surface noise and cartridge/ phono pre questions

Kevvwill, I have to admit I was quite surprised when I heard how dramatic this phenomena can be.

Turns out that it has to do with the propagation delay (the length of time it takes the signal to propagate from input to output, something that all audio circuits have) and how that interacts with feedback.

If there is a small tick at the input, it moves through the circuit with the rest of the signal, and at the output is fed back to the input, out of phase with the original. Problem is that by this time the tick, which is very short duration, has dropped in amplitude (signal level) and the 'fed back' signal may well be most of what remains. That then moves through the circuit again and the process repeats, with each iteration the tick loosing some amplitude and changing its phase. The effect is a ringing phenomena which lengthens the time duration although frequency is not affected.

Now if you have no feedback, the tick moves through the circuit without ringing. What you find out through comparison is that a lot of ticks are so small that they are not really noticed, but can be distorted by this ringing process to become quite audible.

IOW, cleaning is important but is not the whole story.

Ralph,

Thank you offering a better technical description of this phenomena. I described the effect as "ringing" but by explaining the mechanism that produces it you also better described its sound. A feedback loop extends the tick in time by means of linked, phase-shifted repetitions... that's EXACTLY what I've heard.

A tick heard through a zero-feedback preamp like mine or the ones you build sounds like a single pluck of an acoustic instrument with a minimal sound box, a small clavichord for example... plink. The same tick heard through another preamp will sound like a pluck of a heavily amped and reverbed electric guitar... PLONNNKKK!!!

I agree with these other points:
- cleaning is essential and in fact primary, since playing uncleaned records may permanently damage your vinyl
- great tip from Rotarius regarding getting VTA (SRA, actually) and azimuth right; doing this with a fine-line stylus will indeed reduce surface noise more than is possible with an eliptical one

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It's certainly true that some LP surface noises simply cannot be eliminated (without using techniques that also mask the music) but these can be minimized both in number and in obtrusiveness.

As most of you can relate, the more I'm drawn to the vinyl, the less tolerable I've become of the pop's, click's and tick's.

The better you get at playing vinyl, the fewer of them you'll have to tolerate and the more tolerable the remaining ones will become. As you upgrade practices and equipment it gets better, not worse. :)

Have to agree...analogue surface noise comes with the territory...all things being equal...it is more dependent on quality of vinyl than associatted equipment...and if I have a noisey copy of something that infringes on my listening experience I simply buy another, hopefully improved copy...I don't bother with excessive cleaning methods...the effort and cost doesnt justify the means...just my experience...

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.

Jonathon, I'm guessing that you've not heard one of our preamps in probably 15 years! Time to hear one again :)

I agree that its possible to use feedback without surface noise issues, but- its a lot harder to do. In addition, 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. Of course there can be a lot of variables in any design; I am stating this out of working with many circuits over a period of decades- there are always circuits out there that are exceptional.

With regards to feedback I have yet to hear one that really does it right (no excess 'surface' noise, no compression of dynamics, no brightness), although IMO this is a subject for another thread.