Cartridge Loading.....Part II

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 

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?

@lewm 

Ground.

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. 

@intactaudio 

Transformers transform impedance. Further, to prevent ringing they must be properly loaded at their output, to something called 'critical damping' where a squarewave input to the transformer results in minimal overshoot.

If not loaded the transformer can 'ring' with excess harmonics- its making distortion. Your ears will respond to that as sensing it as loudness and yes, it might even hurt if the volume is up a bit.

Because transformers transform impedance its not necessary to load the cartridge directly with a low impedance in order to achieve a low impedance- you can do that on the output side of the transformer as well. IOW putting a lower impedance load on the output will reduce the load impedance the cartridge sees. This is because the only isolation a transformer offers is galvanic and DC; it does not offer impedance isolation. 

This loading issue is one reason I avoid SUTs- you do have to manage their care and feeding. If the SUT is designed for a specific cartridge, the correct load is probably 47K (but will be different if a different cartridge is used). Its really important to keep interconnect cable capacitance to a minimum. But if you simply have enough gain and your phono section has no worries with the RFI that will be generated by the cartridge/cable interaction, then you have no worries- its plug and play. 

I have been following this thread with great interest, and have been learning a significant amount!

Is there any chance the major contributors to this thread will be designing, producing, and selling a new type of phono stage in the near future?  

I certainly hope so!