odd distortion below 50 Hz

AllanMarcus

AllanMarcus

New member
Hello,

I'm comparing some amps and using (among others) the Stereophile Test CD 1. This CD has tracks with warble tones at various frequencies. At 50 Hz and below, the Mainline does not sound like the three other amps I'm comparing (Magni 3, Eddie Current Black Window 2, Dennis Had Dragron Inspire). With the other three amps, I just hear the warble tone at the correct frequency. With the Mainline, I hear the warble tone, but I hear something else, maybe it's some kind of harmonic distortion.

Anyone else with test tones able to hear this too? If not, any ideas?

Link to explanation of the CD
https://www.stereophile.com/content/stereophile-test-cd-tracks-20-31
 
Maybe this explains what is going on -

"Footnote 6: After the Test CD had been mastered, I checked the modulation frequency of these warble tones with an FFT analyzer and realized that it was closer to 14Hz rather than 5Hz. Note that as well as the higher modulation frequency than specified, the warble tone is not particularly pure, FM sidebands appearing around its second and third harmonics. Although the "buzz" of higher harmonics is faintly audible on these tracks—we had considered digitally low-pass filtering these tracks when we compiled the master tape, but weren't able to due to lack of time—the strongest harmonic, the second, lies just under 30dB down from the fundamental, ie, around the 3% level.
Read more at https://www.stereophile.com/content/stereophile-test-cd-tracks-20-31#F0lcCl6xPdgHufdg.99"
 
Doc B. said:
Maybe this explains what is going on -

"Footnote 6: After the Test CD had been mastered, I checked the modulation frequency of these warble tones with an FFT analyzer and realized that it was closer to 14Hz rather than 5Hz. Note that as well as the higher modulation frequency than specified, the warble tone is not particularly pure, FM sidebands appearing around its second and third harmonics. Although the "buzz" of higher harmonics is faintly audible on these tracks—we had considered digitally low-pass filtering these tracks when we compiled the master tape, but weren't able to due to lack of time—the strongest harmonic, the second, lies just under 30dB down from the fundamental, ie, around the 3% level.
Read more at https://www.stereophile.com/content/stereophile-test-cd-tracks-20-31#F0lcCl6xPdgHufdg.99"
Click to expand...

Wow, interesting.  How come I can hear it with the mainline, but not with the other three amps?
 
Doc B. said:
Maybe this explains what is going on -

"Footnote 6: After the Test CD had been mastered, I checked the modulation frequency of these warble tones with an FFT analyzer and realized that it was closer to 14Hz rather than 5Hz. Note that as well as the higher modulation frequency than specified, the warble tone is not particularly pure, FM sidebands appearing around its second and third harmonics. Although the "buzz" of higher harmonics is faintly audible on these tracks—we had considered digitally low-pass filtering these tracks when we compiled the master tape, but weren't able to due to lack of time—the strongest harmonic, the second, lies just under 30dB down from the fundamental, ie, around the 3% level.
Read more at https://www.stereophile.com/content/stereophile-test-cd-tracks-20-31#F0lcCl6xPdgHufdg.99"
Click to expand...

I tried the low frequency tones from test CD 2 and 3, and I get the same results. The tones sound quite different on the Mainline compared to the other amps. On the other amps, all I hear is the low rumble. On the mainline, I hear additional information.

I can't post the tones, since they are proprietary, but you can here it with this test tone

https://www.youtube.com/watch?v=bslHKEh7oZk

My set up allows be to change a switch and flip between amps, so it's really easy to AB test.
I can hear the difference with the 50Hz wav file here too.
https://www.kicker.com/test-tones
 
What headphones, impedance setting, and listening level?

With power hungry headphones and the amp making full power, I wouldn't be surprised if some audible harmonics crept up.
 
Caucasian Blackplate said:
What headphones, impedance setting, and listening level?

With power hungry headphones and the amp making full power, I wouldn't be surprised if some audible harmonics crept up.
Click to expand...

I was using the Utopia at a moderate listening level. When I get home tonight, I will try some other headphones.
 
Yep, I tried other headphones, and the odd harmonic is still there. :-(

It just manifest a little differently depended on headphone. Any ideas? Just a function of the amp, or might I might have problem?
 
I got curious and played the 50Hz tone on my Mainline with HD650's, but didn't notice anything unusual. Then I played the tone on my Crack with HD600's and also didn't hear anything unusual. I tried again on my 02 transistor amp and still nothing. What does the extra noise sound like? Is it very distinctive?


-Raymond
 
Doc B. said:
Does the impedance switch have any effect on it?
Click to expand...
A little, but not much. It's most pronounced on the utopia

It's still hard to hear with the test tone I posted. It's most noticeable with the test tones from the stereophile 1 disk. While they admit there were issues with those test tones, why they sound different with the Mainline compared to other amps is the curious issue.

Since I cannot post them, I can probably send them to an individual for just a quick test. PM me if interested.
 
Raymond P. said:
Just a thought, but are terminals 20 and 30 biased properly?


-Raymond
Click to expand...

Yep. Just checked them last weekend

I listened through my HD6xx and can easily hear the distortion. I had a friend listen too, and he easily heard it. Again, this is with the stereophile warble, not the tone I posted.

When listening to the same track on three other amps, the odd distortion isn’t there.
 
It's a single-ended triode (SET) amp, with no feedback, so it does have a bit of distortion - mostly second harmonic. It is more audible with low frequency sine waves because the ear's sensitivity increases rapidly with frequency in that range, and the distortion harmonics are higher frequency - mostly twice the fundamental. It is less audible with the Crack because the Mainline signal voltage at the tube output is much greater due to the output transformer.

A number of artificial signals have been documented in the literature which bring out low-level distortions like this. These small distortions are usually not audible with music, which is almost never a sine wave, and almost never a single, unvarying tone. SETs have many audible virtues, but low measured distortion is not one of them.
 
Paul Joppa said:
It's a single-ended triode (SET) amp, with no feedback, so it does have a bit of distortion - mostly second harmonic. It is more audible with low frequency sine waves because the ear's sensitivity increases rapidly with frequency in that range, and the distortion harmonics are higher frequency - mostly twice the fundamental. It is less audible with the Crack because the Mainline signal voltage at the tube output is much greater due to the output transformer.

A number of artificial signals have been documented in the literature which bring out low-level distortions like this. These small distortions are usually not audible with music, which is almost never a sine wave, and almost never a single, unvarying tone. SETs have many audible virtues, but low measured distortion is not one of them.
Click to expand...

Excellent explanation! Many thanks.

To my ear the Mainline sounds wonderful. I was just curious about this anomaly.
 
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