surface noise and cartridge/ phono pre questions

Although I fully agree with Ralph's (and Doug's) observations that the phono stage has a significant impact on the perception of surface noise, I do not agree that the use of global NFB has any bearing on the outcome.

FWIW, I have used both Ralph's MP-1 phono preamp (zero NFB) and my own Connoisseur phono stage (uses global NFB, the amount of which has been dialed in by ear) in the same audio system, and I did not feel that there was any advantage to Ralph's design when it came to the reducing the sonic impact of surface noise.

I will acknowledge that Ralph's preamp sounded quite pleasing to the ear. Nice work!

My own findings are that the surface blemishes on an LP that cause the perception of surface noise exist in a frequency range that extends up to 150kHz~300kHz. Since these imperfections are not part of the intended manufacturing process of the LP, they are not subject to any amplitude limit, and therefore can be quite large in amplitude.

If the amplifier circuit was not designed to handle fast, high-amplitude impulses without clipping, distorting or ringing, the circuit will most likely require some time to settle down after it has been hit with a big, wide-band transient, and the longer the circuit settling time, the more likely it is that the ear will hear it.

I find that phono circuits that are tolerant of RF noise and don't change in sound much when the input load resistor is changed are usually good about keeping surface noise low.

In every case, what is needed is an amplifier circuit that can handle fast, high-amplitude signal energy without clipping, distorting or ringing (although the frequency bands for surface noise are different from those for RF energy or loading resonances). If your phono stage can cleanly amplify a 1+MHz square wave of decent amplitude, chances are that it won't have problems with RF, will be fairly insensitive to sonic changes with input loading resistances, and will also suppress LP surface noise rather well.

The performance of the circuit is far more of a concern than what technology it uses.

hth, jonathan carr

PS. It is not difficult to design an NFB circuit that continues to amplify linearly out to 100MHz and beyond.

Ralph: The last time I could do a one-on-one comparison was more like 5 years back. And since then, I have also learned how to get the perceived s/n ratio down by a clearly perceivable margin, through more work on power supplies, parasitics optimization, circuit stability, more comprehensive analyses and so on (smile).

>Time to hear one again :)

Probably true for both of us (smile).

>I have found that universally the use of feedback will cause the circuit to take on a hardness or brightness that is not part of the original signal.

My findings are different. My findings are that there is an appropriate amount of global NFB for a given circuit, and this amount of NFB is best dialed-in by listening. FWIW, I think that 0dB generally sounds OK, but increasing the NFB up to about 20dB does not sound so OK. With more NFB than 20dB, things start sounding OK again, but there will be a point above at which the sound starts worsening again. Your ears will tell you where that point is.

The general trend that I have found is that the more intrinsically linear and stable the circuit is, the more global NFB can applied without damaging the sound. Also, the parasitics arising from the physical construction of the circuit are just as important as the topology.

I should add that I do use non-NFB topologies for certain sections or functions in my designs, and these sections have a clear effect on the sound. It's not like I am anti-zero NFB (grin).

Rather, I don't have any particular feelings regarding NFB, either for or against. I consider it as just another tool. If global NFB makes sense in the context of the topology and it gives better results, I use it. If it doesn't, I don't.

I agree with you that it is easy to make a global NFB circuit that sounds hard, has an unnatural "sheen", polite dynamics, compressed front-to-back depth and so on.

But it doesn't need to be so.

Thank you for a polite, sensible discussion.

kind regards, jonathan

Ralph: I agree with everything that you have written above.

All distortion is not equal to the ears, and higher odd-order distortion sounds increasingly obnoxious.

Also, the character of the noise floor has a significant bearing on far into it the ear can hear. Completely random "soft" white noise, and the ear can hear deep into the noise floor. 20dB sounds about right, and correlates to what I have been told by Keith Johnson. But if there are harmonics, supply or signal-related rectification artifacts along for the ride, the ear's ability to hear into the noise floor stops much earlier.

If the designer doesn't pay attention to these areas, low-level detail is impaired while IMD or inharmonics typically creep into the noise and distortion products, and the result is a circuit that invariably does not sound natural or "believable".

Excellent post! And I hope that you have a good show, too.

kind regards, jonathan