Grado loading

[doctorcilantro]

Hi all,

I'm trying to calculate, based on the following information, the load on my

cart in my setup. I have wired my tonearm in a balanced configuration as

described on PureVinyl site, AudioKarma, etc. and I utilize the following.

Linear Gain AEA TRP ribbon pre:

36K input impedance

Grado Reference Statement (.5mv):

Inductance: 2mH

Resistance: 2

Input Load: 47K

Here is an archived AA post in which the poster was using the higher output

cart. I'm not sure about the capactance of my cable; it is very short maybe 1ft

and, as mentioned, terminates with XLR (3rd pin no ground or connection to

shield - although I have tried that). I get about -70db noise floor with about

70-75db of linear gain applied - this is prior of course to RIAA EQ which is

done via a VST plugin in Samplitude. The AEA TRP ribbon pre is listed at 18K

input impedance but I have verified it is actually 36K from the manufacturer. I

have had the system offline for some time due to a move, but it sound really

good before; right now I am just curious as to the resulting electrical specs..

I'm also planning to look into how the Grado low-output moving iron specs

differ from those of a true MC because lately I have been reading about (here)

an argument about using MC carts at 47K which would open up the possibility of

trying some other carts with this mic preamp.

"GRADO

Inductance: 45mH

Resistance: 475

Input Load: 47K

I cannot reproduce here the VISUAL results of the program used to render the

following calculation, but here are the results. They will, as have other real

world tests, throw into question the rationale that drives manufacturers to

provide a standard 47k load for MM cartridges, as well as the recommendation

that users so load ALL MM cartridges (The Shure V15VxMR, for example, would not

behave anything like we see here; their engineers seem to have designed it to

work into a 47k load). Here are the results and something I think you might

find worthwhile and relatively inexpensive to try out:

GRADO (45mH / 475d)

With 200pF cable & stray capacitance and 47K load resistor, output starts

to rise at ~10KHz to +1dB at 20KHz and a +9dB peak at 50KHz;

With 300pF cable & stray capacitance and 47K load resistor it gets worse.

Output now starts to rise at ~10KHz to +2dB at 20KHz and a +10dB peak at 42KHz;

With 200pF cable & stray capacitance and 10K load resistor things look much

better. Output is pretty flat to 20KHz and rolls off to –3dB at ~45KHz;

With 300pF cable & stray capacitance and 10K load resistor things look

better yet. Output is ruler flat to 20KHz and rolls off to –3dB at ~46KHz. In

this scenario, the added capacitance actually helps.

Whether or not your ears will hear the peak (it is ultrasonic & outside the

audible range), it is probably not good anyway. You WILL hear a difference, and

I think one which you will find favorable, at least based on what information

we presently have about your system and what you are now hearing as

"wrong."

If you place a 13K resistor across the phono input plug, it will be in parallel

with the existing 47K which is then 10K. It will let you experiment without

modifying the preamp."

from http://db.audioasylum.com/cgi/m.mpl?forum=vinyl&n=196869&highlight=grado+impedance+47k&r=&session=

[LarryC]

DC: I'm trying to figure out if I have a worthwhile comment. Mdeneen might have something to add. How do you get your RIAA equalization with a microphone preamp?

I'm under the impression that low-impedance carts that specify 47k (MM's, some MC's) are designed to sound best with that. Impedance loading with the preamps I've had is done with a second pair of phono jacks wired in parallel with the ones labeled "phono." (Actually, with that arrangement, you could plug the phono into the "loading" jacks and it wouldn't make any diff.) Anyway, since they are in parallel, the total resistance is lower than the smallest resistance. I use a pair of 100 ohm plugs in my loading jacks, so the total resistance is minutely less than 100 ohms.

I, like most LOMC owners, do trial and error to see what SOUNDS best. I myself would never choose based on supposed measurements. No doubt the kind of tweaking you're doing could sound better, but I suggest making your ears the final judge.

I have no idea how a low-impedance MM fares in all that. Based on not much experience with a variety of cartridges, I'm an LOMC guy all the way. It would be interesting to know how the low-output Grado compares. Is capacitance more important with that kind of cart than with LOMC's?

Sorry I can't be of more help.

[doctorcilantro]

Thanks Larry. I do want to try som LOMC if it's even theoretically possibly they will sound good. Luckily, I do have a group of audiophiles in Tampa (Tampa BayListening Society) I could ping for some carts to try.

The way RIAA is implmented is in real-time with n EQ implemeneted in a VST plugin named IIEQ:

IIEQ is a 10 band parametric track EQ especially designed for an optimal balance of CPU usage and sonic control.

Main features:

• Minimum phase IIR EQ.
• 
  Low shelf with gain and and cutoff frequency, high shelf with gain and
  cutoff frequency plus 8 peaking filters with gain, center frequencies
  and Q. The gain can be ± 20 dB.
• Overall output level control with VU meter to avoid overshoots.

The person who implemented it did the math and compared it to the results Robert Orban is getting with PureVinyl. I will post the specs but I have an improved version which I run at 96kHz. The 44kHz version pretty good: .05db from 20hz-18kHz, however it has -.2db to -1.3db from 18-20kHz.

DC

[LarryC]

OK, I can only suggest giving an MC a try. Some sound better than others, though -- I, Gary, Travis, Nu2, MarkD and others I'm probably neglecting might be happy to offer comments on the cart(s) you can try.

I dunno about trying to get an RIAA curve out of a 10-band equalizer. I'd expect it to sound a little ragged to me. Maybe your local audio society can scare up a phono stage with enough gain to loan you at the same time, as it might sound smoother.

I have never heard a LOMM. I go by intuition a lot (as you can tell), and have always imagined having the coils moved directly by the stylus replicates a more direct signal than picking it up indirectly from a moving magnetic flux. But that may not be true, of course.

However, I DO know that high-output, or even "medium output" (like 1.0 or 1.2 mv) MCs can start sounding opaque to my ear. That's why I stick with LOMC's.

I did like a MM Ortofon I heard, an OM-40, I think. Wasn't that what you had, Marty?

[doctorcilantro]

I think the MAX implementation may be better but I have used the VST plugin.

I need to get in touch with the designer and ask him directly about the VST vs. MAX.

Here's some info on the MAX version:

"As seen in picture, I have multiplied the gain coefficients by 10. This gains the filter output signal over +30dB. Harmonic distortion stays still under -100dB but, if you feel it's gained too much then, just change those three first values on each RIAA filter. By dividing values by 10 you'll get the original level (~0dB @ 1kHz (as seen in the background image of tis filter software)). Do not touch into those two last filter coefficients !!

Quote:

RIAA Filter software specs:

44.1kHz

Maximum error from 0.00 Hz to 20000.00 Hz = ~0.224dB

Maximum phase error from 0.00 Hz to 20000.00 Hz = ~+/- 30 degrees

where the "phase error" is computed after a constant delay is added or subtracted to make the phase error equiripple

48kHz

Max error from 0.00 Hz to 20000.00 Hz = ~0.14dB

Max phase error from 0.00 Hz to 20000.00 Hz = ~+/- 24 degrees

88.2kHz

Maximum error from 0.00 Hz to 20000.00 Hz = ~0.0082dB

Maximum phase error from 0.00 Hz to 20000.00 Hz = ~+/- 3 degrees

96kHz

Maximum error from 0.00 Hz to 20000.00 Hz = ~0.0057dB

Maximum phase error from 0.00 Hz to 20000.00 Hz = ~+/- 2.4 degrees"

http://img135.imageshack.us/img135/9095/riaafilterscemazi2.jpg

DC

[thebes]

You are right Larry, that was an Ortofon OM 40. You've also heard a variation on it with the OM 30 sytlus installed. The OM 30 is probably the better all -around cart, but the OM 40 (it's really just a needle change) is more refined, better overall soundstage and more detail.

From my own recent and ongoing experience, dialing in a MC is hard work and hopefully when I'm done, my ZU/Denon103 will beat out the Ortofon. So far the Zu is losing.

[doctorcilantro]

Good news.

There are two (one unpublished) VST plug-ins for digital RIAA and the math is excellent.

http://www.kvraudio.com/get/3550.html

RIAA Filter software specs (of unpublished - but he says the math of the one above is comparable ):



44.1kHz

Maximum error from 0.00 Hz to 20000.00 Hz = ~0.224dB

Maximum phase error from 0.00 Hz to 20000.00 Hz = ~+/- 30 degrees



where the "phase error" is computed after a constant delay is added or subtracted to make the phase error equiripple



48kHz

Max error from 0.00 Hz to 20000.00 Hz = ~0.14dB

Max phase error from 0.00 Hz to 20000.00 Hz = ~+/- 24 degrees



88.2kHz

Maximum error from 0.00 Hz to 20000.00 Hz = ~0.0082dB

Maximum phase error from 0.00 Hz to 20000.00 Hz = ~+/- 3 degrees



96kHz

Maximum error from 0.00 Hz to 20000.00 Hz = ~0.0057dB

Maximum phase error from 0.00 Hz to 20000.00 Hz = ~+/- 2.4 degrees