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how to measure power of pre-amplification ?

Discussion in 'Preamps / Channel Strips' started by vonrozen, Mar 7, 2016.

  1. vonrozen

    vonrozen Active Member

    Dear Audio friends,

    a basic question :

    how do you measure how loud an audio preamp can go ? How loud it can pre-amplify the music ?

    Which is the value that I should be looking for ?


    Thank you very much .


    Kind Regards

    Alexander



    ( PS : background : I am trying to choose between the Audio Research SP 11 and 14 in terms of strength of pre-amplification )
     
  2. Kurt Foster

    Kurt Foster Distinguished Member

    nominal operating level ie -20dB or +4dB or anywhere between and sometimes even higher, and the amount of gain the preamp can apply usually expressed in terms of dBs like, provides 10 dB of gain.

    here at RO, we address recording rather than audiophile playback. hence recording dot org and not audiophile dot org. the differences are night and day. professional production techniques and associated equipment and listening recording enviornments vs. consumer listening systems in a home setting. in recording we don't typically have a need for an audio pre amp. all our gear runs at either -10dB or +4 db line level already. we use mixing consoles or monitor controllers to route and control the signal but it's not exactly the same thing as an audio pre amp for a stereo system. usually when we discuss pre amps were are speaking of microphone pre amps.

    we're happy to help, but you may have better luck getting the answers to the questions you pose in an audiophile forum. i'm not saying you are not welcome here, i 'm saying you will have a better chance getting the information you are seeking from others who share interest in audiophile playback.
     
    DonnyThompson and audiokid like this.
  3. vonrozen

    vonrozen Active Member

    Thank you very much, i understand. So basically these are the vlues to look out for - I guess ?
    Gain At 1kHz, bypass mode:
    Phono input to tape output: 46dB (0.2V output from 1mV input)
    Phono input to main output: 75dB (0.175mV input for 1V output)
    High level input to main output: 29dB (36mV input for 1V output)
     
  4. Kurt Foster

    Kurt Foster Distinguished Member

  5. Boswell

    Boswell Moderator Distinguished Member

    I'm assuming this follows on from the previous threads on your bi-amped Quad system. The standard 520 main amps have an unbalanced input at a basic sensitivity of 500mV rms for full output. The data and outline schematics I have seen on the AM8/16 splitter network that sits in front of the main amps imply unity gain in the passbands, so that would not alter the signal level needed at the input. However, the schematics show the AM8/16 as unbalanced but the XLR input connector as balanced, so it's not possible to say from that information whether or not there is an unbalancing transformer with additional post-attenuation in the actual units, not shown in the schematics. In addition, the splitter network needs 15V d.c. at the input pins for correct biassing, and there is no indication as to how this is achieved.

    Now, it's clarified somewhat by the legend on the rear of the AM8/16 panel. This has the following information: Input impedance: 16KOhm transformer isolated. Input sensitivity at maximum gain 0dBm (0.775V rms). Maximum input +18dBm (6.15V rms). There is also a gain control, with no indication of its operational range.

    Leaving aside the technical point about why they would specify levels in dBm rather than dBu when the unit is bridging and not 600 Ohm terminated, they are my italics on the statement about the transformer. What this indicates is that the AM8/16s must be driven from a professional-specification signal source, and will not work if driven, for example, from a typical domestic hi-fi pre-amplifier.

    To go back to your initial post in this thread, loudness is a perceived acoustic quality, not an electrical value that can be measured. What I think you are asking is the required electrical output level from a pre-amplifier to drive the AM8/16 inputs. From what I have been able to find on the Audio Research pre-amps you mentioned, they have a nominal output of 2V rms, but have both inverting and non-inverting outputs available. If these outputs were fed separately into pins 2 and 3 of the AM8/16 XLR inputs, the amplifiers would receive an effective level of 4V rms, which would probably be a reasonable match for their nominal inputs. The required cabling would be an XLR to 2x phono (RCA) insert cable, or maybe the more common TRS plug to 2x RCA insert cable if you can use the jack input of the AM8/16.
     
    DonnyThompson likes this.
  6. vonrozen

    vonrozen Active Member

    Thank you, Boswell, for this detailed reply. Do you teach electronics ? If you do, I envy your students.
    I am interested both in the electronic principles of amplification and accoustics and also in the pragmatic question of how to optimise a given system.

    Thank you very much for your research on the Audio Research preamps. If "rated output" is one of the factors determining the gain and level of signal, then how would these 2 measurements differ ? ( one taken from Sp 11 one from SP 14 respectively ) :

    2V RMS 1Hz to 100kHz main, direct, inverted outputs. (Maximum 80V RMS at 0.2% THD at 1kHz into 60kOhm load with 2.85V RMS high level input).

    2V RMS 5 Hz to 50 kHz, all outputs; 60K ohm load (main output capability is 50V RMS output at .5% THD at 1kHz into a 100K ohm load with 5V RMS high level input)


    Kind Regards

    Alexander
     
  7. paulears

    paulears Well-Known Member

    I really have no idea what you are actually asking? Calculating the gain applied by each stage in the system can produce a total, but you can't really equate chains that have different characteristics, and measurements. The first example above is the kind of specification seen from laboratory quality amplifiers intended for scientific applications where linearity and wide frequency response that extends to radio frequencies is required. The second one offers a less wide frequency response, less output and more distortion at face value. I'd guess the maximum output voltage is power supply limited. If the first amplifier was driven less hard, so it's output was lower, what would it's distortion figure be?

    One interesting feature is the headroom, as this implies a drift away from the professional +4dB level. I don't want to get into the audiophile nonsense, but studios nowadays pay less attention to absolute levels than they ever did in the past, simply because it's no longer the critical factor in good signal to noise.

    In your first post you wanted to know how 'loud' a preamp can go? Surely the answer is as loud as is necessary to correctly drive the amplifier that follows it. In professional recording and live sound - to enable maximum compatibility, everyone just works around the common standard of +4dB. If the audiophiles want a higher level, that's fine, but the the amplifiers will have correspondingly less gain, if you want roughly the same volume.

    The car enthusiasts have a very similar battle with gearbox ratios and the maximum RPM of the engines.
     
  8. vonrozen

    vonrozen Active Member

    Thank you Paulears ,
    i listen mostly to ECM recordings, and some master tape versions - both recorded extremely precisely, so your comments are very helpful. Thank you !
     
  9. paulears

    paulears Well-Known Member

    In the recording world, now that dynamic range is huge, one recurring problem is gain staging, and we now have the ability to alter gain at multiple points in the chain - so we have a microphone with a pad - so it's output is adjustable to cater for optimum mic performance, then we have pre-amp gain, then perhaps control of gain in the mixer bus arrangements, we have numerous places to insert other gain changing plugins, then the traditional master faders, then maybe digital gain on the recording device. The recording playback device might also have variable output level, then another on the chain leading to eventually coming out of speakers. This is a huge number of stages, all impacting on the technical quality. It's easy to hear when done badly, but once it's done OK, I'm not sure little tweaks of a few dB here and few dB there actually make a difference in practice.

    It's made worse by the way the recording systems handle levels. In the analog days, a few red lights never hurt the recordings, and the compression that happens before distortion was even a 'feature' to be exploited. Then digital came along and any red lights were bad news. Now we have digital mixers where again, a few red lights don't actually seem to be a problem. Digital distortion was ALWAYS bad, because it sounded bad. Now it is many devices, not that nasty sounding. I suspect all that has happened is that with some many digits available, the red lights are actually coming on a little below digital maximum, giving a little headroom that isn't on the books - this little cheat isn't noticed because lopping 3dB of your maximum is not audible.

    When you say your sound sources are recorded precisely - that infers a technical adherence to numbers rather than a sonic one?

    If a recording is made that has it's maximum peak level at -3dB it can sound wonderful, but then what is the point of mathematically increasing the level in mastering - the quality remains exactly the same, but it's precise level is unimportant. 2dB is supposed to be the smallest amount we can detect a change in level - I'm not sure that is detectable on some kinds of music.

    What worries me is that some groups of replay enthusiasts place emphasis on aspects the people that recorded it didn't! It seems pointless to consider preciseness of recording levels to equate to recording quality, when the recordists themselves don't.

    When I record classical piano, these recordings might take place over several weeks, and the differences in recording levels show up when compiling the CD, not before. Then I sit and tweak each up or down a little depending on the piece. This means some of the individual tracks are deliberately quieter. How would this sit in the notion of precise recording levels?

    If the loudest the music indicates when being played is f, the maximum level will be set by the piece with fff as it's maximum. Can you imagine a piece of frantic Chopin being the same volume as a piece of quiet Debusy? So what exactly is a 'precise' recording? I have no idea?
     
    DonnyThompson likes this.
  10. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    I have this image of a guy afraid to drive his Porsche to the store until he memorizes the torque and horsepower curves and all the gear ratios.
     
    Kurt Foster likes this.
  11. paulears

    paulears Well-Known Member

    Yep - that's what I was getting. Music is, well, music!

    I remember being on a sound course with Big Mick, Metallica's sound man. There was a young, keen guy there who said "Mick, how many dB do you have at the mix position?" - he said "no idea, I just turn it up until my teeth rattle!". The guy pressed him further. "What happens when you get to a new big venue and there's not enough bottom end?" He thought for a moment then said "get more". Music, not maths!
     
  12. vonrozen

    vonrozen Active Member

    No, I am just interested in scientific foundations of audio technology and the reproduction of sound. In an age where most people listen to music from the iphone or computer such curiocity is rather a way to keep the foundations and
    and asthetic beauty of the sound alive.
     
  13. vonrozen

    vonrozen Active Member

    PS : By the way, to all who were following my 'adventures' with the Rogers I have just ascquired another pair of LS 5/8 from ebay - John Bell , the seller with the nickname bakabunghippo, in case you know him- in this way I'll be able to match them up
    and align them better musically.
     
  14. paulears

    paulears Well-Known Member

  15. DonnyThompson

    DonnyThompson Distinguished Member

    Specs and "musical" don't always equate to the same thing.

    I do understand and respect that you are looking for the best possible scenario.
    And as far as the technical side of things, there's been some great information given on this thread; both Bos and Paul have offered up some valuable info; I also agree with Boulder, who said:

    It seems to me that you are basing everything on specs alone, and not as much on your ears. And perhaps this is where we studio engineers are separate from audiophiles, in the sense that specs are not always a deal breaker for us.
    Specs are an important facet, as they can provide a baseline set of values to build from, but not at the cost of sacrificing audio quality... I've used "spec perfect" gear over the years that didn't sound good to me, and I've also used more than a few pieces of gear over the years where the specs weren't necessarily all that great, but at the time and in a particular scenario, that gear sounded great within the context of what I was using it for.

    Shooting for "perfection" is commendable, but in the end, it's not likely you'll obtain it. You may be able to get all your equipment "perfectly balanced" in voltage, resistance, gain, etc., and that's fine... but that doesn't necessarily mean that it will sound pleasing to you.

    And, unless your listening environment has been acoustically balanced, there will always be sonic inadequacies involved, no matter what the specs of your equipment are.

    RO is about recording, mixing, mastering, acoustics, and all the equipment and methods involved; and while most of us here would be considered to be "audiophiles" - meaning that we are "critical listeners" - because that's a necessary part of our craft - but we use different equipment, and have a different set of criteria than the majority of "home" audiophiles do. Ours is a technical craft, but it's also an artistic craft as well... and there are times where specs and art don't always necessarily work well together.

    That's not to say you aren't welcome on RO; your posts have been intelligent, and you've been very polite and respectful as well, so you are always welcome here... but you may get better advice by checking out actual audiophile forums than you would here, because our craft is of a different level - not a better level - just a different level. ;)

    -d.
     
  16. paulears

    paulears Well-Known Member

    On recording forums, like this one, it's very common for somebody to post up a track and ask for comments. They are prepared for almost anything as a comment, and often replies go something like, "I absolutely HATE the music, but it's recorded well, but at about 2:30 there's a slight buzz?" On the other hand, comments might be about things hidden in the mix, or over prominent, poor eq choices - weird reverbs, that kind of thing. I get the impression that hi-fi oriented people are listening to something different - things that we, when recording pay little attention to?

    Back when it was new, I was involved in Music Technology as a UK qualification. It was new and untried, but a group of us pioneered it. One BIG question for me was to do with listening to recordings, and how to assess them. We squabbled happily for months until a set of areas was agreed on. Stereo field, noise & distortion, dynamic range, frequency response and similar areas where examiners could listen and then grade. It was hell. Clearly, everyone had very different opinions on these, and while the dreadful stood out from excellent - in the middle ground it was hard. UK exams have to be standardised, so any examiner will give the same answer with the same submission - in the end, the solution was to have lots of examples as guides showing that if you hear this, it gets this. Examiners marking had to be scrutinised by somebody senior and then that persons work had to be scrutinised by somebody else to ensure accuracy. I've been out of it over ten years now - thank goodness, but I'm still in some groups with current examiners and teachers and they are STILL complaining about inconsistency, and rightly so. In the late 90s, recordings were being made on 4 track cassettes mostly, and then people started using digital. Recordings got quieter. It's unfair to penalise a college or school for not having the latest equipment, so a hissy recording could be inept gain staging, or a Tascam 8 track on ΒΌ" tape - examiners had to listen, then check what it was recorded on, and adjust their opinion on categories like noise!

    Some 0f those recordings still sound pretty good, others less so. One year, we were short on examiners as the subject kind of exploded, and as I'd had a few articles published in Sound on Sound, we put an advert in the magazine. Got loads of examiners. In the first meetings, they get treated to examples of real work and give them grades - then the 'real' grade is revealed. Many of these people were pro studio long-termer. They'd listen, and declare it a E grade recording - horrible. Then they'd discover it was a B, and get depressed when they hear a real E!

    We always had a few cheats - recordings clearly produced properly, and they were easy to spot. Usually, in Dad's workplace in downtime - one school said in their excuse that they had verified the student "was present when the recording was made". These recordings were just different to listen to. The kinds of difference we're talking about in this topic I'm not really sure I can hear. I genuinely have tried, but chasing maximum specs is a totally pointless process, if you can't hear it - and that is something I am convinced about. Turning a gain up here, and down there to keep the level the same nowadays is impossible to hear if the adjustment is just a few dB.

    I'm not quite sure about this aligning the speaker musically? Are we not just talking about pointing the HF at our ears from whatever distance we select as 'correct'? What has that to do with being musical? A virtuoso musician playing brass or woodwind hears a totally different sound to the listener via the recording. The only factor that can alter musicality is the player.


    EDIT - I just noticed the Harbeth forum has a little note at the bottom
     
  17. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    Both of those preamps appear to fall into the "way more than good enough" category, going by the specs. Unless the phono input section doesn't like your cartridge or something I don't see having a bad result.

    Not sure how a recording can be "precise", unless they calibrated levels and frequency response with a known acoustic source. Then they'd have include reference signals on the recording and a SMAART rig to calibrate the playback system correctly.
     
  18. Kurt Foster

    Kurt Foster Distinguished Member

    precise? there's no precision in audio!


    no-crying-in-baseball.jpg


    mics aren't precise. no amplifier or speakers are either. the only precise audio is the source.
     
  19. vonrozen

    vonrozen Active Member

    dear Donny and others -

    thanks for your comments

    the problem is that audiophile forums are full of absolutely subjective "crap" , label driven, what I'd call hifi - isoteria. Yopu guys are taking a more critical approach and this is why I prefer to converse with you.

    On the subject : Which books about accoustics and loud speaker design would you recommend to read ? Maths is not my very strong points. I am interested in pictures, ideas and analogies and examples. Sound and our relationship to it interests me a lot .

    Thanks very much

    Kind Regards

    Alexander
     
  20. paulears

    paulears Well-Known Member

    This is now 20 years old - but contains really good explanations and the science. It's not a 'nice' book, but the content is solid!
    http://www.amazon.co.uk/Sound-Reinforcement-Handbook-Gary-Davis/dp/0881889008/ref=sr_1_1?ie=UTF8&qid=1457609306&sr=8-1&keywords=yamaha+handbook+sound
     

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