How does the inability of an amp to drive speakers manifest itself?

Tazman46

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Perhaps this is a foolish question, but if a set of speakers presents a load to an amp that's too demanding, what is it that you hear? Or is it what you don't hear? I had always assumed that it would be obvious distortion but it occurred to me that it could also be a lack of extension and dynamism, extended highs and lows etc. Or do you hear nothing & simply cause long term damage to the amp? Reason I ask is because I'm going for a demo later & depending on what I choose to believe on the net, this situation will potentially arise.

 

SergeAuckland

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It rather depends on how loud one listens. At low volumes, there will be very little if any difference between a severe load and a benign load (Depending on how severe, these things are relative) as at low volumes, the amplifier will be require to give out little current so will still be working within its capabilities. At high volume levels, when the amp runs out of current it will distort, similarly to it running out of volts at clipping. Distortion rises rapidly, and the amplifier's protection may also start clipping the output.

The above assumes that the amplifier has a low output impedance, typical of SS amplifiers, but is incapable of driving high currents. A typical amplifier of this type was the original Quad 405, or earlier SS amps. They had a low output impedance but limited current capability.

Another mechanism is an amplifier with a high output impedance, such as a valve amplifier, especially one with little or no negative feedback. In effects, these amplifiers have a large value resistor in series with the loudspeakers (Large in this context means something like 1-5 ohms, as opposed to the 100-250milliohms of a typical SS amplifier). The effect of this is to form a potential divider with the loudspeaker's impdance so that the voltage across the loudspeaker varies with frequency. This will change the frequency response of the loudspeaker from the design figure to something rather different depending on the value of the amplifier's output impedance, the value of the loudspeaker's impedance curve and its normal voltage-fed frequency response. Amplifiers with high output impedances may or may not also distort more in the bass due to transformer core saturation, which is why the better valve amplifiers have to have physicallylarge and heavy output transformers.

As to damage, amplifiers constantly used at high levels into harsh loads will run hotter and therefore most likely have a shorter life than one run cool, but unless the amplifier is very badly designed, should not be damaged as they should be adequately protected.

S.

 
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JANDL100

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Ill-defined, soggy bass, firstly and also a lack of dynamic pace and drive (aka excitement, pizzaz) are the main things I hear when amps haven't got the grunt to drive a speaker properly.

 

Tazman46

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Well I guess that covers that then! Very comprehensive reply Serge and some very useful info to bear in mind for the future, thanks guys.

 

def

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Can't add to the technical information posted but can add an anecdote.

Last year I had the chance to hear an Audionote system, AN-K - Oto SE - Cd2.1x. It was really engaging and easy to listen to, at one point during the day another standmount was introduced for a bit. Can't remember the brand though do remember looking it up afterward and it was well reviewed. In any case they were much less efficient than the K's. The freq extreme's weren't significantly different, if anything bass was more punchy and they were slightly brighter. Presumeably the new speaker was designed with punchier bass than the K's in mind? It was obvious that the 10w SET Oto wasn't up to driving them as the midrange disappeared, it became recessed and rather lost in the mix. That was a surprise, I'd have expected the freq extremes to have been more affected and took from the experience that the implementation of each of the designs has a bearing on the sound you'll get.

 

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