How important is the pre-amp?

@cheeg raises some interesting points.

Point #1: Wikipedia URL with comment "you can see its not quite that simple". Actually, it is that simple. he wikipedia article manages to make a simple topic complicated by badly explaining older dB references in the context of impedance matching. Impedance matching is, in fact, one of the key jobs of a preamp. Sometimes it’s needed, sometimes not. If only high end products adhered to some standards or conventions (they often don’t) the problem would be far simpler. In fact that article points out just how wildly out of spec most DAC outputs are today. nominal line level - under a volt rms (caveat I always thought it WAS updated to 1V rms, but no matter). Many DACs now put out 2, 3 and even 4V this means a 15V swing at the INPUT to your preamp. Are they mad? Which brings us to point #2.....

No, they are not mad. Just narrow minded (IMNSHO) or maybe lazily practical for heir own product anyway. If, as a manufacturer, you wish to offer a DAC with volume control and claim it's basically a control center for all-digital systems, you need sufficient gain (output level) to drive any amp. And they have gains that run the gamut from low to high.

So what do you do? You put out a crazy high voltage level and then attenuate it. Its a preamp. Let’s say that again, it **IS** a preamp, right there at the output of your DAC. The only question is "is it a good one"? Some are, some are not. Volume control in the digital domain is fraught with issues; so that may be compromised. With R2R DACs its largely impossible except via DSP interpolation. Next, too often the output stage (line circuitry) is an op-amp chip. Unimpressed. To be fair, some are excellent, discrete, even tubes. but most are not.


So the idea that the preamp solves the impedance matching problem, if an output stage exists (even an op-amp) is not really valid. More likely the answer to why this "simpler" chain is not in fact superior is:

1. it’s as simple as you might have thought
2. The volume control is compromised (Nice word for "sucks")
3. The line circuitry is compromised
4. They really DID simplify the signal path, by leaving out the line circuitry and placing the volume control at the output and the impedance is now high (yep seen it)

OTOH i have used high quality digital volume (done in the DAC at 32-bits resolution, with all truncation below the 18/20 bit threshold) and a decent (not all that impressive output stage - essentially what one would have with a preamp too) and it soudns very, very good.

And then there’s the elephant on the table. Most tube components ADD euphonic distortion. We like it. this is not really a bad thing - its also what a Piano’s sounding board does. but in that case technical arguments go out he window,and we pivot to music theory.

And, I kid you not, i have a couple of Pianos to try out.

happy listening, and maybe, playing.
G

smandlej- I have the Townshend Allegri Reference (upgraded from the "standard" Allegri). This was well reviewed by Martin Colloms of Hi-Fi Critic and Raphael Todes of Allegri String Quartet fame, the latter uses the Bartok and found the Reference worked far better for volume control/sound quality. I believe both reviewers now own and use the Refernce in their own systems.
 This unit requires no expensive mains lead or support as it incorporates Max's podium style feet, but you could use one of Max's platforms as well.
From my perspective, this is the most important element of my system, and using it is like upgrading all my source components in one go !! It does require a substantial run-in period and benefits from balanced interconnects between pre and power. Martin uses solid state (Naim) whilst Rapheal uses valves - it works well with either - see millercarbon's post above. 

Good to know. It has been quite a few years since I really dug into this subject. Was really interested in improving my integrated amp which at the time one was using trim pots the other a stepped attenuator, neither one a true preamp just a volume control. The more I learned the more clear it became that transformers are the way to go.   

That was a long time ago so I forget all the technical electrical reasons. But the real reason they've been forgotten is my memory tends to low prioritize the moot point. Because ultimately there will always be someone willing to throw enough time and effort and money at it to solve that part of the problem.  

The one they can't solve is the seemingly simple volume control. For that they always fall back on a resistor of some sort. Trim pot, variable resistor, stepped attenuator- whatever you want to call it is just a resistor used to control volume. These things are inherently lossy and noisy and problematic.  

Transformers on the other hand, where resistors have problems transformers have benefits. There is a reason after all why we use them all over the place for isolation and power conditioning.  

But I gave up on transformers because a) hard (read, expensive) to find good ones and b) even harder to find one with all the taps needed to make a good volume control, and then c) you still need a lot of really high quality switches.  

But yeah, like you say, eliminates an expensive power cord, can be incredibly transparent, and passive, so slight gain in not adding any field noise to the system.  

I read the Colloms and Todes reviews, thanks. In the nearly 30 years since I looked into it there must have been a hundred thousand preamp questions and discussions. Yet I could count on my fingers the times transformers have been discussed. This kind of thing deserves a lot more mention and attention. Transformers, I mean.

"This kind of thing deserves a lot more mention and attention. Transformers, I mean."

https://www.youtube.com/watch?v=3ERuhks3GNk

Transformers on the other hand, where resistors have problems transformers have benefits. There is a reason after all why we use them all over the place for isolation and power conditioning.  

You solve one problem; you introduce others. Then it gets tricky.


The issue with transformers is loading and inter-winding capacitance. All transformers have inter-winding capacitance. If the transformer is not loaded correctly (usually too lightly) the capacitance may play a role causing the frequency response to be less than flat. This coloration can be easily heard.

Transformers are called that because they transform impedance. It goes both ways; you're not insulated on the input from the output. So if the output load is reduced, so is the impedance on the input side.

What happens when you have a transformer with multiple taps used as a volume control? Since you are working with a variable turns ratio, to keep the transformer linear you need to have the correct load at both the input and output. You can't, as a designer, expect to always see the same source impedance and you certainly don't see the same load impedance since all power amps are different in that regard. On top of that you have the moving target of variable turns ratio.


So you'll need a lot of switch contacts, not just the ones to change taps but another set with resistor to correct for the changing load impedance as the turns ratio is varied. 

This has to be done right, else the unit can introduce coloration.