300b lovers

In my next phonostage I want to make an external power supply with a separate filament transformer. But I haven't decided which RIAA schematics to use. There are basically two types of tube phonostages. In one type RIAA is implemented in feedback and another type is passive RIAA.

@alexberger You might consider that since the cartridge is a balanced source that you could have a balanced phono section too, or at least a balanced input. If you run EQ via feedback, you run into the same problem that Norman Crowhurst wrote about nearly 70 years ago. You could avoid it by applying the feedback to the grid of the tube rather than the cathode (you'll need a series resistor with the input to allow the mixing to occur, similar to an opamp circuit). You'll have to recalculate all the EQ values since feedback to the grid behaves quite differently (higher impedance).

Passive feedback can work quite well. Just because you have passive EQ does not mean that you can't run feedback in the associated gain stages. H/K did this with their Citation 1 back in 1958.

The advantage tubes have over solid state in a phono section has to do with the fact that most cartridges are inductors; when in parallel with the capacitance of the tonearm cable, an electrical resonance is formed. That resonance can overload the input of the phono section causing ticks and pops that sound like they are on the LP surface. If your tube phono section is properly designed (easier because the operating voltages are higher), this won't happen; you may discover that LP surfaces are actually a lot quieter than the digital crowd would have you believe.

I rebuilt a number citation 1 preamps over the years for customers.  I didn't care much for the line stage section with the anode follower, and miles of wire and switches, but the phono section was among the best of the vintage gear.  

Hi @atmasphere ,

If you run EQ via feedback, you run into the same problem that Norman Crowhurst wrote about nearly 70 years ago.

Which problem? Can you explain?

I saw Cintation 1 schematics. It is difficult to understand capacitors values and how do switches exactly work from these schematics. The first pair of ecc83 works for gain only (10000 voltage gain with a feedback) . After that we have a passive RIAA. The second pair of ecc83 has feedback that looks like the second part of RIAA (that is active).

Hi @atmasphere ,

I have balanced connection from the cartridge to SUT. But from SUT to phonostage it is SE connection. In my next phonostage project, I plane to put SUT close to the first tube inside the phonostage.

But I think to make balanced first amplification stage of the phonostage can be very helpful.

@alexberger , @lynn_olson mentioned earlier:

Ralph brings up a very good point about feedback: the underlying theory assumes a distortionless summing point. (The summing point is the comparator input between signal input and the sampled output.) Any distortion introduced at this point of the circuit will be amplified without correction, and there is a real possibility of introducing new, higher-order terms that are not present in the forward path of the physical amplifier. Norman Crowhurst mentions this in passing in his Audio magazine articles in the late Fifties.

Actually I read about this in one of his books. The point is that feedback applied to a cathode is going to generate higher ordered harmonics and IMD because the cathode is non-linear, even on a 12AX7. If you can, the thing to do is apply the feedback to the grid of the tube rather than the cathode. This gets tricky if you have two stages of gain as you see in the schematic above! It might also mean you have to have a feedback capacitor to block DC, which isn’t likely to treat the feedback signal very well. You see this technique being used in the line section of the Citation right after the tone controls.

You can do this in an amplifier too, wrapping the feedback around the entire amp circuit. Admittedly tube circuits are often lacking in the Gain Bandwidth Product to prevent distortion rising with frequency, but if the feedback is handled properly to start with overall its a better chance of it working right.

But I think to make balanced first amplification stage of the phonostage can be very helpful.

SUTs can have a balanced output if you like- they don't care. Transformers are very good at going back and forth between balanced and unbalanced. You will have to be careful about loading the SUT properly to maximize its performance. Why stop with a balanced input- balanced (differential) throughout gets you greater power supply immunity and lower distortion overall, as well as lower noise if the gain stages are properly executed.