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an mm/mc phono stage
an mm/mc phono stage
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Old 9th October 2017, 06:14 PM   #11
MarcelvdG is offline MarcelvdG  Netherlands
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I don't agree with that at all, but I'm sure we will never convince each other anyway.
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Old 9th October 2017, 08:47 PM   #12
MarcelvdG is offline MarcelvdG  Netherlands
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Nonetheless, here's my point of view:

On one hand the record surface noise tends to dominate by a very large factor when I play dusty old records from the 1970's using an MM cartridge and a carefully noise-optimised amplifier. Under those circumstances a somewhat less noise-optimised amplifier would do just as well. The difference between the noise with the record playing and not playing is, of course, much smaller with clean brand new records.

On the other hand, a common target for the noise of an amplifier for MC cartridges is around 1 nV/sqrt(Hz) or less. As far as I know MM cartridges typically produce about ten times as much signal voltage as MC cartridges, so you would need around 10 nV/sqrt(Hz) or less for an MM cartridge to get the same signal to noise ratio (I will neglect the thermal noise of the cartridges themselves and of the termination resistors for simplicity, even though that's not necessarily correct).

As explained in the link LME49710 Phono Stage , 1 pA/sqrt(Hz) of noise current has about the same impact as 12 nV/sqrt(Hz) of noise voltage when you have a 500 mH MM cartridge. With the 2 pA/sqrt(Hz) noise current of an AD797 you are therefore likely to get less signal to noise ratio with an MM cartridge than with an MC cartridge, at least with MM cartridges with a fairly high inductance.

Last edited by MarcelvdG; 9th October 2017 at 08:52 PM.
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Old 10th October 2017, 12:20 AM   #13
rjm is offline rjm  Japan
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an mm/mc phono stage
@MarcelvdG

First I'd like to point out that it's a socketed IC and anyone can use whatever op amp they want. I'd probably use NE5534A for both IC, as being a good compromise input op amp for both mm and mc cartridges and a low distortion and low noise driver for the output stage too, though I like the OPA134 sound too - it's just not ideal for mc input stage. Of course the LME49710 is up the task. (I have trialed the LME49710 in my Asus sound card and, iirc, one of my headphone amps and it did not win me over I'm afraid.)

Second, I'm going to have to side with vulejov on this one. In general the noise value of the op amp in an mm phono stage isn't the primary concern like it is for a mc stage. And even in an mc stage, at the end of the day it's whatever sounds best - if that means an op amp with 3 dB or 6 dB higher noise floor, so be it. For an mm stage, with a cartridge impedance of 500-1000 ohms, the total input referred noise doesn't change so drastically depending on the op amp used. Most audio opamps will still give better S/N with a high output mm cart than even the lowest noise op amp with an mc cart and as your own numbers confirm, even for the AD797 the current noise wouldn't be so bad as to make it unusable in that role - the end result is about the same S/N as obtained with mc.
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Last edited by rjm; 10th October 2017 at 12:36 AM.
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Old 10th October 2017, 12:37 AM   #14
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by rjm View Post
He said AD797 was ideal for mc stage. As I read it he didn't suggest it would be good for mm, though as your numbers confirm, the current noise wouldn't be so bad as to make it unusable in that role.
In reality LP surface noise swamps many evils.
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Old 10th October 2017, 04:28 AM   #15
MarcelvdG is offline MarcelvdG  Netherlands
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Originally Posted by rjm View Post
@MarcelvdG

(...) For an mm stage, with a cartridge impedance of 500-1000 ohms, the total input referred noise doesn't change so drastically depending on the op amp used. Most audio opamps will still give better S/N with a high output mm cart than even the lowest noise op amp with an mc cart and as your own numbers confirm, even for the AD797 the current noise wouldn't be so bad as to make it unusable in that role - the end result is about the same S/N as obtained with mc.
You are making the common mistake of underestimating the impedance of an MM cartridge. A 600 ohm plus 500 mH cartridge has an impedance that varies between 600 ohm and 63 kohm from 20 Hz to 20 kHz. When you calculate an RIAA- and A-weighted average, you find about 12 kohm.
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Old 10th October 2017, 09:27 AM   #16
rjm is offline rjm  Japan
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Good point. Though that 63k is only in the extreme treble. Anything below 10k Hz is closer to 10k ohms or less. So, yes, FET input types are favored, but bipolars are far from unusable.
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Last edited by rjm; 10th October 2017 at 09:50 AM.
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Old 10th October 2017, 11:02 AM   #17
Calvin is offline Calvin  Germany
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Hi,

also keep in mind, that the current noise figures in Datasheets typically are lower than the real figures.
Due to the testing procedure its roughly 70% of the real numbers and figures rise with unmatchted impedances at the inputs.
IIrc Mr. J.L.Hood claimed en values of <5nV/sqrHz and in values of <1.5pa/sqrHz to be acoustically transparent in MM applications.

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Old 10th October 2017, 11:43 AM   #18
billshurv is offline billshurv  United Kingdom
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But as you know with DIY 'acoustically transparent' is always trumped by 'satisfyingly excessive'
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Old 10th October 2017, 02:48 PM   #19
Hans Polak is offline Hans Polak  Netherlands
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Originally Posted by billshurv View Post
But as you know with DIY 'acoustically transparent' is always trumped by 'satisfyingly excessive'
Hi Bill, English is an intriguing language and you are like a magician playing with its words.
Its always a pleasure reading your comments.

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Old 10th October 2017, 04:11 PM   #20
billshurv is offline billshurv  United Kingdom
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an mm/mc phono stage
Thank you Hans. I do generally try to respect the truly international audience here but every so often one slips through .

RJM: Sorry for the minor Hijack. Finding the noise discussion interesting, as this (as well as overload margin) never get agreement
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