The preamp should be a pass through (unless you have bass/treble and/or other processing such as RoomEQ..).... best measure would be SINAD - ie: how clean and undistorted the sound passes through.
There can be issues with impedance matching of preamps and poweramps - but I think those have been resolved for years - fairly unusual to come across them with any recent gear.
Power amps on the other hand can sound quite different one to the other - primarily when handling speakers that are outside their rated range (very low impedances) - lots of amplifiers "misbehave" when powering speaker impedances below 4 ohm....
Most speakers don't go so low on impedance - but some of the very best speakers... more "exotic" examples - do.
My Gallo Ref 3.2's go down to 1.6 ohm on the tweeter, and around 3 ohm on the woofer.
When asked to power such speakers, amps that can't provide the necessary current or go unstable at that impedance - will cause various sometimes frequency dependent distortions... they will have a "sound".
When amps are run within their rated capabilities - they don't have a sound. (repeated "blind" tests have shown this!)
But as soon as you own a set of speakers that stresses amps - you will start noticing how different amps can sound..
The easy solution - get an amp rated for 2 ohm, or at the very least rated to stable into 2 ohm loads.
Best would be an amp that is fully specified into 2 ohm or lower loads - and which doubles its output in watts, as the impedance halves. - A "perfect" amp would put out double the W into 4 ohm that it does at 8 ohms and would double again into 2 ohms.
Very good amps will go 1.7x into 4ohms and then a further 1.5x into 2 ohms
My Crown amps go 440W 8ohms, 775W 4 ohms and 1200W 2 ohms.... they are way over the top for the level of power I use - I normally use no more than 4W - but running them "stress free" means they don't "misbehave" and they act as a "straight wire with gain" - and also I have enormous amounts of headroom - the system is (in theory) capable of 110db peaks... but I have never seen the LED Meters on the amps rise to 10W let alone go any further.
In todays marketplace - I expect a preamp (or prepro, or AV Receiver...) - to achieve a SINAD of at least 90db... preferably in the mid 90's or higher.
I would like to see similar performance from the power amp - but I know that my own amps only achieve SINAD in the mid 70's... I have also Quad 606 power amps that probably do a bit better (mid 80's?) - but the sound is very clean in my system... and getting power amps that both sound good with my speakers, and are within my budget is .. difficult. The Benchmark AHB2 has great potential, as do some of the latest Class D amps - but right now, I am enormously satisfied with my systems sound... Dirac has done magic here! and I do not feel the need to upgrade.
There can be issues with impedance matching of preamps and poweramps - but I think those have been resolved for years - fairly unusual to come across them with any recent gear.
Power amps on the other hand can sound quite different one to the other - primarily when handling speakers that are outside their rated range (very low impedances) - lots of amplifiers "misbehave" when powering speaker impedances below 4 ohm....
Most speakers don't go so low on impedance - but some of the very best speakers... more "exotic" examples - do.
My Gallo Ref 3.2's go down to 1.6 ohm on the tweeter, and around 3 ohm on the woofer.
When asked to power such speakers, amps that can't provide the necessary current or go unstable at that impedance - will cause various sometimes frequency dependent distortions... they will have a "sound".
When amps are run within their rated capabilities - they don't have a sound. (repeated "blind" tests have shown this!)
But as soon as you own a set of speakers that stresses amps - you will start noticing how different amps can sound..
The easy solution - get an amp rated for 2 ohm, or at the very least rated to stable into 2 ohm loads.
Best would be an amp that is fully specified into 2 ohm or lower loads - and which doubles its output in watts, as the impedance halves. - A "perfect" amp would put out double the W into 4 ohm that it does at 8 ohms and would double again into 2 ohms.
Very good amps will go 1.7x into 4ohms and then a further 1.5x into 2 ohms
My Crown amps go 440W 8ohms, 775W 4 ohms and 1200W 2 ohms.... they are way over the top for the level of power I use - I normally use no more than 4W - but running them "stress free" means they don't "misbehave" and they act as a "straight wire with gain" - and also I have enormous amounts of headroom - the system is (in theory) capable of 110db peaks... but I have never seen the LED Meters on the amps rise to 10W let alone go any further.
In todays marketplace - I expect a preamp (or prepro, or AV Receiver...) - to achieve a SINAD of at least 90db... preferably in the mid 90's or higher.
I would like to see similar performance from the power amp - but I know that my own amps only achieve SINAD in the mid 70's... I have also Quad 606 power amps that probably do a bit better (mid 80's?) - but the sound is very clean in my system... and getting power amps that both sound good with my speakers, and are within my budget is .. difficult. The Benchmark AHB2 has great potential, as do some of the latest Class D amps - but right now, I am enormously satisfied with my systems sound... Dirac has done magic here! and I do not feel the need to upgrade.
