Question about driving a power amp

[SlowpokeJoe]

Hi there. I have to admit that I do not know what is meant when a pre-amp or drive pedal has "enough output to drive a power amp"

Is there something about specific about power amps that require a hotter input? Is this something that is limited to specific power amps like tube powered ones or is this implying that some pre-amp pedal has enough output to drive a power amp to breakup?

Thanks in advance!

 

[gtirard]

Don't know the technical details, but I will try to give you a rough overview

An Amp is made of 2 parts: a preamp and a power amp
The preamp takes an instrument level input signal, and send back a line level signal to the power amp.
Both parts are integrated together, but in a regular amp gain and EQ knobs correspond to the preamp stage, and master volume to the power amp stage.

There are very few standalone power amp, like the Sumo or Baby Sumo. You can't plug a bass directly in them because they need a line level signal, not instrument level.

It's pretty normal that a preamp "has enough output to drive a power amp" because it's basically what a preamp is supposed to do, but not all drive pedals do.

Some amps allow you to bypass their internal preamps if they have a "line in" input, you can plug your preamp pedal.

By the way, I wonder if the internal preamp of an active bass (Stingray or L-2000 for example) have enough output to drive a power amp?

 

[Wasnex]

Most power amps require a line level signal, which is much hotter than instrument level. The specs you need are the Sensitivity rating of the power amp and the max output of whatever device you plan to use to drive it.

Sensitivity tells you the signal level that is required to drive the amp to full power. Some amps have a switch that allows you to choose different Sensitivity levels.

For example, here is the QSC PL3 sensitivity specs.

This shows the amp has two settings, 1) 26dB and 2) 32dB.

When set to 26dB, the PL325 requires 3.28V RMS to drive it to full power. If you change the setting to 32dB, the amp requires 1.6V RMS to reach full power.

Here are the results from an online calculator that show the equivalent dBu and dBv levels.

Here is the calculator's address if you want it:
dB dBu dBFS dBV to volts audio conversion digital - calculator volt to dBu and dBV dB mW SPL dB decibels 0 dBFS - convert dB volt normal decibels relatioship relation explanation analog audio absolute level true rms convertor converter decibel to dbfs converter calculation online attenuation loss gain ratio reference audio engineering sound recording dBFS dBVU 0 dB audio logarithm level converter peak to peak p-p impedance voltage pro consumer audio digital analog recording level - sengpielaudio Sengpiel Berlin

Mesa Subway Preamp/DI

Ref: 070543-Subway_DI-Preamp_download.pdf (imgix.net)

The Preamp Output (XLR balanced) has a max level of +12dBU. If the PL325 is set to the 26dB, it requires 12.54dBu to reach full power. You might want to use the 32dB setting, which only requires 6.3dBu for full power.

 

[teemuk]

Roughly simplified, amps have voltage gain x and output signal voltage y at which they produce their rated output power.

You figure out amp's "input sensitivity", z, or input signal voltage at which the amp produces its rated output power from

z=y/x

If input signal level is lower than specified input sensitivity the amp isn't "driven" to produce it's full output power.

 

[Crater]

Is there something about specific about power amps that require a hotter input?

In the context of TalkBass, this almost always is referring to rack-mount power amps that are used for sound reinforcement/PA use, like those made by Crown, QSC and others. It's the most practical way to build a 1,000 watt (or more) bass rig for those that need it.

 

[SlowpokeJoe]

Most power amps require a line level signal, which is much hotter than instrument level. The specs you need are the Sensitivity rating of the power amp and the max output of whatever device you plan to use to drive it.

Sensitivity tells you the signal level that is required to drive the amp to full power. Some amps have a switch that allows you to choose different Sensitivity levels.

For example, here is the QSC PL3 sensitivity specs.
View attachment 4632937

This shows the amp has two settings, 1) 26dB and 2) 32dB.

When set to 26dB, the PL325 requires 3.28V RMS to drive it to full power. If you change the setting to 32dB, the amp requires 1.6V RMS to reach full power.

Here are the results from an online calculator that show the equivalent dBu and dBv levels.
View attachment 4632938

Here is the calculator's address if you want it:
dB dBu dBFS dBV to volts audio conversion digital - calculator volt to dBu and dBV dB mW SPL dB decibels 0 dBFS - convert dB volt normal decibels relatioship relation explanation analog audio absolute level true rms convertor converter decibel to dbfs converter calculation online attenuation loss gain ratio reference audio engineering sound recording dBFS dBVU 0 dB audio logarithm level converter peak to peak p-p impedance voltage pro consumer audio digital analog recording level - sengpielaudio Sengpiel Berlin

Mesa Subway Preamp/DI
View attachment 4632940

View attachment 4632939

Ref: 070543-Subway_DI-Preamp_download.pdf (imgix.net)

The Preamp Output (XLR balanced) has a max level of +12dBU. If the PL325 is set to the 26dB, it requires 12.54dBu to reach full power. You might want to use the 32dB setting, which only requires 6.3dBu for full power.

This is super helpful and exactly the detail I was missing, thanks!