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Old 16th October 2019, 03:42 PM   #21
adason is offline adason  United States
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Have you heard it?
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Old 16th October 2019, 06:42 PM   #22
solhaga is offline solhaga  Sweden
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Yes, just tested -43 dB second harmonic using REW and I can hear it.
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Old 16th October 2019, 07:00 PM   #23
adason is offline adason  United States
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Ok.

You can hear it when playing full range, fine. But second harmonic of 10kHz is 20kHz. Can you hear it? Just saying.
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Old 16th October 2019, 07:30 PM   #24
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by sgrossklass View Post
It is rather clever in that the coupled inductors cancel out magnetic flux from DC current (perfectly in theory, more or less imperfectly in practice).
The way they are connected, there is no cancellation (fortunately, as otherwise the output signal would be shorted to the supplies), which means that the core will have to bear all of the DC magnetization.
The advantage of coupled coils is that the values are multiplied by 4 instead of 2, but winding the same number of turns in a single coil has the same result, except it is a bit more perfect.

Using air-cored transformer or chokes for that business is by no means impossible (as I demonstrated somewhere in tube forum, but the idea was flamed by iron-lovers), but it requires a substantial number of kilograms of copper (tens, ideally) to work really well.
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Old 16th October 2019, 10:13 PM   #25
moschfet is offline moschfet  Germany
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Quote:
Originally Posted by solhaga View Post
I have given it a thought, but I lack the knowledge to take it to the next step.

Do you have any simple circuitry in mind?
Yes, here a simple circuit I made only for you

It is running in AB/C Bias and a simulated performance of ~ 0.002% THD. Easy to change to pure AB and much better THD, if required.
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Old 17th October 2019, 05:11 AM   #26
solhaga is offline solhaga  Sweden
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Thanks for your efforts, moschfet.
Sadly it does seem to still have NFB.
As such, I think my class A Trimodal does it better.

Quote:
Originally Posted by solhaga View Post
The AMTs also have an outstanding resolution, but they are at the same time merciless in revealing any amplifier's misconduct.

My current 25 W class A amplifier is no exception.

I've been suggested to give up on complex amplifiers and instead try something simpler and with no feedback at all.
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Old 17th October 2019, 06:51 AM   #27
Circlomanen is offline Circlomanen  Sweden
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Quote:
which means that the core will have to bear all of the DC magnetization.

Using air-cored transformer or chokes for that business is by no means impossible
This is an air cored coil. 2 x 100 meters of 1,5 mm2 cable makes a nice 2 x 16 mH inductance, which is more then enough to get a -3 dB point driving a 6 ohm load of 1,5 Hz with feed forward error correction. I suspect the distortion increases in the bass due to the low inductance, but it is nothing I have heard while driving bass horns with a similar amp.

Quote:
The advantage of coupled coils is that the values are multiplied by 4 instead of 2, but winding the same number of turns in a single coil has the same result, except it is a bit more perfect.
This is one of the benefits of this design. This fact partly explains the great but surprising performance from such a cheap and small bifilar coil.

Global negative feedback might do well in simulation, but it never sounds as good as feed forward error correction in my experience. There is usually a very hard to explain and hard to quantify "immediate effortless presence" with amps that does not rely on global negative voltage feedback for its low THD and high damping factor.


Quote:
You can hear it when playing full range, fine. But second harmonic of 10kHz is 20kHz. Can you hear it?
http://www.firstwatt.com/pdf/art_dist_fdbk.pdf

A great article explaining why low levels measured THD with a simple sinewave into a dummy load is of very little importance for the sound of amplifiers.
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Old 17th October 2019, 07:16 AM   #28
Circlomanen is offline Circlomanen  Sweden
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If you want to use a higher voltage (32 Volts) from the power supply and lower distortion then you could experiment with cascode operation. This will lower distortion and lower the gate capacitance seen from the preamp.

http://www.firstwatt.com/pdf/art_cas_amp.pdf

This is what Nelson Pass has to say about cascode operation of gain devices.

I have never tried this so I can not recommend it based on my own experience. It would be quite interesting experiment though....
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File Type: jpg SLAPSCASCODE-circuit.JPG (78.2 KB, 282 views)
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Old 17th October 2019, 07:22 AM   #29
solhaga is offline solhaga  Sweden
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Does M2/M3 still have to be closely matched then?
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Old 17th October 2019, 07:37 AM   #30
Circlomanen is offline Circlomanen  Sweden
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Yes!

http://www.firstwatt.com/pdf/art_matching.pdf

A simple way to get a close enough matching of devices.

This is the way I measure the Device Under Test. I monitor the current draw on my lab power supply and the Voltage over Drain to Source with a multimeter.
Remember to clamp the devices to a heat sink.
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Last edited by Circlomanen; 17th October 2019 at 07:46 AM.
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