audio line out voltage

Good grief, what happened to 0 dBm? What happens when you slam 10 volts RMS into a vU meter? Is nothing sacred any more?

John

Okay. This is nothing special fidelity-wise. Trimming the supplies to

+/-8 or +/-9V (+/-6.5 or +/-7.5V swing) would make quite a difference.

No, maybe the 16 high speed buffer amplifiers, but the chip is bipolar in any case. The data sheet limit is 40mA @+/-12V, for just one set of supplies, which is almost a watt. Quite a bit for a little TQFP.

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Stuck with the chip.... if necessary I'll put a heatsink on it. I'm probably being overly conservative in any case, but a supply reduction would also give a bit more regulator headroom.

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Speff, If it is a XLR input, the voltage is balanced. So the peak voltage is only

+/-4V. I use the INA2137 to convert it to single ended(-6dB) and then DRV135 to amplify and balance it again.

--
ciao Ban
Bordighera, Italy

Yes - if don't want the input to clip it has to handle at least that much.

That much level is commonly available from pro-sumer gear. It's what you can typically get from op-amps running on +/- 15V supplies.My own stuff with true balanced out actually clips @ +28dBu ( ~ 19.5 V rms )

Are you looking at a very low power design ? If you're simply dealing with hi-fi you can get away with less I expect.

Graham

Where did you get that idea Ban ?

Most pro-audio outputs are driven by an op-amps supplied from somewhere between

+/- 15V and 18V

The 8V that Spehro mentioned is a typical rms voltage from such an output. The peak voltage in this case would be about 11V.

Where an output is true balanced both pins 2 and 3 of the XLR are actively driven - providing a pk-pk output of say 25V.

The INA ( and other ) solutions are simply expensive playthings. You can do the same with an op-amp and a few Rs.

Graham

Thats about normal for pro equipment, it gives about 20dB headroom which seems to be the professional standard requiremnt.

quite a lot of prosumer stuff is tascam level , say about -10. so you could get away with +10dBfor your clip level.

Where is the " power dissipation", I hope you are not terminating everything in 600R

martin

Hello Spehro,

As others have already mentioned I also believe it could be that much. I just rigged up an FFT via audio card and when I connected the line-out of a commercial receiver to it everything saturated, big time. On the scope it looked like more than 10Vpp. So I had to attenuate a lot.

Regards, Joerg

Hello John,

Yes, I was shocked, too. I mean, in my case this could have fried the built-in sound card which would have been a real pain.

What really irks me is that there is no information about that at all. The receiver simply said "LINE OUT" next to the jack. No level. The laptop is so skimpily documented that all I could find after 1/4hr of forensic efforts was that it has an Analog Devices Codec in there. That's it. Great. No data whatsoever.

I am not too surprised though considering that most people I know who own a house never had any technical drawings for that house. Luckily we do.

Regards, Joerg

What is the IC, it seems a bit thirsty, maybe there is a better option or approach. I'm a bit out of touch with whats avaialble, but Graham et al will be able to suggest something. Are there space/PCB limitations?

martin

Agreed, 4 10K's and a TL072, CMRR not as good as a INA, but quite adequate, generally. But running the internals at -10dBu will prolly be OK. Depends on what the customer wants.Most mixers I've come across never "mix" at 0dB internally, mainly for headroom maximisation

martin

Hello Martin,

Nah, it goes in waves. The next step would probably be a straight capacitor from the plate of an ECC81 to the line out jack and a 10M bleeder. That should be good for a 75Vpp swing or so.

Regards, Joerg

No-one uses VU meters any more. They always had serious limitations and drawbacks anyway. The extra headroom has actually always been there. The first mixing desk I built ran off a regulated 30V single rail supply for example.

Bear in mind that 0VU = +4dBu - the same in voltage terms as the dBm(600R). So +3dB on a VU meter is already +7dBu. Add to that the fact that a VU meter is a slow response instrument and doesn't register peak voltages and ..... I suspect you see where I'm going with this.

And indeed no-one uses dBm in audio any more except where it's actually needed ( which is rarely ). All modern specs use the dBu which is a voltage reference of 0.775V and all equipment is 'voltage matched'.

Another reason for high levels is digital recording. It makes sense for the digital clip point to be similar to the clip point of the electronics driving it. 0dBFS is often around +19dBu. There is sadly no accepted norm however.

Graham

The ones I'm using can take a differential 4V pk signal IIRC. Prabably the same as your 2.8V rms ?

Actually it's fine. The resistive pad affects the noise as well as the signal ! ;-)

The operating levels are set by accepted practice.There's good reason for this if you look at a noise analysis. -10dBV working is *always* noisier for example.

Well.... Certain audio companies can already offer you a 10kW amplifer. The audiophools wouldn't have anything to do with them though since they're transistor and don't have enough of the *right kind* of distortion to suit them - lol !!

Graham

between

The

the

Absolutely so.

Some older ones ( esp ecially ) do and Mackie got a bad rep for doing exactly that too.

I typically go for -6dB on the mix bus ref external levels. Indeed I sometimes run channels at -6dB too. Gives *plenty* of headroom.

The only mixer I've ever come across that ran at -10dB ( both channels and mix bus ) was the Neve 51 series for the broadcast market. Those European broadcasters can be very fussy.

Graham

On Thu, 04 Aug 2005 14:34:10 -0700, in sci.electronics.design John Larkin wrote: snip

it increases its accuracy...........................

martin

snip postings by fine people

Bleeding F*ing stupid, all the audio ADCs' I've seen run off 5v rails, Wavefront, Ashai, etc and clip at about 2.8V, diff in (from memory, mine, not the computers)

Good (insert appropriate deity) who the hell designs these I/F's when you have to pad it down anyway to feed the ADC

Next we'll be having 3 phase DC supplies from the audiophoole society

martin

Shit Joerg You beat me to it I'd just hit send when I thought about ECC83s

3rd phase, phasor current aka filaments!

Analogue ready.......................... bed time, to sober up!

martin

as

if

transistor

I'lll prooly give a reply when I'm sodbder, dudu, keyboorrsd cant spel writre, but think weeere in the same ballpark,

hic

martin