IEC vs RIAA

I’ve just been playing around with pre-amp settings for a new Concorde Century cartridge.

Switching to IEC instead of RIAA curve seems to give it a slightly more natural sound. It’s hard to explain, but perhaps the upper mids and highs are easier on the ears and overall it’s a bit clearer and more natural sounding.

So from a quick Google, apparently the IEC filter cuts very low frequencies.

Assuming this is not just my imagination, could the fact that cutting the extreme lows improves the sound quality, point to some kind of setup problem, like a tonearm resonance or some other vibration issue?

 

IEC adds a small amount of very low frequency reduction to help deal with flapping from warps and a number of us would say as often there is little nor no information below 20 to 30 hz all you achieve by not is causing the amplifier to work harder than it needs to.

I regarding using IEC as a plus in the real world.

 

A subsonic filter is always a good idea. IEC just codifies that 20Hz slow-rolloff cut. A filter even steeper, thereby cutting even more sub-bass while having less affect on music, may be desirable.

Low frequency rumble and resonance has more audible effects than just wasting your amplifier power fluttering your subwoofer..

A thorough analysis, showing how the resonance creates sidebands in higher frequency tones from modulation, and surprisingly recommending a higher resonance frequency, is this paper presented at AES 1977 "Audible Effects of Mechanical Resonances". http://www.laudioexperience.fr/wp-c...-Resonances-in-Turntables-AN17-233-1977-1.pdf

 

Very interesting. Never thought to use it but now it's time to experiment.

 

Some definitions:
RIAA equalization - Wikipedia

IEC RIAA curve[edit]
In 1976, an alternative version of the replay curve (but not the recording curve) was proposed by the International Electrotechnical Commission, differing from the RIAA replay curve only in the addition of a pole at 7950 μs (approximately 20 Hz).[13] The justification was to reduce the subsonic output of the phono amplifier caused by disk warp and turntable rumble.

This so-called IEC amendment to the RIAA curve is not universally seen as desirable, as it introduces considerable amplitude and—of more concern—phase errors into the low-frequency response during playback. The simple first-order roll-off also provides only very mild reduction of rumble,[11] and many manufacturers consider that turntables, arm, and cartridge combinations should be of sufficient quality for problems not to arise.

Some manufacturers follow the IEC standard, others do not, while the remainder make this IEC-RIAA option user selectable. It remains subject to debate some 35 years later.[2] This IEC Amendment was withdrawn in June 2009, though.

Discussion:
Cut and Thrust: RIAA LP Equalization

 

That’s a pretty clear explanation. You would think that the intermodulation distortion, being vibration induced, happens before the signal gets from the turntable to the pre-amp. So I don’t quite understand how the filter reduces its effect.

Reading that paper you wonder if it’s worth attaching the auxiliary counterweight, and rebalancing the arm, to see what difference that makes to the rumble frequency and in turn the sound quality.

This hobby is an absolute delight.

 

That is right - you cannot mask the intermodulation distortion producing signal sidebands separated by the warp/ripple frequency using a low cut filter. The only way of dealing with that is by using a damper. SME, from the V15IV right through until they ceased manufacture of cartridges, had a damper right at the cartridge - the carbon fiber brush at the front was mounted on a viscous damped hinge. The other (or complementary) technique is damping at or near the arm pivot. SME do that in some of their arms even now with a rod moving in very thick silicone oil. Alternatively, some rather expensive arms use eddy current damping at the arm pivot.

 

Interesting. So any improvement in sound must come from not sending subsonic information to the power amp.

I’m beginning to understand how people end up spending a lot on a tonearm.

 

Could you describe your results of using it? Similar to what the OP said about a more natural sound?

 

My unipivot tonearm has the pivot spike sitting in a cup where you put damping fluid. I stopped using it when I read people saying their similar set ups sounded better without it, but now that the weather is warming up (and the fluid would be less likely to thicken with cold), I'm thinking maybe I should try it again. I don't think I ever noticed any difference ... but what would you suggest based on your experience?

 

Best way to describe it as being a bit more natural and tighter sounding.
Phase issues is something that can also come into play when people compare doing the EQ in the feedback loop as distinct from passively but I cannot say that using IEC has ever been something I felt 'ugh, that's wrong'. Perhaps some people have different experiences?
I found it was a help with recording on cassette where the record amp would attempt to put subsonic frequencies from warps on to tape

 

I have a Musical Fidelity phono preamp with a IEC switch as you do. Any phase issues using that function?

 

The IEC curve introduces another capacitor into the signal path to form a high pass filter, that is why many designers don't follow the IEC extension. If it helps to make it sound better, then use it. If not, then don't.

 

Huh, I have the Musical Fidelity as well. M1 Vinl. What does a phase issue sound like?

 

Well, in the high frequencies out-of-phase usually sounds kind of ... fuzzy and unclear. Think of crisp hits on a cymbal that smear and screw up. An analogy might be your eyes going out of focus.

During the psych era of the '60s lots of bands used phasing effects to make the sounds "dream - like" and hazy. It can work in short bursts for effect, but if it's not deliberate you end up just wondering "what's wrong?" It's a lot more noticeable in the high frequencies in my experience.

I think it can show up as a delay between two signals also. But I'm no expert by any means. Just, as an end user, or listener, I'd describe out-of-phase as out-of-focus.

I pressed the IEC button on the phone pre earlier this morning and I couldn't really notice a difference.

 

The IEC amendment is a first order cut at 20Hz, so is really not a good cut at all. At typical cartridge compliance/arm mass resonance of (say) 10Hz, the attenuation is less than 10dB. If you really want subsonic filtering, you either:

(a) Need a much more serious cut - say 3rd order - to attenuate subsonic information resulting from the above resonance or
(b) A crossfeed filter. These recognise that record companies record anything under 100Hz in mono anyway. So a crossfeed allows every valid signal through, but cancels out any vertical signal from ripples and warps. There are two of these around that I know of. One is the KAB, and the other is the Devyniliser.

There is a shed load of information on this forum about the KAB unit in particular. Some say it adds more electronics to the audio chain and is a bad idea, and others celebrate the lack of speaker drive unit pumping.

I use the Devyniliser, FWIW, and I leave it switched in permanently for records, CD's, NAS files and Tidal. It's presence is impossible to percieve. As in all these things YMMV!

 

In this case, we are talking about the 20Hz 1st-order subsonic filter applied by the additional pole of the IEC EQ spec, which has an included high-pass.

Being so low, and LPs having no music there, there is little effect. It is a slight delay in the arrival of waveforms, and dubious if one could ever hear it. Just your speakers have 10x the group delay.



The phase shift is not anything like the sound of a "phaser" effect as described above, which mixes varying delayed signal with the original, to cause intentional cancellation.

 

I have to say I'm pretty confused here about what effect IEC has, or doesn't have on the resulting sound.

Frankly, I can't say I've been bothered by any rumble with my set up. I don't have but a few records that have some minor to moderate warps...

But being a bit obsessive (and about to engage in a big dubbing to tape project) I would like, maybe in some simple consensus statements, whether or not to deploy IEC on my phono pre is advisable or not. As I posted, I didn't really hear anything different when I used it this morning...

 

One of the reasons that the IEC amendment was introduced was to prevent overload of the phono input stage. Back when it was introduced in 1976, all phono stages were discrete; it was before even half way decent opamps were available. The problem is that the open loop gain of the typical three-transistor phono stage was around 10,000 (80dB), give or take, and a 12V unipolar supply. With moving magnet cartridges you need a gain of around 100 (40dB) times at 1kHz to give an output of 500mV from a 5mV input. However the RIAA characteristic calls for a 20dB boost at low frequency, which only leaves an overload margin of 10 (20dB). So any warps or ripples can push the phono stage toward, or into, overload. In addition the typical 12V supply also limited overload margin to ~10-15dB

There were innovations to overcome the warp overload problem. The original Cambridge Audio P40 in the late 60's had a flat buffer around which the volume control was wrapped. So it was impossible to overload the phono stage that followed until the power amp clipped. The problem with that approach is that noise was around 10-14dB worse. But it got around warp clipping at a stroke, and the Bailey 1968 design incorporated a second order low frequency filter into the input stage feedback loop. So good designers certainly recognized the issue.

The IEC amendment mitigated warp clipping, and also mitigated cone pumping, for phono stage designs that were less innovative.

However, technology moves on. Phono stages are now designed using opamps with open loop gain of order 130dB and +/-18V rails, or discrete designs that recognize the warp overload issue. Which is why, I suggest, the IEC amendment was dropped in 2009.