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Old 1st April 2013, 06:59 AM   #2351
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Quote:
Originally Posted by rjm View Post
An educated guess, based off the datasheets:

For an input impedance of 470 ohms, 2134 is 8.5 nV/sqrtHz, 5532 works out to 5.8 nV/sqrtHz, while the 4562 slips in at 3.9 nV/sqrtHz. For completeness, the OPA27 is at 4.2 and the LT1115 at 3.1 nV/sqrtHz.

As expected, at these low impedances the trusty 5532 is more than up for the task. The 2134 is a better choice to sit behind a 100k volume pot for example, its my go to model for headphone amps and preamps.

Douglas Self was basically right though: at the end of the day you can't go far wrong with the 5532-5534. Just a really nice, easy to implement balance of low noise, low distortion, good drive, and stability.
As we all expected, if you calculate, the FET OP2134 should be the noisiest, followed indeed by the good 'old bipolar NE5532 and the very low noise LM4562.

However in practice, things get a bit different:
NE5532:
Click the image to open in full size.

OPA2134:
Click the image to open in full size.

LM4562:
Click the image to open in full size.

The NE5532 comes out best, followed closely by the OPA2134. The LM4562 is actually worse than both for this application. You can find the full measurements for all opamps here.

This is actually quite funny, because lots of people around here do not recommend the NE5532, because the impedance of an MM cart would be to high. It's not. It's a perfect choice. I do note however that the frequency response of the OPA2134 is "flatter". This causes a bit more bass to come through, and a bit less treble. This might explain why people call the OPA2134 a "warmer" sounding opamp.

NE5532 frequency response:
Click the image to open in full size.

OPA2134 frequency response:
Click the image to open in full size.
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Old 1st April 2013, 08:07 AM   #2352
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Interesting, but lets not get ahead of ourselves.

The predicted difference in the noise baseline is only about 3 dB between each successive op amp. That's difficult to see on an axis scaled to 140 dB and difficult to calculate given the many spurious peaks at 1-10 kHz.

Also, we must think about the PSRR - different op amps have very different values, one can be letting in some power supply noise where the other is not. That could reverse the measured result from the predicted trend.

Finally, a big one: GBWP. The LM4562 is a fast sucker, 55 Mhz. Too fast for this circuit really, it may likely have additional noise pickup from RFI interference and instability too.

Though I would add that from where I stand, all three would be acceptable and the choice would largely come down to preference.

I do not recommend using op amps with bandwidths over 10 MHz in any of my circuit boards, however. I have not tested for stability and the boards / power supply are not designed to operate at such high frequencies.
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Last edited by rjm; 1st April 2013 at 08:17 AM.
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Old 1st April 2013, 08:26 AM   #2353
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Quote:
Originally Posted by vulejov View Post
Except incorrect attempt adding two values ​​so far we have no evidence that the RIAA curve is correct..
vulejov, you can find the correct RIAA eq. curve here, together with the formula to calculate it. To get in the inverse, simply flip it by taking the negative of the dB value.

To anticipate your question "how can I know if it is correct or not?" let me answer that for you now:

Because I said so.

If you have further doubts, feel free to make your own investigations, ask the RIAA, whatever, I am not willing to waste any more of my time with this.
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Old 1st April 2013, 11:07 AM   #2354
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Quote:
Originally Posted by rjm View Post
vulejov, you can find the correct RIAA eq. curve here, together with the formula to calculate it. To get in the inverse, simply flip it by taking the negative of the dB value.

To anticipate your question "how can I know if it is correct or not?" let me answer that for you now:

Because I said so.

If you have further doubts, feel free to make your own investigations, ask the RIAA, whatever, I am not willing to waste any more of my time with this.
It is easy to find the correct curve.. but it is not my point..

I thought about your answer based on any calculations that makes me sure that the RIAA curve in your preamp is correct, and that these are not simulations ..

Attempted addition of capacitance and resistance before a page is simply not correct..
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Old 2nd April 2013, 01:10 AM   #2355
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impedance of a resistor Zr(f) = R
impedance of a capacitor Zc(f) = 1/(i 2 pi (f) C) [or "j" if you prefer]
impedance of a resistor and capacitor in series, Ztot(f) = Zr(f) + Zc(f)

So tell me, what is not correct about the above?

For everyone else reading: the beauty of impedances is that inductive and reactive elements in a network (inductors and capacitors, respectively) can be manipulated mathematically just like simple resistors, adding as Z1+Z2 in series and as Z1*Z2/(Z1+Z2) in parallel. The only difficulty is that the numbers themselves are complex, with real and imaginary parts. If you can use a computer to handle complex numbers, the analysis is straightforward.

Now I'm curious. I mean, I thought everyone who was into DIY knew this, even if they weren't inclined to work through it themselves.

Quick show of hands please: "the impedance of a capacitor is -j/([omega]C" .... "Yeah, I knew that" Y/N?
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Last edited by rjm; 2nd April 2013 at 01:21 AM.
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Old 2nd April 2013, 05:56 AM   #2356
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We are still at the theoretical hearsay ..
I'm only interested in particular circuit, and values ​​associated with it..
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Old 2nd April 2013, 06:06 AM   #2357
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Originally Posted by vulejov View Post
We are still at the theoretical hearsay ..
I'm only interested in particular circuit, and values ​​associated with it..
What RJM is writing is plain math. If you're doubting that (and the associated formula) - then maybe you should start to rewrite every electrical law?..
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Old 2nd April 2013, 07:11 AM   #2358
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Here you go, use plain math and without simulation please calculate for me gain in this phono.. just 3 values.. 20 Hz, 1 kHz and 20 kHz.. thanx..
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Old 2nd April 2013, 09:27 AM   #2359
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I'm not going to. It's not for me or anyone here to do that work for you, especially when LTSpice gives the result with no effort!! If you doubt it, it's your job to prove its wrong, not ours to show you its right!!

Here at least is the approach:

Divide the circuit into three parts: the input inverting op amp, output inverting op amp, and output RC filter. I will note the impedance of any capacitor as Z(C1) etc., defined as above.

First stage gain: easy, as it can be assumed flat over the audio band, so gain1=R3/(R1+Zin), Zin is the DC resistance of the cart.

Second stage, gain2= |[Z(C3)||R7||R8+Z(C2)]/R4| (two bars '||' means "in parallel with", single bar, e.g. |M| means (magnitude of M).

Third stage gain3= |(R9||Zout)/(Z(C1)+R9||Zout)| Zout is the impedance of the following stage, not shown on your schematic.

Total circuit gain in dB is 20 log (gain1*gain2*gain3).

The above leaves out some high frequency components but will be sufficient to allow you to calculate the gain up to 20 kHz with sufficient accuracy.

At least I think that's right. It's been a while and I do make mistakes.

Good luck!
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Last edited by rjm; 2nd April 2013 at 09:30 AM.
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Old 2nd April 2013, 10:10 AM   #2360
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Quote:
Originally Posted by rjm View Post
Second stage, gain2= |[Z(C3)||R7||R8+Z(C2)]/R4| (two bars '||' means "in parallel with", single bar, e.g. |M| means (magnitude of M).

Third stage gain3= |(R9||Zout)/(Z(C1)+R9||Zout)| Zout is the impedance of the following stage, not shown on your schematic.

Total circuit gain in dB is 20 log (gain1*gain2*gain3).

The above leaves out some high frequency components but will be sufficient to allow you to calculate the gain up to 20 kHz with sufficient accuracy.

At least I think that's right. It's been a while and I do make mistakes.

Good luck!
What is Z in second stage?

Zout is 10k..
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