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Headroom in phono preamps

roh

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#1
Hello I am new here.

I am looking into buying a new phono preamp in the sub 200E category.

I read a lot about all the important technical considerations and I am quite annoyed that companies specify their products with different metrics (I noticed that gain can be specified in at least three different ways), some of them even omitting a lot of them (looking at you Pro-Ject).

The one specification that I look into now is overload margin / maximum input level. The 'overload margin' is specified in db , maximum input level in mV @1kHz. As it’s known this parameter is very important so that the musical peaks are not distorted and also the clicks and pops are more noticeable when they distort. These clicks have much higher voltages than the music so I would prefer the headroom to be at least 90mV to accomodate for them with higher output cartridges..

Often none of these parameters is specified, then I usually look at the voltage of the power source as it seems to be linked directly to this metric.(18V-24V seems to be the necessary minimum).
Other important parameter that affect headroom is RIAA feedback or passive implementation, with passive giving usually higher headroom. Unfortunately most of the companies do not specify it.


Thing is (and this is the main point of this long post) that I don't have enough technical knowledge to understand the matter completely and I am not able to convert overload margin to maximum input level or vice versa and compare the preamps. Anybody would care to enlighten me?
What is equivalent of 20dB Overload margin to Maximum input level for Schiit Mani for example?

Meanwhile lets look at some of the options that actually specify headroom value:

Schiit Mani http://www.schiit.com/products/mani
Overload Margin: >20dB
Gain: 30dB , 42dB, 48 dB (MM modes)
Power Supply: 16V AC

Fono Mini A2D http://www.rega.co.uk/fono-mini-a2d.html
Maximum input level: 70mV @ 1kHz
Input sensitivity: 5mV for 500mV output (this is 100x gain, so 40 db)
Power requirements: 24V AC

Art DJ PRE II http://www.artproaudio.com/product/djpre-ii-phono-preamplifier/
Maximum Input Level 40mVrms @1kHz
Maximum Gain 45dB
Power Requirements 7-12V DC or 9-12V AC
Note: has really small input level, and I noticed that you measured only 20mV, that is pitiful. probably because of the low voltage.

Musical Fidelity V90-LPS https://www.musicalfidelity.com/v90-lps/
Input overload margin: 15dB (MM)
Gain 3.0mV (MM) for 300mV out = 40db
Power 12v DC

Camridge Audio ALVA SOLO https://www.cambridgeaudio.com/en/products/hi-fi/phono-preamp/solo
Overload margin >30dB
Gain 39 dB
Power ?

Graham Slee Gram Amp 2 Communicator https://www.hifisystemcomponents.com/phono-preamps/gram-amp-2-communicator-mm-phono-preamp.html
Maximum input level 58mV
Gain 2mv to 224 mv (=41db)
Power 24V DC

Edwards Audio Apprentice MM http://www.talkelectronics.com/edwards-audio-apprentice-mm-phono-stage/
Max Input 75mV
Gain 40 dB
Power ?

NAD PP2e https://nadelectronics.com/product/pp-2e-phono-preamplifier/
Input overload 102 mV
gain 35 dB
Power 23.5V DC


Outside of the class:

NJC Audio reference preamp http://www.njc-audio.co.uk/html/phonopreamp.html
Maximum input level: 93mV (high gain setting)
Gain 5mV for 620mV (high gain setting=41,8dB)
Power ?

Actidamp Mk4 czech diy op amp based preamp with passive two stage RIAA
Maximum input level: 250mV @ 1kHz
EDIT: Gain 46x = 33,3dB
Power: 25V-35V

Onkyo A-15 phono amp stage of amp https://www.hifiengine.com/manual_library/onkyo/a-15.shtml
Maximum input level: 150mV
Gain ?
Power ?

For reference good info about the subject can be found here:
https://pdfs.semanticscholar.org/0bbe/af7a1d26ebe48b5bb418a5013396e15437f1.pdf
http://www.hi-fiworld.co.uk/vinyl-lp/70-tests/106-phono-stage-tests.html?start=2
http://www.renardson-audio.com/phono-1.html
http://www.sengpielaudio.com/calculator-amplification.htm
https://www.kabusa.com/frameset.htm?/phonpre.htm
https://www.hifisystemcomponents.com/blog/analogue-audio-needs-headroom.html
 
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roh

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#2
Nobody cares to explain me?

What is equivalent of 20dB Overload margin to Maximum input level for Schiit Mani for example?
 

restorer-john

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#3
Welcome to ASR!

Yes, you are correct about overload characteristics on phono RIAA preamplifers being rather important. You are also obviously looking at the extremely poor (on the whole) overload characteristics of modern 'phono stages'.

There's been plenty of discussion here on inadequate overload margins on gear. Just type into our superb search 'overload' and member 'sergeauckland' and you will find a ton of useful stuff. ;)

https://www.audiosciencereview.com/...djpre-ii-phono-preamps.3457/page-2#post-83785

I'll do the overload margins and numbers for you in the morning, the Mani has 4 gain settings so the common ~2.5mV standard is the goto. Trouble is, we don't know if it's 20dB across the range or just at 1KHz.
 

roh

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#4
I'll do the overload margins and numbers for you in the morning, the Mani has 4 gain settings so the common ~2.5mV standard is the goto. Trouble is, we don't know if it's 20dB across the range or just at 1KHz.
Hello, let's say it's at 1KHz, what would be the maximum input level?

And please if you can explain how the conversion calculation works, so I can do it myself in the future.

Many thanks
 

SIY

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#5
That is a remarkably useless set of specs. Gain and overload are both strong functions of frequency, so need to be specified as such in order to be useful.

My own phono stages have ridiculously high overload margins, but aren't exactly priced at this level. :D At some point, if someone sends me an inexpensive phono preamp, I'll run measurements and show how (IMO) it really ought to be done.
 

roh

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#6
If the manufacturer even bothers to publish it, the gain and max input level is usually specified at 1kHz.

Examples:

Rega Fono Mini

  • Input sensitivity: 5mV for 500mV output
  • Input loading: 47k +100pF
  • Maximum input level: 70mV @ 1kHz
  • Output Impedance: 100 Ohms
  • Signal to noise ratio: 78dBA ref 5mV
  • Power requirements: 24VAC 85mA
  • Input for full scale digital output: 7.5mV
  • Dimensions W 102mm x D 125mm x H 30mm
Nad PP2

  • Input Impedance (R and C) 47 kilohm + 200pF
  • Gain at 1kHz 35dB
  • Input sensitivity (ref. 200mV output) 2.5mV
  • Signal to Noise Ratio 80dB (A weighted, with cartridge connected)
  • Input overload (20Hz/1kHz/20kHz) 10/102/950mV
  • Rated Distortion (THD 20Hz – 20kHz) <0.03%
  • RIAA response accuracy ±0.3dB

Actidamp MK4
see picture

1551273438381.png
 

sergeauckland

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#7
Overload margin quoted at 1kHz is sufficient, and should be at least 20dB over the cartridge's output at 5cm/sec recorded velocity.

The reason it doesn't need to be quoted at other frequencies is the RIAA equalisation. As the frequency drops, the amplifier's gain increases, thus decreasing the overload margin. BUT, the recorded level also decreases by the same amount, thus maintaining the effective overload margin. Ditto for higher frequencies, the overload margin increases, but so does the level on the LP, by the same amount. With HF, there's also the natural decrease in level with increasing frequency once much past 1kHz, so the overload margin at higher frequencies will seldom be a problem.

If you have a phono stage with accurate RIAA equalisation and a >20dB overload margin with your intended cartridge then that's fine. The problem does occur that the overload margin is quoted with, say, a 2.5mV sensitivity and one uses a cartridge with 5mV output, immediately taking 6dB out of the overload margin.

S.
 

SIY

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#8
Serge, I disagree that 1kHz specification is sufficient- how that overload changes with frequency is highly dependent on the phono stage topology, and there are quite a few variations on that theme. And of course, it becomes more critical at higher frequencies because the highest level signals are almost always impulse-like ticks and pops. What also is important (but never specified) is the manner of overload and recovery; does the stage just clip the excessive signal or does something like blocking occur? I design my phono stages specifically to have essentially instantaneous recovery, but a lot of the so-called classic phono preamps have absolutely dreadful recovery characteristics.
 

sergeauckland

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#9
Serge, I disagree that 1kHz specification is sufficient- how that overload changes with frequency is highly dependent on the phono stage topology, and there are quite a few variations on that theme. And of course, it becomes more critical at higher frequencies because the highest level signals are almost always impulse-like ticks and pops. What also is important (but never specified) is the manner of overload and recovery; does the stage just clip the excessive signal or does something like blocking occur? I design my phono stages specifically to have essentially instantaneous recovery, but a lot of the so-called classic phono preamps have absolutely dreadful recovery characteristics.
I assumed instant recovery and not blocking, as any phono stage that latches up or otherwise doesn't clip cleanly, in my opinion isn't fit for use, and you're probably right that there are some phono stages of 'boutique' manufacture that perform less than well. I also didn't assume passive equalisation, as I can't imagine why anyone would choose to do it that way, except that in certain 'audiophile' circles, passive is good, feedback is bad. Passive HF equalisation has the benefit of reducing noise, but noise is the least of the problems with phono stages using modern devices.

Impulsive signals are indeed the main reason why a high overload margin and clean recovery are needed.

S.
 

SIY

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#10
Well, there's many cans of worms, mostly because of all the tradeoffs needed in design. So one example: a fully active all-in-one-go circuit will have a high common mode signal at the first stage. It takes some clever design work to get around that. I think Doug Self deals with this extensively in one of his books. Another example: optimal device choice for the input stage will be very dependent on what the source is. Take an AD797, which is a very low distortion chip with really impressive input voltage noise. Hook a conventional moving magnet cartridge to it and... oh my, it's very noisy. And then there's the issue of moving magnet loading, which is both critical and rarely specified accurately. Worse yet, it is often frequency dependent. Put transformers into the equation and we now have two more complications to consider, the secondary load and the primary source impedance.

I maintain that good phono stage design is one of the hardest problems in audio electronics. It's a solvable problem (see Self and also Burkhart Vogel's excellent books, and, cough cough, my published phono preamp designs) but far more often done suboptimally than done correctly.

Clean recovery is indeed important and also not always done well, especially when we hit common-mode limits. The poor performance is not limited to fashion audio, there's a lot of "normal" equipment where the details are not attended to very well. Comprehensive phono stage measurement would be a useful thing, but as far as I'm aware, no one is doing that for publication (hence my offer a few posts back).
 

sergeauckland

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#11
It's a solvable problem (see Self and also Burkhart Vogel's excellent books, and, cough cough, my published phono preamp designs) but far more often done suboptimally than done correctly.
I'd like to see those. Any chance of a link?

S
 

SIY

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#12
The ones in Linear Audio are not publicly linked but can be purchased from the magazine's site. Part 1. Part 2. I think this is my better design.

My earlier MC preamp was published at diyAudio- search for "His Master's Noise." I'd probably change a few things now (like reduce the idle current in the first two stages), but even as-is, it's a good preamp, and one I still use.

Both of these have pretty fine measured performance in all the parameters I think are important.

edit: link to MC preamp.
 

roh

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#13
Thanks for the discussion, but I am still in the dark about the relationship between maximum input level and overload margin...

What is better? Maximum input level: 70mV @ 1kHz of Rega Fono mini or Overload Margin: >20dB of Schiit Mani?

Many thanks
 

SIY

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#14
Since the specs are written badly (and are hugely incomplete), you have to do a lot of guessing. Let's look at the Schitt number- the 20dB has to be referenced to something, and measured at a specific frequency, but they don't say what. And since they have several gain settings, that's another monkey wrench. Let's guess that it's 2.3 mV and 1 kHz, which is their setting for "standard MM," and a complete guess as the what frequency the rating is for. 20 dB above that is 23 mV. So in that sense the Rega is "better."

But my main point is that you can't use a badly defined and very incomplete spec to make a judgement here. So don't. :cool:
 

SIY

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#16
Well, again, the issue tends to be at higher frequencies. So the spec here has little or no meaning. Whenever a dB spec is tossed out, the very first question is "dB with respect to what, exactly"?

If someone wants to pop for the $129 and send one to me for measurement, I'll see how the overload looks over the audible range, as well as some other critical measurements left off their specs.
 

sergeauckland

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#17
This is crazy low... I was under impression that 20 dB is quite OK value..
20dB relative to 2.3mV is just about OK, IF your cartridge provides only 2.3mV at 1kHz and 5cm/sec recorded velocity. Many MM cartridges provide a lot more than 2.3mV, so if your cartridge provides, say, 4.6mV, then your just about adequate 20dB becomes an inadequate 14dB.

S.
 

roh

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#18
OK, currently I have AT91 that's 3.5mV at 1 kHz, 5 cm/sec with plans to upgrade probably to Nagaoka MP110 5.0 mV at 1 kHz, 5 cm/sec.

Could you recommend me a preamp with good enough headroom in the sub 200E category? I am not an audiophile but I dislike pronounced technical imperfections like distorted clicks or syllables. I understand that I cannot get rid of the them completely on vinyl, I just want to minimize them within reason.
 

SIY

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#19
The other parameter you need to consider for that cartridge is the input capacitance of the preamp. It's generally not specified, and when it is, it's nearly always specified incorrectly. A-T wants 100-200pF, and of course that also includes the capacitance of the cables. So you'll need a very low input C (actual, not specified) in order to get a flat frequency response.
 
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