Bridged mc7270 -power guard lights

[GenghisKhan]

Current set up
Aerial Acoustics Model 8 bi amp mode
estimated B-weighted sensitivity of 85dB/2.83V/m. According to Stereophile.

Krell HTS 7.1 preamp
Mac Air, Apple Music
SMSL SU9 DAC
2 MC7270 amps bridged mono for mid/ highs Speaker plugs. Should be a minimum of 540 wpc at 0.02 THD. 1 MC7270 recapped and tested at George Meyers at 340wpc. 2nd MC7270 is new to my system, not yet tested but seems to operate correctly.

Yamaha Mx-D1 amp, connected balanced to Krell and connected to bass input on Speakers.
500Wpc into 4 and 8 ohms (24dBW, 27dBW, respectively) at 0.1% THD. Frequency response: 1Hz–100kHz, ±3dB; 20Hz–20kHz, ±0.5dB. THD at 10W/1kHz: <0.003%. Input sensitivity: 1.3V. Input impedance: 25k ohms. Damping factor: 200 (equivalent to an output impedance of 0.04 ohms), no frequency specified. Channel separation: 100dB. Dynamic range: 120dB or greater (IHF-A, low-pass filter at 20kHz). According to Stereophile. Just had it repaired by George Meyers.

So even with 85dB efficiency, I should have plenty of power to drive these speakers.

I got the power guard lights to flash on both MC7270 (simultaneously) at 30 volume setting on my Krell. Did not sound distorted.

With the MC7270 running bridged, am I exceeding 540wpc into the mid and highs?

That seems extremely unlikely.

Thanks in advance for sharing your knowledge.

Roy

 

[damacman]

Read up on what Power Guard is and how it works from the OM of the 7270. What impedance are the speakers? What taps are you using.

 

[GenghisKhan]

Read the OM on Power guard before I posted.
Nominal 6 ohms dipping to 4 ohms.
Using 2 ohm taps as per OM since bridged. Otherwise would be using 4 ohm taps.

 

[whoaru99]

With the MC7270 running bridged, am I exceeding 540wpc into the mid and highs?

Unless you think both amps are broken or not hooked up correctly, the evidence at hand suggests that's the case.

 

[damacman]

Switch to the 4 Ohm taps, report back.

 

[GenghisKhan]

Will try that.

 

[GenghisKhan]

Well I tried that. Didn't really make a difference. However, to sanity check my hearing, I used an online microphone app and measured one of my test tracks, Dire Straits "Money for Nothing".
Setup - Yamaha Mxd1 connected to bass taps on Aerial Acoustics Model 8
1 MC7270 connected to mid/high taps. No bridging this time. Using 4 ohm taps on MC7270 in stereo, 0.75V setting. Yamaha input sensitivity is 1.25v so picked this over the 2.5v setting.

Played at level 10 on my Krell. Seemed like moderate volume. Looked at the app and was shocked - peak volume of 103db, avg of 98db.

Increased volume to 20.
App shows peak volume of 108db.

Infreased volume to 30
App shows peak volume of 119 db. I think that's a tad higher than reference level.

My 60 year old ears are shot.

I didn't realize I was playing music this loud. I'm sure the speaker is working way overtime.

 

[damacman]

Keep in mind that the Power Guard lamps indicate that the distortion threshold between input and output has been exceeded, not that the amplifier has exhausted its power reserves. For example, running an 8 Ohm load on the 4 Ohm taps results in 3dB less output power capability as compared to using the 8 Ohm taps. So the PG lamps could be flashed at 3dB less power than the amp is capable of given the mismatch. When using amplifiers to amplify part of the speaker (bottom / top), one really needs to look at the impedance plot of the speaker when choosing the correct taps to maximize the relationship between amplifier, speaker, and power delivery. Let's say that you were using the amps at 80Hz and up and the impedance curve showed a dip to 4 Ohms at 125Hz which was 1/6 of an octave wide but above 8 Ohms in all other places - the 8 Ohm taps would be the ideal choice or you would sacrifice power at every other place except said dip.

Little know fact about the autoformers is that they also manage heat - hence McIntosh's conservative recommendations in the manuals about how to select taps. The MC7270 has about 1/5 of the heat sink mass as a similarly powerful direct coupled amplifier.

Clean music being played loud is often staggeringly so. I've measured 105dB A-Weighted at my listening location a time or two, which is so loud that it's scary. About half a song, and you simply can't do that any longer. I've seen peaks of 120dB C-Weighted, also crazy. Meanwhile, the bridged 2600s just mock me.

I find that McIntosh amplifiers do LOUD as good as anything I've ever heard. Rock on.