It looks to me as though it sees a short, as spec'd. Either that or tell me how to modify the above circuit.
Best phono stage?
- Thread starter awsmone
- Start date
This circuit isn't right. To correct it, R2 must arrive at the + input and R1 is in fact the cartridge itself.In that case, the cart sees nearly a dead short across frequency. Well under 1 ohm at 20k, and maybe 5e-4 at 20 Hz. Note I chose this low noise opamp with 20k input Z, as you can see, but it could be anything.
As far as the 1000 ohm input, what I was getting at is that as far as the cartridge is concerned, its not driving 0 ohms, its driving something much higher as you can see from looking at the circuit. The fact that it is part of the input connection known as virtual ground doesn't mean its driving zero ohms or ground! That is why I included the Wiki article as this fundamental concept has to be understood.
I know its confusing. Actually the load that the cartridge sees is probably a lot higher. The feedback resistor does reduce that somewhat, and then the input of the opamp itself is in parallel with that, but most opamps can have a fairly high input impedance, so I think we can ignore that. The actual load will be a function of the gain of the opamp circuit, which will have a lot to do with the feedback value and the impedance of the cartridge itself. Again, the gain of the opamp is essentially the ratio of the input resistor (in this case, the cartridge) vs the feedback value. Since you do need some gain (and a gain of 100 is going to be way to low, but calculating with the benefit of the doubt in mind) if you have a 10 ohm cartridge the feedback value is 1000. If anything, that is the value that is loading the cartridge since the output impedance of the opamp is going to be pretty low. But a gain of 100 is impractical, so its likely the feedback resistor is going to be a bit higher. The higher it is, the higher the actual impedance the cartridge is driving, despite the fact that the intersection of the cartridge and the feedback resistor is virtual ground.
IMHO nothing is confusing here. The current to voltage converter using an ideal operational amplifier and a single feedback resistor has zero impedance input - input impedance is just the ratio between a change in voltage and a change in current at the input of the circuit, not at an imaginary point of a mathematical model . As amplifiers are not ideal the input impedance will not be exactly zero, but always a very low value. If the input has close to zero input impedance than the cartridge should see an almost short. virtual or real. The gain of the circuit will depend on the cartridge impedance because the current injected in the input depends on it - when we model a cartridge as a current source the cartridge current is equal to voltage cartridge divided by cartridge resistance (or we measure it in short condition).
This approach has been used by several audio manufacturers in the past, such as Meitner or recently Channel -d-lino https://www.stereophile.com/content/channel-d-lino-c-20-phono-preamplifier-measurements
Wether we like or not the sound of such loading is just a question of preference.
This approach has been used by several audio manufacturers in the past, such as Meitner or recently Channel -d-lino https://www.stereophile.com/content/channel-d-lino-c-20-phono-preamplifier-measurements
Wether we like or not the sound of such loading is just a question of preference.
@Atmasphere , R1 is effectively a 0 Ohm resistor. It's only there to show the input Z of the opamp in the plot. The Rser (cartridge) is 1 and the feedback resistor is 1000. This maintains the ratio you specified previously and doesn't need modification. Your suggested modification doesn't really make sense in the context of virtual ground which feeds back to the inverting input and grounds the non-inverting input. Ralph, could you draw the exact circuit you're proposing?
Yes, that's the virtual ground. I've never asked what a virtual ground is. Circling back, then, it does seem clear that the impedance seen by the cart is not 1000 ohms, but some impedance roughly approximating a dead short.
A way of testing this would be to short out the output of the cartridge in this circuit and see how it works. If the circuit works normally then you are correct. If it does not work correctly then the virtual ground is not in fact the same as a dead short.
This is just silly. Clearly the current of the cartridge will be flowing according to Kirchhoff's law: the better ground will get the most current. If the ratio is 100mohm : 0.1mohm, then the virtual ground will be getting 0.1% of the current.
What would be much more reasonable is to play a record and using a voltmeter/scope try to observe the voltage output of the cartridge while being plugged into a virtual ground.
What would be much more reasonable is to play a record and using a voltmeter/scope try to observe the voltage output of the cartridge while being plugged into a virtual ground.
Yeah, Kirchoff's law has been ignored in some previous commentary. 
Here are the (same) results with and without a 0.1ohm Ralph shunt resistor.
Here are the (same) results with and without a 0.1ohm Ralph shunt resistor.
If I can just point out something here- as far as I know, so far nothing here has had anything to do with violations of Kirchoff's Law. The second thing is a 0.1 ohm resistor is not a
To simulate that, put a wire in the schematic from the output of the cartridge to ground. Let's see how it works then. In the simulation, am I seeing this correctly, that the cartridge is being depicted as a 1 ohm resistor? To be more accurate, it should be an inductor of 5 ohms impedance or so.
Folks, we have a small competition going:
Who is up for the iPhono3 Black Label #NoiseFloorChallenge?
Have a chance to win a prestigious iFi audio PowerStation for taking part! Find out more and how to enter HERE!
Entries close 31st July!
Who is up for the iPhono3 Black Label #NoiseFloorChallenge?
Have a chance to win a prestigious iFi audio PowerStation for taking part! Find out more and how to enter HERE!
Entries close 31st July!
I received my Accuphase C47.
It's above my old Esoteric E-03, but C47 is very sensible with the turntable.
With VPI Classic 3, the differences between E-03 and C47 are very similar...
With Technics SL-1000R , the differences between E-03 and C47 are enormous...
For the two turntables, I use Audio-Technica ART-9.
It's above my old Esoteric E-03, but C47 is very sensible with the turntable.
With VPI Classic 3, the differences between E-03 and C47 are very similar...
With Technics SL-1000R , the differences between E-03 and C47 are enormous...
For the two turntables, I use Audio-Technica ART-9.
It depends on where you live. In UK that would be most likely our office. If you're interested, please send me a PM.
congrats - I luv my C-37. they are similar, so the C-47 will take about 100 hours to fully open up.
It would be very interesting to compare sound of C-37 to C-47.
Thanks.
I can not compare it with the C37.
I listen it all the day...
It's a very exceptional phono preamplifier. No doubt, better than my Esoteric...
Hi guys - loving this discussion. Just thought I'd drop this in here in case anyone's interested. I'm not offended, if not! Lol. Micheal Johnson of Audiophlia recently reviewed our new iPhono3 Black Label. Check it out here.
Anyone given the iPhono3 BL a spin yet? I'd be eager to hear your thoughts?
Cheers, Vic.
Anyone given the iPhono3 BL a spin yet? I'd be eager to hear your thoughts?
Cheers, Vic.
