Have Passive Preamps Finally Come of Age?

@tweak1 Agreed regarding impedance matching. My ATC active monitors have built-in amps having an input impedance of 10-kOhms. While this is typical for pro amp gear, it’s on the low side for consumer gear. And the adage of sticking with gear from the same manufacturer is wise advice with regard to ATC. Their preamps have output impedances set at 10-Ohms. (Typical pro preamps have output impedances of 600-Ohm or less, so I gather.)

All prior comments were when using my Oppo 105  either direct for SACD/DVD-As, or as a transport, after getting the Audio Alchemy DDP-1 DAC/PRE

A couple days ago I decided to dust off my Pioneer PD 65 CD player (inverted platter) Stage 3,modified over a decade ago by Musical Concepts; with outboard power supply: it is a CD player with optical output

After a brief warmup, the music is so much richer, adding texture to the entire musical spectrum

My Hattor does not provide for toslink,  so it was a good thing I bought the AA DDP-1, or I would have never known what I was missing with the Oppo

Hattor definitely going up for sale now


 

And Al, a source that gives out 2v is always going to give it out, even if you shunt half of it to ground because you don’t need it, because you have more gain to make it back up in the active preamp.
So the distortion from the source "whatever it is remains the same", it does not change because you’ve decided not to use half of it and shunted the other half to ground.
If anything shunting more of it to ground "could" make it distort more.

Not all volume controls shunt signal to ground. The traditional volume control does not shunt any at all. But there are shunt-style controls and they have been around for a while. They rely on a resistor (usually of significant value) to be always in series with the source (meaning that the resistor affects the sound if not carefully chosen), so that when the volume control is shunting the resulting output, the source will not distort or be bandwidth limited (IOW not significantly loaded).

Now if the shunting element is the same value as the series element, the signal loss will be 6db, as the the output voltage will be cut in half (at full volume). With most passive systems this sort of loss is unacceptable- so the shunt portion of the circuit (the actual control) is often much higher than the series element; as much as 10 times higher.
The question then is - what is the value of the composite control? For example George has stated quite often that his controls are "10K" (10,000 ohms). If its a shunt-style system as one might infer from the above quote (since many controls don’t shunt the signal at all), then is he stating that the series element is 10K? That would allow for the minimum value that the source (DAC or CDP) would be 10K. This would imply then that the *actual* control is considerably higher.

At any rate, the statement in the quote above is problematic. The designer of a passive control I would **assume** would not want to give an active preamp an edge over his product by loading the source in such a way as to increase its distortion! If this point is ignored by some passive designers, it may explain why good active line stages can easily outperform passive devices in some cases.

FWIW, no *active* preamp loads a source in the way George describes; the volume control (if at the input of the preamp, which is a good place to put it) is chosen to be a value such that no source of any type will be challenged by it!! Again, this is just common sense engineering (much like the built-in headroom that all competent preamps and sources have), and I hope that this is obvious to anyone reading these words.

Your right MrD, they can’t.
By reducing that 2v source with the volume control of an active pre, just shunts part of the 2v to ground. The other part being used (just as "distorted") goes to the preamps volume control then the gain stage, "yes" to be boosted back up again along with it’s own distortion and noise also.

Some preamps are designed to have very large s/n ratios and very large voltage swing range capacity. And then the designer of the circuit puts sort of passive volume pot implementation -in front of the high gain preamp.

This results in a far higher level of signal quality on the output, with regard to preserving micro differentials in the signal, which is where the music works for the human ear.

Some tube preamps are more likely to be designed this way.

Or, some preamps are designed so the volume pot is part of the gain of the circuit itself. I believe that Charles Hansen made a version of such topology in one of his SS preamp designs with Ayre. A switched resistor circuit where each step was optimized to provide best low distortion and highest fidelity. Not just individually gain stepped but individually tweaked on each of those steps. (IIRC, that is, source: interview with Charles at Stereophile)

The one you take note of, which is the most common way to do it for reasons of a ’safe’ output section, is also the lowest fidelity way to set up a preamp circuit topology. The variable loading does indeed change the transient distortion of the circuit, and it also makes the output sensitive to the cables and input it is hooked up to, in a way that is non linear as one scales through the volume settings.

eg, the one with the hardset gain with the input being varied via a potentiometer at the start, that one... is more linear and constant in it’s coloring of the sound signature, as one scales through volume level changes.

Most (IIRC) of the Bruce Moore tube circuits were set up this way. There is a danger aspect as a potential problem, though, as the full gain is always applied to the amplifier inputs, via the preamp circuit.

So, one can accept the cheaper and easier to implement most common design method (volume pot just before preamp output) and have mediocre warmed over sound quality ...or.... play in the danger zone and have a notably greater fidelity in ways that matter to the human ear. (volume pot at front of full gain output)

I made that trade off over 30 years ago, and the cost of problems that have happened, over the years, was less than the cost of one single item upgrade. Less safeties on the system (DC or full scale output issues, etc) and more care needs to be taken, but....the pay off was over 30 years long, on a key point in fidelity. And that was priceless. Priceless.

Essentially, it’s too bad more people don’t understand how fidelity in audio works. It’s not all that simple...

After trying every active topology known to humanity, I gave up at the max grade preamp level of a customized three chassis MFA ’Venusian’ preamp (capable of over 165Vp-p into a standard load!), and went to passive circuits, but only when done via the liquid metal design we have on offer. IMO and IME, active is too complex and too colored. All of them. 100% of them. Even simple buffer circuits, even though they generally exhibit the least harm...

In my personal experience a liquid metal passive preamp defeats all passive contenders and all active contenders at any price. At the $10k level, at the $50k level, no matter. Additionally, the consensus on that, within the group of people who have actually heard such an arrangement of gear with the given specific passive preamp... is rather high. With zero caveats. How? the liquid metal does not behave like wire, on the fundamental scale of what impedance in dynamic living terms --actually is. high correlation on it being best? let me clarify: Hundreds of people, and maybe three or five disagree.

But this as a pronouncement, depends upon the rest of the system, all the individual components, of source, cables, speakers, etc.....and most importantly...the wiring of the mind and ears of the listener....all being up to snuff, in being at the center of the maelstrom of what each is required to do with regard to projecting and exhibiting (and hearing/realizing) the maximum fidelity of signal.

it may explain why good active line stages can easily outperform passive devices in some cases.

"In some cases" Very few.
With 10k passives only if output to input impedance matching is below 1:10 ratio, which in most cases it’s not, unless the amp is 25k or lower.

Some preamps are designed to have very large s/n ratios and very large voltage swing range capacity. This results in a far higher level of signal quality on the output, with regard to preserving micro differentials in the signal

An active preamps gain stage after the volume control cannot give micro detail a "leg up" compared to the original, as the gain stage amplifies everything up, so it remains the same. But it does include it’s own noise and distortions on top of the original, which may give the illusion of detail.

Nelson Pass on passives pres:
"What could be better? Hardly any noise or distortion added by these simple passive parts. No feedback, no worrying about what type of capacitors – just musical perfection."

"Just musical perfection" Which means "true to the source" nothing added or taken away.

Cheers George