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

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...

There aren't vintage tube preamps that support the standard, as they are all single-ended. The oldest tube preamps that support the standard were made by us- we made the first balanced line preamps back in 1989. IIRC Rowland was the second to make such, but to my knowledge didn't support the standard. One easy way to tell is whether RCA outputs are provided in addition to the XLR outputs; if they are then the standard likely isn't supported. This is because RCAs do reference ground, where as an XLR connection should not- they are inherently incompatible.


As best I can tell the Pass Labs preamps don't support the standard.


Now you don't *have* to have a preamp that supports the standard; if the preamp doesn't it can still sound just fine, you just don't get all the benefits that are supposed to come with balanced line- lack of ground loops, immunity to interconnect cable artifacts and the ability to run really long cables (although the benefit is there even if the connection is only 1 foot long).