Cartridge Loading.....Part II

Why not?  Consider the case to be an ideal current amp with the appropriate series resistance added so the input impedance is 5Ω.   In this gedanken world the ideal voltage amp and the ideal current amp sound identical.

It seems to me that you are still conflating virtual ground and actual ground as the same thing! As I said before this leads to confusion.

So the 'Why not?' is the same as before: Because that 5 Ohms is a virtual 5 Ohms instead of a real 5 Ohms. The cartridge is not loaded at an actual 5 Ohms. 'Virtual Ground', again, is opamp parlance for a point in the circuit that exists at the same potential as ground but isn't actually ground.

I recommend that you read up on opamp operation since this seems to be the hanging point. Here's a short tutorial opamp virtual ground.

If you don't want to do that, just keep in mind that 'virtual ground' isn't the same as actual ground. So the cartridge would not be loaded at an actual 5 Ohms even though the virtual ground is 5 Ohms.

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

(1) what if the input device is a discrete transistor or a tube, not an op amp?

@lewm 

Then there won’t be a virtual ground. So right away its a voltage amplifier not a current amplifier.

(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?

Opamps have nearly infinite gain when open loop; the feedback resistor and the input resistor thus define the gain of the circuit and the virtual ground is formed at the intersection of the input resistor and the feedback resistor (see my prior posts for more information).

There is no connection between actual ground and virtual ground; the latter is created as a result of the feedback meeting the input signal. So there isn’t (as in the case of 5 Ohms) 5 Ohms between the ground and the virtual ground. In fact the actual impedance is much higher.

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.

The tricky bit is that in a transimpedance input, the cartridge itself is the input resistor. This means that the actual impedance load on the cartridge varies with the impedance of the cartridge itself- and with it, the gain of the circuit. As I pointed out earlier, the lower the impedance of the cartridge the higher the gain of the circuit.

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.

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. 

@lewm I don't know how you could specify the input impedance; it varies with the impedance of the cartridge! Again, the gain of the circuit is defined by the ratio of the feedback resistor vs the input resistor (which is literally the cartridge itself). So if the cartridge is 30 Ohms and the feedback resistor is 300 Ohms, the gain of the circuit would be 10 or 10dB. If the cartridge were only 15 Ohms that would mean the gain of the circuit is 20 (16dB). So the input impedance can only be defined by the fact that a virtual ground is present. But you have another problem, which is that with almost any opamp made you run out of Gain Bandwidth Product over about 20dB of gain or thereabouts.

This means that with cartridges that have a very low impedance the circuit may lose neutrality. Personally I would prefer to have the gain be a set thing so that the cartridge would not be able to affect the phono section in that manner.