Carver Crimson 275 Review (Tube Amp)

[SIY]

I'm not sure that I agree about 'which is the case for modern amplifiers'; most of them I've encountered are not. But I have encountered a few; IMO what this seems to come down to is exactly how low the distortion actually has to be before its masked- and that is lower than many think. This is entirely due to the fact that the ear uses the higher ordered harmonics to sense sound pressure, so if the amp adds any of its own, it can be detected. In a nutshell we've been hearing this for the last 50 years and most of it simply has to do with the simple fact (or maybe not so simple) that most amps simply lack the gain bandwidth product needed to really control distortion. If you graph distortion vs frequency then you see how this bears out; it can be a mistake to only look at the distortion curve at one (usually low, like 100Hz) frequency. Almost any amp with feedback has enough feedback at 100Hz

Well, it's true that it's easier to hear 0.1% of 15th harmonic at a 100 Hz fundamental. But actual physical modern amps? The last commercial solid state amp on my bench (a Class D unit) showed <0.01% across the audio spectrum with a loudspeaker load. Looking at some of Stereophile's measurements, this is not at all unusual.

Here's the first one I grabbed at random (because I've heard of the brand!), which is an old-fashioned AB type:

(from https://www.stereophile.com/content/rotel-michi-m8-monoblock-power-amplifier-measurements)

And, of course, non-gross levels of HD above, say, 6-7 kHz (3rd is ultrasonic for folks of our generations, 4th and higher are for anyone) don't impact sound. I would certainly not consider levels like this "gross."

So, it would be good to have some actual controlled listening behind some of the audibility assertions.

Side note: I would have grabbed some results from one of your interesting tube amps, but Stereophile seems to have skipped over you. And I don't have one here to test, though it's tempting to build one just for curiosity sake.

Edit: plus ca change, plus c'est la meme chose

 

[Blumlein 88]

It is also true that 2nd and 3rd harmonic in speakers will tend to mask higher harmonics in amps if they should have them.

 

[Selmerdave]

Anyone of my age will have been very familiar with valve amplification, as that's all there was! There's no dispute possible that valve amplification is inferior to solid state in frequency response, distortion and noise, not least power consumption, although the better ones are good enough for transparency so with those, there's no audible difference, and therefore no reason to have them except for the attractive looks.
So, apart from looks, the sonic justification for valved amplification can only be those amplifiers that are not transparent, usually with high output impedance affecting the loudspeaker's frequency response, or high distortion . Noise is seldom an issue even with valves. What somebody prefers has little to do with technical performance, but where technical performance is a criterion, there's zero justification for valves.

S

60 years ago there were tube amps with bandwidth from below 20 Hz to above 50kHz at distortion levels a fraction of a percent. There has been lots of development since then. Of course there are SS amps that measure better, but to suggest that this is limiting and audibly inferior is a stretch.

 

[egellings]

Still, I look askance at hiding a walnut output transformer in a can that oversized. I proudly display my iron on my home brews. Don't need no damn cans to hide 'em!

 

[audioholic63]

I feel like watching some of Amir's YouTube videos should be compulsory viewing for new members so that they at least had an idea what this site is about and would likely be less perplexed by members responses to them.

Really? I mean...Science is in the name so...

 

[restorer-john]

Still, I look askance at hiding a walnut output transformer in a can that oversized. I proudly display my iron on my home brews. Don't need no damn cans to hide 'em!

They are really good looking amps.

 

[egellings]

Two stacked boxes in the middle are the preamp--power supply below the preamp proper.

 

[atmasphere]

I might add that if the distortion in any of my tube amplifiers were high enough to be heard as such, I'd have no problem with dropping them like a bad habit. None of them have reached that level, so all's good.

If you were able to hear a difference between one of those amps to another, there is a good chance it was because of the distortion signature of each. That is literally what the 'sonic signature' is, so long as the amp is able to act as a voltage source. The ear pays a lot more attention to tonality caused by distortion than it does to actual FR errors as long as the latter is not too gross.

Well, it's true that it's easier to hear 0.1% of 15th harmonic at a 100 Hz fundamental. But actual physical modern amps? The last commercial solid state amp on my bench (a Class D unit) showed <0.01% across the audio spectrum with a loudspeaker load. Looking at some of Stereophile's measurements, this is not at all unusual.

Here's the first one I grabbed at random (because I've heard of the brand!), which is an old-fashioned AB type:
View attachment 181793

(from https://www.stereophile.com/content/rotel-michi-m8-monoblock-power-amplifier-measurements)

And, of course, non-gross levels of HD above, say, 6-7 kHz (3rd is ultrasonic for folks of our generations, 4th and higher are for anyone) don't impact sound. I would certainly not consider levels like this "gross."

So, it would be good to have some actual controlled listening behind some of the audibility assertions.

Side note: I would have grabbed some results from one of your interesting tube amps, but Stereophile seems to have skipped over you. And I don't have one here to test, though it's tempting to build one just for curiosity sake.

Edit: plus ca change, plus c'est la meme chose
View attachment 181802

When you see that rising distortion with frequency, that's a sign that the amp might have insufficient gain bandwidth product to support the feedback. When the GBP is used up the feedback goes down and so the distortion goes up. Amps like this tend to sound bright and most of the solid state amps of the last 50 years have this issue- we've all heard it (I don't so much anymore... might be getting too old for this and amps these days are a lot better). And just so we're clear, keep in mind that the ear has a range of in excess of 120dB, so its means of gauging sound pressure through the higher ordered harmonics makes its more sensitive to them than almost anything else. Put another way, you hear this sort of thing as a coloration- in this case, bright and possibly harsh. It is possible to build amps that have enough GBP; two examples are the Benchmark and the Soulution. Most amps simply aren't built with this sort of goal in mind; instead they are built to make money. Put another way you can intend to make an amp that sounds and measures well if you set your mind to it; you have to understand how the ear/brain system works and apply the engineering to that end.

Some class D amps of the self-oscillating variety can get around this problem in an elegant manner. The things that cause distortion in that kind of class D (dead time and non-linearities in the encoding scheme) tend to cause lower ordered harmonics. Their distortion vs frequency can be much nicer- no rising distortion at higher frequencies, so no brightness.

 

[Doodski]

Still, I look askance at hiding a walnut output transformer in a can that oversized. I proudly display my iron on my home brews. Don't need no damn cans to hide 'em!

What is that silver colored box above the pre? It looks like General Radio gear.

 

[egellings]

That is a Krohn-Hite model 4001A low distortion sine wave oscillator. It was used in testing the amplifiers and the preamp below it.

 

[egellings]

My nun-handles are tickled by it. It is a line stage and a phono preamp stage--all tubes, of course.

 

[MakeMineVinyl]

That is a Krohn-Hite model 4001A low distortion sine wave oscillator. It was used in testing the amplifiers and the preamp below it.

Doesn't Krohn-Hite make Bluetooth speakers now?

Just kidding......

 

[levimax]

If you were able to hear a difference between one of those amps to another, there is a good chance it was because of the distortion signature of each. That is literally what the 'sonic signature' is, so long as the amp is able to act as a voltage source. The ear pays a lot more attention to tonality caused by distortion than it does to actual FR errors as long as the latter is not too gross.

I have found the opposite to be true, FR errors are much easier for me to hear than distortion. When I try the "online distortion tests" with music it has to get to 5% or more before I can hear it while I can easily hear a 1 or 2 dB FR change. Do you have any documentation to back up your assertion that the ear is more sensitive to distortion than FR?

When you see that rising distortion with frequency, that's a sign that the amp might have insufficient gain bandwidth product to support the feedback. When the GBP is used up the feedback goes down and so the distortion goes up. Amps like this tend to sound bright and most of the solid state amps of the last 50 years have this issue- we've all heard it (I don't so much anymore... might be getting too old for this and amps these days are a lot better). And just so we're clear, keep in mind that the ear has a range of in excess of 120dB, so its means of gauging sound pressure through the higher ordered harmonics makes its more sensitive to them than almost anything else. Put another way, you hear this sort of thing as a coloration- in this case, bright and possibly harsh. It is possible to build amps that have enough GBP; two examples are the Benchmark and the Soulution. Most amps simply aren't built with this sort of goal in mind; instead they are built to make money. Put another way you can intend to make an amp that sounds and measures well if you set your mind to it; you have to understand how the ear/brain system works and apply the engineering to that end.

Again my experience does not agree. Amps with rising distortion over 10 Khz are almost universal for the reasons you state i.e. less feedback for high frequencies. However I find the second harmonic of a 10 Khz fundamental completely inaudible as it is 20 Khz which is beyond my hearing. Can you link to any documentation about the sensitivity of human hearing to ultrasonic content. I have seen some and most is inconclusive at best.

 

[Doodski]

That is a Krohn-Hite model 4001A low distortion sine wave oscillator. It was used in testing the amplifiers and the preamp below it.

Those are apparently rare. I googled and googled and found a 4001R model but it was a super tiny pic. I think I may have run across Krohn-Hite stuff in the past but not 100% sure. I used to work part time at a metrology lab that sold and serviced test gear and I have vague memories of that time for some stuff. So many pieces of gear in so little time.

 

[egellings]

I did a recap on the power supply and in general cleaned & tested everything else, and it appears to work. I sent it to a metrology lab to have it measured, and it meets its specs.

 

[Doodski]

I did a recap on the power supply and in general cleaned & tested everything else, and it appears to work. I sent it to a metrology lab to have it measured, and it meets its specs.

It's always nice when a piece of gear specs out when under cal.

 

[MakeMineVinyl]

I did a recap on the power supply and in general cleaned & tested everything else, and it appears to work. I sent it to a metrology lab to have it measured, and it meets its specs.

I remember the brand, and I'm sure I've used it in the past along with the usual suspects HP, Sound Technology General Radio ,Bruel & Kjaer etc. I remember the whiz-bang HP 8903 analyzer I talked Altec Lansing into buying for me as having a sine wave generator with .001% THD, remarkable at the time. I can almost whistle a tone that pure compared to today's digital ones.

 

[SIY]

When you see that rising distortion with frequency, that's a sign that the amp might have insufficient gain bandwidth product to support the feedback. When the GBP is used up the feedback goes down and so the distortion goes up. Amps like this tend to sound bright and most of the solid state amps of the last 50 years have this issue- we've all heard it.

Or it might not. Look at the values- they are really low. I'd want to see some evidence that an amp with distortion rising to, say, 0.05% at 20kHz will sound "bright" or audibly different than an amp with distortion ten or a hundred times lower.

 

[LTig]

I have found the opposite to be true, FR errors are much easier for me to hear than distortion. When I try the "online distortion tests" with music it has to get to 5% or more before I can hear it while I can easily hear a 1 or 2 dB FR change.

I agree - and this is exactly what many amps with low negative feedback deliver when connected to a typical loudspeaker: due to the high output impedance you get kind of a small loudness curve which even in a blind test many people would prefer.

 

[restorer-john]

Two stacked boxes in the middle are the preamp--power supply below the preamp proper.

I saw that. I meant the power amps either side look nice. Too many people make ugly wooden cases for their tube amps- I like the plain neat metal chassis and well finished transformers sitting proud.