Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Analogue Source Turntables, Tonearms, Cartridges, Phono Stages, Tuners, Tape Recorders, etc.

The LM4562 in a phono stage.
The LM4562 in a phono stage.
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 14th May 2019, 07:03 PM   #21
dirkwright is offline dirkwright  United States
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
Quote:
Originally Posted by loepke72 View Post
Maybe it was the way I was attempting to make a comparative noise measurement by reading the level at a specific frequency on a spectrum display. Perhaps a screen shot of the measured noise will help:
For RIAA EQ circuit noise measurements, all you need to do is add in the source resistance (about 1k ohm) of a phono cartridge to get a more accurate idea of actual system noise. You should find that your circuits have a SNR of around 90 dB this way, which is basically silent since the record surface noise (plus TT noise) is at best -70dB. Since the MM source impedance is around 1k ohm, then generally speaking an FET input op amp will give better results for noise. I like the OPA627 myself.
  Reply With Quote
Old 14th May 2019, 09:01 PM   #22
MarcelvdG is online now MarcelvdG  Netherlands
diyAudio Member
 
Join Date: Mar 2003
Location: Haarlem, the Netherlands
Quote:
Originally Posted by dirkwright View Post
For RIAA EQ circuit noise measurements, all you need to do is add in the source resistance (about 1k ohm) of a phono cartridge to get a more accurate idea of actual system noise. You should find that your circuits have a SNR of around 90 dB this way, which is basically silent since the record surface noise (plus TT noise) is at best -70dB. Since the MM source impedance is around 1k ohm, then generally speaking an FET input op amp will give better results for noise. I like the OPA627 myself.
You are implicitly neglecting the cartridge inductance, but the impact of the cartridge inductance is anything but negligible. If you want to minimize the integrated RIAA- and A-weighted noise, you can take the cartridge impedance at 3852 Hz as a good estimate of the effective source impedance. For a 1 kohm and 500 mH cartridge, that boils down to about 12 kohm. Modern FET op-amps and the NE5534A have a good noise match to impedances of that order.
  Reply With Quote
Old 14th May 2019, 10:42 PM   #23
Mark Tillotson is offline Mark Tillotson
diyAudio Member
 
Join Date: May 2018
Location: Cambridge UK
(The vast majority of) MM cartridges are massively inductive, about 1/2 a henry, that dominates their source impedance above a few 100Hz.


To measure system noise, just plug the cartridge in. The RIAA curve partially cancels the rise of noise with frequency from the current noise into the inductance. The voltage noise from the preamp falls with frequency (as seen at the output) as it follows the RIAA curve, so is unlikely to make much hiss by comparison to the inductance/current noise, although the flicker noise portion ought to be prominent in the noise spectrum.
Its even more complex than this, as the 47k load resistance voltage noise will be more prominent at high frequencies too, as its effectively in parallel with the rising cartridge impedance. Synthesized load circuitry can address this latter issue.
  Reply With Quote
Old 14th May 2019, 10:46 PM   #24
scott wurcer is offline scott wurcer  United States
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: Belmont MA
Why not just measure the noise with the cartridge as a source impedance?
__________________
"The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed all-important."
  Reply With Quote
Old 15th May 2019, 03:28 PM   #25
billshurv is offline billshurv  United Kingdom
diyAudio Member
 
billshurv's Avatar
 
Join Date: Mar 2014
The LM4562 in a phono stage.
Quote:
Originally Posted by Mark Tillotson View Post
(The vast majority of) MM cartridges are massively inductive, about 1/2 a henry, that dominates their source impedance above a few 100Hz.


.

I'm am edge case as mine go from 40mH to over 1000mH. Scott of course has a Grado which is so low you wonder how it works.



Very glad we are all being honest that there is only one sensible way to measure noise for MM stage
  Reply With Quote
Old 15th May 2019, 06:34 PM   #26
MarcelvdG is online now MarcelvdG  Netherlands
diyAudio Member
 
Join Date: Mar 2003
Location: Haarlem, the Netherlands
Quote:
Originally Posted by scott wurcer View Post
Why not just measure the noise with the cartridge as a source impedance?
That's the best you can do when you want to compare different phono amplifiers for a given cartridge. It can be inconvenient when you want to compare a phono amplifier's measured performance with calculated or simulated data, because you have to take into account the frequency-dependent ESR of the cartridge somehow.
  Reply With Quote
Old 25th May 2019, 04:53 PM   #27
DouglasSelf is offline DouglasSelf  United Kingdom
diyAudio Member
 
DouglasSelf's Avatar
 
Join Date: Mar 2007
Location: London
Quote:
Originally Posted by scott wurcer View Post
Why not just measure the noise with the cartridge as a source impedance?
The main problem is that cartridges are very sensitive to ambient magnetic fields. (Not electrical fields, except in one notorious case) As related in Electronics for Vinyl, I found a mumetal screening can to be the minimum shielding for reliable noise measurements. Two nested cans (as sometimes used for MC step-up transformers) is better.
__________________
Douglas Self
  Reply With Quote
Old 27th May 2019, 07:53 PM   #28
Mark Tillotson is offline Mark Tillotson
diyAudio Member
 
Join Date: May 2018
Location: Cambridge UK
Quote:
Originally Posted by SY View Post
You understand that this is about a hundred times lower than the intrinsic noise of the phono cartridge, thus totally insignificant?

Surface noise, surely, cartridge noise is mainly resistive Johnson noise (plus stray pick-up). High current noise in the amp will be multiplied by a high impedance cartridge, but that's not intrinsic cartridge noise.
  Reply With Quote
Old 27th May 2019, 10:03 PM   #29
billshurv is offline billshurv  United Kingdom
diyAudio Member
 
billshurv's Avatar
 
Join Date: Mar 2014
The LM4562 in a phono stage.
The point is that it's not hard to design a preamp with less noise that a 1k resistor! So, hum aside the cartridge noise will dominate in a competant design.
  Reply With Quote
Old 28th May 2019, 07:13 AM   #30
MarcelvdG is online now MarcelvdG  Netherlands
diyAudio Member
 
Join Date: Mar 2003
Location: Haarlem, the Netherlands
Don't forget the thermal noise of the 47 kohm termination resistor, it is usually of the same order as or greater than the cartridge's thermal noise.

If you want a good example of an incompetent design, look up the moving-magnet version of the Elektor Supra 2.0:

Supra 2.0 | Elektor Magazine

It uses a fortune's worth of ultra low noise op-amps to arrive at the highest noise moving-magnet amplifier ever, all because the designers didn't take the effect of current noise and cartridge inductance into account.
  Reply With Quote

Reply


The LM4562 in a phono stage.Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Anyone built the phono pre from LM4562/LME49720 datasheet? Tolu Analogue Source 52 16th April 2018 07:26 AM
LM4562 Phono Pre Amp Circuit Board eldarvanyar Parts 2 3rd April 2013 02:40 PM
Hum hunting - LM4562 Phono stage JesseG Solid State 11 1st January 2013 01:22 PM
DIY phono to replace roksan phono stage seroxatmad Analog Line Level 6 14th December 2008 05:46 PM
Should I replace Scott 222C phono stage with Claret phono stage? Bing Yang Analogue Source 0 22nd August 2005 06:41 AM


New To Site? Need Help?

All times are GMT. The time now is 04:51 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki