Cartridge Loading.....Part II

Take a 30Ω cartridge and hook it up to an ideal voltage amp and then another one and hook it up to an ideal current amp.  Now take two AC microamp clamps and monitor the current output of each cartridge.  Will the currents be the same for each cartridge?

@intactaudio I doubt it but that depends on the actual impedance that the cartridge is driving in either case.

@atmasphere @intactaudio 

I think it depends on the individual design.

I know from first hand knowledge that the BMC & ESE Nibiru react differently with different cartridges, but on the other hand I have a bespoke current sensing device built by a cartridge manufacturer that is superb across all my cartridges ranging from 3-40ohms internal impedance.

Also I recall the current sensing Goldmund PH2 sounding superb across a variety of cartridges, regardless of internal impedance of the MC.

@dover  This needs to be distilled to the most basic level to get a solid foundation that everyone can build upon equally.  It is not about any particular design and more about cartridge loading as a whole.  My belief is that the nature of current amplification vs. voltage amplification (namely how they load a cartridge with low vs high impedance) has a dramatic effect on the mechanical behavior of the cartridge and may be partially responsible for the differences heard between the two types.

I am not suggesting that all current amps are the same but if we can separate them conceptually from their voltage amp brethren then a discussion about how each loads a cartridge can ensue.  

dave

My belief is that the nature of current amplification vs. voltage amplification (namely how they load a cartridge with low vs high impedance) has a dramatic effect on the mechanical behavior of the cartridge and may be partially responsible for the differences heard between the two types.

 It also has a lot to do with distortion and RIAA EQ differences. Separating those out might be a bit difficult.

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?

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?

I am not suggesting that if the load is a fraction of the cartridge impedance the stage must be a current amplifier.... I am suggest  as a whole current amplification stages will typically load a cartridge with less than its internal impedance.  

A good example of this is the situation I outlined above where I used an 8Ω cartridge through a 1:8 SUT and then directly compared it to a 1:40 with a 2.2Ω resistor across the cartridge directly.  The 2.2Ω resistor was chosen so that the output of the SUT in both cases was identical as was the frequency response from 10Hz to 100kHz.  (±1dB).  In this case even though the cartridge load was 1/4 the internal impedance when using the 1:40,  I would still consider it a voltage gain stage. 

The result of this experiment was interesting.  Even thought the gains of both situations were the same, the 4.7kΩ load through the 1:8 sounded a good 2dB louder.  When the gain of the 2.2Ω load was bumped 2dB suddenly it was preferred and then going back to the 4.7kΩ 1:8 at the same +2dB level started to hurt my ears. 

dave