2. amps-preamps
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To Pre or not to Pre? Here is my take

I remember reading in many places that the best preamp was no preamp.
Finally in a position to test that theory
Equipment is Esoteric sa-50 player, audio research ls preamp, bat vk600 power amp.
So I let everything warm up for an hour or so and then played some Halestorm through the system, for anybody who is not familiar with Mz. Hale, she is similar to a smoother Bonnie Tyler with more range! Her band has some great bass guitar and drum work as well so a nice little workout.
After 5 or 6 tracks I changed over to powering the bat direct from the esoteric using the same cardas xlr interconnects.
It only took a couple of tracks to confirm
I just did not like this sound, bass was much thinner, soundstage not as wide, vocals not as smokey for want of a better description of Mz. Hales style.
Hooked the ls2 back and joy was restored!

What does this tell me?
That absolutely the ls preamp is adding its own sonic signature to the mix, in theory that equates to probably a more "dirty" sound but to me this was the sound I preferred.
Now obviously no 2 people listening to the same gear are going to hear the same thing, its impossible!
However playing MY music on MY gear the sound going through the ls2 preamp was the sound that I wanted to hear, end of story.
Oh trust me its not just value perception, I sort of WANTED the sound to be better without the ls2 preamp as I could have then sold it and spent the money elsewhere! 
I know there will be lots of people who can spout theories to prove I am full of it but the only judge of the sound of your own system worth worrying about is yourself!
uberwaltz
atmasphere's avatar
atmasphere
12,138 posts
 
The issue is not what is the output impedance; whether its tube or transistor. The issue is that the output impedance can vary with frequency while not at the same time having flat frequency response into a certain load impedance. IOW the output impedance curve is not the same as the frequency response!

I've seen this phenomena with solid state CD players and tuners- its really a function of that output coupling cap. You can't always make it as big as you want; the cap construction itself often restricts what is the largest expedient value to use for best sound. This is particularly true of film caps; electrolytic coupling caps have a bad reputation but they do allow for much higher values.

We avoid the issue altogether in out preamps by using a direct-coupled output we patented. In such as case the output impedance curve looks exactly like the frequency response curve.
georgehifi's avatar
georgehifi
10,312 posts
 
The issue is that the output impedance can vary with frequency
Yes of course it can, especially if capacitor coupled. (best cap is no cap)
This is why my system is direct coupled from dac convertor chips output pins all the way to the speakers, with dc servos at each stage taking care of any dc offset.

Cheers George  
atmasphere's avatar
atmasphere
12,138 posts
 
^^ That's pretty unusual (and also explains why a PVC works in your system)!

The problem you often run into with direct-coupling is the circuit can have bandwidth that can go lower than the actual power supplies (which always have some sort of time constant, unless powered by batteries). 

If this is not handled correctly IMD will rise and there can be low frequency instability.

On top of that servos can be a bit tricky (we use servos as well in our direct-coupled outputs) and in general will have some sonic artifact unless there are at least two poles in the servo's bandwidth.

If the servo isn't right, IMD can rise and there will be low frequency instability.

If you can change the servo IC or other component in the servo without changing their values and that results in a change to the sound of the system you know you have a problem.

This is why 99% of everything out there uses coupling caps.
georgehifi's avatar
georgehifi
10,312 posts
 
This is why 99% of everything out there uses coupling caps.
In tube equipment yes, but try to tell designers like Dan Agostino, John Curl, Jeff Rowland etc etc. that. They are firmly in the direct coupled camp, from go to whoa.

I've seen many problems with pieces of even expensive equipment that is capacitor coupled, in that one the cap is not big enough in uF(microfarads), two they use poor quality caps, three sometimes they use electrolytic caps, even I've seen bi-polar electrolytics used.
Still the best cap is no cap. 

Cheers George
atmasphere's avatar
atmasphere
12,138 posts
 
^^ I’m not arguing that point; we don’t use coupling caps at the outputs of our preamps or amps as I previously stated.

But most sources use them whether we like it or not.
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