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Old 5th February 2013, 01:15 PM   #41
PMA is offline PMA  Europe
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Maybe there is a misunderstanding, I have not said your OPA627 was a fake. I only would like to warn against cheap parts. It is based on experience that a colleague of mine bought 100 pcs of OPA627 at a very cheap price, and all fakes. There was completely different chip inside with different parameters. Yours looks OK, but it may be hard to tell from the photo. Unfortunately I do not remember who was the seller of the fakes, I will try to find out.
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Old 5th February 2013, 03:08 PM   #42
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Thanks ALD, your eyes are better than mine! That was pretty much what I assumed. With the right type and lead spacing I should be able to source everything from multiple vendors.
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Old 5th February 2013, 05:09 PM   #43
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Quote:
Originally Posted by PMA View Post
..It is based on experience that a colleague of mine bought 100 pcs of OPA627 at a very cheap price, and all fakes. There was completely different chip inside with different parameters...
I'm sure mine are fine as I got them a long time ago from reputable sources however others might want to know ...

Anyway back to OPENAMP1

PMA knows this (because he's always going ooooon and ooooon about EMI! ;-) ) but others might not:

OPA627 EMI RR in+.png


OPA627 EMI sources .4 to 5GHz.png


http://www.ti.com/lit/an/sboz016a/sboz016a.pdf


PMA could you talk more about your interesting R2(150ohm)/C2(100pF) filter (10MHz) right at the input? I wonder what that could be about ...

Cheers,
Jeff

PS "EMI and OPA627" could be it's own thread too! Application outside of RIAA ...
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Old 5th February 2013, 05:21 PM   #44
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Next, when PMA has the time, perhaps he could talk more about the other interesting aspect of his input stage: diodes

PMA OPENAMP1 input diodes and R2C2 filt.png

Would it have something to do with "diode clamping":

diode clamping.png

From:

http://www.ti.com/lit/an/sboa011/sboa011.pdf

Or are the diodes for safety ...

Cheers,
Jeff

Last edited by AudioLapDance; 5th February 2013 at 05:25 PM.
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Old 6th February 2013, 03:03 AM   #45
mlloyd1 is online now mlloyd1  United States
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re: diodes

diodes to the rails like that are usually done for protection against transients

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Old 6th February 2013, 03:09 AM   #46
mlloyd1 is online now mlloyd1  United States
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jeff:

see post #33

Quote:
Originally Posted by AudioLapDance View Post
...
PMA could you talk more about your interesting R2(150ohm)/C2(100pF) filter (10MHz) right at the input? I wonder what that could be about ...
btw, for the new folks, you can get some good discussion of somewhat similar and other RIAA preamp designs in ADI "app note" here

note: big file but definitely worth downloading and study.
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Old 6th February 2013, 06:31 AM   #47
PMA is offline PMA  Europe
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Quote:
Originally Posted by AudioLapDance View Post

PMA could you talk more about your interesting R2(150ohm)/C2(100pF) filter (10MHz) right at the input? I wonder what that could be about ...
Well, yes. As now, it serves mostly as an RFI filter.

It might, theoretically, improve RIAA response for very high frequencies, if we speak about phono preamp alone, driven from pure voltage source. As you know, non-inverting network has gain no smaller than +1 (0dB), until the opamp's bandwidth will roll it off. R2 C2 could be calculated to allow the RIAA curve continue in attenuation even for very high frequencies above audio band. Please see the attached image.

But, in a real world the phono preamp amplifies signal not from the voltage source, but from MM cartridge, which has output impedance like resistance and inductance in series. Usually 500 ohm - 2000 ohm + 300 mH - 1500 mH. This inductance makes thus a roll-off even for high frequencies and the role of R2C2 described hereabove is valid only in case of voltage source signal (= zero output impedance). And we cannot increase C2 too much because we would load the cartridge by high capacitance and create high resonance peak somewhere between 15 - 20 kHz. We also cannot increase R2 too much for we would get increased noise.

I hope this helps a little, for more on theory please study literature about phono cartridges, like Vogel (The Sound of Silence). Walt Jung's publications are also excellent and may help, but I am not sure if he works with cartridge models in his considerations.
Attached Images
File Type: png R2C2en.PNG (65.8 KB, 247 views)
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Last edited by PMA; 6th February 2013 at 06:38 AM.
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Old 6th February 2013, 07:16 AM   #48
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Quote:
Originally Posted by PMA View Post
It is based on experience that a colleague of mine bought ....
I have some additional info, it was 2 years ago and "OPA627's" were in TO-99 metal cans and they had input bias current several orders higher than specified in the datasheet. So he did not use them and started complaints/warranty return procedure. I agree that in case of discussion on this subject we should start a separate thread, but I cannnot promise I would be able to spend much time there.
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Old 6th February 2013, 05:46 PM   #49
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Now, I just got into vinyl a couple of months ago, so correct any newbie mistakes ...

Quote:
Originally Posted by PMA View Post
...But, in a real world the phono preamp amplifies signal not from the voltage source, but from MM cartridge, which has output impedance like resistance and inductance in series. ...
I've looked at cart loading before for I didn't include the inductance ...
Wow PMA you weren't kidding! Totally different!

Ortophon tells me my OM5E has:

Internal impedance, DC resistance 750 Ohm
Internal inductance 450 mH
And recommends 200-600pF/47k loading (300pF/47k)

I threw those into the Tina circuit of the 627 and then added just the input of the OPENAMP1:

OPA627 PMA riaa input.zip
Had to zip it, hope it unzips ok

And then simmed (1 to 100k) with the RF filt (150/100pF)

OM5E w RFfilt.png
3.4dB boost at 10.2k

And then simmed with no RF filt

OM5E no RFfilt.png
2.4dB boost at 10.9k

I didn't think it would matter being a 10Mhz filter but I guess it interacts with all the other input parameters:

PMA riaa input.png

So a 3.4dB boost at 10.2k with the RF filter but only a 2.4dB boost at 10.9k with out! Wow, wasn't expecting that ... but it is a small difference.

Hmmmm

Last edited by AudioLapDance; 6th February 2013 at 05:49 PM.
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Old 6th February 2013, 05:58 PM   #50
PMA is offline PMA  Europe
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Yes, but you would need to sim the whole circuit, to get the correct answer. This was done, for various Rs, Ls, R2 and C2.
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