Common amp myths

[JmJ]

It implies an input, yes. As I said before, it inputs directly on the mixer.

think about it. when you go to have an injection, the medication that comes out of the needle, is injected. It's been outputted, but it's called an injection.

If you have an injection there is a "from" & a "to".
The act of sending something out could never be considered an injection. The word injection means something is going in. Whatever comes out of your needle is ejected. If it makes it into your system it was injected.

 

[MtManiac]

It implies an input, yes. As I said before, it inputs directly on the mixer.

think about it. when you go to have an injection, the medication that comes out of the needle, is injected. It's been outputted, but it's called an injection.

Something that is "put out," or "outputted" would be ejected, wouldn't it? The needle is injected into your arm (or butt), the doctor or nurse ejects the medicine from the needle. They call it an injection to differentiate it from just squirting the medicine all over the room, which would be just an ejection. Anyway, semantics... like spelling these days, important to some, less important to most.

DI refers to going from the head to the mixer, or from the bass to a DI box to a mixer, rather than having a cabinet mic'ed, which would be an indirect input to the mixer - indirect because you would be converting an acoustic sound wave to electricity twice, rather than just once.

 

[joebingo]

If it makes it into your system it was injected.

Rather like a bass signal, making it into the sound system?

 

[billfitzmaurice]

If I raise the frequency to 200hz still maintain 10V and it will have the same excursion?

Excursion goes down as frequency goes up, once you're above the area where a reflex alignment affects it. SPL will remain the same even though impedance has dropped and power has therefore increased. By the same token go higher in frequency and impedance will rise, power will drop, but if voltage remains constant so will SPL within the region of flat response.

You'd need to be sure to measure and add adjust the voltage at each frequency.

Ones does to account for non-linearity in the amp response and the output of the tone generator. A really good power amp will run ruler flat throughout its passband.

 

[bongomania]

A ideal DI has nothing to do with your signal until the signal reaches the DI. It has everything to do with preparing your signal for insertion into a mic pre. It is not a "direct out" from the bass because that implies no change from the bass to whatever you're plugging into. It is a "direct inject" because you cannot directly inject your bass output into a mic pre.

 

[JmJ]

Rather like a bass signal, making it into the sound system?

Exactly! Injected into the PA FROM your "DI"

 

[anonymous02282011]

phew . . . a little out a breath. I've got a quick question for the experts:

Amp X is rated at 300Watts at 20Hz.

Amp Y is rated at 300Watts at 1kHz.

Will Amp X be perceived as louder?

 

[bongomania]

Not enough data.

 

[anonymous02282011]

Not enough data.

What I'm asking is in lieu of the "watts are watts" fact.

In other words, every amplifier has a wattage rating, but is there a standard frequency at which those watts were rated, and if it were rated at a different frequency, would the rating change?

If that doesn't help, let me know what other parameters I'm missing . .. and thanks!

 

[bongomania]

Yes, rating at different frequencies can easily change the rating. After all, E=IR, and if the R (load) changes then the E (watts) changes too. (The load changes at different frequencies.) Edit: see below, I realized I was using the wrong formula to illustrate.

I'm not sure if there is an industry standard, but IIRC 1KHz seems common.

Since you asked about "loudness", we have already established that wattage is no measure of loudness. And the "missing data" is how each of those amps perform at the frequency stated for the other amp (ignoring the multitude of other factors that come into the picture once we leave simple equation land.)

 

[JmJ]

A ideal DI has nothing to do with your signal until the signal reaches the DI. It has everything to do with preparing your signal for insertion into a mic pre. It is not a "direct out" from the bass because that implies no change from the bass to whatever you're plugging into. It is a "direct inject" because you cannot directly inject your bass output into a mic pre.

Can you translate this into English for us linguistically challenged folks?

 

[anonymous02282011]

I thought it was V=IR where V is volts, I is current (amps) at R is resistance (ohms) . . . . but I understand what you're saying. So an amp that puts out 300W at 20Hz be, hmmm, searching for the right words . . . . "more powerful" or "perceived to be louder" than an amp rated 300W at 1kHz . . . ?

 

[shakerattleroll]

I thought it was V=IR where V is volts, I is current (amps) at R is resistance (ohms) . . . . but I understand what you're saying. So an amp that puts out 300W at 20Hz be, hmmm, searching for the right words . . . . "more powerful" or "perceived to be louder" than an amp rated 300W at 1kHz . . . ?

E is the symbol for volts.

 

[bongomania]

Whoops! *slaps forehead* wrong formula. *sheepishly looks around* The correct one for wattage is P=IV (power = current times voltage), however the current and voltage are measured into a certain load, and that load changes at different frequencies. And I can't remember how to mathematically describe that.

 

[bongomania]

Can you translate this into English for us linguistically challenged folks?

People were arguing quite senselessly about whether a DI is an input or an output. Obviously it is both. However its main function is to prepare a signal for being put into a specific type of input (a mic pre).

Regardless, there are inputs and outputs at every junction, and every "in from" came from an "out to" if you look at it that way, which means semantically it's 6 of one, half dozen of the other except that the industrywide perspective is "how do you get a bass signal into a mic pre".

 

[bongomania]

I thought it was V=IR where V is volts, I is current (amps) at R is resistance (ohms) . . . . but I understand what you're saying. So an amp that puts out 300W at 20Hz be, hmmm, searching for the right words . . . . "more powerful" or "perceived to be louder" than an amp rated 300W at 1kHz . . . ?

No, sorry I misled you there in a mathematical sense. Regardless, the relationship directly involving signal frequency is not the wattage, but the load. And like a dummy I'm sitting here forgetting what the formula is, or if there even is a formula given the different design of different loads.

 

[fdeck]

Just to make sure I've got this right.
Say I have an deal, perfectly tuned cabinet - completely flat response, but with a typical impedance curve, say 20Ω at 100hz, and 8Ω at 200hz. I put 10V on the speaker terminals at 100hz, it has X-excursion.
If I raise the frequency to 200hz still maintain 10V and it will have the same excursion?

I think the danger in saying a constant voltage input is when the amplifier has a higher output impedance. Say 1Ω (as an extreme example). When the driver impedance is 20Ω the effect is 1/20, when it's 8 ohms it's 1/8. You'd need to be sure to measure and add adjust the voltage at each frequency.

I think Bill nailed it. The way I think of it is that flat on-axis response from an ideal piston translates into constant acceleration, not constant excursion, at all frequencies. In frequency space, acceleration is the excursion times the square of the angular frequency, so excursion goes down as the frequency goes up. As it turns out, an ideal cone speaker has the same frequency relationship above resonance when driven by an ideal voltage source.

In terms of measuring and adjusting the voltage at each frequency, a modern amp does that with negative feedback. Both tube and solid state power amps do this, but in different ways.

Overdriving the amp effectively turns off the feedback mechanism, so that the amp becomes current limited by the internal impedance of the circuit.

 

[roflol]

phew . . . a little out a breath. I've got a quick question for the experts:

Amp X is rated at 300Watts at 20Hz.

Amp Y is rated at 300Watts at 1kHz.

Will Amp X be perceived as louder?

amp x is going to be reallllllll loud if it can put out 300watts of volume at 20hz

 

[slyjoe]

Watts is not a measure of volume, as has been pointed out in about the last 100 posts.

 

[seamonkey]

Excursion goes down as frequency goes up, once you're above the area where a reflex alignment affects it. SPL will remain the same even though impedance has dropped and power has therefore increased. By the same token go higher in frequency and impedance will rise, power will drop, but if voltage remains constant so will SPL within the region of flat response.
...

The lights have been turned on! Thanks.