300b lovers

@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.

A quick note on the complex variable equalization seen on the Citation I: people who had extensive 78 and pre-1955 LP collections had records with wildly varying equalization curves. By the time of the stereo LP in 1958, the world had settled on the RIAA system, and all stereo LP’s use RIAA equalization (to the best of my knowledge).

Some early (mono) LP’s had the EQ marked on the label or the record jacket, or the record company was known for having a house curve, but 78’s were notorious for being all over the place, with no industry standard at all. Even the speeds vary, with 78 rpm merely being the industry average.

So the super complex switching on the Citation is mostly aimed at the record collector with a lot of mono records, both 78’s and LP’s. Some folks even kept their old mono preamp, just to play their old records with varying equalization. For that matter, tone controls were very important in the Fifties, and were still important in the Sixties.

Few speakers were flat, and record companies usually had a house sound, or actually several house sounds, depending on the musical genre. Elvis on a 45 single was not going to be treated like an RCA Red Seal classical record, and the early Beatles sound was very different on the US Capitol release than the Parlophone release in the UK. Tone controls were standard on all hifi equipment back then.

Just to boggle your mind a bit more, cutting a lacquer master from the two-track master tape meant the engineer "riding the controls" as the cutterhead neared the center of the record. Different engineers would have their own interpretation of what the master should sound like ... so yes, there can be many "master records", not just one.

P.S. If I'm reading that schematic right, the Center Channel output, although correctly summed from Left and Right, is actually out of phase with both of them. (The 12AT7 inverts phase.)

Hi @lynn_olson

I use EAR834p schematics in my current DIY phonostage.

The second ecc83 miller capacity for RIAA equalization.

It is also using small capacitors in the RIAA equalizer. So I use air capacitors in this RIAA.

I also use the LCLC power supply filter plus 2x 0A2 voltage regulators.

In general I like the sound of this phonostage.

The main drawback of this schematics is output cathode follower that drives feedback and output RCA cable. When I tried different ECC83 tubes in this position - speed, dynamics and bass were changed dramatically.

If I reuse this schematics for the new project I am thinking about adding an output buffer. It can be 6SN7 with the output transformer.

https://www.diyaudio.com/community/threads/ear834p-is-not-what-it-seems.313042/

I did something more similar to this (just without a fixed bias for the second ECC83):

http://www.romythecat.com/Forums/ShowPost.aspx?PageIndex=2&postID=9161#9161

Still out in the weeds dept.;

Many phono preamp designers over the last 70 years have ignored the significance of a magnetic cartridge whose output is in parallel with the capacitance of the tonearm cable.

Any time an inductor is in parallel with a capacitance there is an associated resonance whose frequency is set by the values of the cap and the inductor. With high output MM cartridges this peak is generally about 20dB and centered just inside or just above the audio band. If the phono section does not have good HF overload characteristics, this peak will overload the input of the phono section resulting in ticks and pops.

Just to boggle your mind a bit more, cutting a lacquer master from the two-track master tape meant the engineer "riding the controls" as the cutterhead neared the center of the record.

We never had to do this with any of the LPs we cut. We used the Westerex 3D cutter with 1700 series electronics, all stock. We found that the old myth about loss of bandwidth towards the end of the LP was just that. We cut 30KHz tones in the inner grooves that played back fine on our Technics SL1200 with Grado Gold (we used that setup to know if the groove we'd cut was playable on an average machine). I think a lot of that myth got started in the 1960s when stereo cartridges really just weren't all that good.