9v vs 18v and headroom

[robin watson]

Here's a question I've had loping around my mind for... probably years:

You read about preamps, acoustic guitar preamps, etc., having, for example, 18v power supplies (as opposed to 9v power supplies) to give you extra headroom, right? I site this only as example.

How can this really be the case when said preamp will only kick out, say, a couple of volts (line level) at most anyway? If you take a miniscule voltage, from say a piezo element, and boost it to line level - little more than a couple of volts at most - where does the extra headroom come from in an 18v driven preamp? It's surely has little or nowt to do with power supply voltage, but instead a combination of other design factors.

The question came to mind again upon reading about a D-Tar acoustic pickup, which has a dual 9v (18v) power source, claiming to have more dynamics/headroom over typical 9v types.

I don't really see the logic.

R

 

[mshilarious]

No, it's the power supply.

A nominal line-level signal at +4dBu is 1.3VRMS. That's RMS; peak-to-peak will be 2.828 times that, or 3.6V.

OK, you have 9V, right? Not really. Most circuits aren't rail-to-rail, which means you'll give up a little bit of the power supply. Let's say 7-8V then.

OK, is 8V enough? For +4dBu, yes. But you need headroom above that, at least 12dB if not 15dB, to drive a converter to 0dBFS. And you really want a few extra dB of analog headroom on top of 0dBFS, just to make sure your preamp isn't distorting before you converter clips. OK, now you need more like 18V peak-to-peak.

If you have an electrically balanced output, you can get an extra 6dB "free"; that way you can achieve about +17dBu out of a 9V supply rather than +11dBu. But +17dBu isn't usually regarded as enough headroom for a pro-level signal, and electrically balanced outputs aren't as common, partially due to cost, and maybe due to the fact that manufacturers know a lot of people will be connecting to unbalanced gear, and maybe they don't want the output level to vary according to the type of connection used . . .

If you are using -10dBV nominal level ("consumer level") gear, 9V is plenty.

 

[Light]

The other part of the issue is that you are dealing with a musical signal, not just sine waves. You are going to get momentary peaks which are quite a lot higher than the nominal signal strength - that is where an 18V system is most helpful.



Light

"Cowards can never be moral."
M.K. Gandhi

 

[robin watson]

Thankyou for the replies. I won't claim to have grasped everything there, but I get the gist of it.

R