PREAMP technology shift? - Class A/ balanced/ dual-mono/ zero feedback vs. op-amp preamps


Hi all,

Even ten years ago, it seemed that the gold standard for solid-state pre's was some combination of the following: Class A/ balanced/ dual-mono/ zero feedback or local feedback. Recently, I'm seeing a range of well-reviewed preamps, from Schiit to Rowland, that use new, high-quality op-amps in their circuitry, to the extent that I wonder if op-amp technology, and chip manufacture in general, has gotten so good that some of the tried and true markers of quality (MOSFETs, Class A circuits) are not in the market except at the highest end, and even then, see also Jeff Rowland. Or is it that manufacturers are looking to less power hungry designs (I know that Rowland was concerned with meeting EU efficiency requirements in its latest designs) and so avoid Class A, for example? Does balanced/ dual mono seem less popular because separates are now less popular than integrateds? Have we reached a tipping point with op amps?

Here's a more focused question. I have invested in a 90s era Aragon 8008BB dual mono balanced amp, but i have an early 90s B&K PRO10-MC that I want to upgrade. Is a Schiit Freya a significant step up, or should I save my pennies for a used, more recent example of the old school: Classe CP500, Pass Labs Aleph or X2.5, or a good example of a newer approach, the Rowland Capri. Speakers are Vandersteen 2CE Sigs. Input impedance of the Aragon is 44 kOhms. 

The tube route is always possible, but I like the idea of a no-fuss, very clean ss, valuing sound stage and detail over warmth, which makes me think I'd be wasting my time with the Freya (any shoutouts for its JFET stage?)

Thanks for letting me wonder aloud!

Paul


paulburnett
This is a great discussion, and I am grateful for all of the advice and opinions. Concerning impedance matching: I know about the 20x rule of thumb for input impedance of the amp over the output impedance of the preamp. If the dividend is in the ballpark, does the impedance ratio really matter to the sound? What does a low or high ratio roughly within the "20 times ballpark" sound like? 

I shouldn't rule out tubes. I've never A/Bed good tubes vs. good solid state in my system, and I probably should. I just know I would be neurotic about swapping out tubes, worrying about when to do so. It's bad enough worrying about needles! ha ha

 
Actually that rule of thumb is 10:1. So if your output impedance is 500 ohms you should be able to drive 5000 ohms.

There is a complication with many tube preamps, which is the size of the coupling capacitor at the output. The issue is that to drive 10K ohms (which most of the time is the minimum you will see in a consumer amplifier) you need a pretty big coupling cap at the output of the preamp if you want it to play bass (which, if the cutoff is 20Hz, will be 10uf; but to avoid phase shift in the audio passband 100uf is recommended. You're not going to find very many preamps with a value that large- they usually split the difference in some way with 20uf being about as big as you're going to see). The bigger the cap, the more its going to color the sound so every manufacturer has a different hedge on this. For example, ARC noticed that many solid state amps have a higher input impedance and so limited their preamps to about 30K as a minimum in order to play bass right. Tube amps are generally 100K and some tube preamps have commensurately smaller coupling caps as a result, ignoring the solid state market altogether.

If there are tests available that plot the output impedance, you can see this by looking at the 20Hz output impedance as opposed to 1000Hz which is usually the frequency at which this spec is usually stated. The frequency plot will tell the story- often the 20Hz value will be multiples of the 1000Hz value. If its still 1/10th of the input impedance of the amp you're OK.

We avoided this by developing a direct-coupled output circuit for which we got a patent. As a result the output impedance vs frequency curve is a flat line- the same at 10 or 20Hz as it is at 1000Hz or 10KHz.
Just wanted to mention a few of you have given misinfo on the schiit freya, it doesn’t only have a Jfet solid state output stage, it actually has 3 seperate output stages you can change between. Fully passive, jfet buffered 0 gain, and tube gain output
Specifications may or may not (most often not) relate to great sound quality.  2 different preamplifiers that measure similarly will often sound very different. If you limit your search to "big name brands" you may be missing some opportunities.  To say nothing of the fact that as your original question regarding "design philosophies & circuit topology" render a cloudy picture. As a sidebar the audio industry has no standards on anything.  Even preamplifier output values.  IMO preamplifiers are the weakest link in the audio chain.  I have always been a proponent of full-featured preamplifiers and single ended vs balanced.  Unless you have no aspiration on spinning vinyl then you can simplify your search to line level devices.  Concentrate on in home auditions and listen. 
Good luck!