I'm designing an internal sound system for an outdoor sound sculpture. Audio quality is important but volume (sound pressure) is what's most critical in this application.

I have auditioned many speakers, and settled on these outdoor speakers. They are rated at 175W Peak. I initially tested them through this cheap $25 100W amp I picked up from China. Quality wasn't great, but sound pressure wasn't bad (it was quite "loud", which for this application is most important). Given that I was under driving the speakers, I figured I should pick up a more powerful amp so the cheap amp wouldn't fail after continuous max volume playback.

I picked up this 1000W Peak 500W RMS rack-mount amp and connected the same speakers. Sound quality was way better across the spectrum but I was shocked that when cranked to max, the sound pressure was significantly lower than with the cheap amp.

I am really new to this side of audio and this contradicts everything I've learned so far (or thought I've learned) about driving speakers. Can you shed some light on what might be going on? (And more importantly, HOW can I get more output i.e.: perceived volume, out of these speakers?)

  • What was the speaker impedance? also are you aware that the "pro" amp you are using will only drive 125W into an 8-ohm speaker in stereo mode? This is only twice the RMS power of the cheaper amp. The pro amp isn't as grunty as you might think it is. Check the specs of the "pro" amp. "Power Ratings: RMS Power Output: Stereo 8 Ohm: 2 x 125W, Stereo 4 Ohm: 2 x 250W, Bridged 4 Ohm: 1 x 500W"
    – Mark
    Commented Nov 21, 2018 at 12:33
  • thanks @Mark. Yes, I'm aware of the specs on the larger amp, but I would still have expected a noticeable difference in overall "loudness". The speaker impedance is 8ohm, and the speakers' RMS is roughly 125W, so it should suit the amp perfectly.
    – Tom Auger
    Commented Nov 21, 2018 at 16:24

4 Answers 4


Don’t have time to write a really good answer right now, but max output power is not the same as gain. I couldn’t find either a gain (in dB) or an input sensitivity (in volts) for those amps, but I presume the pro amp just doesn’t have the same gain structure as the cheap one. Like user25885 says, it’s set up for a “hotter” input. The only way to get the same output level is to use a preamp. But if you give it that hotter input it will be able to get louder without clipping.


I was shocked that when cranked to max, the sound pressure was significantly lower than with the cheap amp.

Did you measure this, or was this perceived loudness? If it's perceived loudness: distorted sound can be perceived as louder than clean sound. And this is a big difference between cheap and expensive amplifiers: the expensive one will be able to supply a clean signal at its rated power, the cheap one will start distorting at lower power levels.


Does "cranked to the max" mean that your dials are at the end, or does it mean that the amp starts distorting? If it is the former: professional equipment usually takes signal levels of +4dB while consumer equipment takes signal levels of -10dB. If you have that kind of level mismatch, you won't be able to get from the amp what it can deliver.

Also notice that usually you should have the amp rated higher than the speakers (here you have the speakers at 175W while the amp is, at the speakers' impedance, rated at 125W). The reason is that one can usually hear when (professional) speakers are close to maxing out and can dial the sound down. If your (solid state) amp is maxing out, in contrast, you have very little time before your tweeters are gone. Probably not if your speakers are rated at least 10 times as powerful as the amp, but then that would be sort-of pointless, too.

  • Interesting note about the +4dB vs. -10dB. I'm coming straight out of a Mac / PC line out so I'm not sure, but would wager on the -10dB side.
    – Tom Auger
    Commented Nov 22, 2018 at 5:08

Can you get the clip indicators on the amp's front panel to light?

If not, which inputs are you using? If the jack/XLR combo sockets, try the phono sockets.

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