Pre-amplifer for magnetic phono pick-up.

[Murphyv310]

Yes.........  But only for an AC input or if the feedback resistor was DC coupled back the the valves grid.
On the positive half cycle of the input the anode will correspondingly have a negative half cycle and during that time the anode volts will drop and vice versa...... standard valve theory and behaviour.
If Rx (feedback resistor is DC coupled) then any rise in grid volts the Anode will drop this will be reflected back to the grid pulling it down and then the anode will rise. With the right values gain could approach zero if I'm not mistaken and we would have the perfect virtual earth circuit?
In Kalee's amp the fact he has put an RC network into the circuit will only effect the AC conditions of the amp giving different amounts of feedback at certain frequencies for the purposes of RIAA Equalisation, so in effect how can you call this a true virtual earth circuit.
 
Mark I understand the theory well enough but what I don't get is how this circuit can be called virtual earth?

[Mike Watterson]

Surely to get a "current" amplifier we need to be thinking of grounded Grid or Base in a transistor amp?

No, those are voltage amplifiers, they have no current gain at all as cathode current = anode current, and approximately emitter current = collector current. The gain is thus approximately RLoad / Rin as long as that is lower then the device gain.
Common emitter is a current amplifier and voltage amplifier, the input is almost a fixed DC voltage.
Emitter or Cathode followers are "current amplifiers" with voltage gain = 1

However a current amplifier usually implies the input is connected differently. There are also "Norton" op-amps, such as LM3900 that need input resistors to convert voltages to currents.

[Murphyv310]

Hi.
So looking at Kalee's circuit again C4,C5, R8 & R9 form the basis of the "Virtual earth". I'll be honest I've never even thought of it in that way, perhaps why I found it odd.
Oh well you live and learn

[pwdrive]

This might also help:

http://www.learnabout-electronics.org/Do...ckTone.pdf

Lawrence.

[Mark Hennessy]

In light of your most recent reply, I've chopped away most of what I typed. However, this is worth leaving:

In Kalee's amp the fact he has put an RC network into the circuit will only effect the AC conditions of the amp giving different amounts of feedback at certain frequencies for the purposes of RIAA Equalisation, so in effect how can you call this a true virtual earth circuit.

Well, Kalee's feedback network isn't all that different to a single resistor. I think of it as a single resistor that has a different value at different frequencies

Perhaps this shows that you are thinking in terms of DC when considering a virtual earth circuit. And that's almost certainly because op-amp circuits always work down to DC. But here, we have to consider that the DC conditions are quite separate to the AC signal conditions.

So yes, at DC, it is not a virtual earth. But at AC, it is.

[Mike Watterson]

Though usually in HiFi applications we don't use op-amps to DC, really only in instrumentation like thermocouples etc. Or maybe Video.

[Mark Hennessy]

That's a bit of a generalisation.

Plus, basic op-amp explanations always assume working to DC so the reader doesn't have to differentiate between AC and DC. In my experience of teaching electronics, that subject (AC+DC) is one of the hardest things to come to terms with. You might not think it because it's second-nature to you, but trust me...

[Mike Watterson]

No, I believe you!

[Murphyv310]

Thanks Chaps.
Just didn't look at it this way before. All falls into place now

[Kalee20]

Wow... Yes I'd certainly meant 'virtual earth' at the second stage grid, from a signal point of view, blatantly not true at DC! But assuming there's a high (AC) voltage gain in the second stage, it's a reasonable approximation. After all, even with a modern op-amp, the gain is still finite...

DC coupled would be nice, down side is the relative incompatibility of anode and grid voltages, unless some considerable level shifting is done (which usually involves either negative supply voltages, or rather high positive ones). We're lost without PNP valves...

Of course, for a phono preamp, the response doesn't need to extend to DC, in fact the RIAA curve has a LF turnover frequency of 20Hz if I remember correctly - hence the 0.01μF / 1MΩ right on the input.