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The diyAudio First Watt M2x
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Old 12th July 2019, 06:18 PM   #2331
ItsAllInMyHead is offline ItsAllInMyHead
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Originally Posted by MEPER View Post
Less AC at output with shorted input is better. 500 uV noise is rather high. 0 VAC at output is much better. 0 VAC is the optimal goal which means "no noise". You should not try to get 500 uV if you have 0 uV. Usally a very good amp has 100 uV or less noise (AC). Probably you have some uV noise…...even your DMM reads 0. I have 100 uV (0.1 mV) resolution on my DMM and my DMM reads 0.0 mV at output which means less than 50 uV noise…..but DMM are not very accurate at these small noise levels…..so it could also be 70uV.....but probably less than 100 uV.
Understood with thanks. I know I shouldn't be trying to increase the noise, but given all the conversations around shielding the Edcors etc. etc. etc., I thought I'd have some noise that would register on my DMM as a registered AC voltage. It's great to know that others have achieved noise in the range below which a common DMM may register. That is extremely useful (and indicates that I may have done a few things right in the build).

I still go by my ears which indicate no audible noise with my ear pressed to the grills with 98dB/2.83V/1m speakers. So that's the good news.

I'm trying to learn some of the simpler measurement techniques and appreciate all the insight. Once I get the basics down, I may try some more sophisticated measurements.
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Old 12th July 2019, 06:27 PM   #2332
MEPER is offline MEPER  Denmark
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A good way to measure noise is using the Arta or REW software and external sound card. This also shows where the noise is if it is 50/60 Hz, 100/120 Hz or something else. It is necessary to know if you want to do something about it. But with no audible noise with 98 dB speakers with ears close to the units indicates very low noise. I like to measure everything before I connect amp to speakers. I have MoFo's running with BA3-pre so not in absolute need for another amp :-) …..I am thinking of where to "store" MoFo's…...they are 30 kg each…..
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Old 12th July 2019, 06:44 PM   #2333
ItsAllInMyHead is offline ItsAllInMyHead
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Originally Posted by MEPER View Post
A good way to measure noise is using the Arta or REW software and external sound card. This also shows where the noise is if it is 50/60 Hz, 100/120 Hz or something else. It is necessary to know if you want to do something about it. But with no audible noise with 98 dB speakers with ears close to the units indicates very low noise. I like to measure everything before I connect amp to speakers. I have MoFo's running with BA3-pre so not in absolute need for another amp :-) …..I am thinking of where to "store" MoFo's…...they are 30 kg each…..
Thank you for confirming. I've read the recent REW thread and a bit about ARTA. It's all still a bit foreign to me, and it seems that there is still a lot of conversation even among experts re: how to measure properly with both accuracy and consistency. So, until I get a better feeling for how to properly execute the measurements and interpret the results from both ARTA and REW, I'm waiting. I also hope to learn how to use REW as a potential tool for understanding some of the oddities in my room and how to compensate for or alleviate them.

A friend of mine has an analog scope and potentially a distortion analyzer he can let me use. His family is also pretty well-versed in some of the "art" and science behind audio (and other signal) measurements. In return for a few meals, some excellent camaraderie, and a beverage or two, I hope to have them teach me a thing or two in practicum.

After building the M2x, I took a pause and just listened for a while with friends. I never want to lose sight of the fact that although I am thoroughly enjoying learning about amps and how to build them... the end goal is the music.

Now... the itch hit again, and I wanted to see what else I could learn.
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Old 12th July 2019, 08:18 PM   #2334
MEPER is offline MEPER  Denmark
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Now I also got a measurement on 2nd M2X mono block. Have attached both results for comparation. Think they are similar enough in performance to call it "stereo". Next step is to set it up for a listening test.....maybe in the weekend...….but hard to believe Korg + M2X can beat BA-3 + MoFo? ….let's "see" :-)
Attached Images
File Type: jpg m2x_4w_out.jpg (256.6 KB, 255 views)
File Type: jpg m2x_4w_out2.jpg (246.5 KB, 256 views)
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Old 12th July 2019, 08:22 PM   #2335
xrk971 is offline xrk971  United States
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The diyAudio First Watt M2x
Hi Meper,
Your measurements are very similar to what I got. About 0.06%THD and dominant H2 and H3 of about the same amounts with some higher order stuff but not much.
Your PSU noise looks very good.
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Old 12th July 2019, 09:14 PM   #2336
MEPER is offline MEPER  Denmark
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Ok.....this is with 5.3 V RMS out in 6.8 ohm. The supply is an old school CRCLC with 4-pole Jensen as last caps. Since both channels looks identical in distortion and also match what you got then everything should be ok......even that M2 in theory should be 3rd harmonic dominant. 2nd harmonic probably from Edcor?
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Old 12th July 2019, 11:15 PM   #2337
Mark Johnson is offline Mark Johnson  United States
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Quote:
Originally Posted by MEPER View Post
Now I also got a measurement on 2nd M2X mono block.
Since the M2x has different FFT footprints when different daughter cards are installed, it's Best Practice to mention exactly which daughter card was used for the measurement.

And in the specific case of the "Tucson" daughter card, where many different opamps can be used, it's Best Practice to mention exactly which opamp was used on your particular Tucson daughter card.
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Old 13th July 2019, 07:41 AM   #2338
MEPER is offline MEPER  Denmark
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The input board is: TUCSON using the thru-hole OPA604 chip.
Think I will label the measurement file and include the input board name. Then interesting to see if distortion pattern change when input board changed.
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Old 13th July 2019, 06:06 PM   #2339
xrk971 is offline xrk971  United States
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Another option for an M2X compatible input board is now available - it’s based on the PCA (Pocket Class A) JFET input and MOSFET source follower. It will provide about 12dB gain so no need for a separate preamp. Board also works on the Yarra preamp of course. JPS64 did a great job on the layout and this board has a built in CRCRC to help clean up the positive rail. Provides choice of 2SK209, BF862, and 2SK170 input JFET footprints. The latter two, becoming almost unobtainium. Each one has a different setup so follow values on table provided.

Click the image to open in full size.

I have tried it on an M2X yet, but should work well. I have used it in the Aksa Lender chassis to drive the M2X (Austin) and that sounds great.

Click the image to open in full size.
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Last edited by xrk971; 13th July 2019 at 06:12 PM.
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Old 13th July 2019, 07:03 PM   #2340
Mark Johnson is offline Mark Johnson  United States
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If the new board is an inverting amplifier like the PCA was, be sure to notify users to swap the (+) and (-) plugs of their loudspeaker cables. Just as Nelson Pass alerts builders of the Amp Camp Amp to swap (+) and (-). {Amp Camp Amp is another inverting amplifier.}

PCA consisted of a JFET input stage (inverting) with a MOSFET source follower (noninverting) output stage.
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