300b lovers

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.

Before getting lost in the weeds on balanced vs single-ended phono preamp design, it might be useful to review the general types and look at their advantages and disadvantages.

1) The most common is a high-gain stage with RIAA frequency-shaping feedback wrapped around it. This dates back to an early Fifties RCA application book. Today, it’s what you get when you buy a $200 solid-state preamp ... a modern high-gain opamp with a feedback loop wrapped around it. It has the merit of low cost and simplicity, and if done in the Fifties style seen in many preamps, a traditional sound many like.

The drawback is using a low-current device like a 12AX7, which typically runs at 0.5 mA current, which does not have enough current to drive a moderately long cable and the reactance of the feedback loop at the same time. This leads to the preamp creating slew distortion with record pops and mistracking, which exaggerates their audibility.

2) A new/old approach is splitting the RIAA equalization in two, using it as a passive filter between the first tube (for the first filter) and second tube, and a second passive filter between the second tube and the third tube. The RIAA filter is usually split in two to avoid overload and noise problems that build up with a single passive RIAAA filter with a 40 dB attenuation loss between tube sections.

This passive-filter approach requires a judicious balance between noise buildup (mostly a problem in the first section) and overload, which can easily happen if the stylus starts mistracking (which is much more common than you might expect).

3) One of the more offbeat new/old approaches is a passive LCR filter between sections, using well-shielded inductors as part of the RIAA network. This is usually a pretty exotic part, and the first stage needs enough linear current to drive the highly reactive LCR network. I have heard this type of preamp and was startled by its naturalism and lack of phono preamp coloration. But they are exotic and difficult to design.

I should add that phono cartridges are often blamed for phono preamp coloration, which mimics mistracking and common types of cartridge coloration. Most phono preamps, whether solid-state or vacuum-tube, are actually quite colored and prone to HF distortion, making many records sound shrill and distorted. The best ones reveal surprisingly quiet record surfaces as well as open and natural high frequencies.

Before doubling the complexity of the phono preamp by using a balanced circuit, it first has to have a noise floor lower than the tape hiss recorded on the LP record, and more seriously, be free of slewing distortion and overload. This is subjective, but I hear clear and obvious overload on most preamps I hear at hifi shows. The exhibitor may blame the phono cartridge or the record, but a preamp swap will reveal the distortion is actually in the preamp itself, not the cartridge. Although phono cartridges are often flawed, many phono preamps make them sound much worse than they really are.

It may be a crude standard, but above all else, components should never audibly overload on any record, no matter how badly it is mastered. It does no good to have an expensive hifi system that can only play a handful of audiophile-approved discs that have been very carefully mastered. It should be the other way around: the preamp should accept ANY disc without breaking up, distorting, or becoming shrill. That’s much more important than pushing the noise level 3 dB lower than any record ever made.