THD in tubes and SS amps

mtrycrafts

mtrycrafts

Seriously, I have no life.
Pat D said:
Most tube amps have a high output impedance, usually from about 1 to 4 or 5 ohms, whereas most solid state amps have a low output impedance, less than 0.1 ohm. Hence, tube amps do not have as flat a frequency response into most speaker loads, which are not simple resistors. E. Brad Meyer in "The Amp/Speaker Interface: Are your loudspeakers turning your amplifier into a tone control?," Stereo Review, June 1991, p. 53-56, showed some measurements of a good solid state amp and a much more expensive tube amp into two different speaker loads. I am not absolutely sure of the history here but I think Audio magazine and Stereophile began testing the frequency responses of amplifiers into a standard simulated speaker load sometime after that. Anyway, the biggest difference in the signal seems to be in the frequency response rather than in distortion.

Here are the measurements of one tube amp that no doubt will sound different into most speaker loads:

http://www.soundstagemagazine.com/measurements/atmasphere_ma1_mkii2/

Check out Chart 1. This amp is pretty flat into a resistor, but the green trace shows its response into the simulated speaker load: definitely not flat. The characteristics of the simulated speaker load are shown in the explanation of the measurements BHK Labs makes for Soundstage :

http://www.soundstagemagazine.com/measurements/test_amplifiers.htm

Now, that amp has an output impedance of 10.5 ohms, so it is somewhat extreme. However, most tube amps do not have a flat response into the simulated speaker load or into the load of most speakers, so of course they are likely to sound different than amps (tube or SS) with a low output impedance.

Most solid state amps have an output impedance of less than 0.1 ohms, so their frequency response into a speaker load is almost as flat as it is into a resistor.

The old Audio magazine included the measurement of an amp into the simulated speaker load and so does Stereophile. You can easily check out the measurements of amps in reviews on the Soundstage and Stereophile sites to see the results for various amps when driving the simulated speaker load.

The difference in frequency response into different speaker loads seems to me to be the biggest difference in the signals coming from amps with low and high output impedances rather than the distortion. Whether you like the results, of course, is up to your personal preference, but there is nothing magic about it.
The Damping factor below 1 certainly doesn't help matters :(
 
Rip Van Woofer

Rip Van Woofer

Audioholic General
Good stuff, PatD. Hey, one way to look at it is like this: With those tube amps you get a built-in (if nonadjustable) equalizer (AND space heater) FREE! :D
 
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P

Pat D

Audioholic
Unpredictable.

Rip Van Woofer said:
Good stuff, PatD. Hey, one way to look at it is like this: With those tube amps you get a built-in (if nonadjustable) equalizer (AND space heater) FREE! :D
I think it is better to look at the more likely explanations for audible differences rather than looking at less likely ones.

It is surely possible for someone to calculate the frequency response of a known amplifier into a known speaker load (probably easier to just measure it, though) but for an ordinary consumer, the effects of a high output impedance in the outputs of an amplifier on the frequency response into a given speaker is unpredictable.

Tubes sure can look pretty, though!
 
P

Pat D

Audioholic
mtrycrafts said:
The Damping factor below 1 certainly doesn't help matters :(
Hey, I've noticed you over here :)

We have done so much traveling in the past year we probably won't be taking too many trips for a while. I don't know when we'll be able to get out to see you again. But if you get up into our neck of the woods, you would be welcome to visit.
 
mtrycrafts

mtrycrafts

Seriously, I have no life.
Pat D said:
Hey, I've noticed you over here :)

We have done so much traveling in the past year we probably won't be taking too many trips for a while. I don't know when we'll be able to get out to see you again. But if you get up into our neck of the woods, you would be welcome to visit.

Yes, I thought it best to just depart quietly and found this place, the right one:) I think you will like it here. We have a small number of the old crowd here, maybe more on the way.
You are welcome most anytime. You may not have much reason to come this way though unless you like the weather ;)
If I am in that part of the country, certainly will give you a buzz, good friend.
 

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