RCAguy
Member
First may I commend this site and its founder\admin for a fine service to several audio communities, ranging from newbies to audiophiles to professionals. I myself am in the latter category – a professional audio engineer since 1958 (my first paying gig after graduating RCA Institutes, and since with a BSEE, long-time member of AES and SMPTE, researcher\presenter globally at ASA\CAA and Deutche Verein, author of two books – you get the picture).
However, I have a different interpretation re some of your measurements that I think will be useful for members…
Even though “we’ve always used 1w” is often the admonition, your arbitrary 5w tests for speaker amplifiers is no less valid, maybe more! But up to 0.05% SINAD such as for the Behringer A500 is nothing to sneeze at, because in most circumstances it will be inaudible, which is ultimately what matters. [SINAD is signal to total noise & distortion, sometimes THD+N that is a measure of audio fidelity.] Here’s a “gedanken test” (thought experiment) with a warning that it contains arithmetic (nothing more).
Beginning with listening and working backwards, say you follow the SMPTE cinema\studio mastering standard 85dB SPL reference, 20dB below full scale (FS), which allow for undistorted individual channel peaks of 105SPL, or about 108 with five (5) surround channels driven. Home listeners typically use 10dB lower (75SPL), perceived as “concert level” for acoustic classical, jazz, or movie sounds (there is no reference for pop\rock, as even hearing damage is not the limit). The 20dB between “reference level” and “FS” is intended as “headroom” for preserving the dynamics that occur in the natural, lifelike sounds of music, speech, and ambience.
Now insert a quality (monitor-grade) loudspeaker with a relatively high sensitivity of 91SPL for 1w listening in an acoustically controlled space at 1m – that’s ~2w at 1.4m “near-field” listening, ~5w at 2.5m (8ft) “mid-field.” For these three levels, each power amplifier channel would need to deliver (all channels driven) 20dB more power, or 100w, 200w, or 500w to pass the 105SPL peaks without clipping. A more common 88dB sensitivity speaker for home use (75SPL reference) would need 1/10 the power but ½ the sensitivity, or power levels of 20, 40, or 100w listening at 1m, 1.4m, or 2.5m – less power in a typically live, acoustically uncontrolled room.
The ultimate test is whether SINAD is audible. That will be limited by the environmental noise of the listening space. My control room and studio are both below 30SPL unweighted, or ~25 A-weighted, but ISO:226 (Fletcher-Munsen) applies mostly below 200Hz, so we’ll use unweighted in this example. 30SPL noise is 55 dB below the 85SPL reference, or 45dB below the nominal home listening level of 75SPL, and assumes an unusually quiet house. We can discern signals 15~20dB below noise, so to be inaudible, amplifier artifacts for typical home listening need to be -65dB below 1w, 2w, or 5w by their respective listening distances. -65dB is less than 0.05%. So if SINAD is <0.05%, the amplifier is not perceived to be adding bogus colorations to the sound. And especially not when you factor in that the individual SINAD components would need to be heard, and that another link in the audio chain distort and mask other links’ artifacts much more (speakers being the main culprits, along with acoustic issues).
So for the Behringer A500 power amplifier channel you measures THD+N about 0.047%. In a typical home environment it would be as good as needed. And while the measured spikes for its power supply could be better, individual even and odd order harmonic spikes are mostly down >80dB – very likely inaudible even in a quiet and acoustically-controlled studio. SINAD for most amplifiers appear best at highest power – why they are often the only figures a manufacture's marketing dept publishes – but as long as they cover peaks without clipping, it’s the 1\2\5w percentages that matter more, because most of the time that’s what we are hearing.
Keep up the good work! – Robin Miller (author of The Better Sound of the Phonograph)
However, I have a different interpretation re some of your measurements that I think will be useful for members…
Even though “we’ve always used 1w” is often the admonition, your arbitrary 5w tests for speaker amplifiers is no less valid, maybe more! But up to 0.05% SINAD such as for the Behringer A500 is nothing to sneeze at, because in most circumstances it will be inaudible, which is ultimately what matters. [SINAD is signal to total noise & distortion, sometimes THD+N that is a measure of audio fidelity.] Here’s a “gedanken test” (thought experiment) with a warning that it contains arithmetic (nothing more).
Beginning with listening and working backwards, say you follow the SMPTE cinema\studio mastering standard 85dB SPL reference, 20dB below full scale (FS), which allow for undistorted individual channel peaks of 105SPL, or about 108 with five (5) surround channels driven. Home listeners typically use 10dB lower (75SPL), perceived as “concert level” for acoustic classical, jazz, or movie sounds (there is no reference for pop\rock, as even hearing damage is not the limit). The 20dB between “reference level” and “FS” is intended as “headroom” for preserving the dynamics that occur in the natural, lifelike sounds of music, speech, and ambience.
Now insert a quality (monitor-grade) loudspeaker with a relatively high sensitivity of 91SPL for 1w listening in an acoustically controlled space at 1m – that’s ~2w at 1.4m “near-field” listening, ~5w at 2.5m (8ft) “mid-field.” For these three levels, each power amplifier channel would need to deliver (all channels driven) 20dB more power, or 100w, 200w, or 500w to pass the 105SPL peaks without clipping. A more common 88dB sensitivity speaker for home use (75SPL reference) would need 1/10 the power but ½ the sensitivity, or power levels of 20, 40, or 100w listening at 1m, 1.4m, or 2.5m – less power in a typically live, acoustically uncontrolled room.
The ultimate test is whether SINAD is audible. That will be limited by the environmental noise of the listening space. My control room and studio are both below 30SPL unweighted, or ~25 A-weighted, but ISO:226 (Fletcher-Munsen) applies mostly below 200Hz, so we’ll use unweighted in this example. 30SPL noise is 55 dB below the 85SPL reference, or 45dB below the nominal home listening level of 75SPL, and assumes an unusually quiet house. We can discern signals 15~20dB below noise, so to be inaudible, amplifier artifacts for typical home listening need to be -65dB below 1w, 2w, or 5w by their respective listening distances. -65dB is less than 0.05%. So if SINAD is <0.05%, the amplifier is not perceived to be adding bogus colorations to the sound. And especially not when you factor in that the individual SINAD components would need to be heard, and that another link in the audio chain distort and mask other links’ artifacts much more (speakers being the main culprits, along with acoustic issues).
So for the Behringer A500 power amplifier channel you measures THD+N about 0.047%. In a typical home environment it would be as good as needed. And while the measured spikes for its power supply could be better, individual even and odd order harmonic spikes are mostly down >80dB – very likely inaudible even in a quiet and acoustically-controlled studio. SINAD for most amplifiers appear best at highest power – why they are often the only figures a manufacture's marketing dept publishes – but as long as they cover peaks without clipping, it’s the 1\2\5w percentages that matter more, because most of the time that’s what we are hearing.
Keep up the good work! – Robin Miller (author of The Better Sound of the Phonograph)