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Old 21st February 2013, 07:50 AM   #251
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Thanks for all this work Jeff, judging by all these sim's it's good with or without the cap, up to the individual what he like the sound of.
But I can't believe I'm hearing a few degrees of phase shift from 5khz to 20khz. It's one of those mysteries of audio where a 1st order HF roll off filter at 100khz sounds for some reason a tad brighter than no filter?

Cheers George
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Old 21st February 2013, 12:13 PM   #252
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You might be hearing reduced 'grunge' caused by better bandlimiting as 'brighter' as its probably making b*gger all difference in the audio band. But lopping off the HF will reduce the overall IMD and give a clearer, more dynamic sound. This is how I'm hearing the difference in tweeter output in my chipamps - the bad ones sound 'dull' which has nothing to do with the FR, rather the IMD from RF in the signal and on the PSU rails.
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Old 22nd February 2013, 03:14 AM   #253
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Abraxalito's got the technical explanation of what I was trying to say.

If you're hearing something, it probably isn't the direct and very small change in the frequency response. Rather it's the high frequency being mixed down into the audio band--that's IMD!

If you remember how radios work (audio on a high freq carrier freq) its the same concept: two frequencies (f1 audio and f2 carrier) mixing in a controlled and linear fashion and producing multiple copies at different frequencies (f1-f2 and f1+f2). You create a 3rd signal (f1+f2) that's your high freq carrier and your audio signal mixed together--it's high freq (good for transmitting with a small antenna) and has your info in it.
Send.
Receive.
Reverse the process in the same controlled and linear fashion to separate the signals (f1-f2) and get your audio f1. ... Wow, that takes me back to the labs of first year ... Ah, what was I sayin .... oh ya ...

When the mixing is done poorly or unintentionally it's often non-linear and now you also get small copies at 2x (f1-f2) and 2x (f1+f2), and 3x (f1-f2) and 3x (f1+f2), 4x ..., 5x ....

What this means for us is that you get copies of high freq Mhz stuff all over the place and some can mix down quite far when (f1-f2) is large and the multiplier is large (20x) and the copies are ... loud. What this subjectively sounds like is more difficult ...

Here are a few danger freqs of interest:
OPA627 EMI sources .4 to 5GHz.png


OPENAMP1 - MM phono preamp open project

Last edited by AudioLapDance; 22nd February 2013 at 03:26 AM.
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Old 22nd February 2013, 03:53 AM   #254
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So you can see why RF and EMI is so nasty and PMA keeps harping on about it.

Which brings me to the difference between your 1704 circuit and the 1541 circuit.

It has a sort of input filter cap (only):
AD844 iv TDA1541 input cap only.png

Which shaves off some RF and lots of EMI before it even gets to the 844 let alone your 3rd order filter!
AD844 iv TDA1541 input cap 1p to 10n.png

We'd have to figure out a good value for your circuit.
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Old 22nd February 2013, 04:16 AM   #255
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Quote:
Originally Posted by AudioLapDance View Post
So you can see why RF and EMI is so nasty and PMA keeps harping on about it.
PMA isn't the only one - its my major hobby-horse too Thanks for putting all the mathematical meat on the bones of my writing.
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Old 22nd February 2013, 04:33 AM   #256
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Here's why your 3rd order filter may have trouble with RF/EMI:

OPA627 EMI RR in+.png

... and things get even stranger because you've probably got multiple feedbacks going back to both the pos and neg inputs ...
c o m p l e x

Remember the previous chart, sources at .4, .9, 1.8, 2.4, 3.6 and 5GHz
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Old 22nd February 2013, 05:02 AM   #257
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So what sort of opamp (fet or bi-polar, fast slow, best settling time ect) would be the better 3rd order L/P opamp to use, at the moment I am using an AD825 which gives to my ear the better sound out of the hundreds I have so far tried.
My FR is is only about .1db down at 19.99k using 560 cap and 3rd order L/P, noise on the output is virtualy nil, 1khz square wave looks perfectly square with the tinyest bit of ringing on the leading and trailing edge (gib phenonmen)
Cheers George
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Old 22nd February 2013, 05:19 AM   #258
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AD825 is by all accounts a good one - its rated by JC for SQ and Walt Jung found it didn't give RF related problems in his regulator (whereas AD797 did).

As a suggestion you could try one of the Barrie Gilbert style 'active feedback amplifiers'. That would be AD830, AD8129 and AD8130. Be warned though they're not particularly low noise - the AD8129 is the lowest but needs a gain of 20dB at least. In my experience the AD830 (the only one I've tried so far) is very RF-immune in that it doesn't give nasty sibilance when hung off the output of my TDA1387 DAC with only 1kohm and 1nF protecting it from hundreds of mV of RF.
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Last edited by abraxalito; 22nd February 2013 at 05:21 AM.
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Old 22nd February 2013, 05:32 AM   #259
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Sorry, Dude, I shouldn't have been so dramatic!

Didn't mean to imply there is anything wrong with your circuit. Fact is, most circuitry has to deal with this and does to a varying degree.

I was just trying to illustrate what abraxalito was talking about and why there could be issues with RF/EMI and how it actually affects the audio band. ... and why there may be solid technical reasons for the 560p cap and its rolloff.

Neat thing here is how it connects back to that radio mixing idea. RF/EMI is a fact of life (in general) and especially so with the 'staircases' in current DAC I/V, so you can either try to filter it out of your circuits and/or build circuits that can handle it.

That is, handle it linearly and just pass it through without mixing it all up with the audio.

"Mixing it all up": literally and technically: non-linear mixing producing 2x, 3x, 4x, 5x, (f1+f2) and (f1-f2)...

That's why high speed parts make sense:
AD825: High speed, 41 MHz,−3dB bandwidth, 125 V/μs slew rate, 80 ns settling time

But high speed parts need to be laid out and bypassed properly ...

Enter abraxalito on hobby horse:

riding horse.jpg

Last edited by AudioLapDance; 22nd February 2013 at 05:55 AM.
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Old 22nd February 2013, 03:22 PM   #260
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Yet another reason to put your gear inside of RFI resistant metal boxes. Most of those "pretty" boxes they sell to hobbyists do not shield RF because they are anodized aluminum. Here's what a more professional RFI resistant box looks like. This is from Par-Metal (I am only a satisfied customer) and contains a SV oscillator (not quite finished yet).
Par-Metal
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