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Modulus-86: Composite amplifier achieving <0.0004 % THD+N.
Modulus-86: Composite amplifier achieving <0.0004 % THD+N.
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Old 22nd September 2014, 08:26 AM   #21
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by fas42 View Post
I would be interested in the difference in 19+20kHz IMD behaviour between the 2, for different loads and power levels ...
Any particular reason why? I'm just curious.

I'll have to check to see if the Audio Precision SYS-2712 I have access to has the IMD option. I think it does. Stay tuned...

~Tom
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Old 22nd September 2014, 08:44 AM   #22
fas42 is offline fas42  Australia
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Because in my experience the behaviours of amplifiers when driving difficult loads, in terms of power and current, at high frequencies is very telling. This is the area where an amplifier relying on global feedback has the least available to it, because of the shaping by compensation to maintain stability - from the graphs you've already shown it appears, by extrapolating, that at around 100kHz the performance of the composite will be about the same as a raw LM3886. So, I'm curious as to whether in the real world there is any benefit to your composite approach for IMD behaviour in the audio band when the amplifier is "stress tested" ...

Thanks for responding!
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Old 22nd September 2014, 05:58 PM   #23
tomchr is offline tomchr  Canada
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My guess is that you will at least get the benefit of the loop gain available at 20 kHz. The unity loop gain occurs at about 1.5 MHz for the Modulus-86, so I'd expect good performance quite a bit past the audio band.

I've been wanting to measure IMD for a while. I may have to wait for work to quiet down a bit before getting to it, however. Stay tuned.

~Tom
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Old 22nd September 2014, 06:02 PM   #24
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Tom,

I'm not seeing "0.0004%" on the graphs. Am I not seeing it?
I am seeing things in the range of 0.004 and somewhat below.
Not saying that the performance is bad at all. Just not seeing that.

Afaik, most amps are better in THD at lower levels than high.
And similarly better at lower freqs than high.
Yours seems better at almost max output!

The question of the "first watt" is an interesting and important one.
I'd think that most class A amps and most high-ish bias class AB amps do well there. The good ones, that are already good.

An idea that might be interesting to explore with the 3886 (and given time I would love to try) would be one of the classic (now classic) feedforward distortion reduction schemes. The barrier to these has typically been the cost of a complete amplifier in order to do the trick. Here the cost is nominal.

As far as "better" amps? I think there are a range of designs that reach into this "below 0.001%" THD. Seems to me a few are on here.

Only by way of reference is a random test of my own Symphony No.1 amplifier. Keep in mind that this design is now 25 years old.
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Old 22nd September 2014, 06:10 PM   #25
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Just thinking out loud, the addition of Hawksford would probably pull the measured distortion down more - in the above amp.
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Old 22nd September 2014, 08:07 PM   #26
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by bear View Post
I'm not seeing "0.0004%" on the graphs. Am I not seeing it?
Attached is the THD+N vs power plot that I also attached to Post #1. I have annotated it to show <0.0004 % THD+N.

Quote:
Originally Posted by bear View Post
Afaik, most amps are better in THD at lower levels than high.
And similarly better at lower freqs than high.
Yours seems better at almost max output!
You are correct that, in theory, amplifiers distort less at low signal levels than at high. However, in practice, THD+N is measured. Recall, THD+N = Total Harmonic Distortion + Noise. The noise floor is independent of signal level. When the signal level is decreased, the harmonics fall below the noise floor. The result is a rise in THD+N at lower power levels. You'll notice the same on your amp.

Your Symphony-1 is nice, but my Modulus-86 is nearly an order of magnitude better...

~Tom
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File Type: png MOD86_0p00035THD.png (95.2 KB, 2162 views)
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Old 22nd September 2014, 08:58 PM   #27
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Suddenly I feel naked and perplexed.

How does one measure the vanishing low distortion of the opamp precceeding the 3886, since the noise floor is not likely to move... I ought to know this.
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Old 22nd September 2014, 09:40 PM   #28
Mark Whitney is offline Mark Whitney  Netherlands
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What might be confusing things is that the other graphs seem to show 0.0007% @ 1kHz 25W and 0.003% @ 1kHz 1W.

Still a great result.
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Old 22nd September 2014, 10:11 PM   #29
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by bear View Post
How does one measure the vanishing low distortion of the opamp precceeding the 3886, since the noise floor is not likely to move... I ought to know this.
The THD of the LME49710 is measured by reducing the loop gain of the amp in the test circuit. It is assumed that the distortion tracks inversely with the loop gain. This is described pretty well in the LME49710 data sheet, actually.

Quote:
Originally Posted by Mark Whitney View Post
What might be confusing things is that the other graphs seem to show 0.0007% @ 1kHz 25W and 0.003% @ 1kHz 1W.
For the THD+N vs POWER, the measurement is done with a 1 kHz test signal and 22 kHz measurement bandwidth. This lowers the noise floor (the noise is integrated over 22 kHz rather than 80 kHz). Hence, the +N component of the THD+N is reduced and the THD+N lower. In my view, this is a more realistic measure of the THD. With a 1 kHz test signal, the measurement still includes 21 harmonics, so it's more than fair there.

For measurement at 20 kHz, I have to open up the measurement bandwidth to 80 kHz to get a reasonable number of harmonics represented. Unfortunately, this also opens up the noise bandwidth. There's no way around that... The nearly 4x larger bandwidth means that the integrated noise is now, roughly, 4x higher. Hence, the +N part of the THD+N is higher and the resulting plot shows higher THD+N. In the THD+N vs FREQUENCY plots, I have to use 80 kHz bandwidth to get an accurate measurement at 20 kHz. That's why the THD+N vs FREQUENCY plots show higher THD+N than the THD+N vs POWER plots. It's the difference in the noise floor that you see.

It would probably help if I showed a spectrum of the harmonics (assuming they even pop up above the noise floor). I'll add that to my to-do list.

~Tom
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Modulus-86, 186, 286, & 686: 40-240W (8Ω), <-120dB THD. HP-2 Headphone Amp: 660mW, <-130dB THD. Taming the LM3886.
Neurochrome : : Audio - www.neurochrome.com - Engineering : : Done : : Right
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Old 24th September 2014, 04:41 AM   #30
tomchr is offline tomchr  Canada
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I noticed I forgot to include a title/legend on the oscilloscope shots of the transient response in Post #19. Please find annotated scope shots attached.

~Tom
Attached Images
File Type: jpg Transient_1nF_8R_600px.jpg (122.2 KB, 2073 views)
File Type: jpg Transient_10nF_8R_600px.jpg (120.7 KB, 2020 views)
File Type: jpg Transient_47nF_8R_600px.jpg (121.8 KB, 1971 views)
File Type: jpg Transient_100nF_8R_600px.jpg (122.8 KB, 561 views)
__________________
Modulus-86, 186, 286, & 686: 40-240W (8Ω), <-120dB THD. HP-2 Headphone Amp: 660mW, <-130dB THD. Taming the LM3886.
Neurochrome : : Audio - www.neurochrome.com - Engineering : : Done : : Right
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