there would be very little reason to buy one amp over another, as the only difference would be power ratings and potential spl's. As most all of us frequenting this forum know, such is simply not the case.
On top of that, Mr Berning also stated the following: "Yes, if the output impedance of the amplifier was 50 ohms, and a very underdamped speaker reached a 50 ohm impedance, there would be a sharp peak in the system response". While MANY speakers do have a 50 ohm peak in the low frequency region, especially vented designs with large woofers, most amps do not come anywhere close to having a 50 ohm output impedance. That makes this analogy useless in the real world.
Getting back to the real world and applying a high level of logic to what Mr Berning did state, let's refer back to the Stereophile article i previously referenced. As was seen in that articles test results, we could definitely see that as the output impedance of the amp was brought closer to the input impedance of the speaker i.e. reducing the "damping factor", the response became more peaky / less linear. That's because of the lack of "impedance buffer" that i previously mentioned. In an audio amplifier, the response would get even more ragged looking / non-linear if the output impedance of the amp actually matched the input impedance of the speaker. This would be due to having a near zero damping factor / impedance buffer.
As such, Mr Berning was not exagerating in his 50 ohm - 50 ohm example and Stereophile's test results tend to confirm this. Applying further logic to what Mr Berning stated, he is basically telling us that the response of his own amp, with a measured output impedance of 5 - 10 ohms, will produce very peaky, non-linear response when driving a speaker with a nominal impedance of 5 - 10 ohms. As poor as the Sonic Frontiers amp performed with a "damping factor" of 2.7 into an 8 ohm load, the Berning amp would have a "damping factor" of near ZERO at 8 ohms over part of the audible bandwidth. How linear do you think that would be?
Obviously, we are talking about another "specialty" product here, which i don't have a problem with. People buying such a product should be familiar with what they are buying, so long as the manufacturer and sales people are honest and ethical about the performance potential of the product and what it takes to make it work as they planned.
In that respect, Mr Berning has been honest and forthright about at least one of his products, and we should thank him for contributing to this thread. His contributions allowed us to dig deeper into this subject and actually make some real world comparisons in an apples to apples format, courtesy of the Stereophile test results. Sean
>
Those that are interested in such subjects may want to look at some "reasonably unbiased" test results as conducted and published by Stereophile on this very subject. The article titled "Questions of impedance interaction" has several different graphs documenting the linearity of four different amplifiers driving three very different speakers.
Remember, these graphs are of the actual amplitude linearity / loading characteristics / frequency response of the amp going into the speaker, NOT the measured response AFTER the speaker output. In effect, you are seeing how various amps deal with various loads, not the actual output of the speakers as driven by those specific amps.
Given the phenomenally distorted / non-linear frequency response / loading characteristics of some of these amps, the output response of the speakers would be even more distorted / non-linear. After all, the speaker can only try to reproduce what is fed into it, and if the amplifier is feeding a phenomenally non-linear signal into the speaker, one can only expect further distortions after the speaker.
As can be seen in the test results, the two low output impedance solid state amps ( Hafler & Aragon ) remain extremely consistent in amplitude output ( linearity of frequency response ), regardless of the load they are presented with. While these would commonly be referred to as a high damping factor design, one can see the benefits of using a very low source output impedance as compared to a higher ( speaker ) load impedance. That's because the difference in impedance between the source and the loudspeaker load acts as an "impedance buffer".
This "impedance buffer" (or "higher damping factor") reduces the potential for the varying impedance that the speaker presents to "load down" or "modulate" the output of the amplifier. This can be seen as output remains quite consistent, regardless of frequency or the impedance that these amps are presented with.
The Melos tubed amp results are kind of tricky. In the text of the article, Mr Norton states the following: "Since the Melos 400 also had a relatively low output impedance for a tube amplifier (at 0.43 ohms at low and mid frequencies, rising to 1.2 ohms at 20kHz, from its 8 ohm tap), I took that opportunity to run some frequency-response measurements using an actual loudspeaker as the load for the amplifier".
While this would seem to be a pretty stable design as far as tubed amps are concerned, and the results somewhat confirm that other than a sloping high frequency response, the results are somewhat misleading. That is, the Melos might have an output impedance ranging from .43 ohms up to 1.2 ohms as frequency varies, but this is on the 8 ohm tap. If you re-read the text pertaining to the actual testing on this amp, the results that Norton chose to publish were based on the 4 ohm tap of this amp.
This is somewhat deceiving and most people aren't technically sharp enough to catch something like this. One has to question why the tests and wording were published in the manner that they were. This is why i said that the test results were "reasonably unbiased". I don't know the reasons of why Mr Norton did or worded things the way that he did, but the text and test results were not consistent pertaining to this one specific brand of product.
Using the approach that Norton did i.e. choosing a lower impedance tap on the output transformer, the transformer itself and the tubed output stage sees less stress. In doing so, there's more of an " impedance buffer" built into this approach, hence the pretty decent output linearity / loading characteristics presented by this amp under this set of test conditions. As a side note, distortion was probably also drastically reduced due to the reduction in output stage modulation by using the tap that he did.
Now, take a look at the results of the tubed Sonic Frontiers amp. This amp has an output impedance of above 3 ohms, resulting in a damping factor of less than 2.7 into an 8 ohm load. As can clearly be seen, the lack of "impedance buffer" or "damping factor" is extremely evident. The amplitude linearity / frequency response into any of these loads is quite poor, especially when compared to the low output impedance SS designs.
As can be seen, the varying impedance of the speaker directly coincides with the varying loading characteristics of the amp. In some cases, the linearity of this amp is inferior to the frequency response of what many would consider even a "decently flat" loudspeaker. Driving the Martin-Logan's, the actual frequency response of the amp looks more like a scenic shot of hills and valleys than a piece of "reference grade audiophile approved" electronics.
With this set of speakers, the Sonic Frontiers amps is down well over -5 dB's by 20 KHz with a massive depression in the warmth, lower midrange and midrange frequencies. All of this with an amplifier that has an output impedance of slightly over 3 ohms. Obviously, this type of situation is an extreme example of why "system synergy" becomes SO important in certain installations.
Obviously, this type of performance puts the Sonic Frontiers ( and other amps like it ) into the "specialty category". That is, they are FAR from being universal in application. This is due to the inability to drive various loads with any reasonable form of amplitude linearity. As a side note, it should be noted that a lack of amplitude linearity is NOT measured as a part of THD, IMD, etc..., but is a measure of "frequency response tolerance". In effect, it is a distortion all to itself.
For sake of reference and comparison, the test results that i've seen for one of Mr Berning's products showed the amp as having an output impedance of appr 5 - 10 ohms or so ( can't remember the specifics ). This could be why he included the following in his response: "In practice, the system frequency response flatness will likely have the similar degrees of deviation for all amplifiers from zero to 10 ohms output impedance".
All i can say to that is "HA HA HA". If such were the case, all amps would sound pretty similar on any given set of speakers because the SYSTEM frequency response would all be equally flat. As was verified in the Stereophile test results, this is obviously NOT true. If such were the case, there would be very little reason to buy one amp over another, as the only difference would be power ratings and potential spl's. As most all of us frequenting this forum know, such is simply not the case.
On top of that, Mr Berning also stated the following: "Yes, if the output impedance of the amplifier was 50 ohms, and a very underdamped speaker reached a 50 ohm impedance, there would be a sharp peak in the system response". While MANY speakers do have a 50 ohm peak in the low frequency region, especially vented designs with large woofers, most amps do not come anywhere close to having a 50 ohm output impedance. That makes this analogy useless in the real world.
Getting back to the real world and applying a high level of logic to what Mr Berning did state, let's refer back to the Stereophile article i previously referenced. As was seen in that articles test results, we could definitely see that as the output impedance of the amp was brought closer to the input impedance of the speaker i.e. reducing the "damping factor", the response became more peaky / less linear. That's because of the lack of "impedance buffer" that i previously mentioned. In an audio amplifier, the response would get even more ragged looking / non-linear if the output impedance of the amp actually matched the input impedance of the speaker. This would be due to having a near zero damping factor / impedance buffer.
As such, Mr Berning was not exagerating in his 50 ohm - 50 ohm example and Stereophile's test results tend to confirm this. Applying further logic to what Mr Berning stated, he is basically telling us that the response of his own amp, with a measured output impedance of 5 - 10 ohms, will produce very peaky, non-linear response when driving a speaker with a nominal impedance of 5 - 10 ohms. As poor as the Sonic Frontiers amp performed with a "damping factor" of 2.7 into an 8 ohm load, the Berning amp would have a "damping factor" of near ZERO at 8 ohms over part of the audible bandwidth. How linear do you think that would be?
Obviously, we are talking about another "specialty" product here, which i don't have a problem with. People buying such a product should be familiar with what they are buying, so long as the manufacturer and sales people are honest and ethical about the performance potential of the product and what it takes to make it work as they planned.
In that respect, Mr Berning has been honest and forthright about at least one of his products, and we should thank him for contributing to this thread. His contributions allowed us to dig deeper into this subject and actually make some real world comparisons in an apples to apples format, courtesy of the Stereophile test results. Sean
>
