what pre amp for ATC SCM 40v2 spkrs + 250.5 Pass Labs pwr amp

@atmasphere   

To this end, its really helpful if the output impedance be constant across the band rather than rising at lower frequencies

Would the output impedance of the preamp be static across the band if by design it's not dependent upon feedback?

No. The variable is what is used to couple the output of the preamp to the amp. If a coupling capacitor is used you can expect the output impedance to rise as frequency goes down.


There are zero feedback designs that have a ruler flat output impedance curve- exactly the same as their frequency response.

@atmasphere  Ah gotcha.  So a transformer-coupled design would be more desirable on this front?  I appreciate the education that your posts always bring.

So a transformer-coupled design would be more desirable on this front?

Obviously the design of the transformer is paramount, but in a nutshell, yes, if you can get the bandwidth. FWIW the venerable Ampex 351-2 (which is a studio tape machine from the 1950s) made a lot of the recordings in the Mercury and RCA Living Stereo catalog. It clearly could make bass :) So this approach definitely works. And there are a few tube preamps that use or have used output transformers; we made one called the P-2 years ago, EAR made one and Backert Labs makes one right now. We developed a direct-coupled output for our preamps (which we patented) that supports the balanced standard.

Many thanks for the responses and  @atmasphere,

I'll need to further research to digest the ideas you have expressed. It does sound like I'll have a hard time finding an ideal since manufacturers don't market with these concepts, let less explain them. 

So basically I need to:
1) check the output impedance vs frequency graph to be flat
2) relative to whatever load (value) used to measure. 
3) make sure it is AES 48 standardized 

Sorry the last paragraph:
"To this end, the output of the preamp can't reference ground- instead the complete signal is set up so that the non-inverting output references its opposite and vice versa; ground is ignored and used only for shielding. Traditionally this was done with an output transformer when tubes were king and still is with solid state because of the grounding issue. Regardless there are tube preamps that support the standard. "

Means that old tube amps are more likely to be what I'm looking for,
and solid state has always been more common of what I'm looking for?

I think to sum it up, these specs that you speak of seems like the ideal pre-amp "in general", as oppose to the "best" pre amp for my scenario?
I mean I'm assume everyone wants what I want of course lol
 (wider soundstage, more bass, more detail)....?

Now the simplest solution, I suspect is just to grab a Pass Labs pre-amp from the same era...