RMS, P-P etc...

[Chuck H]

One of my buttons got pressed.

Watts are Watts - a formally defined Standard International Scientific Unit for Power, unlike a lot of other Standard International (SI) Units even Americans use Watts.

For a Laser Beam, Watts it is calulated as Joules/Second, the rate of delivery of ENERGY.
For Electrical Equipment, Power is still the rate of delivery of ENERGY, it's units are WATTS, and it is calculated from the vector product of the RMS value of the voltage and the RMS value of the current (sometimes called TRUE Power).
Apparent Power is calculate by the scalar product of the RMS value of the voltage and the RMS value of the Current and it's units are VA (VoltAmps) not Watts.

Watts RMS has no meaning. Watts Peak has no meaning, Watts Peak Music Power has no meaning.
Being undefined their units are also undefined but whatever they are they are NOT Watts.

Cheers,
Ian

Too subtle. Tell me how you really feel.?.

Anyone else here know watt Ian's going on about

[lowell]

Okay... thanks for the replies. I had asked this because I'm designing an fx loop for my tube preamp. The preamp was designed to feed an LM3886 PA, so the preamp output is in the 12Vac range... or ~ 30v p-p. I realize this is far too much to feed into guitar fx pedals (instrument/line level). So I'll need to tame the output down to 1Vac max, then have an fx loop recovery stage to boost back to 12Vac. Here's my plan and I appreciate any advice/criticisms.

The LM3886 PA is installed into my guitar speaker cab. The cab is powered via an IEC power cord, and the input to the cab is "instrument level." The PA has +/-32v rails. I plan to add my fx recovery stage physically near the input of the LM3886 so I can use the PSU as well. I'd like to use a jfet or mosfet for the recovery stage. I'll need a gain of ~12. I read that Gain=-1(Rd/Rs). Is this true? Seems too easy haha. I know it's not EXACTLY true, and if a bypass cap, or Cs, definitely will affect gain - if one is used.

So, should I shoot for the Vd on said device to be ~ +15v for ideal clean output? Or should I use the bipolar supply and have Vd be 0v? Not sure it really matters as I'll most likely use in/out coupling caps regardless.

What device would you all suggest? Would this Vds 100v MOSFET work? SUP50N10-21P-GE3 Vishay Semiconductors | Mouser My main question is how to get the Gain to equal 12.

[J M Fahey]

The LM3886 does NOT need 12V RMS or 30Vpp input
Typical gain is 20k/680r or thereabouts (around 30X) because it is not stable with gain <20X or so, even less with 1X gain.
With +/-32V rails it will clip with slightly less than 1Vp , 2Vpp or 700mV RMS .
*Slightly* high for some floor pedals, but if you pad tube preamp out down to, say, 200mV RMS and then follow with 3X gain stage (any run of the mill FET will easily give you that) you'll be fine.

Unless it's *horrible* , almost any any Fet with 4k7 source , 22k drain and fed +15v to +25 Vdc (easy to get from your raw +32V rail) will be perfect there.

[Chuck H]

Following Juan's post, re: 200mV to feed the effect send... A lot of effect loops are set up with a "low" (200mV-ish) or "line" (line level-ish) switch, or even send and return level pots. Players can almost never get their heads around this and often have trouble. Screw that and go with a plug and play loop. My experience has been that most pedal type effects that work well in a loop take line level just fine and those that don't, which are usually distortion pedals, belong up front at the amps input and shouldn't be in the loop anyway. The simple approach I've used is to have the send from the loop putting out a max of 1VAC at max preamp output. Since most gain stages in a signal chain will be padded for one reason or another I found it easy to recover all the gain with a single recovery stage.

I just picked the spot in the chain where I wanted the loop, broke it there and padded to 1VAC, then to the recovery stage which was set up bring the signal back to where it was at that point before the loop was designed in. This just requires eliminating or altering any voltage division that was happening between points A and B before the loop was part of the design. Easy peezy.

[lowell]

Thanks guys! I'll post a real reply when I get a minute. Still some questions.

[R.G.]

"Line level" is also bent quite a lot. It's mostly supposed to be around 1Vpeak, 2V p-p, 774mVrms, but it varies with whom you talk to and what their advertising department says. 775mV rms tends to what most "comsumer" line levels are.