Cambridge Audio 840A V2 Volume Levels

Discussion in 'Hi-Fi Stereo Systems & Separates' started by vatana, Feb 3, 2014.

  1. vatana

    vatana
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    I have recently purchased used V2 840A and am having trouble getting my head round the -db volume control display.

    On my unit I am not getting a meaningful output until the display is showing at least -50db and find that dependant on source I need to run it at anything between -45 to -35db to achieve a good sound.

    In my state of KEF Q55 it is not really bad . They can also listen to -54 . I had Dali Ikon 2 and Dynaudio Excite 14 (both compact speakers) , and the volume is very weak at -54, even weaker than the KEF 's . That seems very strange. I know that Ikon 2 need much efect consumption by a little louder, but this is already extrem here on the Dynaudio as well . They can already loud, but i have to turn at 75 % to 85 % up on the controller.

    This implies that I am using half the available output before getting anything from the speakers, can this be right or is it a characteristic of the class XD design ?

    It is hooked up to a pair of KEF Q55, Dali Ikon 2 and Dynaudio Excite X14 which my previous Yamaha AX-490 drove without a problem with a useable output from almost the first "crack" of the volume control.

    I would be grateful to hear what volume levels other owners owners find they are using to gauge if I have a faulty amp or not !

    Having said that, when it's cranked up to -35db the sound is awesome, its just that with an output rated at 120 watts into 8 ohms I would have expected it to rattle the windows much earlier in the power curve.

    All experiences welcome and appreciated.
     
  2. Don Dadda

    Don Dadda
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    I cannot comment on your speakers but I believe all is as it should be with your amp.
    The Dali are 6ohms and the Dynaudio are 8ohms and would expect the Dali's to be a little louder at the same volume setting. The KEF may be more easier to drive than the other 2 which may attribute it to them sounding better. They are also floorstanders which i'm not sure if or how much that has got to do with things but maybe a contributing factor.

    My main amplifier (see sig) volume indicator is also in db's and for me, everyday listening is around -15 to -10. If i want next door to enjoy what i am playing, it be around +5. And If i want to get on their last nerves its +20 and upwards.
    When i switch my amp on it automatically goes to -25 which is low, so I've never tried -54. I suspect its barely audio-able. I will try that tonight and see

    What would be handy if one could find a comparison chart of what db's equals the 'O'clock numbering of the more standard volume indication. If you have a SPL meter at hand then you can work that out yourself.

    Hopefully others will chime in with their views.
     
  3. vatana

    vatana
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    Ok, thank you very much.

    I was at my HiFi dealer today and we compared my 840A and the new one 851A, and the result is the same. At -54 dB which is at 11 o'clock on volume control, both are very queit! At 3-4 meters range you can allmost not hear the speakers.

    It has something with impendens of the speakers to do. Dali and Dynaudio has impendence by 85-86 dB, and KEF has 91 dB, and thats because the KEF are a bit louder at the same volume control as another two.

    He says thats like Cambridge AMP's works. First after 1 and 2 o'clock on the volume control u can hear good loudness.

    It's very strange for me, but everything shold be all right.
     
  4. BlueWizard

    BlueWizard
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    I hate these stupid digital reads out Volume Controls. On an AV Receiver, if one knows the inner details, one can work out what the numbers mean. For example, once you have run Setup on the AV Receiver, the Volume Control is then calibrated. The numbers have meaning. At 0dB, or Reference Level, the volume should be about 85dB average. Though others might be able to correct or verify this.

    However, in a stereo amp, those numbers are not calibrated to your external speakers as they are in an AV Receiver. So, one can only assume the number represent some internal reference within the amp. So, these are not referencing acoustic levels, but internal electrical levels. But ... what do the number mean? Who knows?

    Next there are about 10 different kinds of db rating, all relative to some other reference, so we don't know which dB rating they are using.

    One assumes that 0dB is really the maximum level of the amp. When you go above 0dB, the amp is clipping. Keep in mind that amps clip on peak signals much earlier than most people would assume, but that clipping is minor to insignificant. So, while I have no proof, I suspect, that 0dB is the point at which a steady (sinewave) input signal will push the amps outputs just to the clipping point. Though who know what 0dB actually means. That's my best guess, especially since we have not external reference to calibrate to as in an AV Receiver.

    Now we deal with the next problem. The old fashioned, fixed movement Potentiometer (variable resistor) volume controls are not linear because our ears response to sound is not linear. They use what is called an Audio Taper, meaning the rate of change in resistance is not linear, in other words, 50% (12 o'clock) on the volume control is not half the resistance of the volume control. There are also Logarithmic taper, and reverse taper controls.

    But how do you implement a non-linear taper in a digital volume control with what I assume is very very limited computing power on the volume control? I suspect when you spin the volume control it sends out a pulse train, which simply means a series of pulses, a counter counts the pulses and either counts up or counts down depending on the direction you turned the control. The number on the counter controls a amplifier stage which in turn controls the voltage level on the signal. Likely the polarity of the pulse train determine whether the counter counts up or down.

    In a sense, this is a very limited Digital to Analog converter, on one end is a number, on the other end is a proportional voltage. Using basic analog circuits, this is actually very easy. A simple Resistive Ladder is summed together on an Op-Amp, and the output controls a Voltage Controlled Amplifier stage. To count in linear binary, the resistance values are easy. They are simply multiples of TWO, so if the first resistor is 1k ohms the next is 2k, then next 4k, the next 8k, and you have a perfectly linear Binary to Analog conversion. But, now try a Logarithmic conversion? Scale the resistors, so you get a perfect logarithmic output? That's actually not that hard. Now scale the resistors, so that the response you get perfectly mimics how the ear responds to sound levels. Yes, I'm sure it can be done, but it is not as easy as linear binary or logarithmic which have very precise mathematical determinations.

    So, what you get is someone's opinion of what is the equivalent of an Audio Taper. One that I feel is not very accurate. Then the matter is compounded by the fact that the dB read out is not referenced to any external source, like the sound output coming out of your speakers. If the AV Receiver Read Out says -20dB, the means -20dB below the Reference Level of 85dB. So ... simple, 85dB - 20dB = 65 dB. Though keep in mind casual close conversation is about 70dB.

    It would be nice if one could find an explanation from Cambridge Audio or other manufacturers of what the read out meant. It would also be nice if you had the option of using a common -dB to +dB read out on the volume control or instead a 0% to 100% read out. Especially if the 0% to 100% made some effort to track the old mechanical Volume Controls.

    It would be nice if someone could take a SPL Meter (loudness meter) and calibrate the readings on the display. But those readings would only have meaning for that specific amp with those specific speakers, any other amp and/or any other speakers would require it to be tested again.

    Life was so simple back when the dial your turned was the actual device that was directly controlling he volume.

    I think with these new fangled amps, the best you can do is let you ears be your guide, then over time remember which numbers correspond to which listening levels.

    Did I mention that I really hate the digital read out volume controls, especially on Stereo amp?

    Steve/bluewizard
     
    Last edited: Feb 3, 2014
  5. Don Dadda

    Don Dadda
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    Wasn't it just..

    Be easier to get a spl meter or there's probably an android app out there for a smartphone if you have one. It won't be completely accurate but it will put you in the same ball park at least....yup, there is so may give it ago when I have the time.

    But you may have a point with the referencing of these numbers. 0db on my CA streamer volume output is its max. On my amp, its max is +42db. So I have no idea what referencing they are following. Ok, one is purely digital volume and the other is not, but you would of thought they would of followed the same guidelines to save confusion.

    I just tried my amp at -54db and it may as well have been switched off. I had to be right up close to hear anything and would be no where near the 11'o'clock position on my other amp that has a 'conventional' volume dial indicator.
    Does The 840a volume dial continuously spins full rotations until desired volume is reached where as on a conventional volume dial starts at 7 'O' clock and finishes at 5 'O'clock?
     
  6. vatana

    vatana
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    It's specifically Cambridge AMP "problem". I tried the new one too, 851A. It's the same. Allmost no sound at 11 o'clock! First after 12-1 o'clock comes the power. (speakers with 85 dB sensivity)

    There is option on Cambridge AMP where you can switch from showing volume from dB into procents. It's the same, just another numbers. When i change it to procents, at 11 o'clock is allmost no sound in the speakers. The power on Cambridge AMP's comes very much later on the volume control. But you can use it til 100%. At my another 2 AMP's (Onkyo and Yamaha) you can't turn it over 12 o'clock, it's unpossible, it will damage spaekers or the amp :) At Cambridge you can use it all the way of volume control.

    Thanks a lot to all and sry for my bad english.
     

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