Cartridge Loading.....Part II

Using the ZYX Universe, which has an output of 0.24mV and an internal R of 4 ohms, the gain is as much as you’d want, at the 0db setting. So, if you estimate the current output of the ZYX as 0.24mV/4 ohms or ~60uA (micro-amps), it would seem that any LOMC that makes significantly more current might be pushing too hard. But I also notice there is a certain unpredictability or non-linearity (in the sense of overall SPLs per estimated current) to what really happens. Anyway, there is no problem mating the MCCI with any typical LOMC. (If the current is much less than 60uA or if the voltage output of the unit is not resulting in satisfying drive, then of course one can add +7, +11, or +14db by activating gain circuits under the hood.) I found that the ART7, with 0.12mV output and an internal R of 12 ohms (which calculates to much less current at standard stylus velocity), also can drive the MCCI at the 0db setting very handily, but I think it sounds a bit better at the +7db of gain setting. This is all well and good. The question in my mind remains, is the result really "special" or just very good at the level of any other very good voltage gain phono stage. Or as Cole Porter wrote, "Is it the good turtle soup or merely the mock."

Don’t have a formed opinion yet.

I have two complete systems in two different listening areas in my house. The Steelhead runs a system in my basement based on Beveridge 2SW speakers driven by Beveridge direct-drive amplifiers. The Beveridge outboard woofer system is long gone; I use a pair of Transmission Line cabinets employing KEF B139 woofers for frequencies below 80Hz. That system is single-ended all the way, as is the Steelhead. My upstairs system consists of Sound Lab 845PX speakers with modified crossovers driven by Atma-sphere amplifiers. The preamp is either an Atma MP1 or the 3160 Phonolinepreamp that was conceived and designed by Raul and his engineer friend. The entire system is all balanced from cartridge to speakers. The BMC was made for balanced operation, so I felt constrained to test it in the balanced system. To do that, I had to remove my CDP from its shelf and replace it with the BMC. The BMC goes into a balanced line level input on the 3160, right now. I can already tell the BMC is good, but what would make you think necessarily that the Steelhead would be better? Right now, I cannot make a direct comparison, at any rate.

To satisfy my curiosity and because life is short, I purchased a BMC MCCI Signature ULN. This unit has gain settings of 0, +7, +11, and +14db. My question to anyone is what is the meaning of these db values? At the 0db setting, the unit produces quite a bit of phono voltage gain at its output; I would guesstimate about 60db. Plus, db are units expressing the ratios of voltages, having nothing to do with current. Anyone?

Here’s what I think it might mean. The unit may have a certain baseline voltage gain at its output, but that is dependent upon its interaction with the current fed into it by the cartridge, which will vary depending upon the BMC’s own input impedance (I assume it's greater than zero) and the cartridge’s output voltage and internal impedance. Because of this interdependency with the cartridge, the voltage gain at the output cannot be specified by the manufacturer. (It will be different for every cartridge, and I have already verified that.) So the "0db" must be referenced to whatever is the signal voltage output based on its input current. The higher gain settings must invoke downstream gain stages that add to signal voltage output by the indicated db’s. Is that correct?

I repeat, stay away.

Dear Raul, I recently posted a URL from Wiki that contains a figure which I thought is instructional in understanding Lenz' Law.  Some describe Lenz' Law as the electromagnetic equivalent of Newton's Third Law of Motion. My opinion-less post with the URL preceded the one from Wyn that explains Lenz' Law in words rather than pictures. Other than that, I have been sitting on the side lines. I did not and do not take any position, because I am not qualified at the level of the best contributors, but I am here to learn. So I wonder why I am now the target of your invective. What makes you think I have not been reading the posts by Wyn, Ralph, and Dave, the only 3 people on this thread who are qualified to discuss the subject at hand? 

Cut back on the coffee, take a tranquilizer, get some therapy.  Do whatever the f*** you want but leave me alone, please.

Figure from the Wiki page on Lenz Law. I was driven to read about it after Dave mentioned it.

Ralph, in an ideal current driven phono stage that uses an op amp to sense current, is the coil of the cartridge connected to that virtual ground that you describe? One end of course. If that is the case, where do they connect the other end of the coil? Thanks.

if I am understanding both Ralph and Dave correctly, there is a difference between them in the definition of a current driven phono stage, in that Ralph says it must be an op amp that does the current to voltage conversion, and which can provide a zero ohm virtual ground. Whereas, Dave said that any device with an input impedance much lower than that of the cartridge internal resistance can act as a current driven stage. Is that correct?

Too late to edit my post, but I meant to say, "It connects the cartridge to the emitters of an array of discrete transistors."  Not "It connects the cartridge output to an the emitters of array of discrete transistors."

Sounds like driving a triode via its cathode, but I won't dare go there.

Ralph, I just had a “duh” moment. Sutherland describes the input of his Loco and Little Loco as a “virtual” short circuit. I had heretofore interpreted that word in its more literary sense; virtual meaning “nearly “or “close to”. Thanks to your input, I now see he means it electronically (virtual). I also see that nearly all current drive phono stages do use an op amp input. The BMC MCCI being an exception. It connects the cartridge output to an the emitters of array of discrete transistors.

I haven't kept track of which current driven phono stages on the market use op amps or discrete transistors or tubes in the input I/V stage, although I don't know of any that use a tube or how you could use a tube in that fashion.  However, I would guess that many do use a discrete transistor and so by your definition cannot be current amplifiers.  If they all use op amps, then I stand corrected, and right there I have learned something about them.  However, every one that I have investigated was said to have some finite input Z, greater than zero and notwithstanding the cartridge with which it might be matched. (Especially because most of the advertising suggests you can match them with any cartridge having a "low" internal resistance, low being better than higher.)  But I do get that to be a pure current amplifier, the input impedance ought to be zero. Sticking my neck out in saying this, but I would rather gain understanding than worry about negative feedback, no pun intended.

Ralph, you wrote, "The '0 Ohms' value you see in so many phono sections that have transimpedance inputs probably isn't helping people to understand what is going on. That value is probably the marketing department talking since they probably didn't understand what a virtual ground is."

I have been complaining about that on this forum over and over again.  Only be direct questioning can one find out what the input impedance of these "current driven" phono stages actually is, and it's typically from a few ohms in the best case to as high as 20 ohms.  Even the term "transimpedance" is a marketing ploy.  Almost as bad as "quantum" used to describe an interconnect or a fuse.

Your other responses are helpful but raise other questions in my mind that I will hold in reserve for now. Thanks for your patience.

Ralph, two questions:

(1) what if the input device is a discrete transistor or a tube, not an op amp? 
(2) the only way I can imagine two points separated by 5 ohms but at the same potential is if and when there is no current flowing. How does that work in this case?

Thx

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.