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

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.