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Old 13th November 2010, 02:03 AM   #21
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George: I think you mean FE126?
Probably. I'm not good with numbers, so I became an engineer. Go figure.

Didn't go to college until I was nearly 40 years old. My company paid me to get a degree.
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Old 2nd January 2011, 02:20 PM   #22
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Originally Posted by Allen Wright View Post
This is flat out INCORRECT. P-P cancels even order harmonics generated WITHIN the amp - it DOES NOT affect the music's harmonics, second or otherwise.
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Originally Posted by tubelab.com View Post
A good SE amp will have the second harmonic as its primary THD component.

A good P-P amp will add some second harmonic, and the primary THD component will be a mix of third and some second harmonic. Some higher order harmonics will be added on strong signals.

A well balanced fully differential P-P amp will add nearly no second harmonic and its primary THD component is third harmonic.
I did some measurements of Tubelab SE (SET) and push pull amp Dynaco ST-70 (restored original PC board w/ updated power supply) with my cheap tools (not lab grade) and noticed some differences when fed the same signal. What does this mean about Dynaco ST-70?

Click the image to open in full size.

I also measured Parasound HCA-750A (P-P transistor amp) and noticed the diminished even order harmonics and prominent 3rd, 5th, 7th and 9th order harmonics. So I'm wondering, is this common with push pull amps?

Last edited by Evenharmonics; 2nd January 2011 at 02:22 PM.
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Old 2nd January 2011, 03:34 PM   #23
rknize is offline rknize  United States
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Yes, an SE amp will have a dominant 2nd order harmonic while a PP will typically (though not necessarily) have a dominant 3rd, 5th, etc. The feedback loop on the ST70 helps clean up the rest of the harmonics. The TSE has no feedback at all. Do you have a spectra of the source you are using?
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Old 2nd January 2011, 03:48 PM   #24
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Originally Posted by rknize View Post
Yes, an SE amp will have a dominant 2nd order harmonic while a PP will typically (though not necessarily) have a dominant 3rd, 5th, etc. The feedback loop on the ST70 helps clean up the rest of the harmonics.
But I noticed almost same level of 3rd harmonics from both and yet there is a significant reduction of 2nd harmonics from P-P circuit.

The source I used was 1KHz sine wave that was part of ARTA program I downloaded.
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