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"Output impedance of a preamp is something most people don't think about, but can be influential to the sound. Lefty's rule of thumb (or thump, love that one) of having the preamp output impedance at least 1/10th of the power amp input impedance is sound advice. At this ratio, there will be only 0.83dB of loss in the interconnect. And since this is just a voltage divider loss, it will be constant throughout the passband.
But, to get back to the original question, yes lower is always better. I recall seeing some preamps with as low as 50 ohm spec'd output impedance. Others are much higher. Typically, I've seen 500 as being a common number for SS preamps.
To illustrate what the effects of preamp output impedance has on the sound, I did a quick search of the web for cables. I chose, at random, some Belden cable (9223) which claims 37pF/ft. Let's assume an interconnect is made of 6ft of this, which yields 222pF. If we had an amp with an input impedance of 47k (typical number) and a preamp with an output impedance of 4.7k (1/10th rule), this Belden cable will give a system result of being down 0.06dB @ 20kHz. Keeping everything the same but changing the preamp output impedance to 50 ohms gives us -0.000008dB @ 20kHz. Highly unlikely you'll hear any effect due to the change in output impedance.
But, let's assume that Mr. Audiophile has his amps close to his speakers and must have longer interconnects. Let's also assume he is using an expensive audiophile approved interconnect that happens to have double the capacitance of the randomly found example above. Cable is now 15ft long with 74pF/ft or 1110pF. With this cable the preamp with 50 ohms output impedance is now -0.0002dB @ 20kHz. The 4.7k preamp is -1.28dB @ 20kHz. Finally, let's get extreme and say Mr. Audiophile has an audiophile approved tube preamp with 10k output impedance and a power amp with 100k input impedance to satisfy the "rule." This situation gives us -4.1dB @ 20kHz. In this case, preamp output impedance can play a role in the sound of the system.
To sum it all up, preamp output impedance contributes to the ability to hear differences in cables between preamps and power amps. A preamp with a high output impedance will give the user the greatest possibility to hear differences in cables. A preamp with a very low output impedance will make all cables sound essentially the same. To put it differently, a very low output impedance preamp will be immune to cable effects and a very high output impedance preamp will enable the audiophile to spend countless months and dollars searching out that elusive 'magic' cable."
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