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Amplifier comparison: March Audio P452 (Purifi 1ET400A) vs Buckeye Hypex NC502MP vs AIYIMA A07 (TPA3255) vs Onkyo TX-NR737 (125W Class AB)

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f1shb0n3

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If the matching is done with speakers plugged (it should), no difference at the matching frequency. But since the output impedance is fequency dependant, it could happen at different frequencies.
An eye-opener point for me - never thought I have to measure with speakers plugged-in, I was measuring at amplifier +/- with multimeter. There are so many ways to get stuff wrong..
Thanks!
 
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f1shb0n3

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Major improvement in volume matching, thanks everyone for the help!

What I believed was volume matched by ear measured with oscilloscope showed 0.5dB difference in favor of the Purifi amp. After properly volume matched, all the differences between Onkyo and Purifi I was hearing disappeared and I'm back to square one :rolleyes: The right one this time, crossing fingers :cool:

Damn, those Class D sine is so "digital", look ugly (when zoomed in, not seen that well on pics) and the scope can't get a good "grip" on it. I know - switching amplifier, that's how it works, but seeing those waves can't help but feel "uneasy" about Class D amps.. explained in subsequent posts that it's expected with the 'scope I'm using and not an issue of any kind.

Volume-matched by Vrms:

Frequency, AmplifierOnkyo TX-NR737March Audio P425 (Purifi 1ET400A)
100Hz
Onkyo 100Hz.jpg
Purifi 100Hz.jpg
1kHz
Onkyo 1kHz.jpg
Purifi 1kHz.jpg
 
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tomtoo

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Dont know what it is, but something must be wrong. If the sinewave gets so ugly you would have a hell of distortion. Maybe this a some graphic(resolution) problems on the oszi. You have double as much oszilations on the right screen. Different time base? Yes on the right you have 2ms on the left 1ms.
The graphic with that low resolution screen gets than ugly. Nothing to do with the real signal from the amp. Try on both with lower time base maybe .2 ms then you have a more exact view.
First get them both the same with lower timebase. And not forget your display is only 300*200 pixels. So it not can realy show exact. Espeacaly not with so many oszilations on the screen.
 
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mrmojo2022

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Major improvement in volume matching, thanks everyone for the help!

What I believed was volume matched by ear measured with oscilloscope showed 0.5dB difference in favor of the Purifi amp. After properly volume matched, all the differences between Onkyo and Purifi I was hearing disappeared and I'm back to square one :rolleyes: The right one this time, crossing fingers :cool:

Damn, those Class D sine is so "digital", look ugly (when zoomed in, not seen that well on pics) and the scope can't get a good "grip" on it. I know - switching amplifier, that's how it works, but seeing those waves can't help but feel "uneasy" about Class D amps..

Volume-matched by Vrms:

Frequency, AmplifierOnkyo TX-NR737March Audio P425 (Purifi 1ET400A)
100HzView attachment 182183View attachment 182185
1kHzView attachment 182182View attachment 182184
This is just the switching frequency you are seeing. all class d amps will appeR like this without filtering. the scope is wide enough bandwidth to see the signal. it's not reproduced by a speaker so is audibly irrelevant.

In this review I found on March website you can see a filtered and non filtered square wave


Screenshot_20220127-171122_Office.jpg
 
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tomtoo

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Dont know what it is, but something must be wrong. If the sinewave gets so ugly you would have a hell of distortion.
This is just the switching frequency you are seeing. all class d amps will appeR like this without filtering. the scope is wide enough bandwidth to see the signal. it's not reproduced by a speaker so is audibly irrelevant.

In this review I found on March website you can see a filtered and non filtered square wave

Dont think so, this is different timebase and low resolution of scope screen.
 

restorer-john

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Dont think so, this is different timebase and low resolution of scope screen.

No, it's the sampling rate of the 'scope' the number of bits (scope's A/D) and the interpolation settings for the capture. He needs to either use a decent true RMS multimeter or build a LPF to get rid of the switching noise.

Edit: the scope appears to have a 200kHz bandwidth, so the switching waveform he is seeing is badly aliased and not representative.
 
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f1shb0n3

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Now looking further, the waves of left and right channel look a bit different even - the right channel has a wave going through the wave at 1kHz. Not sure if it's the primitive scope or an indication of something potentially wrong with the one of the channels. Sound like I would benefit from better measuring equipment to figure it out!

I have RME ADI-2 Pro, but don't know how to safely connect amplifier to it nor what software to use for measurement and graphs. I know I need a good dummy resistor and such, will look into all that - pointers would be welcome!
If I can make it work, I can also measure the whole chain - from ADI-2 Pro as source going through MiniDSP SHD and D90 DAC to amplifier output fed back into ADI-2 Pro. Wouldn't this be the holly grail of end-to-end SINAD measurement? :) With a an extra twist that it would be a loop starting and ending at the same place :cool:
 

tomtoo

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No, it's the sampling rate of the 'scope' the number of bits (scope's A/D) and the interpolation settings for the capture. He needs to either use a decent true RMS multimeter or build a LPF to get rid of the switching noise.

First i like to see them with the same settings or the AB with the settings of the class d.
 

storing

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It's just the ultrasonics of the Class D amp
I assume so, but would the that seemingly lower-frequency content (the 6mSec period or so 'beats') be aliasing from those ultrasonics, or something else?
 

solderdude

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It displays the 100Hz signal + averaged switching frequency component.
In reality it will be a 100Hz signal with a several 100kHz signal superposed on it.
The scope plot should either be ignored or a very steep filter should be applied before the 'scope'.
 

tomtoo

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@op

I am realy interested now. Could you pls do screenshots with the same oszi settings?
 

mrmojo2022

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Now looking further, the waves of left and right channel look a bit different even - the right channel has a wave going through the wave at 1kHz. Not sure if it's the primitive scope or an indication of something potentially wrong with the one of the channels. Sound like I would benefit from better measuring equipment to figure it out!

I have RME ADI-2 Pro, but don't know how to safely connect amplifier to it nor what software to use for measurement and graphs. I know I need a good dummy resistor and such, will look into all that - pointers would be welcome!
If I can make it work, I can also measure the whole chain - from ADI-2 Pro as source going through MiniDSP SHD and D90 DAC to amplifier output fed back into ADI-2 Pro. Wouldn't this be the holly grail of end-to-end SINAD measurement? :) With a an extra twist that it would be a loop starting and ending at the same place :cool:
As long as you keep the input to the ADI below it's maximum you can connect it to the amp output with a dummy load.

REW software

Scopes are not good for looking in detail at signals. Even good ones are typically only 8 bit. I'm afraid that one really isn't up to the job.
 

boXem

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As long as you keep the input to the ADI below it's maximum you can connect it to the amp output with a dummy load.

REW software

Scopes are not good for looking in detail at signals. Even good ones are typically only 8 bit. I'm afraid that one really isn't up to the job.
It is not a problem of resolution, 8 bits is more than enough for what is done here. Just that good ol' Shannon is once again in action. Since the signal has a component at ~500 kHz you need to sample it at minimum 1 MHz, more than 5 MHz being reasonable.
 
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f1shb0n3

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As long as you keep the input to the ADI below it's maximum you can connect it to the amp output with a dummy load.

REW software

Scopes are not good for looking in detail at signals. Even good ones are typically only 8 bit. I'm afraid that one really isn't up to the job.
Dummy load ordered. I need to connect to ADI-2's balanced input, right?
Is this the proper way (in series)?
XLR pin 2 --- 8ohm dummy --- amplifier (+)
XLR pin 3 --- amplifier (-)

Correcting myself - should be in parallel with the dummy I believe, not series.

I give up on messing with the scope - it was very useful to volume-match, but to make sure my amplifier is running fine I need a precise measurement.
 
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