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

Due to the fact that solid state amps tend to have a lower input impedance, many tube preamps will not make the best bass into them, even though as transistors go, the Pass Labs amps are quite moderate.

The thing to look at is the output impedance of the preamp. Usually this is specified at 1KHz (1000 cycles per second). And usually that is a benign value. Where it counts is the output impedance at 20Hz. If its even spec'ced, you'll find it to be considerably higher.

If you want the bass to be right, there can't be any phase shift in the bass. To accomplish this, the preamp must go down to at least 2Hz. To this end, its really helpful if the output impedance be constant across the band rather than rising at lower frequencies (the ARC is slightly over double the 1KHz value at 20Hz, climbing considerably higher as frequency drops below this point). ARC specs 0.2Hz as its bottom cutoff, but does not say into what load. The higher the impedance of the load, the lower it can go. Its a good bet this value for the spec is 100KOhms or higher.


One thing to keep in mind- while the Pass Labs amps do support the balanced line standard, neither of the preamps you have in mind do, even though they are balanced. The balanced standard (AES48 which stands for 'Audio Engineering Society file 48')) is there for two reasons- to prevent ground loops and to prevent the interconnect cable from having an artifact. If you've ever auditioned and heard differences between audio cables then you know what I'm talking about.


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.



 

@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...

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).

I see thank you for the further insight. 
So then I kindly ask which SS pre-amps are you aware that meet these ideals that you mentioned earlier? 

Ayre !!!!!

I've been really pleased with how well my VAC Ren MKV preamp pairs with my Pass XA30.8 amp, and hence picked Ralph's brain a bit about whether the preamp design lends itself towards good compatibility - including excellent and tight bass extension.  From the sounds of it, the VAC's transformer- coupled Class A1 balanced triode design is likely responsible for a large piece of the synergy.  I feel the combo is giving me a flavor of the best of SS/tube integration.

Thanks for your input @three_easy_payments, 
which model @tomic601 ? and what kind of system are you running?

my systems are visible on the virtual system page. The reference is an ARC REF5se preamp that while it does not meet the balanced standard that Ralph cites, make decent sound. It drives a 6 m length of Audioquest WEL cable effortlessly into Vandersteen M7 HPA amplifiers. I have used an inexpensive mogami set of XLR wire also with excellent sonic and low noise results.

I have spent more than a few hours listening to the Ayre KX-R preamp and it is an incredible bit of SS engineering. I have owned an Ayre power amp in the past and currently own and use in a recording studio an Ayre A2D converter - anything Ayre touch sounds good and is hyper well engineered.

just my two cents.

Ralph of course makes great gear also!

BTW the REF 3 is an excellent preamp

So then I kindly ask which SS pre-amps are you aware that meet these ideals that you mentioned earlier?

I don't keep track of them but as I mentioned earlier its usually pretty easy to tell if the preamp supports the standard. If it has RCAs in addition to XLRs as main outputs, and does not have either an output transformer or switch to go between the two outputs, then it does not support the standard. You can also ask the manufacturer of course; sometimes they will know but IME sometimes they won't; I've found many that don't seem to know that the standard exists.

But to be clear, stuff can sound just fine if it doesn't meet the standard- the ARC Ref3 is a good example as are the Pass Labs preamps. But you will find that you have to be more careful about the interconnect (which might cost as much as the preamp itself) and you could also run into ground loop issues. The insidious thing about a ground loop is they can cause degradation of the sound without making themselves known by obvious symptoms like a buzz. These are the things that balanced operation is meant to solve!


So if the preamp isn't going to support the standard, IMO/IME you are just as well off exploring single-ended preamps too.


Its the lack of support of the balanced line standard in high end audio that is why there is controversy in high end about which sounds better. All I can say to that is if you hear balanced line done right, there's no going back! But to do it really right, you have to support the standard.

While Ayre has always made excellent sounding products IMO, their preamps don't appear to support the standard- every one that I've seen has a single-ended output beside the balanced output.

@atmasphere  

I've done some research and my findings are rather interesting. 

A) I was aware pro gear is all balanced- as they need to run long cables etc. As such my crown amp only has XLR inputs but they are also NOT AES 48 certified, there is no certification on the amp nor in the owner's manual, nor is it on newer crown amps as shown on their website. 

B) Secondly, I think I've only really seen (in 20years) 1 amp or pre-amp (I don't remember which) that only had XLR connections. 

C) PIN 1 in XLR is ground, if source is RCA then PIN 1 needs to be jumped with PIN 2 as PIN2 then becomes ground (and not balanced anymore). This case is true for my PASS amp 250.5.
Meaning that in differential configuration on my PASS amp PIN 2 and PIN 3 are not connected to ground because PIN 1 is ground.

I also don't understand why you would promote AES48 as a standard that allows for "superior" sound when "almost" no one uses this standard. Pro audio and Audiophile gear has been around for over 40years... 

You also mention IIRC Rowland made a balanced amp but didn't "support the standard" so this means that manufacturers don't need to support the standard in order to make a true balanced design? 

So this basically means there wasn't a problem in the first place, AES 48 is just a stamp of approval (that most manufacturers though following the design chooses not to apply for the certification). And as such brings me to the next point where you say

  "But you will find that you have to be more careful about the interconnect (which might cost as much as the preamp itself) and you could also run into ground loop issues."
Why would one need to spend $1000 on RCA interconnects to avoid a ground loop? when one could just use the XLR and it's shielded.

I'd like to get your input in all this confusion. 
Thanks

AES48 is a specification, **not** a certification. Your Crown supports the standard, as do Pass Labs amps, while their preamps do not.

Most studio gear (but not all) supports the standard. That gear is where the sounds we hear from our stereo comes from. If you run single-ended out of a microphone, that signal won't go very far before its too degraded to be hifi! But run it balanced and direct microphone feed can sound spooky real.


Its easy to build a circuit that is fully balanced and doesn't support the standard. There are not that many ways to build a balanced circuit that *does* support the standard, and because its either expensive or patented (which is another way of saying its expensive) a lot of high end audio manufacturers don't bother.


The standard is there to prevent noise, distortion and buzz from ground loops and to prevent the interconnect cable from imposing an artifact. It was actually invented for telephone use and is what made trans- and inter-continental phone calls possible. But the recording industry realized the benefits immediately and this ushered in the era of High Fidelity.

Now I've noticed that a lot of audiophiles take considerable time and spend considerable money making sure their interconnect cables sound right, but what if there was a way to make **all** cables sound right? Now the cables don't have to cost as much as the preamp! That is what the balanced line system is for, but to implement it and get all the benefits from it you need to observe the requirements the technology has in order to make that happen. Imagine a really nice coat with no zipper or buttons- its not going to be all that warm. Balanced operation is similar- you gotta do everything right to make it work.

B) Secondly, I think I've only really seen (in 20years) 1 amp or pre-amp (I don't remember which) that only had XLR connections.

We started doing it in 1989. 20 years ago it was not nearly as common as it is now. The XLR is the accepted connector for balanced operation; in the early 90s I used to see gear that used dual RCAs for the same task! I think there's been a bit of education going on- you don't see that anymore.

C) PIN 1 in XLR is ground, if source is RCA then PIN 1 needs to be jumped with PIN 2 as PIN2 then becomes ground (and not balanced anymore). This case is true for my PASS amp 250.5.
Meaning that in differential configuration on my PASS amp PIN 2 and PIN 3 are not connected to ground because PIN 1 is ground.

Usually pin 2 is the non-inverting input and so pin 3 is jumped to ground. At that point the connection is single-ended and the shield is now being used to complete the circuit. Pin 3 is usually the inverting input; as you can see a balanced connection has 2 signal pins rather than one. Because of the differential nature of the input circuit on the Pass (or our amps for that matter) those inputs can easily ignore ground. On the preamp side is where it gets tricky. Most make their output with respect to ground; this opens them up to ground loops and suddenly the shield of the interconnect cable becomes critical to its overall artifact.

Why would one need to spend $1000 on RCA interconnects to avoid a ground loop? when one could just use the XLR and it's shielded. 

You can spend $1000/foot on an RCA cable and still have a ground loop- no amount of money in the cable can avoid that! And just using an XLR cable won't get rid of that if the source drives it single-ended. That is what most high end audio manufacturers do- they drive the cable single-ended, but with two single-ended signals, both using the shield to complete the circuit. If its done correctly though, the output of the preamp won't reference ground. This is why in many balanced line products that support the standard, an output transformer is used, since it can float and not reference ground at all. The output of the transformer is merely tied to pins 2 and 3, and pin 1 is simply the chassis ground with no connection to the transformer at all. 


A good transformer is expensive, so when hifi became a thing in the 1950s, that was something that was immediately dispensed with in the consumer gear. Back then RCA had a nice connector that was cheap and had wide bandwidth so that became the defacto standard and is still around today for single-ended operation. So there was a cost thing that is heavily wrapped up in all of this, but IMO/IME balanced does not have to cost any more if we are talking about a high end audio product.