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"Triodity" to cancel loudspeaker distortion
"Triodity" to cancel loudspeaker distortion
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Old 11th June 2019, 04:03 PM   #1
Circlomanen is offline Circlomanen  Sweden
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Default "Triodity" to cancel loudspeaker distortion

THD368Hznormalfas.jpg

THD368Hzfasvänd.jpg

IXTP32-sourcefollower.jpg

Based on Nelson Pass excellent articles about SITs and Triodes I made a little experiment.

With my small IXYS IXTP32P05T based source follower I tested the distortion canceling effects of using the positive going second harmonic to oppose the Bl - and suspension asymmetry of a loudspeaker driver. In this case a cheap 5 inch car coaxial driver in an open baffle.

It is a way to "work the complex load line" of the amp and loudspeaker together.

At 368 Hz the second harmonic dropped from 0,299% to 0,046% with this simple method. I can easily change the phase of the second harmonic with changing polarity of the speaker wire and at the same time flip the phase with my MiniDSP. Since the loudspeaker varies a lot more in its nonlinear behavior with frequency compared to the amp, the effect of this cancelation will also vary a lot with frequency - as seen in the distortion graphs above.

I must say that it is a most worthwhile experiment to do. REW is free and an Umik is cheap, so there is a lot of potential tuning of the harmonic profile of the combination of loudspeaker and amp to be had.

I would recommend everyone who is interested in this to download those two distortion graphs into a new folder so that you can switch fast back and forth between them. It is much easier to see the effects of this experiment that way.


Cheers,
Johannes
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Old 11th June 2019, 04:09 PM   #2
Nelson Pass is offline Nelson Pass  United States
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"Triodity" to cancel loudspeaker distortion
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Old 11th June 2019, 04:10 PM   #3
Circlomanen is offline Circlomanen  Sweden
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8Volt.jpg

10Volt.jpg

12Volt.jpg

15Volt.jpg

Here is the harmonic profile and distortion levels for different amounts of current through the IXTP32P05T and the total voltage from the power supply.

By varying the voltage from the power supply and the distortion profile of the amp it is easy to optimize the distortion canceling effect of the opposing asymmetry to whatever measures best or sounds best to you.

It is kind of a cheap mans second harmonic adjustment similar to the one of the First watt SIT1.
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Old 11th June 2019, 04:18 PM   #4
Nelson Pass is offline Nelson Pass  United States
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"Triodity" to cancel loudspeaker distortion
I will give you another one.

Small amounts of DC bias, applied by some DC offset from the power
amplifier will also adjust the distortion of the driver.

I have applied as much as a volt, and have not lost a driver yet.

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Old 11th June 2019, 05:06 PM   #5
Circlomanen is offline Circlomanen  Sweden
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Thanks Nelson Pass!

One volt DC-offset across a speaker!!!



The four different distortion measurements above is starting from the top:

(volts across the amp and current through the mosfet)

8 volts and 0,35 ampere.

10 volts and 0,53 ampere.

12,2 volt and 0,74 ampere.

15,6 volts and 1,05 ampere .

All the measurements are with the positive going second harmonic reinforcing the asymmetric second harmonic of the loudspeaker.
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Old 11th June 2019, 08:20 PM   #6
PRR is offline PRR  United States
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> One volt DC-offset across a speaker!!!

1V in 3.2 Ohms is 0.312 Watts.

Car speakers are built to take most of 16 Watts (kids playing LOUD).

You are not even 2% of the way to disaster.

There are several sources of 2nd in a real speaker (an ideal moving-coil speaker would have none, though this is impractical). One big cause is a short coil (midrange) assembled off the center of the magnetic gap (lengthwise). The real fix is to un-do all the glue and assemble the cone/coil in the magnet properly, either shimming or trimming the spider. Obviously a stray Volt of DC is a lot faster, and you can play the speaker while adjusting the DC for maximum output at minimum THD.
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Old 11th June 2019, 09:07 PM   #7
Alexandre is offline Alexandre
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Very interesting!

Apparently the phase of the second harmonic is everything.

There might be useful ideas here as well (by Eduardo de Lima):

AUDIOPAX - Technical Articles

Quote:
But what are the chances that we can get this complete distortion cancellation, as it is normally called ? Very few, just like there are very few chances that the distortions will fully add so we would have 2.0%. If the difference between the phases of the 2nd harmonics is between 0° and ± 120° there will be an increase in the total system distortion compared to the distortion of one device alone up to a maximum of 2.0%. If the difference is between ± 120° and ± 180° there will be a reduction in the total system distortion down to a minimum of zero! Of course the audibility of different levels of distortion may not follow the same relation as these numbers suggest, but there is a great possibility that the reduction may be more noticeable than the increase.
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Old 11th June 2019, 11:08 PM   #8
radule is offline radule  Serbia
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It seems Satori TW29DN is very suitable for this kind of 2nd harmonic cancelation.

They have very low 3rd, 4th and 5th and raised 2nd.

HD frequency response (315mm distance)

Satori TW29DN | HiFiCompass

But whats happening with IMD? Usualy amplifiers with high second harmonic have high intermodulation distortion.
Does they also canceling?

Last edited by radule; 11th June 2019 at 11:11 PM.
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Old 12th June 2019, 03:48 PM   #9
Nelson Pass is offline Nelson Pass  United States
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"Triodity" to cancel loudspeaker distortion
Cancellation of 2nd harmonic lowers IM associated with that non-linearity,
but doesn't eliminate it completely.
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Old 12th June 2019, 10:18 PM   #10
Mark Tillotson is offline Mark Tillotson
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Alas the distortion of a speaker is primarily related to the cone position (ie due to suspension forces and voicecoil position), not the voltage across the terminals, and the relationship between voltage and cone position varies substantially across resonance. This probably scuppers chances of naive cancellation.

Without a cone position/velocity/acceleration sensor, the next best way to cancel speaker distortion is perhaps sophisticated model and MPC (model predictive control) techniques. Very compute intensive.
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