Amplifier specs confusing

[nstzya]

225 W RMS at 8 Ω （4-channel output)
320 W RMS at 4 Ω （4-channel output）
430 W RMS at 4 Ω (BTL mode, 2-channel output)
650 W RMS at 2 Ω (BTL mode, 2-channel output)
SN Ratio 112 dB A weighted.
Maximum output current per channel 16.5A/ 33A (Bridged)

These are the advertised specs of a D class amp I’m considering. Working backwards from the max output of 16.5 Amps/33 Amps (bridged) per channel, the watts seem awfully conservative. Are they trying to meet a SNR? Other?

 

[DonH56]

Typically they trade power vs. THD. Max current may (probably does) come at higher distortion than in the spec list.

 

[sergeauckland]

There's really no correlation between maximum power, which is limited by the maximum voltage the amplifier can generate, and maximum output current, which is what the amplifier can provide for very short-term peaks into a very low impedance. As Don mentioned, distortion isn't specified for peak current.

The output power of 225 W into 8 ohms means that each amplifier can supply 42.4V

S.

 

[nstzya]

OK, so therein lies my confusion. Working from that voltage of 42.5V, it should then produce 450 watts into 4 ohms provided there is the current available, correct? That’s about 10.5 Amps - well below the max. So what would be going on that it would only be rated for 320W? And then bridged only 430W?

 

[DonH56]

Thermal management (heat dissipation), desire to hit a certain THD/SNR/SINAD specification below max current output, etc. Look at amplifier distortion charts here and elsewhere and noticed how it rises steeply as full power is reached and exceeded. If they want to rate 0.01% THD or whatever then max current probably does not happen until 1% or 10%. Protection circuits might cut in if they tried to rate at max current; that is likely a short-term (burst) specification, whereas power tends to be for "continuous" operation. As @sergeauckland said, the specs are not and do not need to be tightly coupled/correlated. Different test conditions, different goals/targets for the specs, and so forth.

 

[nstzya]

I may have found the problem. Think there is a typo in the specs for bridged modes. In a review, I saw specs listed as:

225 W RMS at 8 Ω （4-channel output)
320 W RMS at 4 Ω （4-channel output）
430 W RMS at 8 Ω (BTL mode, 2-channel output)
650 W RMS at 4 Ω (BTL mode, 2-channel output)

Now THAT makes more sense to me. I’ll point out the typos to the manufacturer and confirm.

 

[MZKM]

An 8ohm load into a bridged amp is seen as 4ohm by the amp. So, do you do this spec at 4ohm (what the amp sees) or at 8ohm (what the load is)?

Or, it is a typo.

 

[nstzya]

Good point. That may be exactly what it is. I’ll see what manufacturer responds.

 

[DonH56]

If you bridge an amplifier the power should ideally quadruple.