Cartridge Loading.....Part II

The two statements you quoted from me seem to align well with each other and I was unaware that there was a different versions of ohms law for static vs dynamic loads

At any rate, Moncreif threw off his numbers by using a 5 Ohm load

I agree it is an interesting choice but his measured results clearly show signal and if if anything a lower noise floor so I fail to see where the issue is or how it invalidates his results.

In the case of a transimpedance input, feedback is applied to the output of the cartridge. Its a fair amount of feedback too- the more output the cartridge has the more feedback. That is quite a bit different than a simple resistor- you can’t equate a virtual ground with a static resistance- for one thing, you need an opamp to even create a virtual ground.

Ohm’s Law certainly is in play (how could it not be), but the issue here is that while a cartridge has its output at a virtual ground, that is significantly different from having the cartridge drive 0 Ohms (IOW, if it were actually tied to ground). In the case of the former, the actual input impedance is dynamic and isn’t actually 0 Ohms, so amplification can occur because a signal is present. In the case of the latter, the load is actually a short and because the signal is shorted out, no amplification can occur.

’Virtual’ means ’almost or nearly as described, but not completely or according to strict definition.’ If the 0 Ohms of a virtual ground is conflated with the 0 Ohms of actual ground, confusion is the direct result.

Real ground is when a terminal is connected physically to the ground or earth, whereas virtual ground is a concept used in opamps in which a node is assumed to have the potential that of the ground terminal.

 

I say Peter threw off his numbers because he chose a value that no-one would ever use (and a static value at that, no opamp involved, so the load was causing the output of the cartridge to change in a significant way, whereas real-world loads have negligible effect) since its a reasonable expectation that if you are going to use a load on a cartridge, you’d likely start with one that is 10X the source impedance of the cartridge.

I can see using static values lower than that, but not one that’s actually lower than the the source impedance! Barring a good explanation for that, when I read that in his article I found I simply had to take his results in abeyance. I’d have to read the article again (Google defeated my attempts to locate it just now), but IIRC he had some variables left uncontrolled that I felt at the time might affect his results.

One way around this is to ignore that transimpedance inputs exist and simply focus on what happens to the output of any source when the load is a fraction of that of the source. That’s really what I’m getting at here, if Peter was actually suggesting that we use such loads both back then and now the extra gain needed would be impractical.

Dear friends: Looking to the PM charts that @intactaudio shared with us yesterday I took in count that ( for my self. ) I can measure it too due ( as I posted several times in this forum. ) that I own around 20+ different vintage comercial test LPs from: CBS, Shure, JVC, Stereo Review, HiFI, Vanguard, B&K, Ortofon, Audio Technica, etc. etc.

Through them I have 4 tracks IMD dedicated and rigth now I'm talking with some of my audio friends looking for the ritgh phono stage that permits in easy way changes in the cartridge loading because in my unit I can do it unsoldering/soldering resistors . As I said I will do for my self satisfaction and with withness of the gentleman that will share his phono stage for 3-4 hours any day in the afternoon.

So " stupid " I'm that having a solution I did not see way before.

R.

Dear friends: Looking to almost all my vintage LP test recordings I found out that I have some even sealed as one from Micro-Acoustics and the Ortofon serial 003 that yesterday opened and listen to it.

It's not a test tones and the like but the lovely tracks are MUSIC, this LP is an Ortofon collaboration with the label Opus 3 that in the past ( too ) made 3-4 test LPs.

Well, first than all this Ortofon LP is really good centered with no waves you can detect and even that's a vintage recording not only is a great recording but the LP recorded surface is dead silent, even the band that separates the tracks is dead silent.

If you can find out buy it, it's a good investment for you and your room/system set up.

R.

On the subject of phono input impedance as it relates to the discussion between Atma-sphere and Intact Audio, one should also keep in mind that every "current drive" phono stage seems to be a bit different from every other "current drive" phono stage, at least on the subject of input impedance.  Most advertised "current drive" phono stages do have a finite input impedance greater than zero, whether that is virtual zero or whatever.  A couple of years ago, I looked at 4-5 different products and the range of measurable input Z was from a few ohms up to as much as 20 ohms.  A unit with a real 2-ohm input Z will probably react differently to a LOMC with an internal resistance of from 2 to 10 ohms than a unit with a 20-ohm input Z, current, voltage or whatever.  The water gets really muddy.  They want to dumb it down for us consumers.

The topic of this thread is cartridge loading and it only makes sense to look at it from the two terminal perspective of the cartridge without concern of the "technology" used to provide that load.   There are two basic extremes of operation of a cartridge.  It can operate as a current generator where the load value is << the cartridge internal impedance or it can operate as a voltage generator where the load value is >> than the cartridge internal impedance.  There is also a fairly grey area inbetween these two extremes where the load value is ≈ the cartridge impedance.  For this basic discussion I think the two extremes need to conceptually be looked at from the ideal with respect to how the cartridge converts a mechanical movement into an electrical signal.  Since it is the source to load relationship that dictates whether a cartridge generates current or voltage it becomes important to determine if changing the load causes any mechanical or electrical change to the behavior of a cartridge.  

When operating as a voltage generator the load can easily be modified over a fairly wide range and still maintain the basic principles of operation.  I think most will agree that loads  of 10X the cartridge impedance and up have a cartridge operating squarely in the voltage realm and people will start to cry foul as your load approaches  the cartridge internal Z.  This doesn't have anything to do with actual cartridge behavior and everything to do with the type of amplification that follows.  For voltage amplification the unique case where Rsource=Rload nets a 6dB voltage loss and in the case of microvolt level signals that is huge.  Going to the case where Rload is 1/4 that of Rsource the voltage loss will be 18dB which immediately disqualifies that as an option for many.  The problem with that categoric disqualification is that you are trying to make a cartridge operate as a source of current into a voltage amplifier. The problem has nothing to do with the actual load and everything to do with using the wrong tool for the job.  If the goal is to actually load the cartridge with 1/4 the internal impedance then that load should simply be provided by a current amplifier.  If we want to discuss the effects of loading on the behavior of the cartridge we have to assume that the appropriate type of amplification is used.

The first question that needs to be addressed is in a perfect world with ideal amplification, will a 40Ω cartridge sound the same into a 5Ω load as a 47kΩ load?