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Old 28th February 2012, 04:01 PM   #2901
Previously: Kuei Yang Wang
 
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Sy,

Quote:
Originally Posted by SY View Post
The overload margin is really not that bad
REALLY?

Maximum level on LP is 25cm/s. So maximum operating levels are routinely five times as high as the rated cartridge voltage.

Ciao T
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Old 28th February 2012, 04:20 PM   #2902
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Quote:
Originally Posted by SY View Post
The overload margin is really not that bad, though it won't have the stunningly good numbers of a tube circuit. [snip]

I guess if one is getting the kinds of overload resulting from dirt clods and potholes, it's time to clean or just discard the disk!

The OL margins that are brought out in JA measurement sidebars are based on nominal 5mV rms at 1kHz and 50mV rms at 20kHz, although he periodically mentions that there's not likely too many records with even that level at 20kHz. But then he uses that number to determine the overload margin for the unit under scrutiny, so that's been what I've figured on.

In the case of the preamp presented, the first stage gain is (4.7k/100) +1, i.e. times 48, +33.6dB. The output swing typical of an OP37 according to TI with +/-15V rails is +/- 13.4V. So the maximum peak input voltage below clipping to that stage is 279.2mV, or for a sinusoid, 197.4mV rms. So the OL margin based on the above 50mV rms is a factor of 3.948, or 11.9dB, of course improving at lower frequencies.

Now this may be plenty in almost every case, but if you are submitting a product for review and want accolades in the measurement sidebars, this wouldn't cut it.

The voltage noise density of the OP37 is reasonably low, about 3.7nV/rt Hz at 10Hz, 3.2nV/rt Hz at 1kHz, according to TI curves. The 100 ohm divider R at about 1.3nV/rt Hz will not contribute much compared to these. The current noise density according to the datasheet is around 2pA/rt Hz at 10Hz, corners around 70Hz and reaches 400fA/rt Hz midband and above. The current noise of the 47k termination resistor is 594fA/rt Hz. Hence the total parallel noise density from those two sources will be 716fA/rt Hz midband. These parallel-noise contributions will only be somewhat important with high inductance sources at relatively high frequencies, and all of this stuff will be followed by the EQ which will proportionally reduce the HF stuff.

This is about the point in a textbook when the weary author says The calculation of the overall signal-to-noise performance is left as an exercise for the reader, and may then say See Problem 426.


Brad

Edit: these numbers for OL are actually a little optimistic, as I didn't notice the cap multiplier Q's and their ~1 Vbe drops. The OP37 does handle +/-18V btw, although that would be walking near the precipice.

Last edited by bcarso; 28th February 2012 at 04:40 PM. Reason: afterthought
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Old 28th February 2012, 04:34 PM   #2903
bcarso is offline bcarso  United States
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Originally Posted by dvv View Post
Brad, you want to get them before they multiply.
I generally hate people who say Well, it could be worse. I worked with someone at Harman who responded thus whenever I biatched about something, and I thought, Bruce, if we were in tumbrils on our way to the guillotine, and were getting close enough to see heads being lopped off, you'd say Well, it could be worse.

But with the upstairs, It could be Worse. They could be playing loud music with lots of bass to accompany their stomping around.

The previous person was the mother of the then-manager, and I could hear her snore. A rather hefty one, and when she got out of bed I quipped that the mastodon, long believed to be extinct, in fact lives on. Got a lot of complaints from people who were drinking coffee when they read that and had it come out of their nose.
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Old 28th February 2012, 04:38 PM   #2904
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Quote:
Originally Posted by ThorstenL View Post
Sy,
Maximum level on LP is 25cm/s. So maximum operating levels are routinely five times as high as the rated cartridge voltage.
That fits exactly with my point. At 25cm/s, the cartridge will have 25mV output (assuming a typical 5cm/s output of 5mV- a V15 is slightly lower). That's well below the 200mV overload.

Quote:
Originally Posted by bcarso
The OL margins that are brought out in JA measurement sidebars are based on nominal 5mV rms at 1kHz and 50mV rms at 20kHz, although he periodically mentions that there's not likely too many records with even that level at 20kHz. But then he uses that number to determine the overload margin for the unit under scrutiny, so that's been what I've figured on.

In the case of the preamp presented, the first stage gain is (4.7k/100) +1, i.e. times 48, +33.6dB. The output swing typical of an OP37 according to TI with +/-15V rails is +/- 13.4V. So the maximum peak input voltage below clipping to that stage is 279.2mV, or for a sinusoid, 197.4mV rms. So the OL margin based on the above 50mV rms is a factor of 3.948, or 11.9dB, of course improving at lower frequencies.
A few more significant figures than I used, but the same answer.
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Old 28th February 2012, 05:10 PM   #2905
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Quote:
Originally Posted by SY View Post
That fits exactly with my point. At 25cm/s, the cartridge will have 25mV output (assuming a typical 5cm/s output of 5mV- a V15 is slightly lower). That's well below the 200mV overload.

A few more significant figures than I used, but the same answer.
But of course the overload problem is at high frequencies. Again, for most clean and well-preserved disks, it's probably not a big problem. For specsmanship and a good review --- which may or may not translate into customer respect and increased sales --- it might well be a problem.

Although, (and I've only been following them closely in recent times) about the only time I recall seeing a product get panned by JA, it was the 60k$ Vitus (?) phono pre, for which he measured pretty terrible noise. Fremer thought it was the cat's meow iirc.
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Old 28th February 2012, 05:25 PM   #2906
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Hi,

Quote:
Originally Posted by bcarso View Post
The OL margins that are brought out in JA measurement sidebars are based on nominal 5mV rms at 1kHz and 50mV rms at 20kHz
Yet at least in principle you need another 14dB above these levels... Then your overload margin is -3dB!...

And if your cartridge is "loud" it may be -6dB or even -9dB...

And if you use a MC Stepup transformer with a stepup of 30 and a "loud" MC Pickup...

You get the point.

Active EQ has no such issues.

Ciao T

PS, JA also measured no overload margin at HF for my Phono, while I measured plenty, both of us with an AP2. I never did get to the bottom of what happened there... My Phono outputs 30V at 20KHz, no sweat.
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Old 28th February 2012, 05:26 PM   #2907
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Quote:
Originally Posted by bcarso View Post
But of course the overload problem is at high frequencies. Again, for most clean and well-preserved disks, it's probably not a big problem. For specsmanship and a good review --- which may or may not translate into customer respect and increased sales --- it might well be a problem.
Points I also made, so clearly I agree. I designed my pre for far higher margins than were needed just because. Even for not as clean and well-preserved, published data that I've seen doesn't show anything more than 15-20dB above nominal, and even that is a rare occurrence.

Velocity is the key quantity, not frequency- JA chose a 50mV standard at 20k as a torture, not because that's actually something one routinely sees.
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Old 28th February 2012, 06:07 PM   #2908
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Quote:
Originally Posted by ThorstenL View Post
[snip]
PS, JA also measured no overload margin at HF for my Phono, while I measured plenty, both of us with an AP2. I never did get to the bottom of what happened there... My Phono outputs 30V at 20KHz, no sweat.
That sort of thing is very scary. I also asked in a thread I started (and to which you contributed I think, thanks) about measuring parallel noise, and more-or-less answered my own question. JA undoubtedly uses the on-loan SYS2722 which I suspect for RIAA conformance tests has an inverse RIAA network*, but measures noise by shorting the preamp-under-test's input, and afaik has never mentioned parallel noise as such. Similarly, for distortion measurements he uses I presume the Ap generator, which is a rather poor approximation to the impedance of a cartridge most of the time. So distortion induced at the preamp input is likely underestimated.

I wonder if Jan D. happens to remember where the 36dB OL margin "requirement" appeared? Maybe it was on Popa's website and not in LA proper.

Brad

*and how accurate is it? I know people who so completely trust Audio Precision that measurements made any other way are prima facie disbelieved. I told one friend that a DSP-based EQ box from Ashly had highish output noise, which would dominate the noise out of the loudspeakers in a prototype system--- and he flat-out didn't believe me, because it hadn't been measured with an Ap! Of course noise meaurements are fraught with peril, but I've been making them since well before AP was even a gleam in Bruce and Richard's eyes.
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Old 28th February 2012, 06:15 PM   #2909
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http://es.shure.com/idc/groups/tech_...o_v15iv_ug.pdf

page 4 has some measured points over 70 cm/s - presumably intentional music recording velocity
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Old 28th February 2012, 06:28 PM   #2910
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What if to put a vinyl disk into a photo scanner, rotate there, and decipher digitally?
Several rotations can be used for error correction.
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