Input and output impedance (and voltage) for pre and power amp combinations?

[vln]

Can someone enlighten me?

I got about as far as

1) "The output impedance of the preamp should be as low as possible"
2) "The input impedance of the power amp should be as high as possible, preferably above 10 kOhm"

The way I understand it so far is that the power amp needs to be as 'responsive' as possible, which is why its input impedance should be high (correct me if I'm wrong), but why does the output impedance of the pre need to be low? Something to do with damping?

I am looking around for a new pre, and two of the candidates are the ATC SCA-2 (quite pricey though) and the Benchmark DAC1 (the sensible option).

The output impedance of the Benchmark ranges from 43 Ohms to 425 Ohms (depending on the attenuator setting), for the SCA-2 it's 15V into 600 Ohms, according to a HiFi Plus review.

How does voltage suddenly get into this? And 15V strikes me as quite high?

For the ATC speakers, the input sensitivity is 1V and the input impedance is > 10kΩ (according to the ATC website), so seems to meet criterium 2 from above.


Cheers,
Samuel.

 

[Pandapple]

Hi Samuel

With the quality of products you are considering, all these engineering considerations and compatibilities have been taken into account for you. So, you can plug and play.

The output impedance of a preamp needs to be low enough so that the capacitance of long cables does not cause high frequency roll-off. Under 1kohm means you can run 10m cable with the effect at 20khz just hundredths of a dB.

The input impedance of a power amp needs to be high enough so that the preamp can output necessary voltage without distortion. Over 10kohm is an easy load for pretty much any device of any cost.

The Hi-Fi plus review of the SCA-2 is not referring to the output impedance of the preamp. It is instead testing the distortion limit of the preamp. Being a high quality preamp, the ATC can output a massive 15V into a heavy load of 600 ohms. The load of 600ohms means the input impedance of the following device.

The input sensitivity of the power amp/ATC means the input voltage required to drive the power to clipping, or the speaker to its max SPL. Both the Benchmark and ATC can output 1V into 10kohm easily (but then so can the onboard audio from a laptop for that matter ).

Hope that helps

 

[vln]

Thx, that was helpful.

I have misread the specs in the HiFi Plus review, the quoted output impedance of the ATC is 10 Ohm (i.e. lower than the Benchmark), and 15 V into 600 Ohm refers to the pre amp driving a load (as you said in your post).

 

[TheDecameron]

vln,
On a slightly different note, the HDR is a very capable little preamp. I've used it to drive a couple of different power amps, balanced and s/e and it's surprisingly good.