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Review and Measurements of u-turn Pluto and ART DJPRE II Phono Preamps

SIY

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#21
The issue with overload against frequency is the RIAA equalisation that's applied on the cutting side and the opposite on the replay side. This theoretically makes the overall result constant velocity, and as RIAA accuracy can be easily within 1dB and typically rather better than that, measuring the overload at 1kHz gives a decent enough measure of what's important.
That is HIGHLY dependent on the RIAA topology in the preamp. One example- consider a preamp with two gain blocks separated by a passive RIAA filter. The LF will likely not overload the first stage, but the HF very well could. Likewise, the LF could possibly overload the second stage, but the HF is unlikely to. In the real world, I've seen major differences in overload margins as a function of frequency, and that's one of things that's guided my designs.
 

sergeauckland

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#22
That is HIGHLY dependent on the RIAA topology in the preamp. One example- consider a preamp with two gain blocks separated by a passive RIAA filter. The LF will likely not overload the first stage, but the HF very well could. Likewise, the LF could possibly overload the second stage, but the HF is unlikely to. In the real world, I've seen major differences in overload margins as a function of frequency, and that's one of things that's guided my designs.
Indeed, which is why, given that RIAA accuracy is trivially easy to achieve, overload margin matters most. I'm not a fan of passive RIAA eq for the reasons you allude to.

A simple op-amp like the 5532 with 'good enough' EQ is as much as is needed for a phono stage. This is one reason why I find high priced phono stages as laughable as high priced DACs. Both my turntables have integral phono amps, and both are as good as they need to be, and both are a simple op-amp design.

S
 

amirm

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#23
So I still don't know if I should or should not use equalization for measuring headroom. If the worry is pops and glitches, those are not pre-equalized so the generator output should be raw (i.e. not having RIAA emphasis), no?
 

sergeauckland

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#24
So I still don't know if I should or should not use equalization for measuring headroom. If the worry is pops and glitches, those are not pre-equalized so the generator output should be raw (i.e. not having RIAA emphasis), no?
Clicks and pops appear on playback, so get RIAA equalised, so overload margin of a phono stage should be measured with the RIAA eq active, as that's how it will be used. However, clicks and pops tend to be impulsive, so with considerable HF content, which is what's rolled off by the RIAA playback eq. That's why clicks and pops, whilst important, aren't as important as the sheer audio level cut onto LPs. If a phono stage has a poor overload margin, then bad pops and clicks will be made to sound worse, but one with a decent margin, the clicks and pops will be what they are, as they're less likely to overload the phono stage.
Eliminating, or at least, reducing clicks and pops is part of vinyl replay, hence vacuum cleaning machines, as their use makes the phono stage's job easier.


S
 

amirm

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#25
Clicks and pops appear on playback, so get RIAA equalised, so overload margin of a phono stage should be measured with the RIAA eq active, as that's how it will be used.
Looks like I was not clear enough. :) RIAA equalization is always active in these pre-amps. It can't be turned off. So we are good there.

What I do to measure them is to have my generator apply the RIAA pre-emphasis. That way the two (hopefully) cancel out and we see the measurements I posted in my original review.

For this headroom test though, I thought it would make no sense to pre-equalize the output of my analyzer as the pops don't go through that pre-emphasis because they are not in the recording.

So we are in sync on playback equalization always being there. Question is whether I shape my generator output to match an LP recording or leave it flat as I did in my headroom graph I posted.
 

sergeauckland

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#26
Looks like I was not clear enough. :) RIAA equalization is always active in these pre-amps. It can't be turned off. So we are good there.

What I do to measure them is to have my generator apply the RIAA pre-emphasis. That way the two (hopefully) cancel out and we see the measurements I posted in my original review.

For this headroom test though, I thought it would make no sense to pre-equalize the output of my analyzer as the pops don't go through that pre-emphasis because they are not in the recording.

So we are in sync on playback equalization always being there. Question is whether I shape my generator output to match an LP recording or leave it flat as I did in my headroom graph I posted.
To characterise a phono stage as it should be used, then keep the RIAA equalisation on the generator, as that will simulate a 'perfect' LP and tell you what the stage is doing.

Clicks and pops can't be predicted or have any standard level, so there's really no point in trying to measure an effect that could be anywhere as far as levels or frequency spread. The information I would want in choosing a phono stage is how it performs with an RIAA equalised input signal, so that, I think, is what you should use in your measurements of overload, frequency response accuracy, noise and distortion.


With pretty much any LP that's worth playing, the amplitude of clicks and pops is well below that of the audio peaks, (as measured on my PPMs on the audio output of my preamp) so the phono stage's overload capability is unlikely to have any effect on the clicks and pops, provided of course it's enough not to be tripped up by loud modulations.

S
 

SIY

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#27
Several people have measured max expected levels from clicks and pops, so the info is out there. Unfortunately, I'm about a thousand miles from my copy of Morgan Jones's book (where he did some careful measurement of this) so that I can't be more specific. If no-one else pops up with it (so to speak) before I get back from my vacation, I'll post a summary of his data next week.

Yes, you absolutely want the playback EQ in place. In good designs, this is an integral part of managing headroom. In poor designs, this can make things worse.

BTW, Montana is a whole lot nicer than Illinois! I will not be happy for this trip to end.
 

amirm

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#28
BTW, Montana is a whole lot nicer than Illinois! I will not be happy for this trip to end.
Love to be there! Probably hit end of summer to not face the crowds. Enjoy it on our behalf!
 

SIY

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#29
A simple op-amp like the 5532 with 'good enough' EQ is as much as is needed for a phono stage.
Douglas Self's 5534-based designs are excellent, other than the limitation that he didn't go balanced. I have a super-duper AD instrumentation amp waiting for me to get a free weekend to build it into a phono stage; CMR is definitely your friend here.
 

amirm

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#31
Unfortunately, I'm about a thousand miles from my copy of Morgan Jones's book (where he did some careful measurement of this) so that I can't be more specific.
Thanks. Found it:

1528762054342.png


1528762092290.png
 
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#32
Very interesting indeed. I wonder what those very high clicks sounded like. I've never seen anything like that on any LP I'd want to listen to.

Nevertheless, with, say, a 20dB overload margin at 1kHz, that should be near enough 40dB at 20kHz, but what happens after that I don't know, as I've never measured past 20k.

I would expect what happens after 20k depends very much on the design, as I would be surprised if there was much attention to that given by a designer. I shall have a play when I get some time.

S
 
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#34
If I recall correctly, the RIAA eq curve has no ultrasonic limit but it was common to use the Neumann EQ which provides for a cutoff beginning @ 50 KHz at the cutter.
I'm surprised it's as high as 50kHz, as the RIAA on the cutter side boosts HF, so any ultrasonic noise is boosted and adds to heat on the cutter coils, which are pretty fragile. Nevertheless, 50kHz is what was necessary for cutting CD-4 quadraphonic LPs, so that might have influenced the choice of cut-off, although I thought most of those were cut at half-speed for just that reason.

On the playback side, RIAA cuts the HF, so there's no specific need to introduce a further ultrasonic time constant, as the gain keeps dropping off as frequency rises, even if the levels are undefined. Depending on the design, of course, the minimum gain for the whole stage will tend to unity, or just keep dropping.

Getting back to the issue of overload margin, it's one of the reasons I think that passive RIAA eq is a thoroughly bad idea, as the gain has to be excessive so it can be thrown away in the passive EQ, but as is so common in audiophile circles, some manufacturers make a virtue of 'passive' as being better than nasty active. Reminds me of NOS DACs without any filtering!

S.
 
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#35
I'm surprised it's as high as 50kHz, as the RIAA on the cutter side boosts HF, so any ultrasonic noise is boosted and adds to heat on the cutter coils, which are pretty fragile. Nevertheless, 50kHz is what was necessary for cutting CD-4 quadraphonic LPs, so that might have influenced the choice of cut-off, although I thought most of those were cut at half-speed for just that reason.

On the playback side, RIAA cuts the HF, so there's no specific need to introduce a further ultrasonic time constant, as the gain keeps dropping off as frequency rises, even if the levels are undefined. Depending on the design, of course, the minimum gain for the whole stage will tend to unity, or just keep dropping.

Getting back to the issue of overload margin, it's one of the reasons I think that passive RIAA eq is a thoroughly bad idea, as the gain has to be excessive so it can be thrown away in the passive EQ, but as is so common in audiophile circles, some manufacturers make a virtue of 'passive' as being better than nasty active. Reminds me of NOS DACs without any filtering!

S.
Well, Neumann made the cutters, so I assume the coils could take it.
 
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#36
Well, Neumann made the cutters, so I assume the coils could take it.
Whether Neumann or Scully lathes, the cutter heads are very fragile indeed, and it's a sad cutting engineer that has to explain to Management that they have burnt one out. That's why cutting lathes have aggressive HF limiters. In the days of predominantly acoustic music, or even amplified instruments, HF energy rolled off naturally, so wasn't as much of a problem, but once synthesisers became common, coupled with more extensive use of compressors, HF energy rose and started causing problems for cutting lathes. reducing amplitude wasn't really an option, except insofar as some budget compilation LPs got to near 30 minutes a side. The effect wasthat cutting engineers got smarter, and the HF limiters on the lathes got exercised rather more.

S.
 

SIY

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#37
Getting back to the issue of overload margin, it's one of the reasons I think that passive RIAA eq is a thoroughly bad idea, as the gain has to be excessive so it can be thrown away in the passive EQ, but as is so common in audiophile circles, some manufacturers make a virtue of 'passive' as being better than nasty active. Reminds me of NOS DACs without any filtering!
I use passive RIAA in my phono preamps, not for any voodoo reasons but at part of the overall design, and I would happily compare their measurements and performance (especially overload headroom and recovery) with anything commercially available.

Are there other valid approaches which can get good performance using different devices and topologies? Certainly. I don't dismiss any approach out of hand if it's engineered well and the results aren't compromised to satisfy some superstition.
 
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