maximum (peak) line voltage levels

[Ben Cemlyn]

Hi All

I have done some measurements of maximum (peak to peak) line voltage levels for the purpose of amp building and wonder if others have done the same and could provide some more results (which I find are not what might be expected!).

Little backinfo for you, but probably not for discussion on this particular thread: I need to know the maximum peak voltage levels that may occur from a line source for my class A buffer amp designs, so I can run enough quiescent current to minimise distortion into a particular load without running them too hot. These line level Class A BJT Darlington buffers are very simple to build, have a excellent input/output impedance and sound amazingly good... I measure the distortion with REW and a sinewave source with peak values at the maximum expected for a line level source.

The peak to peak values I measured on my 20MHz bandwidth oscilloscope with actual music are higher than the levels quoted in e.g. wikipedia. There must be some maximum that digital sources can provide related to all the fancy DSP, compression etc used with digital recording (somewhat related to the loudness wars) which is above the maximum of the peak sinewave value. For a notoriously loud CD (System of a Down first album) I measured about +/- 3.1V from my Arcam Alpha 7SE. For a very good newer recording (Lana del Ray, Born to Die) I measure up to about +/- 2.4V. I havn't gone through any more CDs, but will do a few more once I get another scope, and do a video for here and/or youtube (my nice old analogue scope died and I'm looking for a new one). I am guessing that analogue sources such as FM radio (which I listen to a lot) and vinyl will have lower peak to peak due to the reduced dynamic range.

So the few measurements I have done show peak voltage considerably above the +/- sqrt (2) V (0 dBV ref level consumer) that one might expect from e.g. the wikipedia page when just considering sinewaves. Anyone got any more data / comments? anyone with a scope and curiousity get connected and tell me what you see!

Regards, Ben

 

[sergeauckland]

A DAC (whether separate or part of a CD player) will have a maximum output at 0dBFS. This is typically 2V from an unbalanced output or 4V from a balanced output. It is the same regardless of whether the signal is a sine wave, heavily compressed music or uncompressed music. 0dBFS is the maximum.

2V RMS is 2.8v peak or 5.6v peak-to-peak.

I've never seen anything other than this on my digital sources, whether on a 'scope or PPM.

S.

 

[Ben Cemlyn]

Many thanks - very helpful. So this matches the maxmum value I measured on "Born to Die" (CD).

I wonder if anyone has measured more from analogue sources? Seems unlikely to me, and it will depend e.g. on phono stage overall gain (there is likely a bit of variation).

Ben

 

[Ben Cemlyn]

...I see I actually put +/-2.4V originally for Born to Die, not 2.8. Likely they didn't max out on that recording...

 

[sergeauckland]

Many thanks - very helpful. So this matches the maxmum value I measured on "Born to Die" (CD).

I wonder if anyone has measured more from analogue sources? Seems unlikely to me, and it will depend e.g. on phono stage overall gain (there is likely a bit of variation).

Ben

Analogue sources have no fixed upper limit, they can be whatever the designer decided was a good number.

A phono stage will typically have a gain of 40dB (100x) for MM and 60dB (1000x) for MC, and a MM cartridge typically outputs 1mV/cm/sec recorded velocity, so the output of the phono stage will be 500mV at the nominal 5cm/sec level of an LP.
However, LPs are cut to any arbitrary level, depending on the cutting engineer and how much music they need to get on a side. Shure measured a number of LPs and found that peak levels could be as high as 20dB (10x) above 5cm/sec so the output of a typical phono stage can be as much as 5V on music peaks. That's why phono stages need to have good overload margin levels, as they may need to output 5V without clipping. Of course that means that the following amp or pre-amp has to be able to take in at least 5V without clipping its input.

Professional levels are higher still. Professional DACs will be set to output +18dBu to +24dBu at 0dBFS, i.e. some 6-12 V before clipping, and Pro analogue equipment is designed for those levels.

S.

 

[DVDdoug]

Yeah... Home audio isn't calibrated and often there is a volume control somewhere in the chain... An output should generally be capable of putting-out above -10dBV and an amplifier should be sensitive enough to put-out full-power at less than -10dBV, and that way everything is compatible. Manufacturers don't necessarily follow these levels/guidelines but usually everything has enough signal and enough gain that it works together.

For a notoriously loud CD (System of a Down first album)

Most CDs are 0dBFS normalized, including the quiet ones so the peaks should be about the same. (Some "inter-sample overs" might create slightly higher peaks on the analog side of the DAC.) But again, there's no standard calibration. You could burn a test CD with a 1kHz, 0dB sine wave, and/or you could make a clipped or square-wave file.

Little backinfo for you, but probably not for discussion on this particular thread: I need to know the maximum peak voltage levels that may occur from a line source for my class A buffer amp designs,

Preamps usually have TONS of headroom. 10V wouldn't be unusual. I've build stuff with op-amps and +/-15V supplies. That gives a theoretical maximum of about 10VRMS and so maybe 8V after allowing for some loss across the op-amps.

 

[Ben Cemlyn]

Thank you Sergeauckland and Doug

Quick follow up Serge: your phono stage numbers e.g. the 5V maximum that might occour - is that rms or peak to peak?
if that's a sinewave average, we are talking +/- 7.07V peaks.

Opinion: rms values at line level aren't very useful! rms voltage is really only useful for calculating average power, which isn't very relevant for line level, at least for my class A stuff... its the peak to peak voltage (and corresponding maximum instantaneous current) that the designer needs to allow for....

Thanks, Ben

 

[Ben Cemlyn]

clarification - this is for the design of unity gain buffer amplifiers running into speaker impedances, not pre-amps - hence my concern about allowing enough voltage headroom and particularly quiescient current without generating loads of heat (the quiescent current / heating juggle would barely be an issue with the pre-amp, even if was designed to run into a relativey low impedance).

 

[sergeauckland]

Thank you Sergeauckland and Doug

Quick follow up Serge: your phono stage numbers e.g. the 5V maximum that might occour - is that rms or peak to peak?
if that's a sinewave average, we are talking +/- 7.07V peaks.

Opinion: rms values at line level aren't very useful! rms voltage is really only useful for calculating average power, which isn't very relevant for line level, at least for my class A stuff... its the peak to peak voltage (and corresponding maximum instantaneous current) that the designer needs to allow for....

Thanks, Ben

Those numbers are RMS voltage, so peak to peak are 2.8x greater. If it's not an oxymoron, I was referring to peak RMS voltages! In other words, what the continuous voltage equivalent would be of the peaks achieved on music signals. That's perfectly useful when using signal level meters as they are calibrated against sine waves, and the maximum indications on music peaks depends on the meter dynamics. A true-peak PPM indicating, say, 2V, will indicate the same on a 2V sine-wave or a music signal peaking to 2V.

When designing an amplifier, especially a phono stage, the maximum design peak-peak voltage required will determine the power supply requirement. +-15v is a common one, as it allows some 28v pp output or 10v RMS which at +22dBu single ended or +28dBu differential is sufficient also for professional applications.

S

 

[Ben Cemlyn]

Thanks Serge. You clearly have a lot of experience and expertise with this stuff