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Yet another autoformer question ...

R

Rick58

Active Member
IIUC, Mc amps are designed for ~2 ohm loads. The autoformers have taps at 2, 4, and 8 ohms, and the amp is rated at 250W into 2, 4, or 8 ohms. (MC252 as an example ...)

http://www.roger-russell.com/mcintosh1.htm#autoformer explains the rationale for the autoformer, but of course I see the 'argument' made elsewhere about Mc autoformer amps 'not doubling down' with ohm-halving of speaker impedance, which I counter with 'not halving down' as the load is doubled ... but this 'counter-argument' actually depends on one changing the tap from 8 to 4 to 2.

So ... my question is what happens when the speaker is connected to say the 4 ohm tap, and the speaker impedance varies from (say) 2 ohms to 16 ohms across the 20Hz - 20kHz frequency range?

I have read ron-c's comment about the amps actually being able to 'double down' at least to half the tap setting, so I believe the amp could actually put out 500W into the 2 ohm part of this load.

What would the power rating be into 8 ohms? 16 ohms? all from the 4 ohm tap ... could it still put out 250W into those impedances? or would it 'halve' to 125W and 62.5W?

Maybe partially answering the question ... I would guess that it depends on the voltage rails.
 
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if I did the math correctly, IF the voltage available was ~32V (allowing 500W into 2 ohms, with a current of 16 amperes), then the power (in watts) available would indeed follow what I stated. The voltage would remain 32V at all impedances, but the actual current would 'halve' with each doubling of the ohms.

But do the autoformers provide some other 'magic' that allows more (i.e., full rated) power to be available from a given 'tap' into higher than normal impedances? That is mostly what I am confused about.
 
I don't believe the output of the autoformer is "dynamic" providing uniform power to a speaker with a changing impedance across the frequency band. I always thought Mac amps were voltage amps but they work with lower voltage rails and higher current capability. The autoformers change the output voltage to match the nominal impedance of the load.
 
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